Merge pull request '[ablegacy] AbLegacy — Auto-demote on comm failure' (#399) from auto/ablegacy/12 into auto/driver-gaps

2026-04-26 08:47:59 -04:00
parent 8ee65a75d2 1e3053c0d8
commit 08a4db2952
18 changed files with 1160 additions and 31 deletions
--- a/docs/Driver.AbLegacy.Cli.md
+++ b/docs/Driver.AbLegacy.Cli.md
@@ -21,6 +21,9 @@ dotnet run --project src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli -- --help
 | `-P` / `--plc-type` | `Slc500` | Slc500 / MicroLogix / Plc5 / LogixPccc |
 | `--timeout-ms` | `5000` | Per-operation timeout — see precedence note below |
 | `--retries` | `0` | Retry count on transient `BadCommunicationError` (PR 9 / #252) |
+| `--demote-failure-threshold` | `3` | **PR ablegacy-12 / #255** — consecutive comm failures before the device is auto-demoted |
+| `--demote-for-ms` | `30000` | **PR ablegacy-12 / #255** — auto-demote cool-down window in ms |
+| `--no-demote` | off | **PR ablegacy-12 / #255** — disable auto-demote entirely (counters still tick) |
 | `--verbose` | off | Serilog debug output |

 Family ↔ CIP-path cheat sheet:
@@ -84,6 +87,37 @@ otopcua-ablegacy-cli probe -g ab://192.168.1.20/1,0
 otopcua-ablegacy-cli probe -g ab://192.168.1.30/ -P MicroLogix -a S:0
 ```

+`probe` output (PR ablegacy-12 / #255) reports both `Health` (driver health
+state) and `Host state`. The latter is sourced from `IHostConnectivityProbe`
+and surfaces `Demoted` when the auto-demote threshold has tripped — a fast
+visual signal that the CLI is short-circuiting future reads against this
+device until the cool-down expires:
+
+```text
+Gateway:      ab://192.168.1.20/1,0
+PLC type:     Slc500
+Health:       Degraded
+Host state:   Demoted
+Last error:   libplctag status -33 reading N7:0
+```
+
+### Auto-demote knobs
+
+```powershell
+# Trip after just one comm failure, hold for 60s.
+otopcua-ablegacy-cli read -g ab://192.168.1.20/1,0 -a N7:0 -t Int `
+    --demote-failure-threshold 1 --demote-for-ms 60000
+
+# Opt out of auto-demote — stresses the link without short-circuiting.
+otopcua-ablegacy-cli read -g ab://192.168.1.20/1,0 -a N7:0 -t Int --no-demote
+```
+
+The CLI is a one-shot test client — auto-demote primarily matters in the
+server-side multi-device deployment, where a single demoted PLC can no
+longer block reads against its healthy peers. Use the CLI flags to
+reproduce a flapping-link scenario locally before tuning the server-side
+`appsettings.json` `Demote` block.
+
 ### `read`

 ```powershell
--- a/docs/drivers/AbLegacy-Diagnostics.md
+++ b/docs/drivers/AbLegacy-Diagnostics.md
@@ -7,10 +7,12 @@ directly without going through a separate diagnostics RPC. Mirrors the AB CIP

 Closes #253 (PR ablegacy-10).

-## The seven counters
+## The nine counters

-Each device managed by the `AbLegacyDriver` exposes seven read-only nodes under
-`AbLegacy/<host>/_Diagnostics/<name>`:
+Each device managed by the `AbLegacyDriver` exposes nine read-only nodes under
+`AbLegacy/<host>/_Diagnostics/<name>`. The first seven shipped in PR ablegacy-10;
+`DemoteCount` + `LastDemotedUtc` arrived with PR ablegacy-12 / #255 (auto-demote
+on comm failure).

 | Name | Type | Semantics |
 |---|---|---|
@@ -21,6 +23,8 @@ Each device managed by the `AbLegacyDriver` exposes seven read-only nodes under
 | `LastErrorCode` | Int32 | Most recent libplctag status code on a failed read; `0` when no error has been seen since the last reset. |
 | `LastErrorMessage` | String | Most recent libplctag error message on a failed read; empty when no error has been seen since the last reset. |
 | `CommFailures` | Int64 | Count of read failures mapped to `BadCommunicationError`. Spans transient libplctag throws + retried-out chains so operators see a single "wire fell off" counter. |
+| `DemoteCount` | Int64 | **PR ablegacy-12** — cumulative auto-demote events for this device. Bumps every time the driver crosses the consecutive-failure threshold and arms a fresh cool-down window. Cumulative across `ReinitializeAsync` (preserved through redeploys) so a flapping link surfaces as a steadily climbing counter. |
+| `LastDemotedUtc` | String | **PR ablegacy-12** — ISO-8601 UTC timestamp of the most recent auto-demotion. Empty string when this device has never been demoted. |

 **Address shape**: `_Diagnostics/<deviceHostAddress>/<name>` —
 e.g. `_Diagnostics/ab://10.0.0.5/1,0/RequestCount`.
@@ -34,10 +38,11 @@ user-config tag node, just under a reserved sibling folder.

 | Trigger | Effect |
 |---|---|
-| `ReinitializeAsync` | Every counter for every device resets to zero, plus `LastErrorMessage` clears to empty. |
-| `ShutdownAsync` | Same as Reinitialize — counters drop with the device map. |
+| `ReinitializeAsync` | Every counter for every device resets to zero, plus `LastErrorMessage` clears to empty. **PR ablegacy-12 exception:** `DemoteCount` + `LastDemotedUtc` survive the reinit so an operator redeploying mid-incident doesn't lose the flapping-link history. |
+| `ShutdownAsync` | All counters drop with the device map (including `DemoteCount`). |
 | Driver process restart | Counters start at zero. |
 | Probe transition Stopped→Running | **No automatic reset** — counters are cumulative across reconnect events so operators can spot intermittent links by watching `CommFailures` keep climbing. |
+| Probe transition Demoted→Running | **PR ablegacy-12** — early-clear of the active demote window, but the cumulative `DemoteCount` stays put. |

 There is no in-process "reset" RPC at the time of writing. If you need to
 clear counters without a redeploy, kick a `ReinitializeAsync` from the Admin
@@ -99,14 +104,85 @@ overview dashboard, plus a faster rate (1 s) on `LastErrorMessage` /
 short-circuit makes every read O(1) — there's no penalty for fast polling
 of the counter itself, only the OPC UA subscription bookkeeping.

