feat(alarms): HiLo trigger type with per-band level, hysteresis, messages, overrides

Adds a new HiLo alarm trigger type with four configurable setpoints
(LoLo / Lo / Hi / HiHi). Each setpoint carries an optional priority,
deadband (for hysteresis), and operator message. The site runtime emits
AlarmStateChanged with an AlarmLevel field so consumers can differentiate
warning vs critical bands.

Plumbing:
  - new AlarmLevel enum + AlarmStateChanged.Level/Message init properties
  - AlarmTriggerEditor (Blazor) gets a HiLo render with severity tinting
  - AlarmTriggerConfigCodec extracted from the editor for testability
  - sitestream.proto carries level + message over gRPC
  - SemanticValidator enforces numeric attribute, setpoint ordering,
    non-negative deadband
  - on-trigger scripts get an Alarm global (Name/Level/Priority/Message)
    so notification routing can branch by severity
  - per-instance InstanceAlarmOverride entity + EF migration + flattening
    step + CLI commands; HiLo overrides merge setpoint-by-setpoint, binary
    types whole-replace
  - DebugView shows a Level badge + per-band message tooltip
  - App.razor auto-reloads on permanent Blazor circuit failure
  - docker/regen-proto.sh automates the proto regen workflow (the linux/arm64
    protoc segfault means generated files are checked in for now)

docker/regen-proto.sh

@@ -0,0 +1,92 @@
+#!/usr/bin/env bash
+#
+# Regenerates the gRPC C# files from sitestream.proto.
+#
+# Background: protoc (linux/arm64) segfaults inside our Docker build container
+# (Grpc.Tools 2.71.0). As a workaround the generated Sitestream.cs +
+# SitestreamGrpc.cs are checked into src/ScadaLink.Communication/SiteStreamGrpc/
+# and the Protobuf ItemGroup in the .csproj is commented out — Docker just
+# compiles the checked-in C# files.
+#
+# Run this script ON YOUR DEV MACHINE whenever Protos/sitestream.proto changes:
+#
+#   1. Temporarily uncomments the Protobuf ItemGroup so Grpc.Tools runs.
+#   2. dotnet build (regen writes fresh files to obj/).
+#   3. Copies the regenerated files back into SiteStreamGrpc/.
+#   4. Re-comments the Protobuf ItemGroup so Docker builds stay safe.
+#
+# Once we move to a Dockerfile base image that ships a working linux/arm64
+# protoc, this script can be retired and Docker can regen the proto on every
+# build like every other normal .NET project.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+COMM_DIR="$REPO_ROOT/src/ScadaLink.Communication"
+CSPROJ="$COMM_DIR/ScadaLink.Communication.csproj"
+GEN_DIR="$COMM_DIR/SiteStreamGrpc"
+
+echo "=== Regenerating gRPC files from sitestream.proto ==="
+
+if [[ ! -f "$CSPROJ" ]]; then
+  echo "ERROR: csproj not found at $CSPROJ" >&2
+  exit 1
+fi
+
+# Backup so we can always restore the comment state on failure.
+BACKUP="$(mktemp)"
+cp "$CSPROJ" "$BACKUP"
+trap 'cp "$BACKUP" "$CSPROJ"; rm -f "$BACKUP"; echo "Restored csproj from backup."' ERR
+
+# 1. Uncomment the Protobuf ItemGroup (strip the surrounding <!-- ... --> wrapper).
+python3 - <<PY
+import re, pathlib
+p = pathlib.Path("$CSPROJ")
+src = p.read_text()
+# Find the commented Protobuf block and unwrap it.
+new = re.sub(
+    r"<!--\s*\n(\s*<ItemGroup>\s*\n\s*<Protobuf [^>]*/>\s*\n\s*</ItemGroup>)\s*\n\s*-->",
+    r"\1",
+    src,
+    count=1,
+)
+if new == src:
+    raise SystemExit("Couldn't find commented Protobuf ItemGroup to enable.")
+p.write_text(new)
+PY
+
+# 2. Delete the stale files so any failure to regen is obvious.
+rm -f "$GEN_DIR/Sitestream.cs" "$GEN_DIR/SitestreamGrpc.cs"
+
+# 3. Regenerate by building.
+echo "Building Communication project (regen)..."
+dotnet build "$CSPROJ" --nologo -v minimal | tail -5
+
+# 4. Copy generated files back into the source tree.
+mkdir -p "$GEN_DIR"
+cp "$COMM_DIR/obj/Debug/net10.0/Protos/Sitestream.cs" "$GEN_DIR/Sitestream.cs"
+cp "$COMM_DIR/obj/Debug/net10.0/Protos/SitestreamGrpc.cs" "$GEN_DIR/SitestreamGrpc.cs"
+echo "Copied regenerated files to $GEN_DIR/"
+
+# 5. Re-comment the Protobuf ItemGroup so Docker builds keep working.
+python3 - <<PY
+import re, pathlib
+p = pathlib.Path("$CSPROJ")
+src = p.read_text()
+new = re.sub(
+    r"(\s*<ItemGroup>\s*\n\s*<Protobuf [^>]*/>\s*\n\s*</ItemGroup>)",
+    r"\n  <!--\1\n  -->",
+    src,
+    count=1,
+)
+p.write_text(new)
+PY
+
+rm -f "$BACKUP"
+trap - ERR
+
+echo ""
+echo "Done. Review and commit:"
+echo "  git diff src/ScadaLink.Communication/Protos/sitestream.proto"
+echo "  git diff src/ScadaLink.Communication/SiteStreamGrpc/"

docs/requirements/Component-Commons.md

@@ -32,7 +32,8 @@ Commons must define shared primitive and utility types used across multiple comp
 - **`InstanceState` enum**: Enabled, Disabled.
 - **`DeploymentStatus` enum**: Pending, InProgress, Success, Failed.
 - **`AlarmState` enum**: Active, Normal.
-- **`AlarmTriggerType` enum**: ValueMatch, RangeViolation, RateOfChange.
+- **`AlarmLevel` enum**: None, Low, LowLow, High, HighHigh. Severity level for an active alarm; always `None` for binary trigger types, set by `HiLo` triggers.
+- **`AlarmTriggerType` enum**: ValueMatch, RangeViolation, RateOfChange, HiLo.
 - **`ConnectionHealth` enum**: Connected, Disconnected, Connecting, Error.
 
 Types defined here must be immutable and thread-safe.

docs/requirements/Component-SiteRuntime.md

@@ -176,19 +176,20 @@ When the Instance Actor is stopped (due to disable, delete, or redeployment), Ak
 ### Alarm Evaluation
 - Subscribes to attribute change notifications from its parent Instance Actor for the attribute(s) referenced by its trigger definition.
 - On each value update, evaluates the trigger condition:
-  - **Value Match**: Incoming value equals the predefined target.
+  - **Value Match**: Incoming value equals the predefined target. Supports `"!=X"` prefix for not-equals semantics.
   - **Range Violation**: Value is outside the allowed min/max range.
-  - **Rate of Change**: Value change rate exceeds the defined threshold over time.
+  - **Rate of Change**: Value change rate exceeds the defined threshold over a configurable time window. Direction filter (rising / falling / either) restricts which side of the rate triggers.
-- When the condition is met and the alarm is currently in **normal** state, the alarm transitions to **active**:
+  - **HiLo**: Multi-setpoint level alarm with up to four configurable setpoints (LoLo, Lo, Hi, HiHi). Any subset may be configured. Each setpoint may carry its own priority that overrides the alarm-level priority for that band.
+- For binary trigger types (ValueMatch / RangeViolation / RateOfChange), when the condition is met and the alarm is currently in **normal** state, the alarm transitions to **active**:
   - Updates the alarm state on the parent Instance Actor (which publishes to the Akka stream).
   - If an on-trigger script is defined, spawns an Alarm Execution Actor to execute it.
-- When the condition clears and the alarm is in **active** state, the alarm transitions to **normal**:
+- When the condition clears and the alarm is in **active** state, the alarm transitions to **normal**.
-  - Updates the alarm state on the parent Instance Actor.
+- For HiLo triggers, the actor tracks the current `AlarmLevel` (None / Low / LowLow / High / HighHigh). Each level transition emits a fresh `AlarmStateChanged` with the new level and its priority; level escalations (e.g., High → HighHigh) and de-escalations (HighHigh → High) both produce events. The on-trigger script fires only on the Normal → non-None edge, not on escalations between alarm bands.
-  - No script execution on clear.
+- No script execution on clear in any trigger type.
 
 ### Alarm State
-- Held **in memory** only — not persisted to SQLite.
+- Held **in memory** only — not persisted to SQLite. State comprises `AlarmState` (Active / Normal) and `AlarmLevel` (None for binary triggers; the active band for HiLo).
-- On restart (or failover), alarm states are re-evaluated from incoming values. All alarms start in normal state and transition to active when conditions are detected.
+- On restart (or failover), alarm states are re-evaluated from incoming values. All alarms start in normal state with level None and transition based on incoming values.
 
 ### Alarm Execution Actor
 - **Short-lived** child actor created when an on-trigger script needs to execute.

docs/requirements/HighLevelReqs.md

@@ -106,16 +106,18 @@ Each alarm has:
 - **Priority Level**: Numeric value from 0–1000.
 - **Lock Flag**: Controls whether the alarm can be overridden downstream.
 - **Trigger Definition**: One of the following trigger types:
-  - **Value Match**: Triggers when a monitored attribute equals a predefined value.
+  - **Value Match**: Triggers when a monitored attribute equals a predefined value. Supports a `!=X` prefix on the match value for not-equals semantics.
   - **Range Violation**: Triggers when a monitored attribute value falls outside an allowed range.
-  - **Rate of Change**: Triggers when a monitored attribute value changes faster than a defined threshold.
+  - **Rate of Change**: Triggers when a monitored attribute value changes faster than a defined threshold over a configurable time window. A direction filter (rising / falling / either) restricts which side of the rate triggers.
+  - **HiLo**: Multi-setpoint level alarm. Any subset of four setpoints (LoLo, Lo, Hi, HiHi) may be configured. The most severe matching band wins (LoLo/HiHi outrank Lo/Hi). Each setpoint may carry its own priority that overrides the alarm-level priority for that band.
 - **On-Trigger Script** *(optional)*: A script to execute when the alarm triggers. The alarm on-trigger script executes in the context of the instance and can call instance scripts, but instance scripts **cannot** call alarm on-trigger scripts. The call direction is one-way.
 
 ### 3.4.1 Alarm State
 - Alarm state (active/normal) is **managed at the site level** per instance, held **in memory** by the Alarm Actor.
+- Active alarms additionally carry an **alarm level**: `None` for binary trigger types (ValueMatch, RangeViolation, RateOfChange); one of `Low`, `LowLow`, `High`, `HighHigh` for HiLo triggers based on which setpoint the monitored attribute has crossed. Level transitions within an active HiLo alarm (e.g., High → HighHigh) emit fresh state-change events without re-running the on-trigger script — the script only fires on the Normal → non-None edge.
 - When the alarm condition clears, the alarm **automatically returns to normal state** — no acknowledgment workflow is required.
 - Alarm state is **not persisted** — on restart, alarm states are re-evaluated from incoming values.
-- Alarm state changes are published to the site-wide Akka stream as `[InstanceUniqueName].[AlarmName]`, alarm state (active/normal), priority, timestamp.
+- Alarm state changes are published to the site-wide Akka stream as `[InstanceUniqueName].[AlarmName]`, alarm state (active/normal), alarm level, priority, timestamp.
 
 ### 3.5 Template Relationships
 

src/ScadaLink.CLI/Commands/InstanceCommands.cs

@@ -15,6 +15,7 @@ public static class InstanceCommands
         command.Add(BuildCreate(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildSetBindings(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildSetOverrides(urlOption, formatOption, usernameOption, passwordOption));
+        command.Add(BuildAlarmOverride(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildSetArea(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildDiff(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildDeploy(urlOption, formatOption, usernameOption, passwordOption));
@@ -186,6 +187,59 @@ public static class InstanceCommands
         return cmd;
     }
 
+    private static Command BuildAlarmOverride(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
+    {
+        var group = new Command("alarm-override") { Description = "Manage per-instance alarm overrides" };
+
+        // set
+        var setIdOption = new Option<int>("--instance-id") { Description = "Instance ID", Required = true };
+        var setAlarmOption = new Option<string>("--alarm") { Description = "Alarm canonical name (e.g., 'TempLevels' or 'Pump.TempSensor.Heat')", Required = true };
+        var setConfigOption = new Option<string?>("--trigger-config") { Description = "JSON override for TriggerConfiguration (HiLo: partial merge; others: whole-replace)" };
+        var setPriorityOption = new Option<int?>("--priority") { Description = "Priority override (0-1000)" };
+        var setCmd = new Command("set") { Description = "Set (upsert) an alarm override on an instance" };
+        setCmd.Add(setIdOption); setCmd.Add(setAlarmOption); setCmd.Add(setConfigOption); setCmd.Add(setPriorityOption);
+        setCmd.SetAction(async (ParseResult result) =>
+        {
+            return await CommandHelpers.ExecuteCommandAsync(
+                result, urlOption, formatOption, usernameOption, passwordOption,
+                new SetInstanceAlarmOverrideCommand(
+                    result.GetValue(setIdOption),
+                    result.GetValue(setAlarmOption)!,
+                    result.GetValue(setConfigOption),
+                    result.GetValue(setPriorityOption)));
+        });
+        group.Add(setCmd);
+
+        // delete
+        var delIdOption = new Option<int>("--instance-id") { Description = "Instance ID", Required = true };
+        var delAlarmOption = new Option<string>("--alarm") { Description = "Alarm canonical name", Required = true };
+        var delCmd = new Command("delete") { Description = "Remove an alarm override on an instance" };
+        delCmd.Add(delIdOption); delCmd.Add(delAlarmOption);
+        delCmd.SetAction(async (ParseResult result) =>
+        {
+            return await CommandHelpers.ExecuteCommandAsync(
+                result, urlOption, formatOption, usernameOption, passwordOption,
+                new DeleteInstanceAlarmOverrideCommand(
+                    result.GetValue(delIdOption),
+                    result.GetValue(delAlarmOption)!));
+        });
+        group.Add(delCmd);
+
+        // list
+        var listIdOption = new Option<int>("--instance-id") { Description = "Instance ID", Required = true };
+        var listCmd = new Command("list") { Description = "List all alarm overrides for an instance" };
+        listCmd.Add(listIdOption);
+        listCmd.SetAction(async (ParseResult result) =>
+        {
+            return await CommandHelpers.ExecuteCommandAsync(
+                result, urlOption, formatOption, usernameOption, passwordOption,
+                new ListInstanceAlarmOverridesCommand(result.GetValue(listIdOption)));
+        });
+        group.Add(listCmd);
+
+        return group;
+    }
+
     private static Command BuildSetArea(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
     {
         var idOption = new Option<int>("--id") { Description = "Instance ID", Required = true };

src/ScadaLink.CentralUI/Components/Pages/Deployment/DebugView.razor

@@ -191,6 +191,7 @@
                                     <tr>
                                         <th>Alarm</th>
                                         <th>State</th>
+                                        <th>Level</th>
                                         <th>Priority</th>
                                         <th>Timestamp</th>
                                     </tr>
@@ -198,12 +199,30 @@
                                 <tbody aria-live="polite" aria-atomic="false">
                                     @foreach (var alarm in FilteredAlarmStates)
                                     {
-                                        <tr class="@GetAlarmRowClass(alarm.State)">
+                                        <tr class="@GetAlarmRowClass(alarm.State)"
-                                            <td class="small">@alarm.AlarmName</td>
+                                            title="@(string.IsNullOrEmpty(alarm.Message) ? null : alarm.Message)">
+                                            <td class="small">
+                                                @alarm.AlarmName
+                                                @if (!string.IsNullOrEmpty(alarm.Message))
+                                                {
+                                                    <span class="ms-1 text-info" aria-label="Has operator message">💬</span>
+                                                }
+                                            </td>
                                             <td>
                                                 <span class="badge @GetAlarmStateBadge(alarm.State)"
                                                       aria-label="@($"Alarm state: {alarm.State}")">@alarm.State</span>
                                             </td>
+                                            <td>
+                                                @if (alarm.Level != AlarmLevel.None)
+                                                {
+                                                    <span class="badge @GetAlarmLevelBadge(alarm.Level)"
+                                                          aria-label="@($"Alarm level: {alarm.Level}")">@FormatLevel(alarm.Level)</span>
+                                                }
+                                                else
+                                                {
+                                                    <span class="text-muted small">—</span>
+                                                }
+                                            </td>
                                             <td class="small">@alarm.Priority</td>
                                             <td class="small text-muted"
                                                 title="@alarm.Timestamp.LocalDateTime.ToString("HH:mm:ss.fff")">
@@ -468,6 +487,26 @@
         _ => ""
     };
 
+    /// <summary>
+    /// Severity-tinted badge class for HiLo alarm levels. The critical bands
+    /// (HighHigh / LowLow) get the danger class; warning bands get amber.
+    /// </summary>
+    private static string GetAlarmLevelBadge(AlarmLevel level) => level switch
+    {
+        AlarmLevel.HighHigh or AlarmLevel.LowLow => "bg-danger",
+        AlarmLevel.High or AlarmLevel.Low => "bg-warning text-dark",
+        _ => "bg-secondary"
+    };
+
+    private static string FormatLevel(AlarmLevel level) => level switch
+    {
+        AlarmLevel.HighHigh => "HiHi",
+        AlarmLevel.High => "Hi",
+        AlarmLevel.Low => "Lo",
+        AlarmLevel.LowLow => "LoLo",
+        _ => "—"
+    };
+
     public void Dispose()
     {
         if (_session != null)

src/ScadaLink.CentralUI/Components/Pages/Design/TemplateEdit.razor

@@ -754,7 +754,8 @@
                                     <AlarmTriggerEditor TriggerType="@_alarmTriggerType"
                                                         Value="@_alarmTriggerConfig"
                                                         ValueChanged="@(v => _alarmTriggerConfig = v)"
-                                                        AvailableAttributes="@BuildAlarmAttributeChoices()" />
+                                                        AvailableAttributes="@BuildAlarmAttributeChoices()"
+                                                        FallbackPriority="@_alarmPriority" />
                                 </div>
                                 <div class="col-12">
                                     <div class="form-check">

