diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/ResolvedTemplateMembers.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/ResolvedTemplateMembers.cs
index abb99456..545aae2e 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/ResolvedTemplateMembers.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Management/ResolvedTemplateMembers.cs
@@ -93,6 +93,16 @@ public sealed record ResolvedTemplateMemberInfo
     /// no single scalar value.
     /// </summary>
     public string? EffectiveValue { get; init; }
+
+    /// <summary>
+    /// For ALARM members only: the effective (inheritance-resolved) trigger
+    /// configuration JSON — for HiLo alarms this is the per-setpoint MERGED
+    /// config (a derived template overriding only <c>hi</c> inherits the base
+    /// <c>lo</c>), identical to what the flattener produces on deploy, so the
+    /// editor can render the effective HiLo thresholds. Null for non-alarm
+    /// members and for alarms with no trigger configuration.
+    /// </summary>
+    public string? EffectiveTriggerConfiguration { get; init; }
 }
 
 /// <summary>
diff --git a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/TemplateInheritanceResolver.cs b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/TemplateInheritanceResolver.cs
index 098aa955..013f8114 100644
--- a/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/TemplateInheritanceResolver.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.TemplateEngine/TemplateInheritanceResolver.cs
@@ -1,12 +1,14 @@
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Management;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+using ZB.MOM.WW.ScadaBridge.TemplateEngine.Flattening;
 
 namespace ZB.MOM.WW.ScadaBridge.TemplateEngine;
 
 /// <summary>
 /// Read-only AUTHORING resolver (M9/T26a). Given a template id and the full
 /// template lookup, computes the EFFECTIVE inherited member set fresh from the
-/// whole inheritance chain (root → leaf, arbitrary depth, cycle-guarded via
+/// whole inheritance chain (root -> leaf, arbitrary depth, cycle-guarded via
 /// <see cref="TemplateResolver.BuildInheritanceChain"/>), annotated per member
 /// with origin (own / inherited-from-X) and lock state, plus a staleness
 /// summary comparing the template's STORED member rows against the resolved set.
@@ -15,16 +17,18 @@ namespace ZB.MOM.WW.ScadaBridge.TemplateEngine;
 /// The inheritance precedence here MIRRORS
 /// <see cref="Flattening.FlatteningService"/>'s <c>ResolveInherited*</c> methods
 /// EXACTLY so the editor preview agrees with what a deploy would produce:
-/// walk base → derived (derived wins); an <c>IsInherited</c> placeholder row on
+/// walk base -> derived (derived wins); an <c>IsInherited</c> placeholder row on
 /// a derived template never shadows the live base value (it only contributes a
 /// row when no ancestor defines the member); a locked member, once seen, is not
-/// overridden by a downstream template; and an ancestor's
-/// <c>LockedInDerived</c> flag is propagated so the editor can render the
-/// member read-only.
+/// overridden by a downstream template; an ancestor's <c>LockedInDerived</c>
+/// flag is propagated so the editor can render the member read-only; and a
+/// partial HiLo alarm override is MERGED per-setpoint against the inherited
+/// trigger config (reusing <see cref="FlatteningService.MergeHiLoConfig"/>) so
+/// the resolved trigger config equals deploy's.
 /// </para>
 ///
 /// <para>
-/// This is inheritance only — composition and instance overrides are the
+/// This is inheritance only -- composition and instance overrides are the
 /// instance flattener's concern (there are no instances in the
 /// template-authoring context). It NEVER mutates stored rows and is not on the
 /// deploy path.
@@ -50,15 +54,25 @@ public static class TemplateInheritanceResolver
         if (!lookup.TryGetValue(templateId, out var self))
             return new ResolvedTemplateMembers { TemplateId = templateId };
 
-        // Root → leaf (derived last). The OWN template is the last element.
+        // Root -> leaf (derived last). The OWN template is the last element.
         var chain = TemplateResolver.BuildInheritanceChain(templateId, lookup);
 