+## Auto-demote on comm failure (PR ablegacy-12 / #255)
+
+When a device fails N consecutive reads or probes the driver marks it
+**Demoted** for a configurable cool-down window. Reads against a demoted
+device short-circuit with `BadCommunicationError` *without invoking
+libplctag* — that's the whole point of the feature: one slow PLC sharing
+the driver thread can't starve faster peers reading from healthy hosts on
+the same `AbLegacyDriver` instance.
+
+### Configuration
+
+Per-device, optional. `null` keeps the documented defaults (auto-demote
+**enabled** with 3 failures / 30 s).
+
+```jsonc
+{
+  "Devices": [
+    {
+      "HostAddress": "ab://10.0.0.5/1,0",
+      "PlcFamily": "Slc500",
+      "Demote": {
+        "FailureThreshold": 3,    // default 3
+        "DemoteForMs": 30000,     // default 30s
+        "Enabled": true           // default true
+      }
+    }
+  ]
+}
+```
+
+| Knob | Default | Notes |
+|---|---|---|
+| `FailureThreshold` | `3` | Consecutive comm failures before the device is demoted. A successful read or probe resets the tally. Terminal failures (`BadNodeIdUnknown`, `BadTypeMismatch`, …) **do not count** — they're config / decoder mismatches, not field outages. |
+| `DemoteForMs` | `30000` (30s) | Cool-down window. Reads while this is active short-circuit; a successful probe clears it early. |
+| `Enabled` | `true` | Set to `false` to keep the diagnostic counters but skip the auto-throttle. The failure tally still ticks but never arms the cool-down. |
+
+### Recovery
+
+Three ways out of Demoted, in order of likelihood:
+
+1. **Probe success** — the per-device probe loop (`Probe.Enabled = true`,
+   default address `S:0`) is the fast path. The next probe iteration after
+   demotion will exercise the wire; on success it clears
+   `DemotedUntilUtc` immediately and transitions the host to `Running`.
+2. **Window expiry** — once `DemoteForMs` elapses the demote marker
+   clears on the next read attempt. The read goes through; if it fails,
+   the failure tally keeps counting from where it left off (so a
+   permanently-down device re-arms the window after one more consecutive
+   failure rather than having to repeat the full threshold).
+3. **`ReinitializeAsync`** — clears `ConsecutiveFailures` +
+   `DemotedUntilUtc` outright. Cumulative `DemoteCount` survives.
+
+### Observability
+
+`DemoteCount` is the headline counter — it bumps once per demotion event,
+not per short-circuited read. A device that flaps every hour for a week
+shows `DemoteCount = ~168` on Friday afternoon, which is the operator
+signal you actually want.
+
+`LastDemotedUtc` is the ISO-8601 UTC timestamp of the most recent
+demotion. Bind it on a per-device tile alongside `DemoteCount` for
+"flapping link" alerting.
+
+### Host-state surface
+
+A demoted device reports `HostState.Demoted` (new in PR ablegacy-12
+on `Core.Abstractions/IHostConnectivityProbe.cs`). Consumers that
+predate the new value (the central `HostStatusPublisher`) safely treat
+it as `Stopped` — no schema migration needed.
+
 ## Cross-references

 - [`AbLegacyDiagnosticTags.cs`](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDiagnosticTags.cs)
  — counter store + read short-circuit
 - [`AbLegacyDriver.cs`](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs)
-  — increment sites in `ReadAsync`, discovery emission in `DiscoverAsync`
+  — increment sites in `ReadAsync`, discovery emission in `DiscoverAsync`,
+  auto-demote bookkeeping in `RecordFailureAndMaybeDemote` + `ProbeLoopAsync`
 - [`AbLegacy-Test-Fixture.md`](AbLegacy-Test-Fixture.md) — `AbLegacyDiagnosticsTests`
-  + collision-rejection contract
+  + `AbLegacyAutoDemoteTests` + collision-rejection contract
 - [AB CIP `_System/` parallel](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipSystemTagSource.cs)
  — same pattern with the CIP-specific six entries (incl. writeable
  `_RefreshTagDb` trigger)
--- a/docs/drivers/AbLegacy-Test-Fixture.md
+++ b/docs/drivers/AbLegacy-Test-Fixture.md
@@ -53,12 +53,31 @@ supplies a `FakeAbLegacyTag`.
  counters: 5 reads (3 ok / 2 fail) → `RequestCount=5`, `ResponseCount=3`,
  `ErrorCount=2`; `LastErrorCode` reflects the most recent libplctag status;
  `RetryCount` increments per retry attempt beyond the first; counters reset
-  on `ReinitializeAsync`; discovery emits exactly 7 diagnostic variables per
-  device under `_Diagnostics/`; collision rejection at `InitializeAsync` for
-  user tags shadowing reserved names or `_Diagnostics/` addresses; the
-  `_Diagnostics/<host>/<name>` short-circuit returns the live snapshot through
-  `ReadAsync` without bumping `RequestCount`; two devices keep counters
-  independent.
+  on `ReinitializeAsync`; discovery emits the canonical diagnostic variables
+  per device under `_Diagnostics/` (now 9 with PR ablegacy-12); collision
+  rejection at `InitializeAsync` for user tags shadowing reserved names or
+  `_Diagnostics/` addresses; the `_Diagnostics/<host>/<name>` short-circuit
+  returns the live snapshot through `ReadAsync` without bumping
+  `RequestCount`; two devices keep counters independent.
+- `AbLegacyAutoDemoteTests` — **PR ablegacy-12 / #255** auto-demote on comm
+  failure: 3 consecutive failures arm the demote window and surface
+  `HostState.Demoted`; subsequent reads short-circuit with
+  `BadCommunicationError` *without invoking libplctag* (verified via
+  `factory.Tags["N7:0"].ReadCount` not advancing); successful read resets
+  the consecutive-failure counter; failure-success-failure pattern doesn't
+  cross the threshold; `DemoteCount` + `LastDemotedUtc` surface via
+  `_Diagnostics/`; `Enabled=false` opts out (failures still count, demotion
+  never fires); `ReinitializeAsync` clears the active window but preserves
+  cumulative `DemoteCount`; cool-down expiry allows the next read through;
+  two devices in one driver — one faulty, one healthy — proves the faulty
+  side's demotion doesn't starve the healthy side; `BadNodeIdUnknown`
+  (terminal) does not count toward the comm-failure tally; DTO JSON
+  round-trip preserves `FailureThreshold` / `DemoteForMs` / `Enabled` at
+  the per-device level; `HostState.Demoted` enum value is wired through
+  `Core.Abstractions`. Companion integration test in
+  `tests/.../IntegrationTests/AbLegacyAutoDemoteTests.cs` runs the
+  two-device-one-unreachable scenario against a live ab_server fixture
+  using `127.0.0.1:1` as the unreachable peer.
 - `RsLogixSymbolImportTests` — ablegacy-11 / #254 RSLogix CSV symbol-import parser:
  canonical 8-row CSV (one row per N/F/B/L/ST/T/C/R) → 8 typed
  `AbLegacyTagDefinition`s with the right `DataType`; header + comment-line
--- a/scripts/e2e/test-ablegacy.ps1
+++ b/scripts/e2e/test-ablegacy.ps1
@@ -39,6 +39,21 @@
  client may have bumped it by more, so the comparison is `>=`). NodeId form:
  ns=<n>;s=AbLegacy/<gateway>/_Diagnostics/RequestCount. Mirrors the
  -SystemConnectionStatusNodeId knob on test-abcip.ps1.
+
+.PARAMETER DiagnosticsDemoteCountNodeId
+  Optional NodeId for the synthetic _Diagnostics/<host>/DemoteCount variable
+  emitted by AB Legacy discovery (PR ablegacy-12 / #255). When supplied, the
+  script runs the auto-demote assertion: kills the simulator container so
+  reads start failing, hammers the user-tag BridgeNodeId at least
+  FailureThreshold times to trip the demotion, then reads the diagnostic
+  counter and asserts the value increased by >= 1. NodeId form:
+  ns=<n>;s=AbLegacy/<gateway>/_Diagnostics/DemoteCount. The simulator
+  must support `docker stop otopcua-ab-server-slc500` for the kill stage.
+
+.PARAMETER FailureThresholdForDemote
+  Failure threshold the server is configured with (default 3). The
+  demote assertion writes/reads N+1 times against the killed simulator
+  to guarantee the threshold trips even if some reads beat the kill.
 #>