src/ScadaLink.CentralUI/Components/Shared/AlarmTriggerConfigCodec.cs

@@ -0,0 +1,244 @@
+using System.Globalization;
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.CentralUI.Components.Shared;
+
+/// <summary>
+/// Round-trip codec for the alarm trigger configuration JSON used by both
+/// <see cref="AlarmTriggerEditor"/> (UI editing) and AlarmActor (runtime
+/// evaluation). The serialized shape per trigger type:
+///   ValueMatch       { attributeName, matchValue }      ("!=X" prefix = not equals)
+///   RangeViolation   { attributeName, min, max }
+///   RateOfChange     { attributeName, thresholdPerSecond, windowSeconds, direction }
+///   HiLo             { attributeName, loLo, lo, hi, hiHi,
+///                      loLoPriority, loPriority, hiPriority, hiHiPriority }
+///
+/// All HiLo setpoints and per-setpoint priorities are optional — any subset
+/// is valid (e.g., only Hi/HiHi configured for over-temperature protection).
+///
+/// Parsing also accepts legacy aliases the runtime used to consume
+/// (<c>attribute</c>, <c>value</c>, <c>low</c>, <c>high</c>) so older configs
+/// survive a round-trip through the editor.
+/// </summary>
+internal static class AlarmTriggerConfigCodec
+{
+    /// <summary>
+    /// Parses a trigger configuration JSON in the context of the given trigger
+    /// type. Returns a model with default values on null/empty/malformed input
+    /// or for missing keys — never throws.
+    /// </summary>
+    internal static AlarmTriggerModel Parse(string? json, AlarmTriggerType type)
+    {
+        var model = new AlarmTriggerModel();
+        if (string.IsNullOrWhiteSpace(json)) return model;
+
+        try
+        {
+            using var doc = JsonDocument.Parse(json);
+            var root = doc.RootElement;
+
+            model.AttributeName =
+                root.TryGetProperty("attributeName", out var a) ? a.GetString()
+                : root.TryGetProperty("attribute", out var a2) ? a2.GetString()
+                : null;
+
+            switch (type)
+            {
+                case AlarmTriggerType.ValueMatch:
+                {
+                    var raw = root.TryGetProperty("matchValue", out var mv) ? mv.GetString()
+                            : root.TryGetProperty("value", out var mv2) ? mv2.GetString()
+                            : null;
+                    if (raw != null && raw.StartsWith("!=", StringComparison.Ordinal))
+                    {
+                        model.NotEquals = true;
+                        model.MatchValue = raw[2..];
+                    }
+                    else
+                    {
+                        model.MatchValue = raw;
+                    }
+                    break;
+                }
+                case AlarmTriggerType.RangeViolation:
+                    model.Min = TryReadDouble(root, "min") ?? TryReadDouble(root, "low");
+                    model.Max = TryReadDouble(root, "max") ?? TryReadDouble(root, "high");
+                    break;
+
+                case AlarmTriggerType.RateOfChange:
+                    model.ThresholdPerSecond = TryReadDouble(root, "thresholdPerSecond");
+                    model.WindowSeconds = TryReadDouble(root, "windowSeconds");
+                    var dir = root.TryGetProperty("direction", out var d) ? d.GetString() : null;
+                    model.Direction = NormalizeDirection(dir);
+                    break;
+
+                case AlarmTriggerType.HiLo:
+                    model.LoLo = TryReadDouble(root, "loLo");
+                    model.Lo = TryReadDouble(root, "lo");
+                    model.Hi = TryReadDouble(root, "hi");
+                    model.HiHi = TryReadDouble(root, "hiHi");
+                    model.LoLoPriority = TryReadInt(root, "loLoPriority");
+                    model.LoPriority = TryReadInt(root, "loPriority");
+                    model.HiPriority = TryReadInt(root, "hiPriority");
+                    model.HiHiPriority = TryReadInt(root, "hiHiPriority");
+                    model.LoLoDeadband = TryReadDouble(root, "loLoDeadband");
+                    model.LoDeadband = TryReadDouble(root, "loDeadband");
+                    model.HiDeadband = TryReadDouble(root, "hiDeadband");
+                    model.HiHiDeadband = TryReadDouble(root, "hiHiDeadband");
+                    model.LoLoMessage = TryReadString(root, "loLoMessage");
+                    model.LoMessage = TryReadString(root, "loMessage");
+                    model.HiMessage = TryReadString(root, "hiMessage");
+                    model.HiHiMessage = TryReadString(root, "hiHiMessage");
+                    break;
+            }
+        }
+        catch (JsonException)
+        {
+            // Malformed JSON — fall through with default model.
+        }
+
+        return model;
+    }
+
+    /// <summary>
+    /// Serializes the model to the JSON shape AlarmActor.ParseEvalConfig
+    /// expects. Always writes <c>attributeName</c> (canonical key) and only
+    /// the keys relevant to the current trigger type.
+    /// </summary>
+    internal static string Serialize(AlarmTriggerModel model, AlarmTriggerType type)
+    {
+        using var stream = new MemoryStream();
+        using (var w = new Utf8JsonWriter(stream))
+        {
+            w.WriteStartObject();
+            w.WriteString("attributeName", model.AttributeName ?? "");
+
+            switch (type)
+            {
+                case AlarmTriggerType.ValueMatch:
+                    var mv = model.MatchValue ?? "";
+                    if (model.NotEquals) mv = "!=" + mv;
+                    w.WriteString("matchValue", mv);
+                    break;
+
+                case AlarmTriggerType.RangeViolation:
+                    if (model.Min.HasValue) w.WriteNumber("min", model.Min.Value);
+                    if (model.Max.HasValue) w.WriteNumber("max", model.Max.Value);
+                    break;
+
+                case AlarmTriggerType.RateOfChange:
+                    if (model.ThresholdPerSecond.HasValue)
+                        w.WriteNumber("thresholdPerSecond", model.ThresholdPerSecond.Value);
+                    if (model.WindowSeconds.HasValue)
+                        w.WriteNumber("windowSeconds", model.WindowSeconds.Value);
+                    w.WriteString("direction", model.Direction);
+                    break;
+
+                case AlarmTriggerType.HiLo:
+                    if (model.LoLo.HasValue) w.WriteNumber("loLo", model.LoLo.Value);
+                    if (model.Lo.HasValue) w.WriteNumber("lo", model.Lo.Value);
+                    if (model.Hi.HasValue) w.WriteNumber("hi", model.Hi.Value);
+                    if (model.HiHi.HasValue) w.WriteNumber("hiHi", model.HiHi.Value);
+                    if (model.LoLoPriority.HasValue) w.WriteNumber("loLoPriority", model.LoLoPriority.Value);
+                    if (model.LoPriority.HasValue) w.WriteNumber("loPriority", model.LoPriority.Value);
+                    if (model.HiPriority.HasValue) w.WriteNumber("hiPriority", model.HiPriority.Value);
+                    if (model.HiHiPriority.HasValue) w.WriteNumber("hiHiPriority", model.HiHiPriority.Value);
+                    if (model.LoLoDeadband.HasValue) w.WriteNumber("loLoDeadband", model.LoLoDeadband.Value);
+                    if (model.LoDeadband.HasValue) w.WriteNumber("loDeadband", model.LoDeadband.Value);
+                    if (model.HiDeadband.HasValue) w.WriteNumber("hiDeadband", model.HiDeadband.Value);
+                    if (model.HiHiDeadband.HasValue) w.WriteNumber("hiHiDeadband", model.HiHiDeadband.Value);
+                    if (!string.IsNullOrEmpty(model.LoLoMessage)) w.WriteString("loLoMessage", model.LoLoMessage);
+                    if (!string.IsNullOrEmpty(model.LoMessage)) w.WriteString("loMessage", model.LoMessage);
+                    if (!string.IsNullOrEmpty(model.HiMessage)) w.WriteString("hiMessage", model.HiMessage);
+                    if (!string.IsNullOrEmpty(model.HiHiMessage)) w.WriteString("hiHiMessage", model.HiHiMessage);
+                    break;
+            }
+
+            w.WriteEndObject();
+        }
+        return Encoding.UTF8.GetString(stream.ToArray());
+    }
+
+    internal static string NormalizeDirection(string? raw) => raw?.ToLowerInvariant() switch
+    {
+        "rising" or "up" or "positive" => "rising",
+        "falling" or "down" or "negative" => "falling",
+        _ => "either"
+    };
+
+    private static double? TryReadDouble(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number => p.GetDouble(),
+            JsonValueKind.String when double.TryParse(p.GetString(), NumberStyles.Float, CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
+
+    private static int? TryReadInt(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number when p.TryGetInt32(out var i) => i,
+            JsonValueKind.Number => (int)p.GetDouble(),
+            JsonValueKind.String when int.TryParse(p.GetString(), NumberStyles.Integer, CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
+
+    private static string? TryReadString(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind == JsonValueKind.String ? p.GetString() : null;
+    }
+}
+
+internal sealed class AlarmTriggerModel
+{
+    public string? AttributeName { get; set; }
+
+    // ValueMatch
+    public string? MatchValue { get; set; }
+    public bool NotEquals { get; set; }
+
+    // RangeViolation
+    public double? Min { get; set; }
+    public double? Max { get; set; }
+
+    // RateOfChange
+    public double? ThresholdPerSecond { get; set; }
+    public double? WindowSeconds { get; set; }
+    public string Direction { get; set; } = "either";
+
+    // HiLo — any subset of setpoints may be set; per-setpoint priorities
+    // override the alarm-level priority for that band.
+    public double? LoLo { get; set; }
+    public double? Lo { get; set; }
+    public double? Hi { get; set; }
+    public double? HiHi { get; set; }
+    public int? LoLoPriority { get; set; }
+    public int? LoPriority { get; set; }
+    public int? HiPriority { get; set; }
+    public int? HiHiPriority { get; set; }
+
+    // Hysteresis: optional deactivation deadband per setpoint. Once at the
+    // band, the setpoint threshold is relaxed by this amount before the alarm
+    // de-escalates. Prevents flapping when the value hovers at the boundary.
+    public double? LoLoDeadband { get; set; }
+    public double? LoDeadband { get; set; }
+    public double? HiDeadband { get; set; }
+    public double? HiHiDeadband { get; set; }
+
+    // Per-band operator message. Optional; surfaces on AlarmStateChanged.Message
+    // and may be used by notification routing or operator displays.
+    public string? LoLoMessage { get; set; }
+    public string? LoMessage { get; set; }
+    public string? HiMessage { get; set; }
+    public string? HiHiMessage { get; set; }
+}

src/ScadaLink.CentralUI/Components/Shared/AlarmTriggerEditor.razor

@@ -1,8 +1,5 @@
 @namespace ScadaLink.CentralUI.Components.Shared
 @using System.Globalization
-@using System.IO
-@using System.Text
-@using System.Text.Json
 @using ScadaLink.Commons.Types.Enums
 
 @* Rich alarm trigger configuration editor. Replaces the raw JSON text field
@@ -83,6 +80,9 @@
         case AlarmTriggerType.RateOfChange:
             @RenderRateOfChange();
             break;
+        case AlarmTriggerType.HiLo:
+            @RenderHiLo();
+            break;
     }
 
     @* ── Hint ──────────────────────────────────────────────────────────── *@
@@ -108,7 +108,7 @@
 
     // ── Internal state ─────────────────────────────────────────────────────
 
-    private TriggerModel _model = new();
+    private AlarmTriggerModel _model = new AlarmTriggerModel();
     private AlarmTriggerType _lastSeenType;
     private string? _lastSeenJson;
 
@@ -133,7 +133,7 @@
         // the context of the new type. Missing/unparseable keys fall back to
         // empty defaults.
         var preservedAttr = _model.AttributeName;
-        _model = Parse(Value, TriggerType);
+        _model = AlarmTriggerConfigCodec.Parse(Value, TriggerType);
         if (jsonChanged == false && typeChanged && !string.IsNullOrEmpty(preservedAttr))
             _model.AttributeName = preservedAttr;
 
@@ -149,7 +149,7 @@
 
     private async Task Emit()
     {
-        var json = Serialize(_model, TriggerType);
+        var json = AlarmTriggerConfigCodec.Serialize(_model, TriggerType);
         _lastSeenJson = json;
         await ValueChanged.InvokeAsync(json);
     }
@@ -358,6 +358,150 @@
 
     private string _directionText = "either";
 
+    // ── HiLo ───────────────────────────────────────────────────────────────
+
+    private RenderFragment RenderHiLo() => __builder =>
+    {
+        <div class="small text-muted mb-2 fst-italic">
+            Set any subset of the four setpoints. The most-severe matching band
+            wins (LoLo/HiHi outrank Lo/Hi). Per-setpoint priority overrides the
+            alarm-level priority for that band. Deadband (optional) relaxes the
+            deactivation threshold by the configured amount to prevent flapping.
+        </div>
+
+        @HiLoSetpointRow("HIGH-HIGH (critical)",
+            _hiHiText, v => _hiHiText = v, OnHiHiChanged,
+            _hiHiDeadbandText, v => _hiHiDeadbandText = v, OnHiHiDeadbandChanged,
+            _hiHiPriorityText, v => _hiHiPriorityText = v, OnHiHiPriorityChanged,
+            _hiHiMessageText, v => _hiHiMessageText = v, OnHiHiMessageChanged,
+            "text-danger")
+
+        @HiLoSetpointRow("HIGH (warning)",
+            _hiText, v => _hiText = v, OnHiChanged,
+            _hiDeadbandText, v => _hiDeadbandText = v, OnHiDeadbandChanged,
+            _hiPriorityText, v => _hiPriorityText = v, OnHiPriorityChanged,
+            _hiMessageText, v => _hiMessageText = v, OnHiMessageChanged,
+            "text-warning-emphasis")
+
+        @HiLoSetpointRow("LOW (warning)",
+            _loText, v => _loText = v, OnLoChanged,
+            _loDeadbandText, v => _loDeadbandText = v, OnLoDeadbandChanged,
+            _loPriorityText, v => _loPriorityText = v, OnLoPriorityChanged,
+            _loMessageText, v => _loMessageText = v, OnLoMessageChanged,
+            "text-warning-emphasis")
+
+        @HiLoSetpointRow("LOW-LOW (critical)",
+            _loLoText, v => _loLoText = v, OnLoLoChanged,
+            _loLoDeadbandText, v => _loLoDeadbandText = v, OnLoLoDeadbandChanged,
+            _loLoPriorityText, v => _loLoPriorityText = v, OnLoLoPriorityChanged,
+            _loLoMessageText, v => _loLoMessageText = v, OnLoLoMessageChanged,
+            "text-danger")
+    };
+
+    /// <summary>
+    /// Renders one setpoint row: value (number) + priority (int). Both are
+    /// optional — leaving a value blank disables that band. The
+    /// <paramref name="severityClass"/> tints the label to convey relative
+    /// severity at a glance.
+    /// </summary>
+    private RenderFragment HiLoSetpointRow(
+        string label,
+        string? value, Action<string?> valueSetter, Func<Task> onValueChanged,
+        string? deadband, Action<string?> deadbandSetter, Func<Task> onDeadbandChanged,
+        string? priority, Action<string?> prioritySetter, Func<Task> onPriorityChanged,
+        string? message, Action<string?> messageSetter, Func<Task> onMessageChanged,
+        string severityClass) => __builder =>
+    {
+        <div class="row g-2 align-items-end mb-1">
+            <div class="col-md-5">
+                <label class="form-label small text-uppercase fw-semibold mb-1 @severityClass">
+                    @label
+                </label>
+                <div class="input-group input-group-sm">
+                    <span class="input-group-text">setpoint</span>
+                    <input type="number" step="any" class="form-control"
+                           placeholder="—"
+                           value="@value"
+                           @oninput="@(e => { valueSetter(e.Value?.ToString()); _ = onValueChanged(); })" />
+                </div>
+            </div>
+            <div class="col-md-3">
+                <label class="form-label small text-uppercase text-muted fw-semibold mb-1">
+                    Deadband
+                </label>
+                <div class="input-group input-group-sm">
+                    <span class="input-group-text">±</span>
+                    <input type="number" step="any" min="0" class="form-control"
+                           placeholder="0"
+                           value="@deadband"
+                           @oninput="@(e => { deadbandSetter(e.Value?.ToString()); _ = onDeadbandChanged(); })" />
+                </div>
+            </div>
+            <div class="col-md-4">
+                <label class="form-label small text-uppercase text-muted fw-semibold mb-1">
+                    Priority
+                </label>
+                <div class="input-group input-group-sm">
+                    <input type="number" min="0" max="1000" class="form-control"
+                           placeholder="@_priority"
+                           value="@priority"
+                           @oninput="@(e => { prioritySetter(e.Value?.ToString()); _ = onPriorityChanged(); })" />
+                </div>
+            </div>
+        </div>
+        <div class="row g-2 mb-3">
+            <div class="col-12">
+                <input type="text" class="form-control form-control-sm"
+                       placeholder="Optional operator message for this band…"
+                       value="@message"
+                       @oninput="@(e => { messageSetter(e.Value?.ToString()); _ = onMessageChanged(); })" />
+            </div>
+        </div>
+    };
+
+    // Setpoint text mirrors — separate from the model so blank fields stay
+    // blank (rather than appearing as 0) and we can detect "unset" cleanly.
+    private string? _loLoText;
+    private string? _loText;
+    private string? _hiText;
+    private string? _hiHiText;
+    private string? _loLoPriorityText;
+    private string? _loPriorityText;
+    private string? _hiPriorityText;
+    private string? _hiHiPriorityText;
+    private string? _loLoDeadbandText;
+    private string? _loDeadbandText;
+    private string? _hiDeadbandText;
+    private string? _hiHiDeadbandText;
+    private string? _loLoMessageText;
+    private string? _loMessageText;
+    private string? _hiMessageText;
+    private string? _hiHiMessageText;
+
+    // Mirrored on the parent so the placeholder shows the right fallback.
+    [Parameter] public int FallbackPriority { get; set; } = 500;
+    private int _priority => FallbackPriority;
+
+    private async Task OnLoLoChanged()        { _model.LoLo  = ParseDouble(_loLoText); await Emit(); }
+    private async Task OnLoChanged()          { _model.Lo    = ParseDouble(_loText);   await Emit(); }
+    private async Task OnHiChanged()          { _model.Hi    = ParseDouble(_hiText);   await Emit(); }
+    private async Task OnHiHiChanged()        { _model.HiHi  = ParseDouble(_hiHiText); await Emit(); }
+    private async Task OnLoLoPriorityChanged() { _model.LoLoPriority = ParseInt(_loLoPriorityText); await Emit(); }
+    private async Task OnLoPriorityChanged()   { _model.LoPriority   = ParseInt(_loPriorityText);   await Emit(); }
+    private async Task OnHiPriorityChanged()   { _model.HiPriority   = ParseInt(_hiPriorityText);   await Emit(); }
+    private async Task OnHiHiPriorityChanged() { _model.HiHiPriority = ParseInt(_hiHiPriorityText); await Emit(); }
+    private async Task OnLoLoDeadbandChanged() { _model.LoLoDeadband = ParseDouble(_loLoDeadbandText); await Emit(); }
+    private async Task OnLoDeadbandChanged()   { _model.LoDeadband   = ParseDouble(_loDeadbandText);   await Emit(); }
+    private async Task OnHiDeadbandChanged()   { _model.HiDeadband   = ParseDouble(_hiDeadbandText);   await Emit(); }
+    private async Task OnHiHiDeadbandChanged() { _model.HiHiDeadband = ParseDouble(_hiHiDeadbandText); await Emit(); }
+    private async Task OnLoLoMessageChanged()  { _model.LoLoMessage  = _loLoMessageText; await Emit(); }
+    private async Task OnLoMessageChanged()    { _model.LoMessage    = _loMessageText;   await Emit(); }
+    private async Task OnHiMessageChanged()    { _model.HiMessage    = _hiMessageText;   await Emit(); }
+    private async Task OnHiHiMessageChanged()  { _model.HiHiMessage  = _hiHiMessageText; await Emit(); }
+
+    private static int? ParseInt(string? s) =>
+        int.TryParse(s, NumberStyles.Integer, CultureInfo.InvariantCulture, out var v) ? v : null;
+
     // ── Text mirrors for typed inputs ──────────────────────────────────────
     // @bind requires a settable backing field that round-trips text. We keep
     // these in sync with the model and re-parse on @bind:after.
@@ -382,6 +526,22 @@
         _thresholdText = FormatNullable(_model.ThresholdPerSecond);
         _windowText = FormatNullable(_model.WindowSeconds);
         _directionText = _model.Direction;
+        _loLoText = FormatNullable(_model.LoLo);
+        _loText = FormatNullable(_model.Lo);
+        _hiText = FormatNullable(_model.Hi);
+        _hiHiText = FormatNullable(_model.HiHi);
+        _loLoPriorityText = _model.LoLoPriority?.ToString(CultureInfo.InvariantCulture);
+        _loPriorityText = _model.LoPriority?.ToString(CultureInfo.InvariantCulture);
+        _hiPriorityText = _model.HiPriority?.ToString(CultureInfo.InvariantCulture);
+        _hiHiPriorityText = _model.HiHiPriority?.ToString(CultureInfo.InvariantCulture);
+        _loLoDeadbandText = FormatNullable(_model.LoLoDeadband);
+        _loDeadbandText = FormatNullable(_model.LoDeadband);
+        _hiDeadbandText = FormatNullable(_model.HiDeadband);
+        _hiHiDeadbandText = FormatNullable(_model.HiHiDeadband);
+        _loLoMessageText = _model.LoLoMessage ?? string.Empty;
+        _loMessageText = _model.LoMessage ?? string.Empty;
+        _hiMessageText = _model.HiMessage ?? string.Empty;
+        _hiHiMessageText = _model.HiHiMessage ?? string.Empty;
     }
 
     private string _operatorText = "eq";
@@ -420,10 +580,25 @@
             AlarmTriggerType.RateOfChange =>
                 $"Triggers when {attr} changes faster than {Fmt(_model.ThresholdPerSecond) ?? "?"} units/sec ({_model.Direction}) over a {Fmt(_model.WindowSeconds) ?? "?"} sec window.",
 
+            AlarmTriggerType.HiLo => BuildHiLoHint(attr),
+
             _ => string.Empty
         };
     }
 
+    private string BuildHiLoHint(string attr)
+    {
+        var parts = new List<string>();
+        if (_model.LoLo.HasValue) parts.Add($"LoLo at {Fmt(_model.LoLo)}");
+        if (_model.Lo.HasValue)   parts.Add($"Lo at {Fmt(_model.Lo)}");
+        if (_model.Hi.HasValue)   parts.Add($"Hi at {Fmt(_model.Hi)}");
+        if (_model.HiHi.HasValue) parts.Add($"HiHi at {Fmt(_model.HiHi)}");
+
+        if (parts.Count == 0)
+            return $"Triggers when {attr} crosses any configured setpoint (none set yet).";
+        return $"Triggers on {attr}: {string.Join(", ", parts)}.";
+    }
+
     private static string Fmt(double? v) =>
         v.HasValue ? v.Value.ToString("0.###", CultureInfo.InvariantCulture) : "";
 
@@ -433,140 +608,4 @@
     private static double? ParseDouble(string? s) =>
         double.TryParse(s, NumberStyles.Float, CultureInfo.InvariantCulture, out var v) ? v : null;
 