-        var attributes = ResolveAttributes(chain, templateId);
-        var alarms = ResolveAlarms(chain, templateId);
-        var scripts = ResolveScripts(chain, templateId);
-        var nativeAlarmSources = ResolveNativeAlarmSources(chain, templateId);
+        // Resolve winners ONCE per member type so projection and staleness share a
+        // single source of truth (the same merged effective state).
+        var attributeWinners = ResolveAttributeWinners(chain);
+        var alarmWinners = ResolveAlarmWinners(chain);
+        var scriptWinners = ResolveScriptWinners(chain);
+        var nativeSourceWinners = ResolveNativeAlarmSourceWinners(chain);
 
-        var staleness = ComputeStaleness(self, attributes, alarms, scripts, nativeAlarmSources);
+        var attributes = Project(attributeWinners, templateId, w => w.EffectiveValue, _ => null);
+        // The DTO EffectiveValue keeps surfacing the priority as the at-a-glance
+        // preview; EffectiveTriggerConfiguration carries the merged trigger config.
+        var alarms = Project(alarmWinners, templateId, w => w.Row.PriorityLevel.ToString(), w => w.EffectiveValue);
+        var scripts = Project(scriptWinners, templateId, w => w.EffectiveValue, _ => null);
+        var nativeAlarmSources = Project(nativeSourceWinners, templateId, w => w.EffectiveValue, _ => null);
+
+        var staleness = ComputeStaleness(
+            self, templateId, attributeWinners, alarmWinners, scriptWinners, nativeSourceWinners);
 
         return new ResolvedTemplateMembers
         {
@@ -73,17 +87,33 @@ public static class TemplateInheritanceResolver
         };
     }
 
-    // ── per-member-type winners (origin-tracking) ──
-
-    /// <summary>The winning row for a member, plus where it came from.</summary>
-    private sealed record Winner<T>(T Row, Template Origin, bool BaseLocked);
+    // -- per-member-type winners (origin-tracking) --
 
     /// <summary>
-    /// Generic base → derived walk that picks the winning row per member name,
+    /// The winning row for a member, plus where it came from. <c>EffectiveValue</c>
+    /// is the resolved scalar/effective value for the member type -- usually the
+    /// winning row's own field, but for HiLo alarms it is the PER-SETPOINT
+    /// MERGED trigger configuration accumulated down the chain (see
+    /// <c>mergeEffective</c> on <see cref="ResolveWinners{T}"/>), so the resolver
+    /// preview equals what the flattener produces on deploy.
+    /// </summary>
+    private sealed record Winner<T>(T Row, Template Origin, bool BaseLocked, string? EffectiveValue);
+
+    /// <summary>
+    /// Generic base -> derived walk that picks the winning row per member name,
     /// replicating FlatteningService precedence:
     /// derived wins; an <c>IsInherited</c> placeholder does not shadow a live
     /// ancestor value; a locked member is not overridden downstream; the
     /// ancestor <c>LockedInDerived</c> flag is tracked.
+    /// <para>
+    /// <paramref name="valueOf"/> extracts the per-member effective value from a
+    /// single row. <paramref name="mergeEffective"/> (optional) computes the
+    /// effective value when a derived row OVERRIDES an existing winner -- for HiLo
+    /// alarms this reuses <see cref="FlatteningService.MergeHiLoConfig"/>
+    /// so a partial override (e.g. just <c>hi</c>) inherits the rest of the
+    /// ancestor config, exactly as deploy does. When null, the winning row's own
+    /// value is used verbatim (whole-replace).
+    /// </para>
     /// </summary>
     private static Dictionary<string, Winner<T>> ResolveWinners<T>(
         IReadOnlyList<Template> chain,
@@ -91,11 +121,13 @@ public static class TemplateInheritanceResolver
         Func<T, string> nameOf,
         Func<T, bool> isInherited,
         Func<T, bool> isLocked,
-        Func<T, bool> lockedInDerived)
+        Func<T, bool> lockedInDerived,
+        Func<T, string?> valueOf,
+        Func<string?, T, string?>? mergeEffective = null)
     {
         var result = new Dictionary<string, Winner<T>>(StringComparer.Ordinal);
 