 param(
@@ -47,7 +62,9 @@ param(
    [string]$Address = "N7:5",
    [string]$OpcUaUrl = "opc.tcp://localhost:4840",
    [Parameter(Mandatory)] [string]$BridgeNodeId,
-    [string]$DiagnosticsRequestCountNodeId
+    [string]$DiagnosticsRequestCountNodeId,
+    [string]$DiagnosticsDemoteCountNodeId,
+    [int]$FailureThresholdForDemote = 3
 )

 $ErrorActionPreference = "Stop"
@@ -245,5 +262,67 @@ finally {
    Remove-Item -Path $importJsonPath -ErrorAction SilentlyContinue
 }

+# PR ablegacy-12 / #255 — auto-demote round-trip. Kill the simulator container,
+# hammer the bridge NodeId past the failure threshold, then assert the
+# DemoteCount diagnostic incremented. Restart the simulator at the end so the
+# next run gets a clean baseline. Gated on -DiagnosticsDemoteCountNodeId so
+# environments without docker-side control of the simulator can opt out.
+if ($DiagnosticsDemoteCountNodeId) {
+    Write-Header "AutoDemote (kill simulator + observe DemoteCount from $DiagnosticsDemoteCountNodeId)"
+    $baselineDemoteOut = & $opcUaCli.File @($opcUaCli.PrefixArgs) `
+        @("read", "-u", $OpcUaUrl, "-n", $DiagnosticsDemoteCountNodeId) 2>&1
+    $baselineDemote = 0
+    if (($baselineDemoteOut -join "`n") -match '(\d+)') { $baselineDemote = [int64]$Matches[1] }
+
+    # Best-effort container kill — prefer the slc500 profile name; fall back to
+    # micrologix / plc5 in case the operator pointed the e2e at a different family.
+    $simContainers = @("otopcua-ab-server-slc500", "otopcua-ab-server-micrologix", "otopcua-ab-server-plc5")
+    $killed = $false
+    foreach ($c in $simContainers) {
+        $stop = docker stop $c 2>$null
+        if ($LASTEXITCODE -eq 0 -and $stop) {
+            Write-Host "Stopped $c"
+            $killed = $true
+            break
+        }
+    }
+    if (-not $killed) {
+        Write-Fail "AutoDemote: no ab_server container found via 'docker stop' — skipping demote assertion"
+        $results += @{ Passed = $false; Reason = "no simulator container to kill" }
+    }
+    else {
+        # Hammer past the threshold. Each read against a now-unreachable simulator
+        # surfaces BadCommunicationError; FailureThreshold consecutive ones trip
+        # the demotion. We add 2 extra to absorb timing slack (one read may be
+        # in-flight when the kill lands).
+        $hammerCount = $FailureThresholdForDemote + 2
+        for ($i = 0; $i -lt $hammerCount; $i++) {
+            & $opcUaCli.File @($opcUaCli.PrefixArgs) `
+                @("read", "-u", $OpcUaUrl, "-n", $BridgeNodeId) 2>&1 | Out-Null
+        }
+
+        Start-Sleep -Seconds 1
+
+        $afterDemoteOut = & $opcUaCli.File @($opcUaCli.PrefixArgs) `
+            @("read", "-u", $OpcUaUrl, "-n", $DiagnosticsDemoteCountNodeId) 2>&1
+        $afterDemote = 0
+        if (($afterDemoteOut -join "`n") -match '(\d+)') { $afterDemote = [int64]$Matches[1] }
+
+        $deltaDemote = $afterDemote - $baselineDemote
+        if ($deltaDemote -ge 1) {
+            Write-Pass "AutoDemote DemoteCount delta $deltaDemote >= 1 after $hammerCount failed reads"
+            $results += @{ Passed = $true }
+        } else {
+            Write-Fail "AutoDemote DemoteCount delta $deltaDemote < 1 (baseline=$baselineDemote after=$afterDemote)"
+            $results += @{ Passed = $false; Reason = "demote delta $deltaDemote" }
+        }
+
+        # Restart the simulator so subsequent test runs have a clean baseline.
+        # Best-effort — if docker-compose isn't on the path the operator can
+        # bring it back manually via the Docker/docker-compose.yml profile.
+        try { docker start (docker ps -aq -f "name=otopcua-ab-server-") | Out-Null } catch { }
+    }
+}
+
 Write-Summary -Title "AB Legacy e2e" -Results $results
 if ($results | Where-Object { -not $_.Passed }) { exit 1 }
--- a/scripts/smoke/seed-ablegacy-smoke.sql
+++ b/scripts/smoke/seed-ablegacy-smoke.sql
@@ -96,7 +96,12 @@ VALUES (@Gen, @DrvId, @ClusterId, @NsId, 'ablegacy-smoke', 'AbLegacy', N'{
      "PlcFamily": "Slc500",
      "DeviceName": "slc-500",
      "TimeoutMs": 500,
-      "Retries": 1
+      "Retries": 1,
+      "Demote": {
+        "FailureThreshold": 3,
+        "DemoteForMs": 30000,
+        "Enabled": true
+      }
    }
  ],
  "Probe": { "Enabled": true, "IntervalMs": 5000, "TimeoutMs": 2000, "ProbeAddress": "S:0" },
@@ -155,7 +160,15 @@ PRINT '      e.g. "ab://<plc-ip>:44818/" and re-run this seed.';
 PRINT '';
 PRINT 'PR ablegacy-10 / #253 — diagnostic counters auto-emit per device under';
 PRINT '   AbLegacy/<host>/_Diagnostics/<name>. No dbo.Tag rows needed — the';
-PRINT '   driver registers them at DiscoverAsync time. Seven counters per device:';
+PRINT '   driver registers them at DiscoverAsync time. Nine counters per device:';
 PRINT '   RequestCount, ResponseCount, ErrorCount, RetryCount, LastErrorCode,';
-PRINT '   LastErrorMessage, CommFailures. See docs/drivers/AbLegacy-Diagnostics.md';
-PRINT '   for the full surface + reset semantics.';
+PRINT '   LastErrorMessage, CommFailures, DemoteCount, LastDemotedUtc. See';
+PRINT '   docs/drivers/AbLegacy-Diagnostics.md for the full surface + reset';