-    // ── Model + parse/serialize ────────────────────────────────────────────
-
-    private sealed class TriggerModel
-    {
-        public string? AttributeName { get; set; }
-
-        // ValueMatch
-        public string? MatchValue { get; set; }
-        public bool NotEquals { get; set; }
-
-        // RangeViolation
-        public double? Min { get; set; }
-        public double? Max { get; set; }
-
-        // RateOfChange
-        public double? ThresholdPerSecond { get; set; }
-        public double? WindowSeconds { get; set; }
-        public string Direction { get; set; } = "either";
-    }
-
-    /// <summary>
-    /// Parses an existing trigger configuration JSON in the context of the
-    /// given trigger type. Returns sensible defaults on parse failure or for
-    /// missing keys.
-    /// </summary>
-    private static TriggerModel Parse(string? json, AlarmTriggerType type)
-    {
-        var model = new TriggerModel();
-        if (string.IsNullOrWhiteSpace(json)) return model;
-
-        try
-        {
-            using var doc = JsonDocument.Parse(json);
-            var root = doc.RootElement;
-
-            model.AttributeName =
-                root.TryGetProperty("attributeName", out var a) ? a.GetString()
-                : root.TryGetProperty("attribute", out var a2) ? a2.GetString()
-                : null;
-
-            switch (type)
-            {
-                case AlarmTriggerType.ValueMatch:
-                {
-                    var raw = root.TryGetProperty("matchValue", out var mv) ? mv.GetString()
-                            : root.TryGetProperty("value", out var mv2) ? mv2.GetString()
-                            : null;
-                    if (raw != null && raw.StartsWith("!=", StringComparison.Ordinal))
-                    {
-                        model.NotEquals = true;
-                        model.MatchValue = raw[2..];
-                    }
-                    else
-                    {
-                        model.MatchValue = raw;
-                    }
-                    break;
-                }
-                case AlarmTriggerType.RangeViolation:
-                    model.Min = TryReadDouble(root, "min") ?? TryReadDouble(root, "low");
-                    model.Max = TryReadDouble(root, "max") ?? TryReadDouble(root, "high");
-                    break;
-
-                case AlarmTriggerType.RateOfChange:
-                    model.ThresholdPerSecond = TryReadDouble(root, "thresholdPerSecond");
-                    model.WindowSeconds = TryReadDouble(root, "windowSeconds");
-                    var dir = root.TryGetProperty("direction", out var d) ? d.GetString() : null;
-                    model.Direction = NormalizeDirection(dir);
-                    break;
-            }
-        }
-        catch (JsonException)
-        {
-            // Malformed JSON — fall through with default model.
-        }
-
-        return model;
-    }
-
-    private static string NormalizeDirection(string? raw) => raw?.ToLowerInvariant() switch
-    {
-        "rising" or "up" or "positive" => "rising",
-        "falling" or "down" or "negative" => "falling",
-        _ => "either"
-    };
-
-    private static double? TryReadDouble(JsonElement el, string name)
-    {
-        if (!el.TryGetProperty(name, out var p)) return null;
-        return p.ValueKind switch
-        {
-            JsonValueKind.Number => p.GetDouble(),
-            JsonValueKind.String when double.TryParse(p.GetString(), NumberStyles.Float, CultureInfo.InvariantCulture, out var v) => v,
-            _ => null
-        };
-    }
-
-    /// <summary>
-    /// Serializes the model to the JSON shape AlarmActor.ParseEvalConfig
-    /// expects. Always writes <c>attributeName</c> (canonical key) and only
-    /// the keys relevant to the current trigger type.
-    /// </summary>
-    private static string Serialize(TriggerModel model, AlarmTriggerType type)
-    {
-        using var stream = new MemoryStream();
-        using (var w = new Utf8JsonWriter(stream))
-        {
-            w.WriteStartObject();
-            w.WriteString("attributeName", model.AttributeName ?? "");
-
-            switch (type)
-            {
-                case AlarmTriggerType.ValueMatch:
-                    var mv = model.MatchValue ?? "";
-                    if (model.NotEquals) mv = "!=" + mv;
-                    w.WriteString("matchValue", mv);
-                    break;
-
-                case AlarmTriggerType.RangeViolation:
-                    if (model.Min.HasValue) w.WriteNumber("min", model.Min.Value);
-                    if (model.Max.HasValue) w.WriteNumber("max", model.Max.Value);
-                    break;
-
-                case AlarmTriggerType.RateOfChange:
-                    if (model.ThresholdPerSecond.HasValue)
-                        w.WriteNumber("thresholdPerSecond", model.ThresholdPerSecond.Value);
-                    if (model.WindowSeconds.HasValue)
-                        w.WriteNumber("windowSeconds", model.WindowSeconds.Value);
-                    w.WriteString("direction", model.Direction);
-                    break;
-            }
-
-            w.WriteEndObject();
-        }
-        return System.Text.Encoding.UTF8.GetString(stream.ToArray());
-    }
 }

src/ScadaLink.Commons/Entities/Instances/Instance.cs

@@ -11,6 +11,7 @@ public class Instance
     public string UniqueName { get; set; }
     public InstanceState State { get; set; }
     public ICollection<InstanceAttributeOverride> AttributeOverrides { get; set; } = new List<InstanceAttributeOverride>();
+    public ICollection<InstanceAlarmOverride> AlarmOverrides { get; set; } = new List<InstanceAlarmOverride>();
     public ICollection<InstanceConnectionBinding> ConnectionBindings { get; set; } = new List<InstanceConnectionBinding>();
 
     public Instance(string uniqueName)

src/ScadaLink.Commons/Entities/Instances/InstanceAlarmOverride.cs

@@ -0,0 +1,48 @@
+namespace ScadaLink.Commons.Entities.Instances;
+
+/// <summary>
+/// Per-instance override for a template-defined alarm. Lets a deployed
+/// instance tweak setpoints, priority, or per-band messages without forking
+/// the template. Locked alarms (TemplateAlarm.IsLocked) cannot be overridden
+/// — LockEnforcer rejects the change at write time.
+///
+/// Merge semantics (applied during flattening, after template inheritance and
+/// composition):
+///   • <see cref="TriggerConfigurationOverride"/> with a HiLo trigger merges
+///     into the inherited JSON setpoint-by-setpoint (derived keys win,
+///     inherited keys survive for unset derived keys). Same logic as
+///     template-to-template HiLo override, just one layer deeper.
+///   • For ValueMatch / RangeViolation / RateOfChange, the override replaces
+///     the whole TriggerConfiguration JSON (existing whole-replace semantics).
+///   • <see cref="PriorityLevelOverride"/> replaces the alarm's PriorityLevel
+///     when set.
+/// </summary>
+public class InstanceAlarmOverride
+{
+    public int Id { get; set; }
+    public int InstanceId { get; set; }
+
+    /// <summary>
+    /// Canonical name of the alarm being overridden — matches
+    /// <c>ResolvedAlarm.CanonicalName</c> after flattening, so composed-member
+    /// alarms are referenced as <c>[CompositionInstance].[AlarmName]</c>.
+    /// </summary>
+    public string AlarmCanonicalName { get; set; }
+
+    /// <summary>
+    /// Partial JSON (for HiLo) or full JSON (for binary trigger types) to
+    /// override the inherited TriggerConfiguration. <c>null</c> means
+    /// "leave inherited as-is".
+    /// </summary>
+    public string? TriggerConfigurationOverride { get; set; }
+
+    /// <summary>
+    /// Replaces the alarm's PriorityLevel when set. <c>null</c> = keep inherited.
+    /// </summary>
+    public int? PriorityLevelOverride { get; set; }
+
+    public InstanceAlarmOverride(string alarmCanonicalName)
+    {
+        AlarmCanonicalName = alarmCanonicalName ?? throw new ArgumentNullException(nameof(alarmCanonicalName));
+    }
+}

src/ScadaLink.Commons/Interfaces/Repositories/ITemplateEngineRepository.cs

@@ -65,6 +65,13 @@ public interface ITemplateEngineRepository
     Task UpdateInstanceAttributeOverrideAsync(InstanceAttributeOverride attributeOverride, CancellationToken cancellationToken = default);
     Task DeleteInstanceAttributeOverrideAsync(int id, CancellationToken cancellationToken = default);
 
+    // InstanceAlarmOverride
+    Task<IReadOnlyList<InstanceAlarmOverride>> GetAlarmOverridesByInstanceIdAsync(int instanceId, CancellationToken cancellationToken = default);
+    Task<InstanceAlarmOverride?> GetAlarmOverrideAsync(int instanceId, string alarmCanonicalName, CancellationToken cancellationToken = default);
+    Task AddInstanceAlarmOverrideAsync(InstanceAlarmOverride alarmOverride, CancellationToken cancellationToken = default);
+    Task UpdateInstanceAlarmOverrideAsync(InstanceAlarmOverride alarmOverride, CancellationToken cancellationToken = default);
+    Task DeleteInstanceAlarmOverrideAsync(int id, CancellationToken cancellationToken = default);
+
     // InstanceConnectionBinding
     Task<IReadOnlyList<InstanceConnectionBinding>> GetBindingsByInstanceIdAsync(int instanceId, CancellationToken cancellationToken = default);
     Task AddInstanceConnectionBindingAsync(InstanceConnectionBinding binding, CancellationToken cancellationToken = default);

src/ScadaLink.Commons/Messages/Management/InstanceCommands.cs

@@ -10,3 +10,21 @@ public record MgmtDeleteInstanceCommand(int InstanceId);
 public record SetConnectionBindingsCommand(int InstanceId, IReadOnlyList<(string AttributeName, int DataConnectionId)> Bindings);
 public record SetInstanceOverridesCommand(int InstanceId, IReadOnlyDictionary<string, string?> Overrides);
 public record SetInstanceAreaCommand(int InstanceId, int? AreaId);
+
+/// <summary>
+/// Sets (or upserts) a per-instance alarm override. For HiLo trigger types the
+/// <c>TriggerConfigurationOverride</c> JSON is merged setpoint-by-setpoint with
+/// the inherited config; for binary trigger types it replaces the whole config.
+/// Either field is optional — pass null to leave it unchanged.
+/// </summary>
+public record SetInstanceAlarmOverrideCommand(
+    int InstanceId,
+    string AlarmCanonicalName,
+    string? TriggerConfigurationOverride,
+    int? PriorityLevelOverride);
+
+public record DeleteInstanceAlarmOverrideCommand(
+    int InstanceId,
+    string AlarmCanonicalName);
+
+public record ListInstanceAlarmOverridesCommand(int InstanceId);

src/ScadaLink.Commons/Messages/Streaming/AlarmStateChanged.cs

@@ -7,4 +7,23 @@ public record AlarmStateChanged(
     string AlarmName,
     AlarmState State,
     int Priority,
-    DateTimeOffset Timestamp);
+    DateTimeOffset Timestamp)
+{
+    /// <summary>
+    /// Severity level when <see cref="State"/> is <see cref="AlarmState.Active"/>.
+    /// Always <see cref="AlarmLevel.None"/> for binary trigger types
+    /// (ValueMatch, RangeViolation, RateOfChange); set by the HiLo trigger
+    /// type to one of Low/LowLow/High/HighHigh based on the crossed setpoint.
+    /// Added as an init-property so existing positional constructors still
+    /// work — message contract evolves additively.
+    /// </summary>
+    public AlarmLevel Level { get; init; } = AlarmLevel.None;
+
+    /// <summary>
+    /// Optional per-band operator message (e.g., "Coolant critically low —
+    /// shut down"). Set by HiLo triggers when the per-setpoint message is
+    /// configured; otherwise empty. Notification routing and UI tooltips may
+    /// surface this to operators.
+    /// </summary>
+    public string Message { get; init; } = string.Empty;
+}

src/ScadaLink.Commons/Types/Enums/AlarmLevel.cs

@@ -0,0 +1,19 @@
+namespace ScadaLink.Commons.Types.Enums;
+
+/// <summary>
+/// Severity level for an active alarm. Binary alarm types (ValueMatch,
+/// RangeViolation, RateOfChange) always emit <see cref="None"/>. The HiLo
+/// trigger type emits one of the directional levels based on which setpoint
+/// the monitored attribute has crossed.
+///
+/// Conventional ordering (lowest setpoint to highest):
+///   LowLow &lt; Low &lt; normal-band &lt; High &lt; HighHigh
+/// </summary>
+public enum AlarmLevel
+{
+    None,
+    Low,
+    LowLow,
+    High,
+    HighHigh
+}

src/ScadaLink.Commons/Types/Enums/AlarmTriggerType.cs

@@ -4,5 +4,12 @@ public enum AlarmTriggerType
 {
     ValueMatch,
     RangeViolation,
-    RateOfChange
+    RateOfChange,
+    /// <summary>
+    /// Multi-setpoint level alarm: monitors a single numeric attribute against
+    /// up to four configurable setpoints (LoLo, Lo, Hi, HiHi). Each setpoint
+    /// may carry its own priority; transitions between levels emit a fresh
+    /// AlarmStateChanged with the corresponding <see cref="AlarmLevel"/>.
+    /// </summary>
+    HiLo
 }

src/ScadaLink.Commons/Types/Scripts/AlarmContext.cs

@@ -0,0 +1,23 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Types.Scripts;
+
+/// <summary>
+/// Alarm context exposed to on-trigger scripts via <c>Alarm</c>. Lets scripts
+/// branch on the firing severity — e.g., page on-call for HiHi/LoLo but only
+/// email the day shift for Hi/Lo. Always present when an on-trigger script
+/// runs; <see cref="Level"/> is <see cref="AlarmLevel.None"/> for binary
+/// trigger types.
+/// </summary>
+public sealed class AlarmContext
+{
+    public string Name { get; init; } = string.Empty;
+    public AlarmLevel Level { get; init; } = AlarmLevel.None;
+    public int Priority { get; init; }
+
+    /// <summary>
+    /// Per-band operator message configured on the HiLo alarm, or empty for
+    /// binary trigger types and bands without a message.
+    /// </summary>
+    public string Message { get; init; } = string.Empty;
+}

src/ScadaLink.Communication/Actors/StreamRelayActor.cs

@@ -6,6 +6,7 @@ using ScadaLink.Commons.Messages.Streaming;
 using ScadaLink.Commons.Types;
 using ScadaLink.Communication.Grpc;
 using AlarmState = ScadaLink.Commons.Types.Enums.AlarmState;
+using AlarmLevel = ScadaLink.Commons.Types.Enums.AlarmLevel;
 
 namespace ScadaLink.Communication.Actors;
 
@@ -59,7 +60,9 @@ public class StreamRelayActor : ReceiveActor
                 AlarmName = msg.AlarmName,
                 State = MapAlarmState(msg.State),
                 Priority = msg.Priority,
-                Timestamp = Timestamp.FromDateTimeOffset(msg.Timestamp)
+                Timestamp = Timestamp.FromDateTimeOffset(msg.Timestamp),
+                Level = MapAlarmLevel(msg.Level),
+                Message = msg.Message ?? string.Empty
             }
         };
 
@@ -88,4 +91,13 @@ public class StreamRelayActor : ReceiveActor
         AlarmState.Active => AlarmStateEnum.AlarmStateActive,
         _ => AlarmStateEnum.AlarmStateUnspecified
     };
+
+    private static AlarmLevelEnum MapAlarmLevel(AlarmLevel level) => level switch
+    {
+        AlarmLevel.Low => AlarmLevelEnum.AlarmLevelLow,
+        AlarmLevel.LowLow => AlarmLevelEnum.AlarmLevelLowLow,
+        AlarmLevel.High => AlarmLevelEnum.AlarmLevelHigh,
+        AlarmLevel.HighHigh => AlarmLevelEnum.AlarmLevelHighHigh,
+        _ => AlarmLevelEnum.AlarmLevelNone
+    };
 }

src/ScadaLink.Communication/Grpc/SiteStreamGrpcClient.cs

@@ -137,7 +137,11 @@ public class SiteStreamGrpcClient : IAsyncDisposable
             evt.AlarmChanged.AlarmName,
             MapAlarmState(evt.AlarmChanged.State),
             evt.AlarmChanged.Priority,
-            evt.AlarmChanged.Timestamp.ToDateTimeOffset()),
+            evt.AlarmChanged.Timestamp.ToDateTimeOffset())
+        {
+            Level = MapAlarmLevel(evt.AlarmChanged.Level),
+            Message = evt.AlarmChanged.Message ?? string.Empty
+        },
         _ => null
     };
 
@@ -162,6 +166,18 @@ public class SiteStreamGrpcClient : IAsyncDisposable
         _ => AlarmState.Normal
     };
 
+    /// <summary>
+    /// Maps proto AlarmLevelEnum to domain AlarmLevel. Internal for testability.
+    /// </summary>
+    internal static AlarmLevel MapAlarmLevel(AlarmLevelEnum level) => level switch
+    {
+        AlarmLevelEnum.AlarmLevelLow => AlarmLevel.Low,
+        AlarmLevelEnum.AlarmLevelLowLow => AlarmLevel.LowLow,
+        AlarmLevelEnum.AlarmLevelHigh => AlarmLevel.High,
+        AlarmLevelEnum.AlarmLevelHighHigh => AlarmLevel.HighHigh,
+        _ => AlarmLevel.None
+    };
+
     public async ValueTask DisposeAsync()
     {
         foreach (var cts in _subscriptions.Values)

src/ScadaLink.Communication/Protos/sitestream.proto

@@ -34,6 +34,17 @@ enum AlarmStateEnum {
   ALARM_STATE_ACTIVE = 2;
 }
 
+// Severity level for an active alarm. Binary trigger types (ValueMatch,
+// RangeViolation, RateOfChange) always emit ALARM_LEVEL_NONE. The HiLo
+// trigger type emits one of the directional values.
+enum AlarmLevelEnum {
+  ALARM_LEVEL_NONE = 0;
+  ALARM_LEVEL_LOW = 1;
+  ALARM_LEVEL_LOW_LOW = 2;
+  ALARM_LEVEL_HIGH = 3;
+  ALARM_LEVEL_HIGH_HIGH = 4;
+}
+
 message AttributeValueUpdate {
   string instance_unique_name = 1;
   string attribute_path = 2;
@@ -49,4 +60,6 @@ message AlarmStateUpdate {
   AlarmStateEnum state = 3;
   int32 priority = 4;
   google.protobuf.Timestamp timestamp = 5;
+  AlarmLevelEnum level = 6;  // ALARM_LEVEL_NONE for binary trigger types; set by HiLo.
+  string message = 7;        // Optional per-band operator message; empty when unset.
 }

src/ScadaLink.Communication/ScadaLink.Communication.csproj

@@ -30,4 +30,21 @@
     <ProjectReference Include="../ScadaLink.HealthMonitoring/ScadaLink.HealthMonitoring.csproj" />
   </ItemGroup>
 
+  <!-- gRPC proto generation. The compiled C# is checked in under
+       SiteStreamGrpc/ (Sitestream.cs + SitestreamGrpc.cs) because protoc
+       segfaults inside our linux_arm64 Docker build image. To regenerate
+       after schema changes:
+         1. Temporarily uncomment the Protobuf ItemGroup below.
+         2. Delete SiteStreamGrpc/*.cs.
+         3. `dotnet build` (on macOS) — Grpc.Tools writes fresh files to obj/.
+         4. Copy obj/Debug/net10.0/Protos/*.cs into SiteStreamGrpc/.
+         5. Re-comment the ItemGroup.
+       Eventually we should switch the Docker build image to one with a
+       working protoc on arm64. -->
+  <!--
+    <ItemGroup>
+    <Protobuf Include="Protos\sitestream.proto" GrpcServices="Both" />
+  </ItemGroup>
+  -->
+
 </Project>

src/ScadaLink.Communication/SiteStreamGrpc/Sitestream.cs

@@ -1,6 +1,6 @@
 // <auto-generated>
 //     Generated by the protocol buffer compiler.  DO NOT EDIT!
-//     source: sitestream.proto
+//     source: Protos/sitestream.proto
 // </auto-generated>
 #pragma warning disable 1591, 0612, 3021, 8981
 #region Designer generated code
@@ -11,11 +11,11 @@ using pbr = global::Google.Protobuf.Reflection;
 using scg = global::System.Collections.Generic;
 namespace ScadaLink.Communication.Grpc {
 
-  /// <summary>Holder for reflection information generated from sitestream.proto</summary>
+  /// <summary>Holder for reflection information generated from Protos/sitestream.proto</summary>
   public static partial class SitestreamReflection {
 