-        // chain is root-first; walk root → derived so the derived template
+        // chain is root-first; walk root -> derived so the derived template
         // (last) wins, mirroring FlatteningService (which walks its
         // most-derived-first chain back-to-front for the same effect).
         foreach (var template in chain)
@@ -122,74 +154,77 @@ public static class TemplateInheritanceResolver
                 }
 
                 var baseLocked = (existing?.BaseLocked ?? false) || lockedInDerived(row);
-                result[name] = new Winner<T>(row, template, baseLocked);
+                // When a derived row overrides an existing winner, give the merge
+                // hook a chance to fuse the two effective values (HiLo per-setpoint);
+                // otherwise the overriding row's own value wins (whole-replace).
+                var effective = existing != null && mergeEffective != null
+                    ? mergeEffective(existing.EffectiveValue, row)
+                    : valueOf(row);
+                result[name] = new Winner<T>(row, template, baseLocked, effective);
             }
         }
 
         return result;
     }
 
-    private static IReadOnlyList<ResolvedTemplateMemberInfo> ResolveAttributes(
-        IReadOnlyList<Template> chain, int selfId)
-    {
-        var winners = ResolveWinners(
+    private static Dictionary<string, Winner<TemplateAttribute>> ResolveAttributeWinners(
+        IReadOnlyList<Template> chain) =>
+        ResolveWinners(
             chain,
             t => t.Attributes,
             a => a.Name,
             a => a.IsInherited,
             a => a.IsLocked,
-            a => a.LockedInDerived);
+            a => a.LockedInDerived,
+            a => a.Value);
 
-        return Project(winners, selfId, w => w.Row.Value);
-    }
-
-    private static IReadOnlyList<ResolvedTemplateMemberInfo> ResolveAlarms(
-        IReadOnlyList<Template> chain, int selfId)
-    {
-        var winners = ResolveWinners(
+    private static Dictionary<string, Winner<TemplateAlarm>> ResolveAlarmWinners(
+        IReadOnlyList<Template> chain) =>
+        // Winner.EffectiveValue carries the PER-SETPOINT MERGED trigger config so
+        // a partial HiLo override (e.g. just `hi`) inherits the rest from the
+        // ancestor -- reusing FlatteningService.MergeHiLoConfig so the resolver
+        // preview equals deploy. Other trigger types whole-replace (mergeEffective
+        // returns the derived row's own config unchanged).
+        ResolveWinners(
             chain,
             t => t.Alarms,
             a => a.Name,
             a => a.IsInherited,
             a => a.IsLocked,
-            a => a.LockedInDerived);
+            a => a.LockedInDerived,
+            a => a.TriggerConfiguration,
+            (existingTrigger, derived) =>
+                derived.TriggerType == AlarmTriggerType.HiLo
+                    ? FlatteningService.MergeHiLoConfig(existingTrigger, derived.TriggerConfiguration)
+                    : derived.TriggerConfiguration);
 
-        // No single scalar value for an alarm; surface the priority as the preview value.
-        return Project(winners, selfId, w => w.Row.PriorityLevel.ToString());
-    }
-
-    private static IReadOnlyList<ResolvedTemplateMemberInfo> ResolveScripts(
-        IReadOnlyList<Template> chain, int selfId)
-    {
-        var winners = ResolveWinners(
+    private static Dictionary<string, Winner<TemplateScript>> ResolveScriptWinners(
+        IReadOnlyList<Template> chain) =>
+        ResolveWinners(
             chain,
             t => t.Scripts,
             s => s.Name,
             s => s.IsInherited,
             s => s.IsLocked,
-            s => s.LockedInDerived);
+            s => s.LockedInDerived,
+            s => s.Code);
 
-        return Project(winners, selfId, w => w.Row.Code);
-    }
-
-    private static IReadOnlyList<ResolvedTemplateMemberInfo> ResolveNativeAlarmSources(
-        IReadOnlyList<Template> chain, int selfId)
-    {
-        var winners = ResolveWinners(
+    private static Dictionary<string, Winner<TemplateNativeAlarmSource>> ResolveNativeAlarmSourceWinners(
+        IReadOnlyList<Template> chain) =>
+        ResolveWinners(
             chain,
             t => t.NativeAlarmSources,
             s => s.Name,
             s => s.IsInherited,
             s => s.IsLocked,
-            s => s.LockedInDerived);
-
-        return Project(winners, selfId, w => w.Row.SourceReference);
-    }
+            s => s.LockedInDerived,
+            s => s.SourceReference);
 