+PRINT '   semantics.';
+PRINT '';
+PRINT 'PR ablegacy-12 / #255 — auto-demote on comm failure: 3 consecutive';
+PRINT '   failed reads / probes mark the device Demoted for DemoteFor=PT30S';
+PRINT '   (30 s); reads against a demoted device short-circuit with';
+PRINT '   BadCommunicationError so one slow PLC can''t starve the driver.';
+PRINT '   Tune via the Demote block on each Devices[] row. DemoteCount +';
+PRINT '   LastDemotedUtc on the _Diagnostics folder surface flapping links.';
--- a/src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IHostConnectivityProbe.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IHostConnectivityProbe.cs
@@ -38,4 +38,16 @@ public sealed record HostStatusChangedEventArgs(
    HostState NewState);

 /// <summary>Host lifecycle state. Generalization of Galaxy's Platform/Engine ScanState.</summary>
-public enum HostState { Unknown, Running, Stopped, Faulted }
+/// <remarks>
+///     <para>
+///         <see cref="Demoted"/> (PR ablegacy-12 / #255) is a soft-stopped state used by drivers
+///         that auto-throttle a host after N consecutive comm failures. Reads are short-circuited
+///         with <c>BadCommunicationError</c> for a configurable cool-down window so one slow PLC
+///         doesn't starve faster peers sharing the same driver. Demoted is *not* the same as
+///         <see cref="Stopped"/> (which means "probe says it's down") nor <see cref="Faulted"/>
+///         (which means "the driver itself is broken"); it's a deliberate driver-side back-off.
+///         Consumers that don't recognize <c>Demoted</c> can safely treat it as <c>Stopped</c>
+///         (see <c>HostStatusPublisher.MapState</c>).
+///     </para>
+/// </remarks>
+public enum HostState { Unknown, Running, Stopped, Faulted, Demoted }
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli/AbLegacyCommandBase.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli/AbLegacyCommandBase.cs
@@ -25,6 +25,34 @@ public abstract class AbLegacyCommandBase : DriverCommandBase
    [CommandOption("timeout-ms", Description = "Per-operation timeout in ms (default 5000).")]
    public int TimeoutMs { get; init; } = 5000;

+    /// <summary>
+    ///     PR ablegacy-12 / #255 — consecutive comm failures before this device is
+    ///     auto-demoted. Reads against a demoted device short-circuit with
+    ///     <c>BadCommunicationError</c> for <see cref="DemoteForMs"/> ms so one
+    ///     unreachable PLC can't starve faster peers sharing the driver thread.
+    /// </summary>
+    [CommandOption("demote-failure-threshold", Description =
+        "Consecutive comm failures before the device is auto-demoted (PR ablegacy-12). Default 3.")]
+    public int DemoteFailureThreshold { get; init; } = 3;
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — auto-demote cool-down window in ms. Reads while
+    ///     this window is active short-circuit with <c>BadCommunicationError</c>;
+    ///     a successful probe clears it early.
+    /// </summary>
+    [CommandOption("demote-for-ms", Description =
+        "Auto-demote cool-down window in ms (PR ablegacy-12). Default 30000 (30s).")]
+    public int DemoteForMs { get; init; } = 30_000;
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — opt out of the auto-demote behaviour. The
+    ///     consecutive-failure tally still ticks (so DemoteCount/LastDemotedUtc
+    ///     stay zero) but reads never short-circuit.
+    /// </summary>
+    [CommandOption("no-demote", Description =
+        "Disable auto-demote on consecutive comm failures (PR ablegacy-12). Default off (auto-demote enabled).")]
+    public bool NoDemote { get; init; }
+
    /// <inheritdoc />
    public override TimeSpan Timeout
    {
@@ -41,7 +69,11 @@ public abstract class AbLegacyCommandBase : DriverCommandBase
        Devices = [new AbLegacyDeviceOptions(
            HostAddress: Gateway,
            PlcFamily: PlcType,
-            DeviceName: $"cli-{PlcType}")],
+            DeviceName: $"cli-{PlcType}",
+            Demote: new AbLegacyDemoteOptions(
+                FailureThreshold: DemoteFailureThreshold,
+                DemoteFor: TimeSpan.FromMilliseconds(DemoteForMs),
+                Enabled: !NoDemote))],
        Tags = tags,
        Timeout = Timeout,
        Probe = new AbLegacyProbeOptions { Enabled = false },
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli/Commands/ProbeCommand.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli/Commands/ProbeCommand.cs
@@ -40,10 +40,19 @@ public sealed class ProbeCommand : AbLegacyCommandBase
            await driver.InitializeAsync("{}", ct);
            var snapshot = await driver.ReadAsync(["__probe"], ct);
            var health = driver.GetHealth();
+            // PR ablegacy-12 / #255 — surface Demoted alongside the probe-driven
+            // HostState. After a one-shot probe the host hasn't been observed
+            // (no probe loop runs in CLI mode), so HostState is typically Unknown
+            // unless the read above tripped the demote threshold.
+            var hostStatus = driver.GetHostStatuses().FirstOrDefault();