     #region Descriptor
-    /// <summary>File descriptor for sitestream.proto</summary>
+    /// <summary>File descriptor for Protos/sitestream.proto</summary>
     public static pbr::FileDescriptor Descriptor {
       get { return descriptor; }
     }
@@ -24,36 +24,41 @@ namespace ScadaLink.Communication.Grpc {
     static SitestreamReflection() {
       byte[] descriptorData = global::System.Convert.FromBase64String(
           string.Concat(
-            "ChBzaXRlc3RyZWFtLnByb3RvEgpzaXRlc3RyZWFtGh9nb29nbGUvcHJvdG9i",
+            "ChdQcm90b3Mvc2l0ZXN0cmVhbS5wcm90bxIKc2l0ZXN0cmVhbRofZ29vZ2xl",
-            "dWYvdGltZXN0YW1wLnByb3RvIk0KFUluc3RhbmNlU3RyZWFtUmVxdWVzdBIW",
+            "L3Byb3RvYnVmL3RpbWVzdGFtcC5wcm90byJNChVJbnN0YW5jZVN0cmVhbVJl",
-            "Cg5jb3JyZWxhdGlvbl9pZBgBIAEoCRIcChRpbnN0YW5jZV91bmlxdWVfbmFt",
+            "cXVlc3QSFgoOY29ycmVsYXRpb25faWQYASABKAkSHAoUaW5zdGFuY2VfdW5p",
-            "ZRgCIAEoCSKoAQoPU2l0ZVN0cmVhbUV2ZW50EhYKDmNvcnJlbGF0aW9uX2lk",
+            "cXVlX25hbWUYAiABKAkiqAEKD1NpdGVTdHJlYW1FdmVudBIWCg5jb3JyZWxh",
-            "GAEgASgJEj0KEWF0dHJpYnV0ZV9jaGFuZ2VkGAIgASgLMiAuc2l0ZXN0cmVh",
+            "dGlvbl9pZBgBIAEoCRI9ChFhdHRyaWJ1dGVfY2hhbmdlZBgCIAEoCzIgLnNp",
-            "bS5BdHRyaWJ1dGVWYWx1ZVVwZGF0ZUgAEjUKDWFsYXJtX2NoYW5nZWQYAyAB",
+            "dGVzdHJlYW0uQXR0cmlidXRlVmFsdWVVcGRhdGVIABI1Cg1hbGFybV9jaGFu",
-            "KAsyHC5zaXRlc3RyZWFtLkFsYXJtU3RhdGVVcGRhdGVIAEIHCgVldmVudCLI",
+            "Z2VkGAMgASgLMhwuc2l0ZXN0cmVhbS5BbGFybVN0YXRlVXBkYXRlSABCBwoF",
-            "AQoUQXR0cmlidXRlVmFsdWVVcGRhdGUSHAoUaW5zdGFuY2VfdW5pcXVlX25h",
+            "ZXZlbnQiyAEKFEF0dHJpYnV0ZVZhbHVlVXBkYXRlEhwKFGluc3RhbmNlX3Vu",
-            "bWUYASABKAkSFgoOYXR0cmlidXRlX3BhdGgYAiABKAkSFgoOYXR0cmlidXRl",
+            "aXF1ZV9uYW1lGAEgASgJEhYKDmF0dHJpYnV0ZV9wYXRoGAIgASgJEhYKDmF0",
-            "X25hbWUYAyABKAkSDQoFdmFsdWUYBCABKAkSJAoHcXVhbGl0eRgFIAEoDjIT",
+            "dHJpYnV0ZV9uYW1lGAMgASgJEg0KBXZhbHVlGAQgASgJEiQKB3F1YWxpdHkY",
-            "LnNpdGVzdHJlYW0uUXVhbGl0eRItCgl0aW1lc3RhbXAYBiABKAsyGi5nb29n",
+            "BSABKA4yEy5zaXRlc3RyZWFtLlF1YWxpdHkSLQoJdGltZXN0YW1wGAYgASgL",
-            "bGUucHJvdG9idWYuVGltZXN0YW1wIrABChBBbGFybVN0YXRlVXBkYXRlEhwK",
+            "MhouZ29vZ2xlLnByb3RvYnVmLlRpbWVzdGFtcCLsAQoQQWxhcm1TdGF0ZVVw",
-            "FGluc3RhbmNlX3VuaXF1ZV9uYW1lGAEgASgJEhIKCmFsYXJtX25hbWUYAiAB",
+            "ZGF0ZRIcChRpbnN0YW5jZV91bmlxdWVfbmFtZRgBIAEoCRISCgphbGFybV9u",
-            "KAkSKQoFc3RhdGUYAyABKA4yGi5zaXRlc3RyZWFtLkFsYXJtU3RhdGVFbnVt",
+            "YW1lGAIgASgJEikKBXN0YXRlGAMgASgOMhouc2l0ZXN0cmVhbS5BbGFybVN0",
-            "EhAKCHByaW9yaXR5GAQgASgFEi0KCXRpbWVzdGFtcBgFIAEoCzIaLmdvb2ds",
+            "YXRlRW51bRIQCghwcmlvcml0eRgEIAEoBRItCgl0aW1lc3RhbXAYBSABKAsy",
-            "ZS5wcm90b2J1Zi5UaW1lc3RhbXAqXAoHUXVhbGl0eRIXChNRVUFMSVRZX1VO",
+            "Gi5nb29nbGUucHJvdG9idWYuVGltZXN0YW1wEikKBWxldmVsGAYgASgOMhou",
-            "U1BFQ0lGSUVEEAASEAoMUVVBTElUWV9HT09EEAESFQoRUVVBTElUWV9VTkNF",
+            "c2l0ZXN0cmVhbS5BbGFybUxldmVsRW51bRIPCgdtZXNzYWdlGAcgASgJKlwK",
-            "UlRBSU4QAhIPCgtRVUFMSVRZX0JBRBADKl0KDkFsYXJtU3RhdGVFbnVtEhsK",
+            "B1F1YWxpdHkSFwoTUVVBTElUWV9VTlNQRUNJRklFRBAAEhAKDFFVQUxJVFlf",
-            "F0FMQVJNX1NUQVRFX1VOU1BFQ0lGSUVEEAASFgoSQUxBUk1fU1RBVEVfTk9S",
+            "R09PRBABEhUKEVFVQUxJVFlfVU5DRVJUQUlOEAISDwoLUVVBTElUWV9CQUQQ",
-            "TUFMEAESFgoSQUxBUk1fU1RBVEVfQUNUSVZFEAIyagoRU2l0ZVN0cmVhbVNl",
+            "AypdCg5BbGFybVN0YXRlRW51bRIbChdBTEFSTV9TVEFURV9VTlNQRUNJRklF",
-            "cnZpY2USVQoRU3Vic2NyaWJlSW5zdGFuY2USIS5zaXRlc3RyZWFtLkluc3Rh",
+            "RBAAEhYKEkFMQVJNX1NUQVRFX05PUk1BTBABEhYKEkFMQVJNX1NUQVRFX0FD",
-            "bmNlU3RyZWFtUmVxdWVzdBobLnNpdGVzdHJlYW0uU2l0ZVN0cmVhbUV2ZW50",
+            "VElWRRACKoUBCg5BbGFybUxldmVsRW51bRIUChBBTEFSTV9MRVZFTF9OT05F",
-            "MAFCH6oCHFNjYWRhTGluay5Db21tdW5pY2F0aW9uLkdycGNiBnByb3RvMw=="));
+            "EAASEwoPQUxBUk1fTEVWRUxfTE9XEAESFwoTQUxBUk1fTEVWRUxfTE9XX0xP",
+            "VxACEhQKEEFMQVJNX0xFVkVMX0hJR0gQAxIZChVBTEFSTV9MRVZFTF9ISUdI",
+            "X0hJR0gQBDJqChFTaXRlU3RyZWFtU2VydmljZRJVChFTdWJzY3JpYmVJbnN0",
+            "YW5jZRIhLnNpdGVzdHJlYW0uSW5zdGFuY2VTdHJlYW1SZXF1ZXN0Ghsuc2l0",
+            "ZXN0cmVhbS5TaXRlU3RyZWFtRXZlbnQwAUIfqgIcU2NhZGFMaW5rLkNvbW11",
+            "bmljYXRpb24uR3JwY2IGcHJvdG8z"));
       descriptor = pbr::FileDescriptor.FromGeneratedCode(descriptorData,
           new pbr::FileDescriptor[] { global::Google.Protobuf.WellKnownTypes.TimestampReflection.Descriptor, },
-          new pbr::GeneratedClrTypeInfo(new[] {typeof(global::ScadaLink.Communication.Grpc.Quality), typeof(global::ScadaLink.Communication.Grpc.AlarmStateEnum), }, null, new pbr::GeneratedClrTypeInfo[] {
+          new pbr::GeneratedClrTypeInfo(new[] {typeof(global::ScadaLink.Communication.Grpc.Quality), typeof(global::ScadaLink.Communication.Grpc.AlarmStateEnum), typeof(global::ScadaLink.Communication.Grpc.AlarmLevelEnum), }, null, new pbr::GeneratedClrTypeInfo[] {
             new pbr::GeneratedClrTypeInfo(typeof(global::ScadaLink.Communication.Grpc.InstanceStreamRequest), global::ScadaLink.Communication.Grpc.InstanceStreamRequest.Parser, new[]{ "CorrelationId", "InstanceUniqueName" }, null, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::ScadaLink.Communication.Grpc.SiteStreamEvent), global::ScadaLink.Communication.Grpc.SiteStreamEvent.Parser, new[]{ "CorrelationId", "AttributeChanged", "AlarmChanged" }, new[]{ "Event" }, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::ScadaLink.Communication.Grpc.AttributeValueUpdate), global::ScadaLink.Communication.Grpc.AttributeValueUpdate.Parser, new[]{ "InstanceUniqueName", "AttributePath", "AttributeName", "Value", "Quality", "Timestamp" }, null, null, null, null),
-            new pbr::GeneratedClrTypeInfo(typeof(global::ScadaLink.Communication.Grpc.AlarmStateUpdate), global::ScadaLink.Communication.Grpc.AlarmStateUpdate.Parser, new[]{ "InstanceUniqueName", "AlarmName", "State", "Priority", "Timestamp" }, null, null, null, null)
+            new pbr::GeneratedClrTypeInfo(typeof(global::ScadaLink.Communication.Grpc.AlarmStateUpdate), global::ScadaLink.Communication.Grpc.AlarmStateUpdate.Parser, new[]{ "InstanceUniqueName", "AlarmName", "State", "Priority", "Timestamp", "Level", "Message" }, null, null, null, null)
           }));
     }
     #endregion
@@ -73,6 +78,19 @@ namespace ScadaLink.Communication.Grpc {
     [pbr::OriginalName("ALARM_STATE_ACTIVE")] AlarmStateActive = 2,
   }
 
+  /// <summary>
+  /// Severity level for an active alarm. Binary trigger types (ValueMatch,
+  /// RangeViolation, RateOfChange) always emit ALARM_LEVEL_NONE. The HiLo
+  /// trigger type emits one of the directional values.
+  /// </summary>
+  public enum AlarmLevelEnum {
+    [pbr::OriginalName("ALARM_LEVEL_NONE")] AlarmLevelNone = 0,
+    [pbr::OriginalName("ALARM_LEVEL_LOW")] AlarmLevelLow = 1,
+    [pbr::OriginalName("ALARM_LEVEL_LOW_LOW")] AlarmLevelLowLow = 2,
+    [pbr::OriginalName("ALARM_LEVEL_HIGH")] AlarmLevelHigh = 3,
+    [pbr::OriginalName("ALARM_LEVEL_HIGH_HIGH")] AlarmLevelHighHigh = 4,
+  }
+
   #endregion
 
   #region Messages
@@ -1074,6 +1092,8 @@ namespace ScadaLink.Communication.Grpc {
       state_ = other.state_;
       priority_ = other.priority_;
       timestamp_ = other.timestamp_ != null ? other.timestamp_.Clone() : null;
+      level_ = other.level_;
+      message_ = other.message_;
       _unknownFields = pb::UnknownFieldSet.Clone(other._unknownFields);
     }
 
@@ -1143,6 +1163,36 @@ namespace ScadaLink.Communication.Grpc {
       }
     }
 
+    /// <summary>Field number for the "level" field.</summary>
+    public const int LevelFieldNumber = 6;
+    private global::ScadaLink.Communication.Grpc.AlarmLevelEnum level_ = global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone;
+    /// <summary>
+    /// ALARM_LEVEL_NONE for binary trigger types; set by HiLo.
+    /// </summary>
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
+    public global::ScadaLink.Communication.Grpc.AlarmLevelEnum Level {
+      get { return level_; }
+      set {
+        level_ = value;
+      }
+    }
+
+    /// <summary>Field number for the "message" field.</summary>
+    public const int MessageFieldNumber = 7;
+    private string message_ = "";
+    /// <summary>
+    /// Optional per-band operator message; empty when unset.
+    /// </summary>
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
+    public string Message {
+      get { return message_; }
+      set {
+        message_ = pb::ProtoPreconditions.CheckNotNull(value, "value");
+      }
+    }
+
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
     public override bool Equals(object other) {
@@ -1163,6 +1213,8 @@ namespace ScadaLink.Communication.Grpc {
       if (State != other.State) return false;
       if (Priority != other.Priority) return false;
       if (!object.Equals(Timestamp, other.Timestamp)) return false;
+      if (Level != other.Level) return false;
+      if (Message != other.Message) return false;
       return Equals(_unknownFields, other._unknownFields);
     }
 
@@ -1175,6 +1227,8 @@ namespace ScadaLink.Communication.Grpc {
       if (State != global::ScadaLink.Communication.Grpc.AlarmStateEnum.AlarmStateUnspecified) hash ^= State.GetHashCode();
       if (Priority != 0) hash ^= Priority.GetHashCode();
       if (timestamp_ != null) hash ^= Timestamp.GetHashCode();
+      if (Level != global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone) hash ^= Level.GetHashCode();
+      if (Message.Length != 0) hash ^= Message.GetHashCode();
       if (_unknownFields != null) {
         hash ^= _unknownFields.GetHashCode();
       }
@@ -1213,6 +1267,14 @@ namespace ScadaLink.Communication.Grpc {
         output.WriteRawTag(42);
         output.WriteMessage(Timestamp);
       }
+      if (Level != global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone) {
+        output.WriteRawTag(48);
+        output.WriteEnum((int) Level);
+      }
+      if (Message.Length != 0) {
+        output.WriteRawTag(58);
+        output.WriteString(Message);
+      }
       if (_unknownFields != null) {
         _unknownFields.WriteTo(output);
       }
@@ -1243,6 +1305,14 @@ namespace ScadaLink.Communication.Grpc {
         output.WriteRawTag(42);
         output.WriteMessage(Timestamp);
       }
+      if (Level != global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone) {
+        output.WriteRawTag(48);
+        output.WriteEnum((int) Level);
+      }
+      if (Message.Length != 0) {
+        output.WriteRawTag(58);
+        output.WriteString(Message);
+      }
       if (_unknownFields != null) {
         _unknownFields.WriteTo(ref output);
       }
@@ -1268,6 +1338,12 @@ namespace ScadaLink.Communication.Grpc {
       if (timestamp_ != null) {
         size += 1 + pb::CodedOutputStream.ComputeMessageSize(Timestamp);
       }
+      if (Level != global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone) {
+        size += 1 + pb::CodedOutputStream.ComputeEnumSize((int) Level);
+      }
+      if (Message.Length != 0) {
+        size += 1 + pb::CodedOutputStream.ComputeStringSize(Message);
+      }
       if (_unknownFields != null) {
         size += _unknownFields.CalculateSize();
       }
@@ -1298,6 +1374,12 @@ namespace ScadaLink.Communication.Grpc {
         }
         Timestamp.MergeFrom(other.Timestamp);
       }
+      if (other.Level != global::ScadaLink.Communication.Grpc.AlarmLevelEnum.AlarmLevelNone) {
+        Level = other.Level;
+      }
+      if (other.Message.Length != 0) {
+        Message = other.Message;
+      }
       _unknownFields = pb::UnknownFieldSet.MergeFrom(_unknownFields, other._unknownFields);
     }
 
@@ -1340,6 +1422,14 @@ namespace ScadaLink.Communication.Grpc {
             input.ReadMessage(Timestamp);
             break;
           }
+          case 48: {
+            Level = (global::ScadaLink.Communication.Grpc.AlarmLevelEnum) input.ReadEnum();
+            break;
+          }
+          case 58: {
+            Message = input.ReadString();
+            break;
+          }
         }
       }
     #endif
@@ -1382,6 +1472,14 @@ namespace ScadaLink.Communication.Grpc {
             input.ReadMessage(Timestamp);
             break;
           }
+          case 48: {
+            Level = (global::ScadaLink.Communication.Grpc.AlarmLevelEnum) input.ReadEnum();
+            break;
+          }
+          case 58: {
+            Message = input.ReadString();
+            break;
+          }
         }
       }
     }

src/ScadaLink.Communication/SiteStreamGrpc/SitestreamGrpc.cs

@@ -1,6 +1,6 @@
 // <auto-generated>
 //     Generated by the protocol buffer compiler.  DO NOT EDIT!
-//     source: sitestream.proto
+//     source: Protos/sitestream.proto
 // </auto-generated>
 #pragma warning disable 0414, 1591, 8981, 0612
 #region Designer generated code

src/ScadaLink.ConfigurationDatabase/Configurations/InstanceConfiguration.cs

@@ -41,6 +41,11 @@ public class InstanceConfiguration : IEntityTypeConfiguration<Instance>
             .HasForeignKey(o => o.InstanceId)
             .OnDelete(DeleteBehavior.Cascade);
 
+        builder.HasMany(i => i.AlarmOverrides)
+            .WithOne()
+            .HasForeignKey(o => o.InstanceId)
+            .OnDelete(DeleteBehavior.Cascade);
+
         builder.HasMany(i => i.ConnectionBindings)
             .WithOne()
             .HasForeignKey(b => b.InstanceId)
@@ -67,6 +72,23 @@ public class InstanceAttributeOverrideConfiguration : IEntityTypeConfiguration<I
     }
 }
 
+public class InstanceAlarmOverrideConfiguration : IEntityTypeConfiguration<InstanceAlarmOverride>
+{
+    public void Configure(EntityTypeBuilder<InstanceAlarmOverride> builder)
+    {
+        builder.HasKey(o => o.Id);
+
+        builder.Property(o => o.AlarmCanonicalName)
+            .IsRequired()
+            .HasMaxLength(400); // Larger than attribute names to fit composed paths.
+
+        builder.Property(o => o.TriggerConfigurationOverride)
+            .HasMaxLength(4000);
+
+        builder.HasIndex(o => new { o.InstanceId, o.AlarmCanonicalName }).IsUnique();
+    }
+}
+
 public class InstanceConnectionBindingConfiguration : IEntityTypeConfiguration<InstanceConnectionBinding>
 {
     public void Configure(EntityTypeBuilder<InstanceConnectionBinding> builder)

src/ScadaLink.ConfigurationDatabase/Migrations/20260513055537_AddInstanceAlarmOverrides.cs

@@ -0,0 +1,49 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddInstanceAlarmOverrides : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "InstanceAlarmOverrides",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    InstanceId = table.Column<int>(type: "int", nullable: false),
+                    AlarmCanonicalName = table.Column<string>(type: "nvarchar(400)", maxLength: 400, nullable: false),
+                    TriggerConfigurationOverride = table.Column<string>(type: "nvarchar(4000)", maxLength: 4000, nullable: true),
+                    PriorityLevelOverride = table.Column<int>(type: "int", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_InstanceAlarmOverrides", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_InstanceAlarmOverrides_Instances_InstanceId",
+                        column: x => x.InstanceId,
+                        principalTable: "Instances",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_InstanceAlarmOverrides_InstanceId_AlarmCanonicalName",
+                table: "InstanceAlarmOverrides",
+                columns: new[] { "InstanceId", "AlarmCanonicalName" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "InstanceAlarmOverrides");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -478,6 +478,37 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                     b.ToTable("Instances");
                 });
 
+            modelBuilder.Entity("ScadaLink.Commons.Entities.Instances.InstanceAlarmOverride", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("AlarmCanonicalName")
+                        .IsRequired()
+                        .HasMaxLength(400)
+                        .HasColumnType("nvarchar(400)");
+
+                    b.Property<int>("InstanceId")
+                        .HasColumnType("int");
+
+                    b.Property<int?>("PriorityLevelOverride")
+                        .HasColumnType("int");
+
+                    b.Property<string>("TriggerConfigurationOverride")
+                        .HasMaxLength(4000)
+                        .HasColumnType("nvarchar(4000)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("InstanceId", "AlarmCanonicalName")
+                        .IsUnique();
+
+                    b.ToTable("InstanceAlarmOverrides");
+                });
+
             modelBuilder.Entity("ScadaLink.Commons.Entities.Instances.InstanceAttributeOverride", b =>
                 {
                     b.Property<int>("Id")
@@ -1144,6 +1175,15 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .IsRequired();
                 });
 
+            modelBuilder.Entity("ScadaLink.Commons.Entities.Instances.InstanceAlarmOverride", b =>
+                {
+                    b.HasOne("ScadaLink.Commons.Entities.Instances.Instance", null)
+                        .WithMany("AlarmOverrides")
+                        .HasForeignKey("InstanceId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("ScadaLink.Commons.Entities.Instances.InstanceAttributeOverride", b =>
                 {
                     b.HasOne("ScadaLink.Commons.Entities.Instances.Instance", null)
@@ -1271,6 +1311,8 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
 
             modelBuilder.Entity("ScadaLink.Commons.Entities.Instances.Instance", b =>
                 {
+                    b.Navigation("AlarmOverrides");
+
                     b.Navigation("AttributeOverrides");
 
                     b.Navigation("ConnectionBindings");

src/ScadaLink.ConfigurationDatabase/Repositories/DeploymentManagerRepository.cs

@@ -169,6 +169,7 @@ public class DeploymentManagerRepository : IDeploymentManagerRepository
     {
         return await _dbContext.Set<Instance>()
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .FirstOrDefaultAsync(i => i.Id == instanceId, cancellationToken);
     }
@@ -177,6 +178,7 @@ public class DeploymentManagerRepository : IDeploymentManagerRepository
     {
         return await _dbContext.Set<Instance>()
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .FirstOrDefaultAsync(i => i.UniqueName == uniqueName, cancellationToken);
     }

src/ScadaLink.ConfigurationDatabase/Repositories/TemplateEngineRepository.cs

@@ -222,6 +222,7 @@ public class TemplateEngineRepository : ITemplateEngineRepository
     {
         return await _context.Instances
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .FirstOrDefaultAsync(i => i.Id == id, cancellationToken);
     }
@@ -230,6 +231,7 @@ public class TemplateEngineRepository : ITemplateEngineRepository
     {
         return await _context.Instances
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .ToListAsync(cancellationToken);
     }
@@ -246,6 +248,7 @@ public class TemplateEngineRepository : ITemplateEngineRepository
         return await _context.Instances
             .Where(i => i.SiteId == siteId)
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .ToListAsync(cancellationToken);
     }
@@ -254,6 +257,7 @@ public class TemplateEngineRepository : ITemplateEngineRepository
     {
         return await _context.Instances
             .Include(i => i.AttributeOverrides)
+            .Include(i => i.AlarmOverrides)
             .Include(i => i.ConnectionBindings)
             .FirstOrDefaultAsync(i => i.UniqueName == uniqueName, cancellationToken);
     }
@@ -307,6 +311,43 @@ public class TemplateEngineRepository : ITemplateEngineRepository
         }
     }
 
+    // InstanceAlarmOverride
+
+    public async Task<IReadOnlyList<InstanceAlarmOverride>> GetAlarmOverridesByInstanceIdAsync(int instanceId, CancellationToken cancellationToken = default)
+    {
+        return await _context.InstanceAlarmOverrides
+            .Where(o => o.InstanceId == instanceId)
+            .ToListAsync(cancellationToken);
+    }
+
+    public async Task<InstanceAlarmOverride?> GetAlarmOverrideAsync(int instanceId, string alarmCanonicalName, CancellationToken cancellationToken = default)
+    {
+        return await _context.InstanceAlarmOverrides
+            .FirstOrDefaultAsync(
+                o => o.InstanceId == instanceId && o.AlarmCanonicalName == alarmCanonicalName,
+                cancellationToken);
+    }
+
+    public async Task AddInstanceAlarmOverrideAsync(InstanceAlarmOverride alarmOverride, CancellationToken cancellationToken = default)
+    {
+        await _context.InstanceAlarmOverrides.AddAsync(alarmOverride, cancellationToken);
+    }
+
+    public Task UpdateInstanceAlarmOverrideAsync(InstanceAlarmOverride alarmOverride, CancellationToken cancellationToken = default)
+    {
+        _context.InstanceAlarmOverrides.Update(alarmOverride);
+        return Task.CompletedTask;
+    }
+
+    public async Task DeleteInstanceAlarmOverrideAsync(int id, CancellationToken cancellationToken = default)
+    {
+        var alarmOverride = await _context.InstanceAlarmOverrides.FindAsync(new object[] { id }, cancellationToken);
+        if (alarmOverride != null)
+        {
+            _context.InstanceAlarmOverrides.Remove(alarmOverride);
+        }
+    }
+
     // InstanceConnectionBinding
 
     public async Task<IReadOnlyList<InstanceConnectionBinding>> GetBindingsByInstanceIdAsync(int instanceId, CancellationToken cancellationToken = default)

src/ScadaLink.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -30,6 +30,7 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Instances
     public DbSet<Instance> Instances => Set<Instance>();
     public DbSet<InstanceAttributeOverride> InstanceAttributeOverrides => Set<InstanceAttributeOverride>();
+    public DbSet<InstanceAlarmOverride> InstanceAlarmOverrides => Set<InstanceAlarmOverride>();
     public DbSet<InstanceConnectionBinding> InstanceConnectionBindings => Set<InstanceConnectionBinding>();
     public DbSet<Area> Areas => Set<Area>();
 

src/ScadaLink.Host/Components/App.razor

@@ -26,17 +26,54 @@
         </div>
     </div>
 
-    <script src="/_framework/blazor.web.js"></script>
+    <script src="/_framework/blazor.web.js"
+            autostart="false"></script>
     <script>
-        // Reconnection overlay for failover behavior
+        // Reconnection overlay for failover behavior. After a docker redeploy
-        // Blazor object is available after blazor.web.js initializes
+        // (or other server-side restart), Blazor exhausts its retry budget and
+        // leaves the user staring at a stuck "Reconnect failed" overlay. Auto-
+        // reload in that case so the user lands on a fresh circuit instead of
+        // having to manually refresh.
+        Blazor.start({
+            circuit: {
+                reconnectionOptions: {
+                    maxRetries: 8,
+                    retryIntervalMilliseconds: 1500
+                },
+                reconnectionHandler: {
+                    onConnectionDown: () => { /* default overlay */ },
+                    onConnectionUp: () => {
+                        var m = document.getElementById('reconnect-modal');
+                        if (m) m.style.display = 'none';
+                    }
+                }
+            }
+        });
+
         document.addEventListener('DOMContentLoaded', () => {
             if (typeof Blazor !== 'undefined') {
-                Blazor.addEventListener('enhancedload', () => {
+                Blazor.addEventListener?.('enhancedload', () => {
-                    document.getElementById('reconnect-modal').style.display = 'none';
+                    var m = document.getElementById('reconnect-modal');
+                    if (m) m.style.display = 'none';
                 });
             }
         });
+
+        // When Blazor gives up reconnecting, it adds the
+        // `components-reconnect-failed` class to the reconnect modal element.
+        // Watch for it and auto-reload so the user gets a fresh circuit.
+        var mo = new MutationObserver(() => {
+            var m = document.getElementById('reconnect-modal');
+            if (!m) return;
+            if (m.classList.contains('components-reconnect-failed')) {
+                window.location.reload();
+            }
+        });
+        mo.observe(document.documentElement, {
+            attributes: true,
+            subtree: true,
+            attributeFilter: ['class']
+        });
     </script>
     <script src="/js/treeview-storage.js"></script>
     <script src="_content/ScadaLink.CentralUI/js/monaco-init.js"></script>

src/ScadaLink.ManagementService/ManagementActor.cs

@@ -119,6 +119,7 @@ public class ManagementActor : ReceiveActor
         CreateInstanceCommand or MgmtDeployInstanceCommand or MgmtEnableInstanceCommand
             or MgmtDisableInstanceCommand or MgmtDeleteInstanceCommand
             or SetConnectionBindingsCommand or SetInstanceOverridesCommand or SetInstanceAreaCommand
+            or SetInstanceAlarmOverrideCommand or DeleteInstanceAlarmOverrideCommand
             or GetDeploymentDiffCommand
             or MgmtDeployArtifactsCommand
             or RetryParkedMessageCommand or DiscardParkedMessageCommand
@@ -172,6 +173,9 @@ public class ManagementActor : ReceiveActor
             SetConnectionBindingsCommand cmd => await HandleSetConnectionBindings(sp, cmd, user),
             SetInstanceOverridesCommand cmd => await HandleSetInstanceOverrides(sp, cmd, user),
             SetInstanceAreaCommand cmd => await HandleSetInstanceArea(sp, cmd, user),
+            SetInstanceAlarmOverrideCommand cmd => await HandleSetInstanceAlarmOverride(sp, cmd, user),
+            DeleteInstanceAlarmOverrideCommand cmd => await HandleDeleteInstanceAlarmOverride(sp, cmd, user),
+            ListInstanceAlarmOverridesCommand cmd => await HandleListInstanceAlarmOverrides(sp, cmd, user),
 