     private static IReadOnlyList<ResolvedTemplateMemberInfo> Project<T>(
         Dictionary<string, Winner<T>> winners,
         int selfId,
-        Func<Winner<T>, string?> effectiveValue)
+        Func<Winner<T>, string?> effectiveValue,
+        Func<Winner<T>, string?> effectiveTriggerConfiguration)
     {
         return winners.Values
             .Select(w => new ResolvedTemplateMemberInfo
@@ -200,7 +235,8 @@ public static class TemplateInheritanceResolver
                 OriginTemplateName = w.Origin.Name,
                 IsLocked = IsLockedOf(w.Row),
                 IsBaseLocked = w.BaseLocked && w.Origin.Id != selfId,
-                EffectiveValue = effectiveValue(w)
+                EffectiveValue = effectiveValue(w),
+                EffectiveTriggerConfiguration = effectiveTriggerConfiguration(w)
             })
             .OrderBy(m => m.Name, StringComparer.Ordinal)
             .ToList();
@@ -224,32 +260,64 @@ public static class TemplateInheritanceResolver
         _ => false
     };
 
-    // ── staleness ──
+    // -- staleness --
 
     /// <summary>
     /// Compares the template's STORED member rows against the freshly-resolved
     /// set and counts how many INHERITED members drift: a freshly-resolved
-    /// inherited member with no stored row, or whose stored placeholder value
-    /// differs from the live resolved value. Own (non-inherited) members never
-    /// count toward staleness — they are authoritative by definition.
+    /// inherited member with no stored row, or whose stored placeholder differs
+    /// from the freshly-resolved (effective) value. Own (non-inherited) members
+    /// never count toward staleness -- they are authoritative by definition.
+    /// <para>
+    /// The comparison key per type is WIDE -- it covers the fields that actually
+    /// matter, not a single scalar -- so a base change that leaves the previously
+    /// compared scalar untouched (e.g. an alarm HiLo threshold change with the
+    /// priority unchanged, or a native source ConnectionName/ConditionFilter
+    /// change with the SourceReference unchanged) still registers as stale:
+    /// </para>
+    /// <list type="bullet">
+    ///   <item><description>Attributes: <c>Value</c>.</description></item>
+    ///   <item><description>Alarms: <c>PriorityLevel</c> + (merged) <c>TriggerConfiguration</c> + <c>Description</c> + <c>OnTriggerScriptId</c>.</description></item>
+    ///   <item><description>Scripts: <c>Code</c>.</description></item>
+    ///   <item><description>NativeAlarmSources: <c>ConnectionName</c> + <c>SourceReference</c> + <c>ConditionFilter</c>.</description></item>
+    /// </list>
     /// </summary>
     private static ResolvedTemplateStaleness ComputeStaleness(
         Template self,
-        IReadOnlyList<ResolvedTemplateMemberInfo> attributes,
-        IReadOnlyList<ResolvedTemplateMemberInfo> alarms,
-        IReadOnlyList<ResolvedTemplateMemberInfo> scripts,
-        IReadOnlyList<ResolvedTemplateMemberInfo> nativeAlarmSources)
+        int selfId,
+        Dictionary<string, Winner<TemplateAttribute>> attributeWinners,
+        Dictionary<string, Winner<TemplateAlarm>> alarmWinners,
+        Dictionary<string, Winner<TemplateScript>> scriptWinners,
+        Dictionary<string, Winner<TemplateNativeAlarmSource>> nativeSourceWinners)
     {
         int differing = 0;
 