            await console.Output.WriteLineAsync($"Gateway:      {Gateway}");
            await console.Output.WriteLineAsync($"PLC type:     {PlcType}");
            await console.Output.WriteLineAsync($"Health:       {health.State}");
+            if (hostStatus is not null)
+            {
+                await console.Output.WriteLineAsync($"Host state:   {hostStatus.State}");
+            }
            if (health.LastError is { } err)
                await console.Output.WriteLineAsync($"Last error:   {err}");
            await console.Output.WriteLineAsync();
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDiagnosticTags.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDiagnosticTags.cs
@@ -40,6 +40,11 @@ public sealed class AbLegacyDiagnosticTags
    public const string DiagnosticsFolderPrefix = "_Diagnostics/";

    /// <summary>Canonical names the diagnostics folder exposes. Keep in lockstep with discovery.</summary>
+    /// <remarks>
+    ///     PR ablegacy-12 / #255 — <c>DemoteCount</c> + <c>LastDemotedUtc</c> ride
+    ///     alongside the original seven so HMIs can spot a flapping device by
+    ///     watching <c>DemoteCount</c> climb without scraping logs.
+    /// </remarks>
    public static readonly IReadOnlyList<string> DiagnosticTagNames =
    [
        "RequestCount",
@@ -49,6 +54,9 @@ public sealed class AbLegacyDiagnosticTags
        "LastErrorCode",
        "LastErrorMessage",
        "CommFailures",
+        // PR ablegacy-12 / #255 — auto-demote on comm failure surface.
+        "DemoteCount",
+        "LastDemotedUtc",
    ];

    private static readonly HashSet<string> DiagnosticTagNameSet =
@@ -130,6 +138,39 @@ public sealed class AbLegacyDiagnosticTags
        Interlocked.Increment(ref c.Retry);
    }

+    /// <summary>
+    ///     PR ablegacy-12 / #255 — record an auto-demotion event: bumps cumulative
+    ///     <c>DemoteCount</c> and stamps <c>LastDemotedUtc</c>. Fires every time the
+    ///     driver crosses the failure threshold and arms a fresh cool-down window —
+    ///     a single flapping link that demotes hourly will surface as a steadily
+    ///     climbing counter, which is the operator-facing signal we want.
+    /// </summary>
+    public void RecordDemote(string deviceHostAddress, DateTime nowUtc)
+    {
+        ArgumentNullException.ThrowIfNull(deviceHostAddress);
+        var c = GetOrCreate(deviceHostAddress);
+        Interlocked.Increment(ref c.DemoteCount);
+        // DateTime is 64 bits — use Interlocked.Exchange on the Ticks field so a
+        // concurrent reader sees a torn-free snapshot. On x86 a 64-bit non-aligned
+        // write isn't atomic; on x64 it is, but routing through Interlocked is
+        // platform-independent + costs almost nothing.
+        Interlocked.Exchange(ref c.LastDemotedUtcTicks, nowUtc.Ticks);
+    }
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — restore cumulative demote bookkeeping after a
+    ///     <see cref="AbLegacyDriver.ReinitializeAsync"/> cycle so an operator
+    ///     redeploying config mid-incident doesn't lose flapping-link history.
+    ///     Sets the counters to absolute values rather than incrementing.
+    /// </summary>
+    public void RestoreDemote(string deviceHostAddress, long demoteCount, DateTime? lastDemotedUtc)
+    {
+        ArgumentNullException.ThrowIfNull(deviceHostAddress);
+        var c = GetOrCreate(deviceHostAddress);
+        Interlocked.Exchange(ref c.DemoteCount, demoteCount);
+        Interlocked.Exchange(ref c.LastDemotedUtcTicks, lastDemotedUtc?.Ticks ?? 0);
+    }
+
    /// <summary>Snapshot the current counters for a device. Returns zeros for unknown hosts.</summary>
    public DiagnosticsSnapshot Snapshot(string deviceHostAddress)
    {
@@ -139,7 +180,8 @@ public sealed class AbLegacyDiagnosticTags
        {
            _counters.TryGetValue(deviceHostAddress, out c);
        }
-        if (c is null) return new DiagnosticsSnapshot(0, 0, 0, 0, 0, string.Empty, 0);
+        if (c is null) return new DiagnosticsSnapshot(0, 0, 0, 0, 0, string.Empty, 0, 0, null);
+        var ticks = Interlocked.Read(ref c.LastDemotedUtcTicks);
        return new DiagnosticsSnapshot(
            Request: Interlocked.Read(ref c.Request),
            Response: Interlocked.Read(ref c.Response),
@@ -147,7 +189,10 @@ public sealed class AbLegacyDiagnosticTags
            Retry: Interlocked.Read(ref c.Retry),
            LastErrorCode: Volatile.Read(ref c.LastErrorCode),
            LastErrorMessage: c.LastErrorMessage ?? string.Empty,
-            CommFailures: Interlocked.Read(ref c.CommFailures));
+            CommFailures: Interlocked.Read(ref c.CommFailures),
+            // PR ablegacy-12 / #255 — auto-demote surface.
+            DemoteCount: Interlocked.Read(ref c.DemoteCount),
+            LastDemotedUtc: ticks == 0 ? null : new DateTime(ticks, DateTimeKind.Utc));
    }