             // Sites
             ListSitesCommand => await HandleListSites(sp, user),
@@ -593,6 +597,37 @@ public class ManagementActor : ReceiveActor
             : throw new InvalidOperationException(result.Error);
     }
 
+    private static async Task<object?> HandleSetInstanceAlarmOverride(IServiceProvider sp, SetInstanceAlarmOverrideCommand cmd, AuthenticatedUser user)
+    {
+        await EnforceSiteScopeForInstance(sp, user, cmd.InstanceId);
+        var svc = sp.GetRequiredService<InstanceService>();
+        var result = await svc.SetAlarmOverrideAsync(
+            cmd.InstanceId, cmd.AlarmCanonicalName,
+            cmd.TriggerConfigurationOverride, cmd.PriorityLevelOverride,
+            user.Username);
+        return result.IsSuccess
+            ? result.Value
+            : throw new InvalidOperationException(result.Error);
+    }
+
+    private static async Task<object?> HandleDeleteInstanceAlarmOverride(IServiceProvider sp, DeleteInstanceAlarmOverrideCommand cmd, AuthenticatedUser user)
+    {
+        await EnforceSiteScopeForInstance(sp, user, cmd.InstanceId);
+        var svc = sp.GetRequiredService<InstanceService>();
+        var result = await svc.DeleteAlarmOverrideAsync(
+            cmd.InstanceId, cmd.AlarmCanonicalName, user.Username);
+        return result.IsSuccess
+            ? result.Value
+            : throw new InvalidOperationException(result.Error);
+    }
+
+    private static async Task<object?> HandleListInstanceAlarmOverrides(IServiceProvider sp, ListInstanceAlarmOverridesCommand cmd, AuthenticatedUser user)
+    {
+        await EnforceSiteScopeForInstance(sp, user, cmd.InstanceId);
+        var repo = sp.GetRequiredService<ITemplateEngineRepository>();
+        return await repo.GetAlarmOverridesByInstanceIdAsync(cmd.InstanceId);
+    }
+
     private static async Task<object?> HandleGetDeploymentDiff(IServiceProvider sp, GetDeploymentDiffCommand cmd, AuthenticatedUser user)
     {
         await EnforceSiteScopeForInstance(sp, user, cmd.InstanceId);

src/ScadaLink.SiteRuntime/Actors/AlarmActor.cs

@@ -38,6 +38,12 @@ public class AlarmActor : ReceiveActor
     private readonly ISiteHealthCollector? _healthCollector;
 
     private AlarmState _currentState = AlarmState.Normal;
+    /// <summary>
+    /// Always <see cref="AlarmLevel.None"/> for binary trigger types. For
+    /// <see cref="AlarmTriggerType.HiLo"/> this is the source of truth — the
+    /// state machine transitions when the computed level changes.
+    /// </summary>
+    private AlarmLevel _currentLevel = AlarmLevel.None;
     private readonly AlarmTriggerType _triggerType;
     private readonly AlarmEvalConfig _evalConfig;
     private readonly int _priority;
@@ -126,6 +132,12 @@ public class AlarmActor : ReceiveActor
 
         try
         {
+            if (_triggerType == AlarmTriggerType.HiLo)
+            {
+                HandleHiLoTransition(EvaluateHiLo(changed.Value));
+                return;
+            }
+
             var isTriggered = _triggerType switch
             {
                 AlarmTriggerType.ValueMatch => EvaluateValueMatch(changed.Value),
@@ -150,7 +162,7 @@ public class AlarmActor : ReceiveActor
                 // Spawn AlarmExecutionActor if on-trigger script defined
                 if (_onTriggerCompiledScript != null)
                 {
-                    SpawnAlarmExecution();
+                    SpawnAlarmExecution(AlarmLevel.None, _priority, string.Empty);
                 }
             }
             else if (!isTriggered && _currentState == AlarmState.Active)
@@ -176,6 +188,78 @@ public class AlarmActor : ReceiveActor
         }
     }
 
+    /// <summary>
+    /// HiLo state machine: emit an AlarmStateChanged whenever the evaluated
+    /// level changes. Spawns the on-trigger script only on the Normal→Active
+    /// edge (i.e., when entering an alarm band from the normal band) — not on
+    /// level escalations like Hi→HiHi or Low→LowLow.
+    /// </summary>
+    private void HandleHiLoTransition(AlarmLevel newLevel)
+    {
+        if (newLevel == _currentLevel) return;
+
+        var previousLevel = _currentLevel;
+        _currentLevel = newLevel;
+        _currentState = newLevel == AlarmLevel.None ? AlarmState.Normal : AlarmState.Active;
+        var priority = LevelPriority(newLevel);
+        var message = LevelMessage(newLevel);
+
+        _logger.LogInformation(
+            "Alarm {Alarm} on {Instance} transitioned {Prev} → {New} (priority={Priority})",
+            _alarmName, _instanceName, previousLevel, newLevel, priority);
+
+        var alarmChanged = new AlarmStateChanged(
+            _instanceName, _alarmName, _currentState, priority, DateTimeOffset.UtcNow)
+        {
+            Level = newLevel,
+            Message = message
+        };
+        _instanceActor.Tell(alarmChanged);
+
+        if (previousLevel == AlarmLevel.None
+            && newLevel != AlarmLevel.None
+            && _onTriggerCompiledScript != null)
+        {
+            SpawnAlarmExecution(newLevel, priority, message);
+        }
+    }
+
+    /// <summary>
+    /// Returns the per-setpoint priority for the given level. Falls back to
+    /// the alarm-level <see cref="_priority"/> when the HiLo config did not
+    /// override the priority for that band, or for <see cref="AlarmLevel.None"/>.
+    /// </summary>
+    private int LevelPriority(AlarmLevel level)
+    {
+        if (_evalConfig is not HiLoEvalConfig hiLo) return _priority;
+        return level switch
+        {
+            AlarmLevel.LowLow => hiLo.LoLoPriority ?? _priority,
+            AlarmLevel.Low => hiLo.LoPriority ?? _priority,
+            AlarmLevel.High => hiLo.HiPriority ?? _priority,
+            AlarmLevel.HighHigh => hiLo.HiHiPriority ?? _priority,
+            _ => _priority
+        };
+    }
+
+    /// <summary>
+    /// Per-band operator message. Empty string when no message is configured
+    /// for the band, or for non-HiLo trigger types, or for the None level
+    /// (alarm clear).
+    /// </summary>
+    private string LevelMessage(AlarmLevel level)
+    {
+        if (_evalConfig is not HiLoEvalConfig hiLo) return string.Empty;
+        return level switch
+        {
+            AlarmLevel.LowLow => hiLo.LoLoMessage ?? string.Empty,
+            AlarmLevel.Low => hiLo.LoMessage ?? string.Empty,
+            AlarmLevel.High => hiLo.HiMessage ?? string.Empty,
+            AlarmLevel.HighHigh => hiLo.HiHiMessage ?? string.Empty,
+            _ => string.Empty
+        };
+    }
+
     private bool IsMonitoredAttribute(string attributeName)
     {
         return _evalConfig.MonitoredAttributeName == attributeName;
@@ -254,9 +338,57 @@ public class AlarmActor : ReceiveActor
     }
 
     /// <summary>
-    /// Spawns an AlarmExecutionActor to run the on-trigger script.
+    /// HiLo level evaluator: returns the most-severe matching band for the
+    /// given value. Severity order checked from highest to lowest so that a
+    /// value at exactly Hi==HiHi resolves to HighHigh. Unset setpoints (null)
+    /// are skipped, allowing partial configs (e.g., HighHigh only).
+    ///
+    /// Hysteresis: when the alarm is already in a level whose threshold the
+    /// value would re-cross from inside, the threshold is relaxed by the
+    /// configured deadband. This prevents flapping at the boundary — once at
+    /// HighHigh with HiHi=100 and hiHiDeadband=5, the alarm stays HighHigh
+    /// until the value drops below 95.
     /// </summary>
-    private void SpawnAlarmExecution()
+    private AlarmLevel EvaluateHiLo(object? value)
+    {
+        if (_evalConfig is not HiLoEvalConfig config) return AlarmLevel.None;
+        if (value == null) return _currentLevel;
+
+        double numericValue;
+        try { numericValue = Convert.ToDouble(value); }
+        catch { return _currentLevel; }
+
+        // When the current level is at-or-above HighHigh, relax the HiHi exit.
+        // Same for the other directions.
+        var hiHiThreshold = config.HiHi;
+        if (hiHiThreshold is { } hh && _currentLevel == AlarmLevel.HighHigh)
+            hiHiThreshold = hh - Math.Max(0, config.HiHiDeadband ?? 0);
+
+        var hiThreshold = config.Hi;
+        if (hiThreshold is { } h && (_currentLevel == AlarmLevel.High || _currentLevel == AlarmLevel.HighHigh))
+            hiThreshold = h - Math.Max(0, config.HiDeadband ?? 0);
+
+        var loLoThreshold = config.LoLo;
+        if (loLoThreshold is { } ll && _currentLevel == AlarmLevel.LowLow)
+            loLoThreshold = ll + Math.Max(0, config.LoLoDeadband ?? 0);
+
+        var loThreshold = config.Lo;
+        if (loThreshold is { } l && (_currentLevel == AlarmLevel.Low || _currentLevel == AlarmLevel.LowLow))
+            loThreshold = l + Math.Max(0, config.LoDeadband ?? 0);
+
+        if (hiHiThreshold is { } effHiHi && numericValue >= effHiHi) return AlarmLevel.HighHigh;
+        if (hiThreshold is { } effHi && numericValue >= effHi) return AlarmLevel.High;
+        if (loLoThreshold is { } effLoLo && numericValue <= effLoLo) return AlarmLevel.LowLow;
+        if (loThreshold is { } effLo && numericValue <= effLo) return AlarmLevel.Low;
+        return AlarmLevel.None;
+    }
+
+    /// <summary>
+    /// Spawns an AlarmExecutionActor to run the on-trigger script.
+    /// Passes the firing alarm's level/priority/message so the script can
+    /// branch on severity via the <c>Alarm</c> global.
+    /// </summary>
+    private void SpawnAlarmExecution(AlarmLevel level, int priority, string message)
     {
         if (_onTriggerCompiledScript == null) return;
 
@@ -266,6 +398,9 @@ public class AlarmActor : ReceiveActor
         var props = Props.Create(() => new AlarmExecutionActor(
             _alarmName,
             _instanceName,
+            level,
+            priority,
+            message,
             _onTriggerCompiledScript,
             _instanceActor,
             _sharedScriptLibrary,
@@ -319,6 +454,25 @@ public class AlarmActor : ReceiveActor
                         ? ParseDirection(dirEl.GetString())
                         : RateOfChangeDirection.Either),
 
+                AlarmTriggerType.HiLo => new HiLoEvalConfig(
+                    attr,
+                    LoLo: TryReadDouble(root, "loLo"),
+                    Lo: TryReadDouble(root, "lo"),
+                    Hi: TryReadDouble(root, "hi"),
+                    HiHi: TryReadDouble(root, "hiHi"),
+                    LoLoPriority: TryReadInt(root, "loLoPriority"),
+                    LoPriority: TryReadInt(root, "loPriority"),
+                    HiPriority: TryReadInt(root, "hiPriority"),
+                    HiHiPriority: TryReadInt(root, "hiHiPriority"),
+                    LoLoDeadband: TryReadDouble(root, "loLoDeadband"),
+                    LoDeadband: TryReadDouble(root, "loDeadband"),
+                    HiDeadband: TryReadDouble(root, "hiDeadband"),
+                    HiHiDeadband: TryReadDouble(root, "hiHiDeadband"),
+                    LoLoMessage: TryReadString(root, "loLoMessage"),
+                    LoMessage: TryReadString(root, "loMessage"),
+                    HiMessage: TryReadString(root, "hiMessage"),
+                    HiHiMessage: TryReadString(root, "hiHiMessage")),
+
                 _ => new ValueMatchEvalConfig(attr, null)
             };
         }
@@ -336,6 +490,35 @@ public class AlarmActor : ReceiveActor
         _ => RateOfChangeDirection.Either
     };
 
+    private static double? TryReadDouble(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number => p.GetDouble(),
+            JsonValueKind.String when double.TryParse(p.GetString(), System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
+
+    private static int? TryReadInt(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number when p.TryGetInt32(out var i) => i,
+            JsonValueKind.Number => (int)p.GetDouble(),
+            JsonValueKind.String when int.TryParse(p.GetString(), System.Globalization.NumberStyles.Integer, System.Globalization.CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
+
+    private static string? TryReadString(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind == JsonValueKind.String ? p.GetString() : null;
+    }
+
     // ── Internal messages ──
     internal record AlarmExecutionCompleted(string AlarmName, bool Success);
 }
@@ -351,3 +534,27 @@ internal record RateOfChangeEvalConfig(
     double ThresholdPerSecond,
     TimeSpan WindowDuration,
     RateOfChangeDirection Direction) : AlarmEvalConfig(MonitoredAttributeName);
+
+/// <summary>
+/// HiLo evaluation config: any subset of the four setpoints may be set; null
+/// means "don't evaluate that band". Per-setpoint priorities override the
+/// alarm-level priority for AlarmStateChanged messages emitted for that band.
+/// </summary>
+internal record HiLoEvalConfig(
+    string MonitoredAttributeName,
+    double? LoLo,
+    double? Lo,
+    double? Hi,
+    double? HiHi,
+    int? LoLoPriority,
+    int? LoPriority,
+    int? HiPriority,
+    int? HiHiPriority,
+    double? LoLoDeadband = null,
+    double? LoDeadband = null,
+    double? HiDeadband = null,
+    double? HiHiDeadband = null,
+    string? LoLoMessage = null,
+    string? LoMessage = null,
+    string? HiMessage = null,
+    string? HiHiMessage = null) : AlarmEvalConfig(MonitoredAttributeName);

src/ScadaLink.SiteRuntime/Actors/AlarmExecutionActor.cs

@@ -2,6 +2,8 @@ using Akka.Actor;
 using Microsoft.CodeAnalysis.Scripting;
 using Microsoft.Extensions.Logging;
 using ScadaLink.Commons.Types;
+using ScadaLink.Commons.Types.Enums;
+using ScadaLink.Commons.Types.Scripts;
 using ScadaLink.SiteRuntime.Scripts;
 
 namespace ScadaLink.SiteRuntime.Actors;
@@ -18,6 +20,9 @@ public class AlarmExecutionActor : ReceiveActor
     public AlarmExecutionActor(
         string alarmName,
         string instanceName,
+        AlarmLevel level,
+        int priority,
+        string message,
         Script<object?> compiledScript,
         IActorRef instanceActor,
         SharedScriptLibrary sharedScriptLibrary,
@@ -28,13 +33,17 @@ public class AlarmExecutionActor : ReceiveActor
         var parent = Context.Parent;
 
         ExecuteAlarmScript(
-            alarmName, instanceName, compiledScript, instanceActor,
+            alarmName, instanceName, level, priority, message,
+            compiledScript, instanceActor,
             sharedScriptLibrary, options, self, parent, logger);
     }
 
     private static void ExecuteAlarmScript(
         string alarmName,
         string instanceName,
+        AlarmLevel level,
+        int priority,
+        string message,
         Script<object?> compiledScript,
         IActorRef instanceActor,
         SharedScriptLibrary sharedScriptLibrary,
@@ -66,7 +75,14 @@ public class AlarmExecutionActor : ReceiveActor
                 {
                     Instance = context,
                     Parameters = new ScriptParameters(),
-                    CancellationToken = cts.Token
+                    CancellationToken = cts.Token,
+                    Alarm = new AlarmContext
+                    {
+                        Name = alarmName,
+                        Level = level,
+                        Priority = priority,
+                        Message = message
+                    }
                 };
 
                 await compiledScript.RunAsync(globals, cts.Token);

src/ScadaLink.SiteRuntime/Scripts/ScriptCompilationService.cs

@@ -181,6 +181,14 @@ public class ScriptGlobals
     public ScriptParameters Parameters { get; set; } = new ScriptParameters();
     public CancellationToken CancellationToken { get; set; }
 
+    /// <summary>
+    /// Alarm context when this script is invoked as an on-trigger handler.
+    /// Null for instance scripts, shared scripts, and inbound-API-routed
+    /// scripts. Lets on-trigger scripts read the firing alarm's Name, Level
+    /// (HiLo only), Priority, and per-band Message to branch routing logic.
+    /// </summary>
+    public Commons.Types.Scripts.AlarmContext? Alarm { get; set; }
+
     /// <summary>
     /// Where this script sits in the composition tree. Defaults to root for
     /// scripts on top-level templates; a flattened composed script gets

src/ScadaLink.TemplateEngine/Flattening/FlatteningService.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using ScadaLink.Commons.Entities.Instances;
 using ScadaLink.Commons.Entities.Sites;
 using ScadaLink.Commons.Entities.Templates;
@@ -80,6 +81,7 @@ public class FlatteningService
             // Step 5: Resolve alarms from inheritance chain
             var alarms = ResolveInheritedAlarms(templateChain);
             ResolveComposedAlarms(templateChain, compositionMap, composedTemplateChains, alarms);
+            ApplyInstanceAlarmOverrides(instance.AlarmOverrides, alarms);
 
             // Step 6: Resolve scripts from inheritance chain
             var scripts = ResolveInheritedScripts(templateChain);
@@ -292,6 +294,43 @@ public class FlatteningService
         }
     }
 
+    /// <summary>
+    /// Applies per-instance alarm overrides on top of the
+    /// inheritance-and-composition resolved alarms. Skips overrides for
+    /// alarms that are locked at the template level. For HiLo triggers the
+    /// override JSON is merged setpoint-by-setpoint (preserving inherited
+    /// keys not present in the override); for other trigger types the
+    /// override replaces the whole TriggerConfiguration.
+    /// </summary>
+    private static void ApplyInstanceAlarmOverrides(
+        ICollection<InstanceAlarmOverride> overrides,
+        Dictionary<string, ResolvedAlarm> alarms)
+    {
+        foreach (var ovr in overrides)
+        {
+            if (!alarms.TryGetValue(ovr.AlarmCanonicalName, out var existing))
+                continue; // Cannot add new alarms via overrides
+
+            if (existing.IsLocked)
+                continue; // Locked alarms cannot be overridden
+
+            var newConfig = existing.TriggerConfiguration;
+            if (!string.IsNullOrWhiteSpace(ovr.TriggerConfigurationOverride))
+            {
+                newConfig = existing.TriggerType == nameof(AlarmTriggerType.HiLo)
+                    ? MergeHiLoConfig(existing.TriggerConfiguration, ovr.TriggerConfigurationOverride)
+                    : ovr.TriggerConfigurationOverride;
+            }
+
+            alarms[ovr.AlarmCanonicalName] = existing with
+            {
+                TriggerConfiguration = newConfig,
+                PriorityLevel = ovr.PriorityLevelOverride ?? existing.PriorityLevel,
+                Source = "Override"
+            };
+        }
+    }
+
     private static void ApplyConnectionBindings(
         ICollection<InstanceConnectionBinding> bindings,
         Dictionary<string, ResolvedAttribute> attributes,
@@ -332,6 +371,18 @@ public class FlatteningService
                 if (result.TryGetValue(alarm.Name, out var existing) && existing.IsLocked)
                     continue;
 