-        differing += CountDrift(attributes,
-            self.Attributes.ToDictionary(a => a.Name, a => (string?)a.Value, StringComparer.Ordinal));
-        differing += CountDrift(alarms,
-            self.Alarms.ToDictionary(a => a.Name, a => (string?)a.PriorityLevel.ToString(), StringComparer.Ordinal));
-        differing += CountDrift(scripts,
-            self.Scripts.ToDictionary(s => s.Name, s => (string?)s.Code, StringComparer.Ordinal));
-        differing += CountDrift(nativeAlarmSources,
-            self.NativeAlarmSources.ToDictionary(s => s.Name, s => (string?)s.SourceReference, StringComparer.Ordinal));
+        differing += CountDrift(
+            attributeWinners, selfId,
+            self.Attributes.ToDictionary(a => a.Name, a => AttributeKey(a.Value), StringComparer.Ordinal),
+            w => AttributeKey(w.EffectiveValue));
+
+        differing += CountDrift(
+            alarmWinners, selfId,
+            self.Alarms.ToDictionary(
+                a => a.Name,
+                a => AlarmKey(a.PriorityLevel, a.TriggerConfiguration, a.Description, a.OnTriggerScriptId),
+                StringComparer.Ordinal),
+            // The resolved (winner) side uses the MERGED effective trigger config.
+            w => AlarmKey(w.Row.PriorityLevel, w.EffectiveValue, w.Row.Description, w.Row.OnTriggerScriptId));
+
+        differing += CountDrift(
+            scriptWinners, selfId,
+            self.Scripts.ToDictionary(s => s.Name, s => ScriptKey(s.Code), StringComparer.Ordinal),
+            w => ScriptKey(w.Row.Code));
+
+        differing += CountDrift(
+            nativeSourceWinners, selfId,
+            self.NativeAlarmSources.ToDictionary(
+                s => s.Name,
+                s => NativeSourceKey(s.ConnectionName, s.SourceReference, s.ConditionFilter),
+                StringComparer.Ordinal),
+            w => NativeSourceKey(w.Row.ConnectionName, w.Row.SourceReference, w.Row.ConditionFilter));
 
         return new ResolvedTemplateStaleness
         {
@@ -258,17 +326,24 @@ public static class TemplateInheritanceResolver
         };
     }
 
-    private static int CountDrift(
-        IReadOnlyList<ResolvedTemplateMemberInfo> resolved,
-        IReadOnlyDictionary<string, string?> storedValuesByName)
+    /// <summary>
+    /// Counts inherited winners that drift from their stored placeholder: a
+    /// winner whose origin is an ANCESTOR (inherited) and whose composite key has
+    /// no stored counterpart, or differs from the stored one.
+    /// </summary>
+    private static int CountDrift<T>(
+        Dictionary<string, Winner<T>> winners,
+        int selfId,
+        IReadOnlyDictionary<string, string> storedKeysByName,
+        Func<Winner<T>, string> resolvedKey)
     {
         int count = 0;
-        foreach (var m in resolved)
+        foreach (var (name, w) in winners)
         {
-            if (!m.IsInherited)
-                continue; // own rows are authoritative — never stale
+            if (w.Origin.Id == selfId)
+                continue; // own rows are authoritative -- never stale
 
-            if (!storedValuesByName.TryGetValue(m.Name, out var storedValue))
+            if (!storedKeysByName.TryGetValue(name, out var storedKey))
             {
                 // Inherited member with no stored placeholder row (e.g. a base
                 // member added after the derived template was created).
@@ -276,10 +351,33 @@ public static class TemplateInheritanceResolver
                 continue;
             }
 
-            if (!string.Equals(storedValue, m.EffectiveValue, StringComparison.Ordinal))
-                count++; // stored placeholder is stale vs. the live base value
+            if (!string.Equals(storedKey, resolvedKey(w), StringComparison.Ordinal))
+                count++; // stored placeholder is stale vs. the live (effective) base value
         }
 