    /// <summary>
@@ -155,7 +200,14 @@ public sealed class AbLegacyDiagnosticTags
    ///     from <see cref="AbLegacyDriver.ReinitializeAsync"/> so a config redeploy starts
    ///     with a clean diagnostic surface.
    /// </summary>
-    public void Reset(string deviceHostAddress)
+    /// <remarks>
+    ///     PR ablegacy-12 / #255 — when <paramref name="preserveDemote"/> is <c>true</c> the
+    ///     cumulative <c>DemoteCount</c> + <c>LastDemotedUtc</c> survive the reset.
+    ///     <see cref="AbLegacyDriver.ReinitializeAsync"/> uses that mode so an operator
+    ///     redeploying a config doesn't lose their flapping-link history; a fresh process
+    ///     start clears them naturally because the dictionary is rebuilt from scratch.
+    /// </remarks>
+    public void Reset(string deviceHostAddress, bool preserveDemote = false)
    {
        ArgumentNullException.ThrowIfNull(deviceHostAddress);
        var c = GetOrCreate(deviceHostAddress);
@@ -166,14 +218,40 @@ public sealed class AbLegacyDiagnosticTags
        Interlocked.Exchange(ref c.LastErrorCode, 0);
        c.LastErrorMessage = string.Empty;
        Interlocked.Exchange(ref c.CommFailures, 0);
+        if (!preserveDemote)
+        {
+            Interlocked.Exchange(ref c.DemoteCount, 0);
+            Interlocked.Exchange(ref c.LastDemotedUtcTicks, 0);
+        }
    }

    /// <summary>Reset every tracked device. Called on full <c>ShutdownAsync</c>.</summary>
-    public void ResetAll()
+    /// <remarks>
+    ///     PR ablegacy-12 / #255 — when <paramref name="preserveDemote"/> is <c>true</c> the
+    ///     cumulative demote counters survive a per-device reset of every other field.
+    ///     The default (<c>false</c>) clears the dictionary outright, which is what
+    ///     <see cref="AbLegacyDriver.ShutdownAsync"/> wants.
+    /// </remarks>
+    public void ResetAll(bool preserveDemote = false)
    {
+        if (!preserveDemote)
+        {
+            lock (_lock)
+            {
+                _counters.Clear();
+            }
+            return;
+        }
+
+        // Preserve mode: keep the dictionary keys + cumulative demote fields, but
+        // zero everything else. Used by Reinitialize to span a config redeploy
+        // without losing flapping-link history.
        lock (_lock)
        {
-            _counters.Clear();
+            foreach (var key in _counters.Keys.ToList())
+            {
+                Reset(key, preserveDemote: true);
+            }
        }
    }

@@ -205,6 +283,11 @@ public sealed class AbLegacyDiagnosticTags
            "LastErrorCode" => snapshot.LastErrorCode,
            "LastErrorMessage" => snapshot.LastErrorMessage,
            "CommFailures" => snapshot.CommFailures,
+            // PR ablegacy-12 / #255 — auto-demote surface. LastDemotedUtc returns
+            // the empty string when no demotion has happened yet, mirroring the
+            // LastErrorMessage convention so HMIs can bind directly to a string.
+            "DemoteCount" => snapshot.DemoteCount,
+            "LastDemotedUtc" => snapshot.LastDemotedUtc?.ToString("o") ?? string.Empty,
            _ => null,
        };
        return true;
@@ -236,6 +319,11 @@ public sealed class AbLegacyDiagnosticTags
        public int LastErrorCode;
        public string? LastErrorMessage = string.Empty;
        public long CommFailures;
+        // PR ablegacy-12 / #255 — cumulative across config redeploys. Cleared only
+        // on full driver process restart (the dictionary is rebuilt from scratch);
+        // ReinitializeAsync uses preserveDemote: true.
+        public long DemoteCount;
+        public long LastDemotedUtcTicks;
    }
 }

@@ -251,6 +339,9 @@ public sealed class AbLegacyDiagnosticTags
 /// <param name="LastErrorCode">Most recent libplctag status code on a failed read.</param>
 /// <param name="LastErrorMessage">Most recent libplctag error message on a failed read.</param>
 /// <param name="CommFailures">Count of read failures mapped to <c>BadCommunicationError</c>.</param>
+/// <param name="DemoteCount">PR ablegacy-12 / #255 — cumulative auto-demote events.</param>
+/// <param name="LastDemotedUtc">PR ablegacy-12 / #255 — UTC timestamp of the most
+///   recent demotion, or <c>null</c> if the device has never been demoted.</param>
 public sealed record DiagnosticsSnapshot(
    long Request,
    long Response,
@@ -258,4 +349,6 @@ public sealed record DiagnosticsSnapshot(
    long Retry,
    int LastErrorCode,
    string LastErrorMessage,
-    long CommFailures);
+    long CommFailures,
+    long DemoteCount,
+    DateTime? LastDemotedUtc);
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
@@ -217,6 +217,20 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover

    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
    {
+        // PR ablegacy-12 / #255 — capture the cumulative DemoteCount + LastDemotedUtc
+        // for every currently-tracked device before we tear down. The Shutdown below
+        // calls ResetAll() which clears the dictionary; the per-host InitializeAsync
+        // below re-EnsureDevice's the slots; we restore the cumulative demote
+        // history so an operator who redeploys mid-incident doesn't lose the trail
+        // of how often this device was flapping.
+        var preservedDemote = new Dictionary<string, (long DemoteCount, DateTime? LastDemotedUtc)>(
+            StringComparer.OrdinalIgnoreCase);
+        foreach (var (host, _) in _devices)
+        {
+            var snap = _diagnosticTags.Snapshot(host);
+            preservedDemote[host] = (snap.DemoteCount, snap.LastDemotedUtc);
+        }
+
        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
        // PR ablegacy-10 / #253 — counters were dropped along with the device map when
        // ShutdownAsync called ResetAll; the InitializeAsync below re-EnsureDevice's each
@@ -224,6 +238,16 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
        // here in case a downstream override of either method skips the cycle.
        _diagnosticTags.ResetAll();
        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
+
+        // PR ablegacy-12 / #255 — restore the cumulative demote history. Only hosts
+        // that survive the redeploy get their counters back; a device removed from
+        // config legitimately drops its history (it isn't being tracked any more).
+        foreach (var (host, (count, lastUtc)) in preservedDemote)
+        {
+            if (count == 0 && lastUtc is null) continue;
+            if (!_devices.ContainsKey(host)) continue;
+            _diagnosticTags.RestoreDemote(host, count, lastUtc);
+        }
    }

    public async Task ShutdownAsync(CancellationToken cancellationToken)
@@ -275,6 +299,38 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
    internal int ResolveRetries(DeviceState device) =>
        device.Options.Retries ?? _options.Retries ?? 0;