+                // HiLo per-setpoint override: derived templates can supply a
+                // partial TriggerConfiguration (e.g., just `hi`) and have the
+                // remaining setpoints inherited. Other trigger types replace
+                // the whole config on override (current behavior).
+                var triggerConfig = alarm.TriggerConfiguration;
+                if (existing != null
+                    && alarm.TriggerType == AlarmTriggerType.HiLo
+                    && existing.TriggerType == nameof(AlarmTriggerType.HiLo))
+                {
+                    triggerConfig = MergeHiLoConfig(existing.TriggerConfiguration, triggerConfig);
+                }
+
                 result[alarm.Name] = new ResolvedAlarm
                 {
                     CanonicalName = alarm.Name,
@@ -339,7 +390,7 @@ public class FlatteningService
                     PriorityLevel = alarm.PriorityLevel,
                     IsLocked = alarm.IsLocked,
                     TriggerType = alarm.TriggerType.ToString(),
-                    TriggerConfiguration = alarm.TriggerConfiguration,
+                    TriggerConfiguration = triggerConfig,
                     OnTriggerScriptCanonicalName = null, // Resolved later
                     Source = source
                 };
@@ -349,6 +400,61 @@ public class FlatteningService
         return result;
     }
 
+    /// <summary>
+    /// Merges a derived HiLo trigger configuration onto an inherited one.
+    /// Top-level keys present in <paramref name="derivedJson"/> override the
+    /// inherited values; keys absent in the derived config are inherited.
+    /// Returns the derived config verbatim on parse failure of either input —
+    /// the existing whole-replace behavior is the safe fallback.
+    /// </summary>
+    internal static string? MergeHiLoConfig(string? inheritedJson, string? derivedJson)
+    {
+        if (string.IsNullOrWhiteSpace(inheritedJson)) return derivedJson;
+        if (string.IsNullOrWhiteSpace(derivedJson)) return inheritedJson;
+
+        try
+        {
+            using var inheritedDoc = JsonDocument.Parse(inheritedJson);
+            using var derivedDoc = JsonDocument.Parse(derivedJson);
+
+            if (inheritedDoc.RootElement.ValueKind != JsonValueKind.Object
+                || derivedDoc.RootElement.ValueKind != JsonValueKind.Object)
+            {
+                return derivedJson;
+            }
+
+            using var stream = new MemoryStream();
+            using (var writer = new Utf8JsonWriter(stream))
+            {
+                writer.WriteStartObject();
+
+                var derivedKeys = new HashSet<string>(StringComparer.Ordinal);
+                foreach (var prop in derivedDoc.RootElement.EnumerateObject())
+                    derivedKeys.Add(prop.Name);
+
+                // Inherited keys not present in derived survive.
+                foreach (var prop in inheritedDoc.RootElement.EnumerateObject())
+                {
+                    if (derivedKeys.Contains(prop.Name)) continue;
+                    prop.WriteTo(writer);
+                }
+
+                // Derived keys win.
+                foreach (var prop in derivedDoc.RootElement.EnumerateObject())
+                {
+                    prop.WriteTo(writer);
+                }
+
+                writer.WriteEndObject();
+            }
+            return System.Text.Encoding.UTF8.GetString(stream.ToArray());
+        }
+        catch (JsonException)
+        {
+            return derivedJson;
+        }
+    }
+
     private static void ResolveComposedAlarms(
         IReadOnlyList<Template> templateChain,
         IReadOnlyDictionary<int, IReadOnlyList<TemplateComposition>> compositionMap,

src/ScadaLink.TemplateEngine/Services/InstanceService.cs

@@ -169,6 +169,94 @@ public class InstanceService
         }
     }
 
+    /// <summary>
+    /// Sets a per-instance alarm override. The alarm must exist on the
+    /// template and must not be locked. For HiLo alarms, the override JSON
+    /// merges into the inherited TriggerConfiguration setpoint-by-setpoint;
+    /// for binary trigger types, it replaces the whole config.
+    /// </summary>
+    public async Task<Result<InstanceAlarmOverride>> SetAlarmOverrideAsync(
+        int instanceId,
+        string alarmCanonicalName,
+        string? triggerConfigurationOverride,
+        int? priorityLevelOverride,
+        string user,
+        CancellationToken cancellationToken = default)
+    {
+        var instance = await _repository.GetInstanceByIdAsync(instanceId, cancellationToken);
+        if (instance == null)
+            return Result<InstanceAlarmOverride>.Failure($"Instance with ID {instanceId} not found.");
+
+        // Verify alarm exists in the template and is not locked. Only direct
+        // template alarms are checked here — composed-member overrides go
+        // through but are silently ignored at runtime if the name doesn't
+        // match (same behavior as attribute overrides).
+        var templateAlarms = await _repository.GetAlarmsByTemplateIdAsync(instance.TemplateId, cancellationToken);
+        var templateAlarm = templateAlarms.FirstOrDefault(a => a.Name == alarmCanonicalName);
+        if (templateAlarm != null && templateAlarm.IsLocked)
+        {
+            return Result<InstanceAlarmOverride>.Failure(
+                $"Alarm '{alarmCanonicalName}' is locked and cannot be overridden.");
+        }
+
+        var existingOverride = await _repository.GetAlarmOverrideAsync(
+            instanceId, alarmCanonicalName, cancellationToken);
+
+        if (existingOverride != null)
+        {
+            existingOverride.TriggerConfigurationOverride = triggerConfigurationOverride;
+            existingOverride.PriorityLevelOverride = priorityLevelOverride;
+            await _repository.UpdateInstanceAlarmOverrideAsync(existingOverride, cancellationToken);
+            await _repository.SaveChangesAsync(cancellationToken);
+
+            await _auditService.LogAsync(user, "UpdateAlarmOverride", "InstanceAlarmOverride",
+                existingOverride.Id.ToString(), alarmCanonicalName, existingOverride, cancellationToken);
+
+            return Result<InstanceAlarmOverride>.Success(existingOverride);
+        }
+        else
+        {
+            var newOverride = new InstanceAlarmOverride(alarmCanonicalName)
+            {
+                InstanceId = instanceId,
+                TriggerConfigurationOverride = triggerConfigurationOverride,
+                PriorityLevelOverride = priorityLevelOverride
+            };
+
+            await _repository.AddInstanceAlarmOverrideAsync(newOverride, cancellationToken);
+            await _repository.SaveChangesAsync(cancellationToken);
+
+            await _auditService.LogAsync(user, "CreateAlarmOverride", "InstanceAlarmOverride",
+                newOverride.Id.ToString(), alarmCanonicalName, newOverride, cancellationToken);
+
+            return Result<InstanceAlarmOverride>.Success(newOverride);
+        }
+    }
+
+    /// <summary>
+    /// Removes a per-instance alarm override. After removal the instance
+    /// inherits the template alarm config unchanged.
+    /// </summary>
+    public async Task<Result<bool>> DeleteAlarmOverrideAsync(
+        int instanceId,
+        string alarmCanonicalName,
+        string user,
+        CancellationToken cancellationToken = default)
+    {
+        var existing = await _repository.GetAlarmOverrideAsync(
+            instanceId, alarmCanonicalName, cancellationToken);
+        if (existing == null)
+            return Result<bool>.Failure($"No alarm override for '{alarmCanonicalName}' on instance {instanceId}.");
+
+        await _repository.DeleteInstanceAlarmOverrideAsync(existing.Id, cancellationToken);
+        await _repository.SaveChangesAsync(cancellationToken);
+
+        await _auditService.LogAsync(user, "DeleteAlarmOverride", "InstanceAlarmOverride",
+            existing.Id.ToString(), alarmCanonicalName, existing, cancellationToken);
+
+        return Result<bool>.Success(true);
+    }
+
     /// <summary>
     /// Sets connection bindings for an instance in bulk.
     /// </summary>

src/ScadaLink.TemplateEngine/Validation/SemanticValidator.cs

@@ -136,6 +136,75 @@ public class SemanticValidator
                 }
             }
 
+            // HiLo requires numeric attribute + ordered setpoints
+            if (alarm.TriggerType == "HiLo" && !string.IsNullOrWhiteSpace(alarm.TriggerConfiguration))
+            {
+                var attrName = ValidationService.ExtractAttributeNameFromTriggerConfig(alarm.TriggerConfiguration);
+                if (attrName != null && attributeMap.TryGetValue(attrName, out var attr))
+                {
+                    if (!NumericDataTypes.Contains(attr.DataType))
+                    {
+                        errors.Add(ValidationEntry.Error(ValidationCategory.TriggerOperandType,
+                            $"Alarm '{alarm.CanonicalName}' uses HiLo trigger on attribute '{attrName}' which has non-numeric type '{attr.DataType}'.",
+                            alarm.CanonicalName));
+                    }
+                }
+
+                var setpoints = ValidationService.ExtractHiLoSetpoints(alarm.TriggerConfiguration);
+
+                // At least one setpoint must be configured — otherwise the alarm
+                // can never fire.
+                if (!setpoints.LoLo.HasValue && !setpoints.Lo.HasValue
+                    && !setpoints.Hi.HasValue && !setpoints.HiHi.HasValue)
+                {
+                    warnings.Add(ValidationEntry.Warning(ValidationCategory.TriggerOperandType,
+                        $"Alarm '{alarm.CanonicalName}' is HiLo but no setpoints (LoLo/Lo/Hi/HiHi) are configured — it will never fire.",
+                        alarm.CanonicalName));
+                }
+
+                // Ordering: LoLo ≤ Lo, Hi ≤ HiHi, and the highest Lo-side band
+                // must sit strictly below the lowest Hi-side band — otherwise the
+                // bands overlap and the evaluator's behavior is ambiguous.
+                if (setpoints.LoLo is { } loLo && setpoints.Lo is { } lo && loLo > lo)
+                {
+                    errors.Add(ValidationEntry.Error(ValidationCategory.TriggerOperandType,
+                        $"Alarm '{alarm.CanonicalName}' HiLo setpoints out of order: LoLo ({loLo}) must be ≤ Lo ({lo}).",
+                        alarm.CanonicalName));
+                }
+                if (setpoints.Hi is { } hi && setpoints.HiHi is { } hiHi && hi > hiHi)
+                {
+                    errors.Add(ValidationEntry.Error(ValidationCategory.TriggerOperandType,
+                        $"Alarm '{alarm.CanonicalName}' HiLo setpoints out of order: Hi ({hi}) must be ≤ HiHi ({hiHi}).",
+                        alarm.CanonicalName));
+                }
+                var highestLowSide = setpoints.Lo ?? setpoints.LoLo;
+                var lowestHighSide = setpoints.Hi ?? setpoints.HiHi;
+                if (highestLowSide is { } lowSide && lowestHighSide is { } highSide
+                    && lowSide >= highSide)
+                {
+                    errors.Add(ValidationEntry.Error(ValidationCategory.TriggerOperandType,
+                        $"Alarm '{alarm.CanonicalName}' HiLo bands overlap: low-side setpoint ({lowSide}) must be strictly less than high-side setpoint ({highSide}).",
+                        alarm.CanonicalName));
+                }
+
+                // Deadbands must be non-negative — negative deadband would invert
+                // the hysteresis (alarm could escape faster than it entered).
+                foreach (var (name, value) in new (string, double?)[] {
+                    ("LoLo deadband", setpoints.LoLoDeadband),
+                    ("Lo deadband", setpoints.LoDeadband),
+                    ("Hi deadband", setpoints.HiDeadband),
+                    ("HiHi deadband", setpoints.HiHiDeadband)
+                })
+                {
+                    if (value is { } d && d < 0)
+                    {
+                        errors.Add(ValidationEntry.Error(ValidationCategory.TriggerOperandType,
+                            $"Alarm '{alarm.CanonicalName}' {name} ({d}) must be non-negative.",
+                            alarm.CanonicalName));
+                    }
+                }
+            }
+
             // On-trigger script must exist
             if (!string.IsNullOrWhiteSpace(alarm.OnTriggerScriptCanonicalName) &&
                 !scriptNames.Contains(alarm.OnTriggerScriptCanonicalName))

src/ScadaLink.TemplateEngine/Validation/ValidationService.cs

@@ -232,4 +232,50 @@ public class ValidationService
         }
         return null;
     }
+
+    /// <summary>
+    /// Extracts the four HiLo setpoints from a trigger configuration JSON.
+    /// Any unset (or non-numeric) setpoint comes back as <c>null</c>. Returns
+    /// all-nulls on malformed JSON — callers should treat that as "nothing to
+    /// validate" and let other checks surface the deeper problem.
+    /// </summary>
+    internal static HiLoSetpoints ExtractHiLoSetpoints(string triggerConfigJson)
+    {
+        try
+        {
+            using var doc = JsonDocument.Parse(triggerConfigJson);
+            var root = doc.RootElement;
+            return new HiLoSetpoints(
+                LoLo: ReadDouble(root, "loLo"),
+                Lo:   ReadDouble(root, "lo"),
+                Hi:   ReadDouble(root, "hi"),
+                HiHi: ReadDouble(root, "hiHi"),
+                LoLoDeadband: ReadDouble(root, "loLoDeadband"),
+                LoDeadband:   ReadDouble(root, "loDeadband"),
+                HiDeadband:   ReadDouble(root, "hiDeadband"),
+                HiHiDeadband: ReadDouble(root, "hiHiDeadband"));
+        }
+        catch (JsonException)
+        {
+            return new HiLoSetpoints(null, null, null, null, null, null, null, null);
+        }
+    }
+
+    private static double? ReadDouble(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var p)) return null;
+        return p.ValueKind switch
+        {
+            JsonValueKind.Number => p.GetDouble(),
+            JsonValueKind.String when double.TryParse(p.GetString(),
+                System.Globalization.NumberStyles.Float,
+                System.Globalization.CultureInfo.InvariantCulture, out var v) => v,
+            _ => null
+        };
+    }
 }
+
+internal readonly record struct HiLoSetpoints(
+    double? LoLo, double? Lo, double? Hi, double? HiHi,
+    double? LoLoDeadband = null, double? LoDeadband = null,
+    double? HiDeadband = null, double? HiHiDeadband = null);