         return count;
     }
+
+    // -- composite comparison keys --
+    // A unit-separator delimiter (U+001F, which never appears in JSON / names /
+    // code) joins the fields; a distinct null marker (U+0000) ensures a null
+    // field never collides with an empty one. The SAME normalization is applied
+    // to both the stored side and the resolved side.
+    private const string KeySep = "\u001F";
+    private const string NullMarker = "\u0000";
+    private static string Mark(string? s) => s ?? NullMarker;
+
+    private static string AttributeKey(string? value) => Mark(value);
+
+    private static string ScriptKey(string? code) => Mark(code);
+
+    private static string AlarmKey(int priorityLevel, string? triggerConfiguration, string? description, int? onTriggerScriptId) =>
+        string.Join(KeySep,
+            priorityLevel.ToString(),
+            Mark(triggerConfiguration),
+            Mark(description),
+            onTriggerScriptId?.ToString() ?? NullMarker);
+
+    private static string NativeSourceKey(string? connectionName, string? sourceReference, string? conditionFilter) =>
+        string.Join(KeySep, Mark(connectionName), Mark(sourceReference), Mark(conditionFilter));
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/TemplateInheritanceResolverTests.cs b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/TemplateInheritanceResolverTests.cs
index 1267b933..443ccf17 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/TemplateInheritanceResolverTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/TemplateInheritanceResolverTests.cs
@@ -290,4 +290,213 @@ public class TemplateInheritanceResolverTests
         }
         Assert.Equal(flat.Value.Attributes.Count, resolved.Attributes.Count);
     }
+
+    // ── I1: HiLo per-setpoint merge — the resolved alarm's effective
+    //    TriggerConfiguration MUST equal the FlatteningService-merged result for
+    //    a derived template that overrides only one setpoint (preview = deploy). ──
+
+    [Fact]
+    public void Resolve_AgreesWithFlatteningService_ForPartialHiLoOverride()
+    {
+        // Base HiLo alarm carries both hi + lo. Derived overrides ONLY hi and
+        // inherits lo. The resolver must surface the MERGED effective trigger
+        // config (lo inherited, hi overridden) — identical to what deploy
+        // produces via the flattener's MergeHiLoConfig.
+        var baseT = new Template("Base") { Id = 1 };
+        baseT.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 10,
+            TemplateId = 1,
+            PriorityLevel = 5,
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":90,\"lo\":10}"
+        });
+
+        var derived = new Template("Derived") { Id = 2, ParentTemplateId = 1 };
+        // Partial override: only hi changes; lo must be inherited from base.
+        derived.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 20,
+            TemplateId = 2,
+            PriorityLevel = 5,
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":95}",
+            IsInherited = false
+        });
+
+        var all = new[] { baseT, derived };
+
+        // Resolver view.
+        var resolved = TemplateInheritanceResolver.Resolve(2, all);
+        var resolvedAlarm = Assert.Single(resolved.Alarms, m => m.Name == "Level");
+
+        // Flattener view: most-derived-first chain, no overrides, no compositions.
+        var instance = new Instance("inst") { Id = 1, TemplateId = 2, SiteId = 1 };
+        var chain = new List<Template> { derived, baseT }; // most-derived first
+        var flat = new FlatteningService().Flatten(
+            instance,
+            chain,
+            new Dictionary<int, IReadOnlyList<TemplateComposition>>(),
+            new Dictionary<int, IReadOnlyList<Template>>(),
+            new Dictionary<int, DataConnection>());
+
+        Assert.True(flat.IsSuccess, flat.IsFailure ? flat.Error : null);
+        var flatAlarm = Assert.Single(flat.Value.Alarms, a => a.CanonicalName == "Level");
+
+        // The resolver's effective TriggerConfiguration MUST equal the flattener's
+        // merged output verbatim — the key regression guard for preview = deploy.
+        Assert.Equal(flatAlarm.TriggerConfiguration, resolvedAlarm.EffectiveTriggerConfiguration);
+
+        // And sanity: the merge actually happened (lo inherited, hi overridden).
+        Assert.Equal(
+            FlatteningService.MergeHiLoConfig("{\"hi\":90,\"lo\":10}", "{\"hi\":95}"),
+            resolvedAlarm.EffectiveTriggerConfiguration);
+        Assert.Contains("\"lo\":10", resolvedAlarm.EffectiveTriggerConfiguration);
+        Assert.Contains("\"hi\":95", resolvedAlarm.EffectiveTriggerConfiguration);
+    }
+
+    // ── I2: staleness must flag drift on the fields that actually matter, not
+    //    just one scalar per type. ──
+
+    [Fact]
+    public void Resolve_StalenessSummary_TrueWhenBaseAlarmTriggerConfigChanged_PriorityUnchanged()
+    {