+    /// <summary>
+    ///     PR ablegacy-12 / #255 — resolve the active <see cref="AbLegacyDemoteOptions"/> for
+    ///     a device. Per-device options win; otherwise the documented defaults (3 failures /
+    ///     30 s / enabled). Returns a non-null record so callers can assume a usable value.
+    /// </summary>
+    internal AbLegacyDemoteOptions ResolveDemote(DeviceState device) =>
+        device.Options.Demote ?? new AbLegacyDemoteOptions();
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — common bookkeeping for one comm failure: bump the
+    ///     consecutive-failure counter and arm the demote window once the threshold is
+    ///     crossed. Returns <c>true</c> when this call tipped the device into Demoted (so
+    ///     the caller can fire <see cref="OnHostStatusChanged"/>); <c>false</c> when the
+    ///     device was already demoted or stayed below the threshold.
+    /// </summary>
+    private bool RecordFailureAndMaybeDemote(DeviceState state, DateTime nowUtc)
+    {
+        var demote = ResolveDemote(state);
+        var consecutive = Interlocked.Increment(ref state.ConsecutiveFailures);
+
+        if (!demote.Enabled || consecutive < demote.FailureThreshold) return false;
+        // Already demoted? Don't re-arm — the original window's expiry is the
+        // operator-facing recovery clock and re-arming on every subsequent failed
+        // read would suppress reads forever on a fully-down device. The probe
+        // loop is what eventually clears the demotion (or the window expiring).
+        if (state.DemotedUntilUtc is { } until && until > nowUtc) return false;
+
+        state.DemotedUntilUtc = nowUtc + demote.EffectiveDemoteFor;
+        _diagnosticTags.RecordDemote(state.Options.HostAddress, nowUtc);
+        return true;
+    }
+
    // ---- IReadable ----

    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
@@ -323,6 +379,45 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
            // double-counting the original attempt as a retry.
            _diagnosticTags.RecordRequest(def.DeviceHostAddress);

+            // PR ablegacy-12 / #255 — auto-demote short-circuit. When the device's demote
+            // window is still active we return BadCommunicationError immediately, without
+            // touching libplctag or its retry loop. That's the whole point of the feature:
+            // one slow PLC sharing the driver thread can't drag down healthy peers. We
+            // don't bump ErrorCount/CommFailures here because this isn't a fresh field
+            // failure — it's the cool-down on a previously-counted one.
+            if (device.DemotedUntilUtc is { } demotedUntil)
+            {
+                if (demotedUntil > now)
+                {
+                    results[i] = new DataValueSnapshot(null,
+                        AbLegacyStatusMapper.BadCommunicationError, null, now);
+                    continue;
+                }
+                // Window expired without an early-clear from a probe success — drop the
+                // marker but don't reset ConsecutiveFailures yet. If this read also
+                // fails the failure tally keeps counting from where it left off, so a
+                // permanently-down device re-arms the window after one more
+                // consecutive failure (vs. having to repeat the full threshold).
+                lock (device.ProbeLock)
+                {
+                    if (device.DemotedUntilUtc is { } stillUntil && stillUntil <= now)
+                    {
+                        device.DemotedUntilUtc = null;
+                        // Mirror Stopped→Running on a probe-driven recovery: leave the
+                        // HostState transition to the probe loop (or the upcoming success
+                        // below); we just clear the cool-down marker so the next read
+                        // dispatches normally.
+                        if (device.HostState == HostState.Demoted)
+                        {
+                            // Surface a transition out of Demoted. The probe loop will
+                            // bring it Running once a probe succeeds; until then leave
+                            // it in Stopped to reflect "we don't actually know it's up".
+                            TransitionDeviceState(device, HostState.Stopped);
+                        }
+                    }
+                }
+            }
+
            // PR 9 — per-device retry loop: on transient BadCommunicationError (libplctag throw
            // OR a non-zero status that maps to BadCommunicationError) retry up to N times. A
            // terminal mapped status (e.g. BadNodeIdUnknown for a missing PLC tag, BadTypeMismatch
@@ -360,6 +455,15 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                            status,
                            $"libplctag status {status} reading {reference}",
                            commFailure: mappedStatus == AbLegacyStatusMapper.BadCommunicationError);
+                        // PR ablegacy-12 / #255 — only comm failures count toward the
+                        // demote tally. A BadNodeIdUnknown / BadTypeMismatch is a config
+                        // / decoder mismatch, not a sign the host is unreachable, so
+                        // demoting on it would punish the operator for a typo.
+                        if (mappedStatus == AbLegacyStatusMapper.BadCommunicationError
+                            && RecordFailureAndMaybeDemote(device, now))
+                        {
+                            TransitionDeviceState(device, HostState.Demoted);
+                        }
                        snapshot = new DataValueSnapshot(null, mappedStatus, null, now);
                        _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead,
                            $"libplctag status {status} reading {reference}");
@@ -385,6 +489,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                        _health = new DriverHealth(DriverState.Healthy, now, null);
                        // PR ablegacy-10 / #253 — successful array read.
                        _diagnosticTags.RecordResponse(def.DeviceHostAddress);
+                        // PR ablegacy-12 / #255 — successful read clears the
+                        // consecutive-failure tally. We do NOT auto-clear DemotedUntilUtc
+                        // here — the demote window is honoured to its full duration so an
+                        // intermittent link that just happened to answer once doesn't
+                        // immediately re-flood the channel. Probe success is the early
+                        // recovery path.
+                        Interlocked.Exchange(ref device.ConsecutiveFailures, 0);
                        break;
                    }