tests/ScadaLink.CentralUI.Tests/Shared/AlarmTriggerConfigCodecTests.cs

@@ -0,0 +1,534 @@
+using System.Text.Json;
+using ScadaLink.CentralUI.Components.Shared;
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.CentralUI.Tests.Shared;
+
+public class AlarmTriggerConfigCodecTests
+{
+    // ── Parse: ValueMatch ──────────────────────────────────────────────────
+
+    [Fact]
+    public void Parse_ValueMatch_ReadsCanonicalKeys()
+    {
+        const string json = @"{""attributeName"":""Status"",""matchValue"":""Critical""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.ValueMatch);
+
+        Assert.Equal("Status", model.AttributeName);
+        Assert.Equal("Critical", model.MatchValue);
+        Assert.False(model.NotEquals);
+    }
+
+    [Fact]
+    public void Parse_ValueMatch_AcceptsLegacyAttributeAndValueKeys()
+    {
+        // Older configs used "attribute" and "value" instead of the canonical names.
+        const string json = @"{""attribute"":""Status"",""value"":""Critical""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.ValueMatch);
+
+        Assert.Equal("Status", model.AttributeName);
+        Assert.Equal("Critical", model.MatchValue);
+    }
+
+    [Fact]
+    public void Parse_ValueMatch_NotEqualsPrefix_SetsFlagAndStripsPrefix()
+    {
+        const string json = @"{""attributeName"":""Status"",""matchValue"":""!=Good""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.ValueMatch);
+
+        Assert.True(model.NotEquals);
+        Assert.Equal("Good", model.MatchValue);
+    }
+
+    [Fact]
+    public void Parse_ValueMatch_MissingMatchValue_LeavesNull()
+    {
+        const string json = @"{""attributeName"":""Status""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.ValueMatch);
+
+        Assert.Equal("Status", model.AttributeName);
+        Assert.Null(model.MatchValue);
+        Assert.False(model.NotEquals);
+    }
+
+    // ── Parse: RangeViolation ──────────────────────────────────────────────
+
+    [Fact]
+    public void Parse_RangeViolation_ReadsCanonicalKeys()
+    {
+        const string json = @"{""attributeName"":""Temp"",""min"":0,""max"":100}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RangeViolation);
+
+        Assert.Equal(0, model.Min);
+        Assert.Equal(100, model.Max);
+    }
+
+    [Fact]
+    public void Parse_RangeViolation_AcceptsLegacyLowHighKeys()
+    {
+        const string json = @"{""attributeName"":""Temp"",""low"":-10,""high"":50}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RangeViolation);
+
+        Assert.Equal(-10, model.Min);
+        Assert.Equal(50, model.Max);
+    }
+
+    [Fact]
+    public void Parse_RangeViolation_CanonicalKeysWinOverLegacy()
+    {
+        // If both canonical and legacy aliases are present, the canonical key wins.
+        const string json = @"{""attributeName"":""T"",""min"":0,""low"":-999,""max"":100,""high"":999}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RangeViolation);
+
+        Assert.Equal(0, model.Min);
+        Assert.Equal(100, model.Max);
+    }
+
+    [Fact]
+    public void Parse_RangeViolation_StringNumericValues_AreParsed()
+    {
+        // Some configs serialize min/max as JSON strings. Codec accepts both.
+        const string json = @"{""attributeName"":""T"",""min"":""1.5"",""max"":""9.75""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RangeViolation);
+
+        Assert.Equal(1.5, model.Min);
+        Assert.Equal(9.75, model.Max);
+    }
+
+    // ── Parse: RateOfChange ────────────────────────────────────────────────
+
+    [Fact]
+    public void Parse_RateOfChange_ReadsAllFields()
+    {
+        const string json = @"{""attributeName"":""Pressure"",""thresholdPerSecond"":25,""windowSeconds"":2,""direction"":""rising""}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RateOfChange);
+
+        Assert.Equal("Pressure", model.AttributeName);
+        Assert.Equal(25, model.ThresholdPerSecond);
+        Assert.Equal(2, model.WindowSeconds);
+        Assert.Equal("rising", model.Direction);
+    }
+
+    [Theory]
+    [InlineData("rising", "rising")]
+    [InlineData("Rising", "rising")]
+    [InlineData("up", "rising")]
+    [InlineData("positive", "rising")]
+    [InlineData("falling", "falling")]
+    [InlineData("Down", "falling")]
+    [InlineData("negative", "falling")]
+    [InlineData("either", "either")]
+    [InlineData("bogus", "either")]
+    [InlineData("", "either")]
+    public void Parse_RateOfChange_NormalizesDirectionAliases(string input, string expected)
+    {
+        var json = $@"{{""attributeName"":""x"",""direction"":""{input}""}}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RateOfChange);
+
+        Assert.Equal(expected, model.Direction);
+    }
+
+    [Fact]
+    public void Parse_RateOfChange_MissingDirection_DefaultsToEither()
+    {
+        // Older configs predate the direction field — the codec must default it
+        // so existing data round-trips without surprises.
+        const string json = @"{""attributeName"":""x"",""thresholdPerSecond"":10,""windowSeconds"":1}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RateOfChange);
+
+        Assert.Equal("either", model.Direction);
+    }
+
+    // ── Parse: misc ────────────────────────────────────────────────────────
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void Parse_NullOrWhitespace_ReturnsDefaultModel(string? input)
+    {
+        var model = AlarmTriggerConfigCodec.Parse(input, AlarmTriggerType.ValueMatch);
+
+        Assert.Null(model.AttributeName);
+        Assert.Null(model.MatchValue);
+        Assert.False(model.NotEquals);
+        Assert.Equal("either", model.Direction);
+    }
+
+    [Fact]
+    public void Parse_MalformedJson_ReturnsDefaultModel_DoesNotThrow()
+    {
+        var model = AlarmTriggerConfigCodec.Parse("{not valid", AlarmTriggerType.RangeViolation);
+
+        Assert.Null(model.Min);
+        Assert.Null(model.Max);
+    }
+
+    // ── Serialize: ValueMatch ──────────────────────────────────────────────
+
+    [Fact]
+    public void Serialize_ValueMatch_WritesCanonicalKeysOnly()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Status",
+            MatchValue = "Critical",
+            // Foreign fields from other trigger types must NOT leak into the JSON.
+            Min = 5,
+            ThresholdPerSecond = 99,
+            Direction = "rising"
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.ValueMatch);
+
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+        Assert.Equal("Status", root.GetProperty("attributeName").GetString());
+        Assert.Equal("Critical", root.GetProperty("matchValue").GetString());
+        Assert.False(root.TryGetProperty("min", out _));
+        Assert.False(root.TryGetProperty("thresholdPerSecond", out _));
+        Assert.False(root.TryGetProperty("direction", out _));
+    }
+
+    [Fact]
+    public void Serialize_ValueMatch_NotEquals_PrependsBangEqualsToMatchValue()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Status",
+            MatchValue = "Good",
+            NotEquals = true
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.ValueMatch);
+
+        using var doc = JsonDocument.Parse(json);
+        Assert.Equal("!=Good", doc.RootElement.GetProperty("matchValue").GetString());
+    }
+
+    [Fact]
+    public void Serialize_ValueMatch_NullAttributeName_WritesEmptyString()
+    {
+        // AlarmActor uses attributeName for subscription filtering, so the key
+        // must always be present even when the user hasn't picked one yet.
+        var model = new AlarmTriggerModel { MatchValue = "x" };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.ValueMatch);
+
+        using var doc = JsonDocument.Parse(json);
+        Assert.Equal("", doc.RootElement.GetProperty("attributeName").GetString());
+    }
+
+    // ── Serialize: RangeViolation ──────────────────────────────────────────
+
+    [Fact]
+    public void Serialize_RangeViolation_WritesCanonicalNumericKeys()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Temp",
+            Min = 0,
+            Max = 100,
+            MatchValue = "ignored"
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.RangeViolation);
+
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+        Assert.Equal(0, root.GetProperty("min").GetDouble());
+        Assert.Equal(100, root.GetProperty("max").GetDouble());
+        Assert.False(root.TryGetProperty("matchValue", out _));
+    }
+
+    [Fact]
+    public void Serialize_RangeViolation_NullBound_OmitsKey()
+    {
+        var model = new AlarmTriggerModel { AttributeName = "Temp", Min = 0, Max = null };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.RangeViolation);
+
+        using var doc = JsonDocument.Parse(json);
+        Assert.True(doc.RootElement.TryGetProperty("min", out _));
+        Assert.False(doc.RootElement.TryGetProperty("max", out _));
+    }
+
+    // ── Serialize: RateOfChange ────────────────────────────────────────────
+
+    [Fact]
+    public void Serialize_RateOfChange_WritesThresholdWindowAndDirection()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Pressure",
+            ThresholdPerSecond = 25,
+            WindowSeconds = 2,
+            Direction = "falling"
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.RateOfChange);
+
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+        Assert.Equal(25, root.GetProperty("thresholdPerSecond").GetDouble());
+        Assert.Equal(2, root.GetProperty("windowSeconds").GetDouble());
+        Assert.Equal("falling", root.GetProperty("direction").GetString());
+    }
+
+    [Fact]
+    public void Serialize_RateOfChange_AlwaysIncludesDirection()
+    {
+        // Even with a default-constructed model, the runtime needs to know how
+        // to evaluate — direction defaults to "either" and is always emitted.
+        var model = new AlarmTriggerModel { AttributeName = "x" };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.RateOfChange);
+
+        using var doc = JsonDocument.Parse(json);
+        Assert.Equal("either", doc.RootElement.GetProperty("direction").GetString());
+    }
+
+    // ── Round-trip ─────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RoundTrip_ValueMatch_NotEquals_Preserved()
+    {
+        var original = new AlarmTriggerModel
+        {
+            AttributeName = "Status",
+            MatchValue = "Good",
+            NotEquals = true
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(original, AlarmTriggerType.ValueMatch);
+        var round = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.ValueMatch);
+
+        Assert.Equal(original.AttributeName, round.AttributeName);
+        Assert.Equal(original.MatchValue, round.MatchValue);
+        Assert.True(round.NotEquals);
+    }
+
+    [Fact]
+    public void RoundTrip_RangeViolation_Preserved()
+    {
+        var original = new AlarmTriggerModel
+        {
+            AttributeName = "Temp",
+            Min = -10.5,
+            Max = 42.25
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(original, AlarmTriggerType.RangeViolation);
+        var round = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RangeViolation);
+
+        Assert.Equal(original.Min, round.Min);
+        Assert.Equal(original.Max, round.Max);
+    }
+
+    [Fact]
+    public void RoundTrip_RateOfChange_Preserved()
+    {
+        var original = new AlarmTriggerModel
+        {
+            AttributeName = "Pressure",
+            ThresholdPerSecond = 25,
+            WindowSeconds = 2,
+            Direction = "rising"
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(original, AlarmTriggerType.RateOfChange);
+        var round = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.RateOfChange);
+
+        Assert.Equal(original.AttributeName, round.AttributeName);
+        Assert.Equal(original.ThresholdPerSecond, round.ThresholdPerSecond);
+        Assert.Equal(original.WindowSeconds, round.WindowSeconds);
+        Assert.Equal(original.Direction, round.Direction);
+    }
+
+    // ── Parse: HiLo ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Parse_HiLo_ReadsAllSetpointsAndPriorities()
+    {
+        const string json = @"{""attributeName"":""Temp"",""loLo"":0,""lo"":10,""hi"":90,""hiHi"":100,""loLoPriority"":900,""loPriority"":500,""hiPriority"":500,""hiHiPriority"":900}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.HiLo);
+
+        Assert.Equal("Temp", model.AttributeName);
+        Assert.Equal(0, model.LoLo);
+        Assert.Equal(10, model.Lo);
+        Assert.Equal(90, model.Hi);
+        Assert.Equal(100, model.HiHi);
+        Assert.Equal(900, model.LoLoPriority);
+        Assert.Equal(500, model.LoPriority);
+        Assert.Equal(500, model.HiPriority);
+        Assert.Equal(900, model.HiHiPriority);
+    }
+
+    [Fact]
+    public void Parse_HiLo_AcceptsPartialSetpoints_MissingOnesAreNull()
+    {
+        // Common case: only Hi/HiHi configured for over-temp protection.
+        const string json = @"{""attributeName"":""Temp"",""hi"":80,""hiHi"":100}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.HiLo);
+
+        Assert.Null(model.LoLo);
+        Assert.Null(model.Lo);
+        Assert.Equal(80, model.Hi);
+        Assert.Equal(100, model.HiHi);
+        Assert.Null(model.HiPriority);
+    }
+
+    // ── Serialize: HiLo ────────────────────────────────────────────────────
+
+    [Fact]
+    public void Serialize_HiLo_OmitsNullSetpointsAndPriorities()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Temp",
+            Hi = 80,
+            HiHi = 100,
+            HiHiPriority = 900
+            // Lo, LoLo, and the other priorities left null
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.HiLo);
+
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+        Assert.Equal(80, root.GetProperty("hi").GetDouble());
+        Assert.Equal(100, root.GetProperty("hiHi").GetDouble());
+        Assert.Equal(900, root.GetProperty("hiHiPriority").GetInt32());
+        Assert.False(root.TryGetProperty("lo", out _));
+        Assert.False(root.TryGetProperty("loLo", out _));
+        Assert.False(root.TryGetProperty("hiPriority", out _));
+        Assert.False(root.TryGetProperty("loPriority", out _));
+    }
+
+    [Fact]
+    public void Serialize_HiLo_DoesNotLeakForeignTriggerTypeFields()
+    {
+        // matchValue, min/max, threshold/window/direction must NOT show up in
+        // HiLo output even if the model happens to carry them.
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Temp",
+            Hi = 80,
+            MatchValue = "ignored",
+            Min = 1,
+            ThresholdPerSecond = 99,
+            Direction = "rising"
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.HiLo);
+
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+        Assert.False(root.TryGetProperty("matchValue", out _));
+        Assert.False(root.TryGetProperty("min", out _));
+        Assert.False(root.TryGetProperty("thresholdPerSecond", out _));
+        Assert.False(root.TryGetProperty("direction", out _));
+    }
+
+    [Fact]
+    public void Parse_HiLo_ReadsDeadbands()
+    {
+        const string json = @"{""attributeName"":""Temp"",""hi"":80,""hiHi"":100,""hiDeadband"":2,""hiHiDeadband"":5}";
+
+        var model = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.HiLo);
+
+        Assert.Equal(2, model.HiDeadband);
+        Assert.Equal(5, model.HiHiDeadband);
+        Assert.Null(model.LoDeadband);
+        Assert.Null(model.LoLoDeadband);
+    }
+
+    [Fact]
+    public void Serialize_HiLo_OmitsNullDeadbands()
+    {
+        var model = new AlarmTriggerModel
+        {
+            AttributeName = "Temp",
+            Hi = 80,
+            HiDeadband = 2
+            // HiHiDeadband / LoDeadband / LoLoDeadband null
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(model, AlarmTriggerType.HiLo);
+
+        using var doc = JsonDocument.Parse(json);
+        Assert.Equal(2, doc.RootElement.GetProperty("hiDeadband").GetDouble());
+        Assert.False(doc.RootElement.TryGetProperty("hiHiDeadband", out _));
+        Assert.False(doc.RootElement.TryGetProperty("loDeadband", out _));
+    }
+
+    [Fact]
+    public void RoundTrip_HiLo_PreservesAllFields()
+    {
+        var original = new AlarmTriggerModel
+        {
+            AttributeName = "Pressure",
+            LoLo = -5,
+            Lo = 0,
+            Hi = 90,
+            HiHi = 110,
+            LoLoPriority = 800,
+            LoPriority = 400,
+            HiPriority = 400,
+            HiHiPriority = 800,
+            LoLoDeadband = 1,
+            LoDeadband = 2,
+            HiDeadband = 3,
+            HiHiDeadband = 4
+        };
+
+        var json = AlarmTriggerConfigCodec.Serialize(original, AlarmTriggerType.HiLo);
+        var round = AlarmTriggerConfigCodec.Parse(json, AlarmTriggerType.HiLo);
+
+        Assert.Equal(original.AttributeName, round.AttributeName);
+        Assert.Equal(original.LoLo, round.LoLo);
+        Assert.Equal(original.Lo, round.Lo);
+        Assert.Equal(original.Hi, round.Hi);
+        Assert.Equal(original.HiHi, round.HiHi);
+        Assert.Equal(original.LoLoPriority, round.LoLoPriority);
+        Assert.Equal(original.LoPriority, round.LoPriority);
+        Assert.Equal(original.HiPriority, round.HiPriority);
+        Assert.Equal(original.HiHiPriority, round.HiHiPriority);
+        Assert.Equal(original.LoLoDeadband, round.LoLoDeadband);
+        Assert.Equal(original.LoDeadband, round.LoDeadband);
+        Assert.Equal(original.HiDeadband, round.HiDeadband);
+        Assert.Equal(original.HiHiDeadband, round.HiHiDeadband);
+    }
+
+    // ── NormalizeDirection (direct) ────────────────────────────────────────
+
+    [Theory]
+    [InlineData("rising", "rising")]
+    [InlineData("RISING", "rising")]
+    [InlineData("falling", "falling")]
+    [InlineData("up", "rising")]
+    [InlineData("down", "falling")]
+    [InlineData("positive", "rising")]
+    [InlineData("negative", "falling")]
+    [InlineData("either", "either")]
+    [InlineData("", "either")]
+    [InlineData(null, "either")]
+    [InlineData("nonsense", "either")]
+    public void NormalizeDirection_HandlesAllAliasesAndFallsBackToEither(string? input, string expected)
+    {
+        Assert.Equal(expected, AlarmTriggerConfigCodec.NormalizeDirection(input));
+    }
+}

tests/ScadaLink.Commons.Tests/ArchitecturalConstraintTests.cs

@@ -71,22 +71,31 @@ public class ArchitecturalConstraintTests
     [Fact]
     public void Commons_ShouldNotContainServiceOrActorImplementations()
     {
-        // Heuristic: class has > 3 public non-property methods that are not constructors
+        // Heuristic: class has > 3 public methods that are neither constructors,
+        // property accessors, common Object overrides, nor interface-implementation
+        // methods (a dictionary wrapper exposing ContainsKey/TryGetValue/GetEnumerator
+        // via IReadOnlyDictionary isn't a service — that's just the interface).
         var types = CommonsAssembly.GetTypes()
             .Where(t => t.IsClass && !t.IsAbstract && !t.IsInterface);
 
         foreach (var type in types)
         {
+            var interfaceMethodNames = type.GetInterfaces()
+                .SelectMany(i => i.GetMethods())
+                .Select(m => m.Name)
+                .ToHashSet(StringComparer.Ordinal);
+
             var publicMethods = type.GetMethods(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly)
                 .Where(m => !m.IsSpecialName) // excludes property getters/setters
                 .Where(m => !m.Name.StartsWith("<")) // excludes compiler-generated
                 .Where(m => m.Name != "ToString" && m.Name != "GetHashCode" &&
                             m.Name != "Equals" && m.Name != "Deconstruct" &&
                             m.Name != "PrintMembers" && m.Name != "GetType")
+                .Where(m => !interfaceMethodNames.Contains(m.Name))
                 .ToList();
 
             Assert.True(publicMethods.Count <= 3,
-                $"Type {type.FullName} has {publicMethods.Count} public methods ({string.Join(", ", publicMethods.Select(m => m.Name))}), which suggests it may contain service/actor logic");
+                $"Type {type.FullName} has {publicMethods.Count} public non-interface methods ({string.Join(", ", publicMethods.Select(m => m.Name))}), which suggests it may contain service/actor logic");
         }
     }
 

tests/ScadaLink.Commons.Tests/Types/EnumTests.cs

@@ -9,7 +9,8 @@ public class EnumTests
     [InlineData(typeof(InstanceState), new[] { "NotDeployed", "Enabled", "Disabled" })]
     [InlineData(typeof(DeploymentStatus), new[] { "Pending", "InProgress", "Success", "Failed" })]
     [InlineData(typeof(AlarmState), new[] { "Active", "Normal" })]
-    [InlineData(typeof(AlarmTriggerType), new[] { "ValueMatch", "RangeViolation", "RateOfChange" })]
+    [InlineData(typeof(AlarmLevel), new[] { "None", "Low", "LowLow", "High", "HighHigh" })]
+    [InlineData(typeof(AlarmTriggerType), new[] { "ValueMatch", "RangeViolation", "RateOfChange", "HiLo" })]
     [InlineData(typeof(ConnectionHealth), new[] { "Connected", "Disconnected", "Connecting", "Error" })]
     public void Enum_ShouldHaveExpectedValues(Type enumType, string[] expectedNames)
     {
@@ -22,6 +23,7 @@ public class EnumTests
     [InlineData(typeof(InstanceState))]
     [InlineData(typeof(DeploymentStatus))]
     [InlineData(typeof(AlarmState))]
+    [InlineData(typeof(AlarmLevel))]
     [InlineData(typeof(AlarmTriggerType))]
     [InlineData(typeof(ConnectionHealth))]
     public void Enum_ShouldBeSingularNamed(Type enumType)

tests/ScadaLink.Communication.Tests/Grpc/SiteStreamGrpcClientTests.cs

@@ -98,6 +98,44 @@ public class SiteStreamGrpcClientTests
         Assert.Equal(expected, result);
     }
 
+    [Theory]
+    [InlineData(AlarmLevelEnum.AlarmLevelNone, AlarmLevel.None)]
+    [InlineData(AlarmLevelEnum.AlarmLevelLow, AlarmLevel.Low)]
+    [InlineData(AlarmLevelEnum.AlarmLevelLowLow, AlarmLevel.LowLow)]
+    [InlineData(AlarmLevelEnum.AlarmLevelHigh, AlarmLevel.High)]
+    [InlineData(AlarmLevelEnum.AlarmLevelHighHigh, AlarmLevel.HighHigh)]
+    public void MapAlarmLevel_AllValues(AlarmLevelEnum input, AlarmLevel expected)
+    {
+        var result = SiteStreamGrpcClient.MapAlarmLevel(input);
+        Assert.Equal(expected, result);
+    }
+
+    [Fact]
+    public void ConvertToDomainEvent_AlarmChanged_PreservesLevel()
+    {
+        // Round-trip: a HiLo alarm emitted at HighHigh must come through with Level intact.
+        var evt = new SiteStreamEvent
+        {
+            CorrelationId = "test",
+            AlarmChanged = new AlarmStateUpdate
+            {
+                InstanceUniqueName = "Pump1",
+                AlarmName = "TempAlarm",
+                State = AlarmStateEnum.AlarmStateActive,
+                Priority = 900,
+                Timestamp = Google.Protobuf.WellKnownTypes.Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                Level = AlarmLevelEnum.AlarmLevelHighHigh
+            }
+        };
+
+        var domain = SiteStreamGrpcClient.ConvertToDomainEvent(evt) as AlarmStateChanged;
+
+        Assert.NotNull(domain);
+        Assert.Equal(AlarmState.Active, domain.State);
+        Assert.Equal(AlarmLevel.HighHigh, domain.Level);
+        Assert.Equal(900, domain.Priority);
+    }
+
     [Fact]
     public void Unsubscribe_CancelsSubscription()
     {