+        // Base alarm threshold changed (TriggerConfiguration), PriorityLevel is
+        // unchanged. The derived placeholder still carries the OLD threshold, so
+        // staleness must flag it even though the priority scalar matches.
+        var baseT = new Template("Base") { Id = 1 };
+        baseT.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 10,
+            TemplateId = 1,
+            PriorityLevel = 5,
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":95,\"lo\":10}"
+        });
+
+        var derived = new Template("Derived") { Id = 2, ParentTemplateId = 1 };
+        derived.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 20,
+            TemplateId = 2,
+            PriorityLevel = 5, // priority matches the base — only the threshold drifted
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":90,\"lo\":10}", // stale threshold
+            IsInherited = true
+        });
+
+        var resolved = TemplateInheritanceResolver.Resolve(2, new[] { baseT, derived });
+
+        Assert.True(resolved.Staleness.IsStale);
+        Assert.Equal(1, resolved.Staleness.DifferingMemberCount);
+    }
+
+    [Fact]
+    public void Resolve_StalenessSummary_TrueWhenNativeSourceConnectionNameChanged_SourceReferenceUnchanged()
+    {
+        var baseT = new Template("Base") { Id = 1 };
+        baseT.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 10,
+            TemplateId = 1,
+            ConnectionName = "newConn",
+            SourceReference = "ns=2;s=Alarm",
+            ConditionFilter = null
+        });
+
+        var derived = new Template("Derived") { Id = 2, ParentTemplateId = 1 };
+        derived.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 20,
+            TemplateId = 2,
+            ConnectionName = "oldConn", // ConnectionName drifted
+            SourceReference = "ns=2;s=Alarm", // SourceReference unchanged
+            ConditionFilter = null,
+            IsInherited = true
+        });
+
+        var resolved = TemplateInheritanceResolver.Resolve(2, new[] { baseT, derived });
+
+        Assert.True(resolved.Staleness.IsStale);
+        Assert.Equal(1, resolved.Staleness.DifferingMemberCount);
+    }
+
+    [Fact]
+    public void Resolve_StalenessSummary_TrueWhenNativeSourceConditionFilterChanged_SourceReferenceUnchanged()
+    {
+        var baseT = new Template("Base") { Id = 1 };
+        baseT.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 10,
+            TemplateId = 1,
+            ConnectionName = "conn",
+            SourceReference = "ns=2;s=Alarm",
+            ConditionFilter = "severity>500"
+        });
+
+        var derived = new Template("Derived") { Id = 2, ParentTemplateId = 1 };
+        derived.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 20,
+            TemplateId = 2,
+            ConnectionName = "conn",
+            SourceReference = "ns=2;s=Alarm", // unchanged
+            ConditionFilter = null, // ConditionFilter drifted
+            IsInherited = true
+        });
+
+        var resolved = TemplateInheritanceResolver.Resolve(2, new[] { baseT, derived });
+
+        Assert.True(resolved.Staleness.IsStale);
+        Assert.Equal(1, resolved.Staleness.DifferingMemberCount);
+    }
+
+    [Fact]
+    public void Resolve_StalenessSummary_FalseWhenAlarmAndNativeSourceInSync()
+    {
+        // Guard against false-positives from the widened comparison: when the
+        // stored placeholders match the resolved (merged) values, nothing is stale.
+        var baseT = new Template("Base") { Id = 1 };
+        baseT.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 10,
+            TemplateId = 1,
+            PriorityLevel = 5,
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":90,\"lo\":10}"
+        });
+        baseT.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 11,
+            TemplateId = 1,
+            ConnectionName = "conn",
+            SourceReference = "ns=2;s=Alarm",
+            ConditionFilter = "severity>500"
+        });
+
+        var derived = new Template("Derived") { Id = 2, ParentTemplateId = 1 };
+        derived.Alarms.Add(new TemplateAlarm("Level")
+        {
+            Id = 20,
+            TemplateId = 2,
+            PriorityLevel = 5,
+            TriggerType = AlarmTriggerType.HiLo,
+            TriggerConfiguration = "{\"hi\":90,\"lo\":10}", // in sync
+            IsInherited = true
+        });
+        derived.NativeAlarmSources.Add(new TemplateNativeAlarmSource("Src")
+        {
+            Id = 21,
+            TemplateId = 2,
+            ConnectionName = "conn",
+            SourceReference = "ns=2;s=Alarm",
+            ConditionFilter = "severity>500", // in sync
+            IsInherited = true
+        });
+
+        var resolved = TemplateInheritanceResolver.Resolve(2, new[] { baseT, derived });
+
+        Assert.False(resolved.Staleness.IsStale);
+        Assert.Equal(0, resolved.Staleness.DifferingMemberCount);
+    }
 }