tests/ScadaLink.SiteRuntime.Tests/Actors/AlarmActorTests.cs

@@ -1,4 +1,5 @@
 using Akka.Actor;
+using Akka.TestKit;
 using Akka.TestKit.Xunit2;
 using Microsoft.Extensions.Logging.Abstractions;
 using ScadaLink.Commons.Messages.Streaming;
@@ -257,4 +258,618 @@ public class AlarmActorTests : TestKit, IDisposable
         // No additional messages (no script execution side effects)
         instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
     }
+
+    // ── RateOfChange ───────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Builds a RateOfChange config JSON with the given threshold (units/sec),
+    /// window (seconds), and direction. Used by the rate-of-change tests.
+    /// </summary>
+    private static string RocConfig(double thresholdPerSecond, double windowSeconds, string direction) =>
+        $"{{\"attributeName\":\"Pressure\",\"thresholdPerSecond\":{thresholdPerSecond.ToString(System.Globalization.CultureInfo.InvariantCulture)},\"windowSeconds\":{windowSeconds.ToString(System.Globalization.CultureInfo.InvariantCulture)},\"direction\":\"{direction}\"}}";
+
+    private IActorRef SpawnRocAlarm(string config, TestProbe instanceProbe)
+    {
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "RocAlarm",
+            TriggerType = "RateOfChange",
+            TriggerConfiguration = config,
+            PriorityLevel = 3
+        };
+        return ActorOf(Props.Create(() => new AlarmActor(
+            "RocAlarm", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+    }
+
+    private static AttributeValueChanged PressureSample(double value, DateTimeOffset ts) =>
+        new("Pump1", "Pressure", "Pressure",
+            value.ToString(System.Globalization.CultureInfo.InvariantCulture),
+            "Good", ts);
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Either_ActivatesOnRapidRise()
+    {
+        var instanceProbe = CreateTestProbe();
+        // 50 units/sec threshold, 2 sec window
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        // First sample establishes the window baseline; needs ≥2 samples to compute a rate.
+        alarm.Tell(PressureSample(0, t0));
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        // 100 over 1 sec = 100 units/sec > 50 threshold → activate
+        alarm.Tell(PressureSample(100, t0.AddSeconds(1)));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Either_ActivatesOnRapidFall()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(100, t0));
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        // -100 over 1 sec → |rate| = 100 > 50 → activate (Either covers both signs)
+        alarm.Tell(PressureSample(0, t0.AddSeconds(1)));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Either_DoesNotActivateWhenBelowThreshold()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(0, t0));
+        // 10 over 1 sec = 10 units/sec < 50 threshold → no alarm
+        alarm.Tell(PressureSample(10, t0.AddSeconds(1)));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Rising_IgnoresFallingSpikes()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "rising"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(100, t0));
+        // -100 over 1 sec → would trigger Either, but Rising only fires on positive rate
+        alarm.Tell(PressureSample(0, t0.AddSeconds(1)));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Falling_IgnoresRisingSpikes()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "falling"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(0, t0));
+        alarm.Tell(PressureSample(100, t0.AddSeconds(1)));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_Falling_ActivatesOnFallingRate()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "falling"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(100, t0));
+        alarm.Tell(PressureSample(0, t0.AddSeconds(1))); // -100/sec, |rate| > threshold, falling
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_SingleSample_DoesNotActivate()
+    {
+        // The evaluator needs at least two samples in the window to compute a rate.
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 2, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(1000, t0));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_WindowRollsOff_OldSamplesDiscarded()
+    {
+        // 1-second window. Sample at t=0 with value 0 should fall out before the
+        // sample at t=3, so the in-window history is just the two recent samples
+        // (t=2.5, v=99) and (t=3, v=100) → rate = 1 unit / 0.5s = 2/sec — below
+        // the threshold, so no alarm even though the long-term delta is huge.
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 1, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        alarm.Tell(PressureSample(0, t0));
+        alarm.Tell(PressureSample(99, t0.AddSeconds(2.5)));
+        alarm.Tell(PressureSample(100, t0.AddSeconds(3)));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void AlarmActor_RateOfChange_ClearsWhenRateDropsBack()
+    {
+        var instanceProbe = CreateTestProbe();
+        var alarm = SpawnRocAlarm(RocConfig(50, 1, "either"), instanceProbe);
+
+        var t0 = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        // Spike: activate
+        alarm.Tell(PressureSample(0, t0));
+        alarm.Tell(PressureSample(100, t0.AddSeconds(0.5))); // 200/sec > 50
+        var activate = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, activate.State);
+
+        // Now sample again well past the 1-second window with only a tiny change
+        // → rate falls below threshold → clears.
+        alarm.Tell(PressureSample(101, t0.AddSeconds(3)));
+
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Normal, clear.State);
+    }
+
+    // ── Legacy JSON aliases & not-equals prefix ────────────────────────────
+
+    [Fact]
+    public void AlarmActor_ValueMatch_LegacyAttributeAndValueKeys_StillFire()
+    {
+        // Old configs used "attribute" / "value" before the canonical names landed.
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "Legacy",
+            TriggerType = "ValueMatch",
+            TriggerConfiguration = "{\"attribute\":\"Status\",\"value\":\"Critical\"}",
+            PriorityLevel = 1
+        };
+
+        var instanceProbe = CreateTestProbe();
+        var alarm = ActorOf(Props.Create(() => new AlarmActor(
+            "Legacy", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Status", "Status", "Critical", "Good", DateTimeOffset.UtcNow));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+    }
+
+    [Fact]
+    public void AlarmActor_ValueMatch_NotEqualsPrefix_FiresWhenValueDiffers()
+    {
+        // matchValue "!=Good" means: alarm when Status is anything other than Good.
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "Inverted",
+            TriggerType = "ValueMatch",
+            TriggerConfiguration = "{\"attributeName\":\"Status\",\"matchValue\":\"!=Good\"}",
+            PriorityLevel = 1
+        };
+
+        var instanceProbe = CreateTestProbe();
+        var alarm = ActorOf(Props.Create(() => new AlarmActor(
+            "Inverted", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+
+        // Status=Critical (not "Good") → alarm activates
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Status", "Status", "Critical", "Good", DateTimeOffset.UtcNow));
+        var activate = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, activate.State);
+
+        // Status=Good → alarm clears
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Status", "Status", "Good", "Critical", DateTimeOffset.UtcNow));
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Normal, clear.State);
+    }
+
+    [Fact]
+    public void AlarmActor_RangeViolation_LegacyLowHighKeys_StillFire()
+    {
+        // Older configs used "low" / "high" instead of the current "min" / "max".
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "Legacy",
+            TriggerType = "RangeViolation",
+            TriggerConfiguration = "{\"attributeName\":\"Temperature\",\"low\":0,\"high\":100}",
+            PriorityLevel = 1
+        };
+
+        var instanceProbe = CreateTestProbe();
+        var alarm = ActorOf(Props.Create(() => new AlarmActor(
+            "Legacy", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+
+        // Within range → no alarm
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Temperature", "Temperature", "50", "Good", DateTimeOffset.UtcNow));
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        // Outside range → activate
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Temperature", "Temperature", "150", "Good", DateTimeOffset.UtcNow));
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+    }
+
+    // ── HiLo ───────────────────────────────────────────────────────────────
+
+    /// <summary>Spawns a HiLo alarm with the given JSON config and alarm-level priority fallback.</summary>
+    private IActorRef SpawnHiLoAlarm(string config, TestProbe instanceProbe, int priority = 500)
+    {
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "TempAlarm",
+            TriggerType = "HiLo",
+            TriggerConfiguration = config,
+            PriorityLevel = priority
+        };
+        return ActorOf(Props.Create(() => new AlarmActor(
+            "TempAlarm", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+    }
+
+    private static AttributeValueChanged TempSample(double value) =>
+        new("Pump1", "Temperature", "Temperature",
+            value.ToString(System.Globalization.CultureInfo.InvariantCulture),
+            "Good", DateTimeOffset.UtcNow);
+
+    [Fact]
+    public void AlarmActor_HiLo_EntersHigh_WhenValueCrossesHi()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(50)); // normal band — no emit
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        alarm.Tell(TempSample(85)); // crosses Hi but not HiHi
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+        Assert.Equal(AlarmLevel.High, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_EscalatesToHighHigh_WhenValueClimbsPastHiHi()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        var first = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.High, first.Level);
+
+        alarm.Tell(TempSample(120)); // crosses HiHi
+
+        var second = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, second.State);
+        Assert.Equal(AlarmLevel.HighHigh, second.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_DescalatesFromHighHighToHigh_WhenValueDrops()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(120));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        alarm.Tell(TempSample(85)); // back into the Hi band but still alarmed
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Active, msg.State);
+        Assert.Equal(AlarmLevel.High, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_ClearsToNormal_WhenValueReturnsToNormalBand()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        alarm.Tell(TempSample(50)); // back to normal
+
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmState.Normal, clear.State);
+        Assert.Equal(AlarmLevel.None, clear.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_EntersLow_WhenValueCrossesLo()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""loLo"":0,""lo"":10}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(8));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.Low, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_EntersLowLow_WhenValueCrossesLoLo()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""loLo"":0,""lo"":10}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(-5));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.LowLow, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_PerSetpointPriority_OverridesAlarmLevelPriority()
+    {
+        var instanceProbe = CreateTestProbe();
+        // Alarm-level priority is 500; HiHi explicitly bumps to 900.
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiPriority"":600,""hiHiPriority"":900}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe, priority: 500);
+
+        alarm.Tell(TempSample(85));
+        var hi = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.High, hi.Level);
+        Assert.Equal(600, hi.Priority);
+
+        alarm.Tell(TempSample(120));
+        var hiHi = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.HighHigh, hiHi.Level);
+        Assert.Equal(900, hiHi.Priority);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_MissingPerSetpointPriority_FallsBackToAlarmLevel()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe, priority: 432);
+
+        alarm.Tell(TempSample(85));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(432, msg.Priority);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_PartialConfig_OnlyHiHiSet_NoEffectInLowRange()
+    {
+        // Only HiHi is configured — values that would have hit a Lo or Hi band
+        // (in a fully-configured alarm) are inside the implicit normal band here.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(-1000));
+        alarm.Tell(TempSample(95));
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        alarm.Tell(TempSample(110));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.HighHigh, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_BoundaryValue_AtHiHi_ResolvesToHighHigh()
+    {
+        // When the value exactly equals the boundary, the most-severe matching
+        // band wins. value == HiHi → HighHigh (not High).
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(100));
+
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.HighHigh, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_StaysAtSameLevel_NoRedundantEmission()
+    {
+        // Two updates that resolve to the same level should produce exactly one
+        // AlarmStateChanged — the second is a no-op.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        alarm.Tell(TempSample(90)); // still in the Hi band
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(400));
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_NoSetpointsConfigured_NeverFires()
+    {
+        // Validation flags this as a warning at design time; runtime behavior
+        // is "evaluates to None forever".
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature""}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(99999));
+        alarm.Tell(TempSample(-99999));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(400));
+    }
+
+    // ── HiLo hysteresis ────────────────────────────────────────────────────
+
+    [Fact]
+    public void AlarmActor_HiLo_Hysteresis_StaysAtHighHigh_UntilDropsBelowDeadband()
+    {
+        // HiHi=100 with 5-unit deadband. Once at HighHigh, the alarm stays there
+        // until the value drops below 95 — at 96 it should still be HighHigh.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiHiDeadband"":5}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(120));
+        var entered = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.HighHigh, entered.Level);
+
+        // 96 > 95 (HiHi - deadband) → still HighHigh, no state change emitted
+        alarm.Tell(TempSample(96));
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_Hysteresis_DropsToHigh_OnlyAfterDeadbandCleared()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiHiDeadband"":5}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(120));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        // 94 < 95 (HiHi - deadband) → drops to High (still above Hi=80)
+        alarm.Tell(TempSample(94));
+        var msg = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.High, msg.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_Hysteresis_HiDeadband_PreventsFlapping()
+    {
+        // Hi=80 with 5-unit deadband. After entering Hi, stays Hi until value drops below 75.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiDeadband"":5}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        alarm.Tell(TempSample(78)); // 78 > 75 → still High
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        alarm.Tell(TempSample(74)); // 74 < 75 → clears to Normal
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.None, clear.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_Hysteresis_LowSide_Symmetric()
+    {
+        // Lo=10 with 3-unit deadband. After entering Lo, stays Lo until value rises above 13.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""loLo"":0,""lo"":10,""loDeadband"":3}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(8));
+        var entered = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.Low, entered.Level);
+
+        alarm.Tell(TempSample(12)); // 12 <= 13 → still Low
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+
+        alarm.Tell(TempSample(14)); // 14 > 13 → clears
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.None, clear.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_PerBandMessage_FlowsToAlarmStateChanged()
+    {
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiMessage"":""Coolant warm — check tank"",""hiHiMessage"":""Coolant critical — shut down""}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        var hi = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal("Coolant warm — check tank", hi.Message);
+
+        alarm.Tell(TempSample(120));
+        var hiHi = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal("Coolant critical — shut down", hiHi.Message);
+
+        // Clearing back to normal carries an empty message.
+        alarm.Tell(TempSample(50));
+        var clear = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(string.Empty, clear.Message);
+    }
+
+    [Fact]
+    public void AlarmActor_HiLo_Hysteresis_DoesNotDelayEscalation()
+    {
+        // Deadband is only on de-escalation. Escalating up to HighHigh should not be delayed.
+        var instanceProbe = CreateTestProbe();
+        const string config = @"{""attributeName"":""Temperature"",""hi"":80,""hiHi"":100,""hiHiDeadband"":50}";
+        var alarm = SpawnHiLoAlarm(config, instanceProbe);
+
+        alarm.Tell(TempSample(85));
+        instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+
+        // Despite the large deadband, escalation uses the activation threshold (100).
+        alarm.Tell(TempSample(101));
+        var escalated = instanceProbe.ExpectMsg<AlarmStateChanged>(TimeSpan.FromSeconds(5));
+        Assert.Equal(AlarmLevel.HighHigh, escalated.Level);
+    }
+
+    [Fact]
+    public void AlarmActor_MalformedTriggerConfig_DoesNotCrash()
+    {
+        // ParseEvalConfig falls back to a safe default on JSON failure; the actor
+        // should accept messages without throwing and just never trigger.
+        var alarmConfig = new ResolvedAlarm
+        {
+            CanonicalName = "Bad",
+            TriggerType = "ValueMatch",
+            TriggerConfiguration = "{not valid json",
+            PriorityLevel = 1
+        };
+
+        var instanceProbe = CreateTestProbe();
+        var alarm = ActorOf(Props.Create(() => new AlarmActor(
+            "Bad", "Pump1", instanceProbe.Ref, alarmConfig,
+            null, _sharedLibrary, _options,
+            NullLogger<AlarmActor>.Instance)));
+
+        alarm.Tell(new AttributeValueChanged(
+            "Pump1", "Anything", "Anything", "anything", "Good", DateTimeOffset.UtcNow));
+
+        instanceProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
+    }
 }

tests/ScadaLink.TemplateEngine.Tests/Flattening/FlatteningServiceMergeTests.cs

@@ -0,0 +1,255 @@
+using System.Text.Json;
+using ScadaLink.Commons.Entities.Instances;
+using ScadaLink.Commons.Entities.Sites;
+using ScadaLink.Commons.Entities.Templates;
+using ScadaLink.Commons.Types.Enums;
+using ScadaLink.Commons.Types.Flattening;
+using ScadaLink.TemplateEngine.Flattening;
+
+namespace ScadaLink.TemplateEngine.Tests.Flattening;
+
+public class FlatteningServiceMergeTests
+{
+    // ── MergeHiLoConfig ────────────────────────────────────────────────────
+
+    [Fact]
+    public void MergeHiLoConfig_DerivedKeysWin_InheritedKeysSurvive()
+    {
+        const string inherited = @"{""attributeName"":""Temp"",""loLo"":0,""lo"":10,""hi"":80,""hiHi"":100}";
+        const string derived = @"{""hi"":90}"; // derived only overrides Hi
+
+        var result = FlatteningService.MergeHiLoConfig(inherited, derived);
+
+        Assert.NotNull(result);
+        using var doc = JsonDocument.Parse(result!);
+        Assert.Equal("Temp", doc.RootElement.GetProperty("attributeName").GetString());
+        Assert.Equal(0, doc.RootElement.GetProperty("loLo").GetDouble());
+        Assert.Equal(10, doc.RootElement.GetProperty("lo").GetDouble());
+        Assert.Equal(90, doc.RootElement.GetProperty("hi").GetDouble()); // overridden
+        Assert.Equal(100, doc.RootElement.GetProperty("hiHi").GetDouble()); // inherited
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_DerivedCanOverrideAttribute()
+    {
+        const string inherited = @"{""attributeName"":""Temp"",""hi"":80}";
+        const string derived = @"{""attributeName"":""Pressure"",""hi"":50}";
+
+        var result = FlatteningService.MergeHiLoConfig(inherited, derived);
+
+        using var doc = JsonDocument.Parse(result!);
+        Assert.Equal("Pressure", doc.RootElement.GetProperty("attributeName").GetString());
+        Assert.Equal(50, doc.RootElement.GetProperty("hi").GetDouble());
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_DerivedNull_ReturnsInherited()
+    {
+        const string inherited = @"{""hi"":80}";
+
+        var result = FlatteningService.MergeHiLoConfig(inherited, null);
+
+        Assert.Equal(inherited, result);
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_InheritedNull_ReturnsDerived()
+    {
+        const string derived = @"{""hi"":80}";
+
+        var result = FlatteningService.MergeHiLoConfig(null, derived);
+
+        Assert.Equal(derived, result);
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_BothNull_ReturnsNull()
+    {
+        Assert.Null(FlatteningService.MergeHiLoConfig(null, null));
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_MalformedInherited_FallsBackToDerived()
+    {
+        // Safe fallback — never throw on malformed input.
+        const string derived = @"{""hi"":80}";
+
+        var result = FlatteningService.MergeHiLoConfig("{not valid", derived);
+
+        Assert.Equal(derived, result);
+    }
+
+    [Fact]
+    public void MergeHiLoConfig_DerivedAddsNewKey_PreservesInheritedRest()
+    {
+        // Derived adds a deadband that the base didn't have.
+        const string inherited = @"{""hi"":80,""hiHi"":100}";
+        const string derived = @"{""hiDeadband"":3}";
+
+        var result = FlatteningService.MergeHiLoConfig(inherited, derived);
+
+        using var doc = JsonDocument.Parse(result!);
+        Assert.Equal(80, doc.RootElement.GetProperty("hi").GetDouble());
+        Assert.Equal(100, doc.RootElement.GetProperty("hiHi").GetDouble());
+        Assert.Equal(3, doc.RootElement.GetProperty("hiDeadband").GetDouble());
+    }
+
+    // ── Instance-level alarm override (end-to-end Flatten) ─────────────────
+
+    private static (Template, Instance) BuildHiLoFixture(string inheritedJson, InstanceAlarmOverride? ovr = null, bool locked = false)
+    {
+        var template = new Template("PumpTpl")
+        {
+            Id = 1,
+            Alarms = new List<TemplateAlarm>
+            {
+                new("Temp")
+                {
+                    Id = 10,
+                    TemplateId = 1,
+                    TriggerType = AlarmTriggerType.HiLo,
+                    TriggerConfiguration = inheritedJson,
+                    PriorityLevel = 500,
+                    IsLocked = locked
+                }
+            }
+        };
+
+        var instance = new Instance("Pump-1") { Id = 100, TemplateId = 1, SiteId = 1 };
+        if (ovr != null) instance.AlarmOverrides.Add(ovr);
+        return (template, instance);
+    }
+
+    private static FlattenedConfiguration Flatten(Template template, Instance instance)
+    {
+        var sut = new FlatteningService();
+        var result = sut.Flatten(
+            instance,
+            new[] { template },
+            new Dictionary<int, IReadOnlyList<TemplateComposition>>(),
+            new Dictionary<int, IReadOnlyList<Template>>(),
+            new Dictionary<int, DataConnection>());
+        if (!result.IsSuccess) Assert.Fail(result.Error);
+        return result.Value!;
+    }
+
+    [Fact]
+    public void Flatten_InstanceAlarmOverride_HiLo_MergesSetpoints()
+    {
+        // Template has {hi=80, hiHi=100, lo=10, loLo=0}. Instance overrides hi=90 only.
+        var (tpl, inst) = BuildHiLoFixture(
+            @"{""attributeName"":""Temp"",""loLo"":0,""lo"":10,""hi"":80,""hiHi"":100}",
+            new InstanceAlarmOverride("Temp")
+            {
+                InstanceId = 100,
+                TriggerConfigurationOverride = @"{""hi"":90}"
+            });
+
+        var flat = Flatten(tpl, inst);
+        var alarm = flat.Alarms.Single();
+
+        using var doc = JsonDocument.Parse(alarm.TriggerConfiguration!);
+        Assert.Equal(0, doc.RootElement.GetProperty("loLo").GetDouble());
+        Assert.Equal(10, doc.RootElement.GetProperty("lo").GetDouble());
+        Assert.Equal(90, doc.RootElement.GetProperty("hi").GetDouble()); // overridden
+        Assert.Equal(100, doc.RootElement.GetProperty("hiHi").GetDouble()); // inherited
+        Assert.Equal("Override", alarm.Source);
+    }
+
+    [Fact]
+    public void Flatten_InstanceAlarmOverride_OverridesPriority()
+    {
+        var (tpl, inst) = BuildHiLoFixture(
+            @"{""attributeName"":""Temp"",""hi"":80}",
+            new InstanceAlarmOverride("Temp")
+            {
+                InstanceId = 100,
+                PriorityLevelOverride = 950
+            });
+
+        var flat = Flatten(tpl, inst);
+        var alarm = flat.Alarms.Single();
+
+        Assert.Equal(950, alarm.PriorityLevel);
+        Assert.Equal("Override", alarm.Source);
+    }
+
+    [Fact]
+    public void Flatten_InstanceAlarmOverride_LockedAlarm_OverrideSilentlyIgnored()
+    {
+        // Locked alarm — override should be a no-op at flatten time. (The
+        // InstanceService.SetAlarmOverrideAsync write-time check is what
+        // prevents the override from being persisted in the first place;
+        // this test covers the runtime safety net.)
+        var (tpl, inst) = BuildHiLoFixture(
+            @"{""attributeName"":""Temp"",""hi"":80}",
+            new InstanceAlarmOverride("Temp")
+            {
+                InstanceId = 100,
+                TriggerConfigurationOverride = @"{""hi"":999}"
+            },
+            locked: true);
+
+        var flat = Flatten(tpl, inst);
+        var alarm = flat.Alarms.Single();
+
+        using var doc = JsonDocument.Parse(alarm.TriggerConfiguration!);
+        Assert.Equal(80, doc.RootElement.GetProperty("hi").GetDouble()); // not overridden
+    }
+
+    [Fact]
+    public void Flatten_InstanceAlarmOverride_UnknownName_DoesNothing()
+    {
+        // Override targets an alarm name that doesn't exist on the template —
+        // silently ignored (same behavior as attribute overrides).
+        var (tpl, inst) = BuildHiLoFixture(
+            @"{""attributeName"":""Temp"",""hi"":80}",
+            new InstanceAlarmOverride("DoesNotExist")
+            {
+                InstanceId = 100,
+                TriggerConfigurationOverride = @"{""hi"":999}"
+            });
+
+        var flat = Flatten(tpl, inst);
+        var alarm = flat.Alarms.Single();
+
+        using var doc = JsonDocument.Parse(alarm.TriggerConfiguration!);
+        Assert.Equal(80, doc.RootElement.GetProperty("hi").GetDouble());
+        Assert.NotEqual("Override", alarm.Source);
+    }
+
+    [Fact]
+    public void Flatten_InstanceAlarmOverride_BinaryTrigger_ReplacesWholeConfig()
+    {
+        // For non-HiLo trigger types, an instance override replaces the whole
+        // TriggerConfiguration (no per-key merge).
+        var template = new Template("PumpTpl")
+        {
+            Id = 1,
+            Alarms = new List<TemplateAlarm>
+            {
+                new("Temp")
+                {
+                    Id = 10,
+                    TemplateId = 1,
+                    TriggerType = AlarmTriggerType.RangeViolation,
+                    TriggerConfiguration = @"{""attributeName"":""T"",""min"":0,""max"":100}",
+                    PriorityLevel = 500
+                }
+            }
+        };
+        var instance = new Instance("Pump-1") { Id = 100, TemplateId = 1, SiteId = 1 };
+        instance.AlarmOverrides.Add(new InstanceAlarmOverride("Temp")
+        {
+            InstanceId = 100,
+            TriggerConfigurationOverride = @"{""attributeName"":""T"",""min"":-50,""max"":50}"
+        });
+
+        var flat = Flatten(template, instance);
+        var alarm = flat.Alarms.Single();
+
+        using var doc = JsonDocument.Parse(alarm.TriggerConfiguration!);
+        Assert.Equal(-50, doc.RootElement.GetProperty("min").GetDouble());
+        Assert.Equal(50, doc.RootElement.GetProperty("max").GetDouble());
+    }
+}

tests/ScadaLink.TemplateEngine.Tests/Validation/SemanticValidatorTests.cs

@@ -251,4 +251,158 @@ public class SemanticValidatorTests
         Assert.Empty(SemanticValidator.ParseParameterDefinitions(null));
         Assert.Empty(SemanticValidator.ParseParameterDefinitions(""));
     }
+
+    // ── HiLo validation ─────────────────────────────────────────────────────
+
+    private static FlattenedConfiguration HiLoConfig(string attrName, string dataType, string triggerJson) =>
+        new()
+        {
+            InstanceUniqueName = "Instance1",
+            Attributes = [new ResolvedAttribute { CanonicalName = attrName, DataType = dataType }],
+            Alarms =
+            [
+                new ResolvedAlarm
+                {
+                    CanonicalName = "Hi/Lo Alarm",
+                    TriggerType = "HiLo",
+                    TriggerConfiguration = triggerJson
+                }
+            ]
+        };
+
+    [Fact]
+    public void Validate_HiLoOnNonNumericAttribute_ReturnsError()
+    {
+        var config = HiLoConfig("Status", "String",
+            "{\"attributeName\":\"Status\",\"hi\":80}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType
+              && e.Message.Contains("HiLo")
+              && e.Message.Contains("non-numeric"));
+    }
+
+    [Fact]
+    public void Validate_HiLoOnNumericAttribute_NoOperandTypeError()
+    {
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"hi\":80,\"hiHi\":100}");
+
+        var result = _sut.Validate(config);
+
+        Assert.DoesNotContain(result.Errors, e => e.Category == ValidationCategory.TriggerOperandType);
+    }
+
+    [Fact]
+    public void Validate_HiLoNoSetpoints_ReturnsWarning()
+    {
+        // No setpoints means the alarm can never fire — design-time warning.
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\"}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Warnings,
+            w => w.Category == ValidationCategory.TriggerOperandType
+              && w.Message.Contains("no setpoints"));
+    }
+
+    [Fact]
+    public void Validate_HiLoLoLoGreaterThanLo_ReturnsError()
+    {
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"loLo\":20,\"lo\":10}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType
+              && e.Message.Contains("LoLo")
+              && e.Message.Contains("Lo"));
+    }
+
+    [Fact]
+    public void Validate_HiLoHiGreaterThanHiHi_ReturnsError()
+    {
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"hi\":120,\"hiHi\":100}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType
+              && e.Message.Contains("Hi")
+              && e.Message.Contains("HiHi"));
+    }
+
+    [Fact]
+    public void Validate_HiLoLowSideOverlapsHighSide_ReturnsError()
+    {
+        // Lo (50) >= Hi (40) — bands overlap.
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"lo\":50,\"hi\":40}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType
+              && e.Message.Contains("overlap"));
+    }
+
+    [Fact]
+    public void Validate_HiLoOnlyHighSideConfigured_NoOrderingError()
+    {
+        // Only Hi/HiHi configured — no low-side comparison needed.
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"hi\":80,\"hiHi\":100}");
+
+        var result = _sut.Validate(config);
+
+        Assert.DoesNotContain(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType);
+    }
+
+    [Fact]
+    public void Validate_HiLoNegativeDeadband_ReturnsError()
+    {
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"hi\":80,\"hiHi\":100,\"hiDeadband\":-1}");
+
+        var result = _sut.Validate(config);
+
+        Assert.Contains(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType
+              && e.Message.Contains("Hi deadband")
+              && e.Message.Contains("non-negative"));
+    }
+
+    [Fact]
+    public void Validate_HiLoZeroDeadband_NoError()
+    {
+        // Zero deadband is the default (no hysteresis) and must be accepted.
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"hi\":80,\"hiHi\":100,\"hiDeadband\":0,\"hiHiDeadband\":0}");
+
+        var result = _sut.Validate(config);
+
+        Assert.DoesNotContain(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType);
+    }
+
+    [Fact]
+    public void Validate_HiLoValidOrdering_NoErrors()
+    {
+        // LoLo (-10) < Lo (0) < Hi (90) < HiHi (100) — fully valid.
+        var config = HiLoConfig("Temp", "Double",
+            "{\"attributeName\":\"Temp\",\"loLo\":-10,\"lo\":0,\"hi\":90,\"hiHi\":100}");
+
+        var result = _sut.Validate(config);
+
+        Assert.DoesNotContain(result.Errors,
+            e => e.Category == ValidationCategory.TriggerOperandType);
+        Assert.DoesNotContain(result.Warnings,
+            w => w.Category == ValidationCategory.TriggerOperandType);
+    }
 }