@@ -398,6 +509,10 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                    _health = new DriverHealth(DriverState.Healthy, now, null);
                    // PR ablegacy-10 / #253 — successful scalar / sub-element / bit read.
                    _diagnosticTags.RecordResponse(def.DeviceHostAddress);
+                    // PR ablegacy-12 / #255 — successful read clears the
+                    // consecutive-failure tally; demote window keeps running
+                    // until a probe success or natural expiry.
+                    Interlocked.Exchange(ref device.ConsecutiveFailures, 0);
                    break;
                }
                catch (OperationCanceledException) { throw; }
@@ -414,6 +529,12 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                        libplctagStatus: 0,
                        errorMessage: ex.Message,
                        commFailure: true);
+                    // PR ablegacy-12 / #255 — exception-driven comm failure counts
+                    // toward the demote tally just like a status-mapped one.
+                    if (RecordFailureAndMaybeDemote(device, now))
+                    {
+                        TransitionDeviceState(device, HostState.Demoted);
+                    }
                    snapshot = new DataValueSnapshot(null,
                        AbLegacyStatusMapper.BadCommunicationError, null, now);
                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
@@ -591,6 +712,14 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
            "Most recent libplctag error message on a failed read; empty when no error has been seen since the last reset.");
        EmitDiagnosticVariable(diag, deviceHostAddress, "CommFailures", DriverDataType.Int64,
            "Count of read failures mapped to BadCommunicationError. Spans transient libplctag throws + retried-out chains so operators see a single 'wire fell off' counter.");
+        // PR ablegacy-12 / #255 — auto-demote surface. DemoteCount is cumulative
+        // across reinit (preserved in ReinitializeAsync); LastDemotedUtc is a
+        // string (ISO-8601 UTC) so HMIs can bind directly without a separate
+        // DateTime decoder. Empty string means "never demoted".
+        EmitDiagnosticVariable(diag, deviceHostAddress, "DemoteCount", DriverDataType.Int64,
+            "Cumulative auto-demote events for this device — bumps every time the driver crosses the consecutive-failure threshold and arms a fresh cool-down window. Survives ReinitializeAsync.");
+        EmitDiagnosticVariable(diag, deviceHostAddress, "LastDemotedUtc", DriverDataType.String,
+            "ISO-8601 UTC timestamp of the most recent auto-demotion; empty when this device has never been demoted.");
    }

    private static void EmitDiagnosticVariable(
@@ -665,7 +794,39 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                state.ProbeInitialized = false;
            }

-            TransitionDeviceState(state, success ? HostState.Running : HostState.Stopped);
+            // PR ablegacy-12 / #255 — probe success is the early-recovery path: clear
+            // any active demote window + reset the failure tally so the next read
+            // dispatches normally. Probe failure participates in the same shared
+            // failure-tally as ReadAsync so a device with no live read traffic still
+            // demotes on a sustained outage.
+            if (success)
+            {
+                bool wasDemoted;
+                lock (state.ProbeLock)
+                {
+                    wasDemoted = state.DemotedUntilUtc is not null;
+                    state.DemotedUntilUtc = null;
+                }
+                Interlocked.Exchange(ref state.ConsecutiveFailures, 0);
+                TransitionDeviceState(state, HostState.Running);
+                _ = wasDemoted; // intentionally observed for future telemetry hooks
+            }
+            else
+            {
+                if (RecordFailureAndMaybeDemote(state, DateTime.UtcNow))
+                {
+                    TransitionDeviceState(state, HostState.Demoted);
+                }
+                else
+                {
+                    // Mid-tally probe failure: surface as Stopped if not already
+                    // Demoted. This preserves pre-PR-12 behaviour for the common
+                    // case (FailureThreshold=3 + a single hiccup ends up Stopped,
+                    // not Demoted).
+                    if (state.HostState != HostState.Demoted)
+                        TransitionDeviceState(state, HostState.Stopped);
+                }
+            }

            try { await Task.Delay(_options.Probe.Interval, ct).ConfigureAwait(false); }
            catch (OperationCanceledException) { break; }
@@ -890,6 +1051,25 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
        public CancellationTokenSource? ProbeCts { get; set; }
        public bool ProbeInitialized { get; set; }

+        /// <summary>
+        ///     PR ablegacy-12 / #255 — running tally of consecutive read / probe failures.
+        ///     Reset on every successful read or probe; tripping
+        ///     <see cref="AbLegacyDemoteOptions.FailureThreshold"/> arms the demote window.
+        ///     Read + written via <see cref="Interlocked"/> because read + probe loops can
+        ///     touch it concurrently.
+        /// </summary>
+        public int ConsecutiveFailures;
+
+        /// <summary>
+        ///     PR ablegacy-12 / #255 — when set, reads against this device short-circuit
+        ///     with <c>BadCommunicationError</c> until the timestamp passes; cleared early
+        ///     by a successful probe. Guarded by <see cref="ProbeLock"/> for the mutator
+        ///     paths (TransitionDeviceState + RecordFailureAndMaybeDemote); reads grab
+        ///     the property without locking — a torn DateTime? read is harmless here
+        ///     because the worst case is one extra dispatched read on an x86 boundary.
+        /// </summary>
+        public DateTimeOffset? DemotedUntilUtc { get; set; }
+
        public void DisposeRuntimes()
        {
            foreach (var r in Runtimes.Values) r.Dispose();
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverFactoryExtensions.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverFactoryExtensions.cs
@@ -45,7 +45,14 @@ public static class AbLegacyDriverFactoryExtensions
                    DeviceName: d.DeviceName,
                    // PR 9 — per-device timeout / retry overrides. Device-level wins over driver-wide.
                    Timeout: d.TimeoutMs is int devMs ? TimeSpan.FromMilliseconds(devMs) : null,
-                    Retries: d.Retries))]
+                    Retries: d.Retries,
+                    // PR ablegacy-12 / #255 — auto-demote knobs.
+                    Demote: d.Demote is null ? null : new AbLegacyDemoteOptions(
+                        FailureThreshold: d.Demote.FailureThreshold ?? 3,
+                        DemoteFor: d.Demote.DemoteForMs is int demMs
+                            ? TimeSpan.FromMilliseconds(demMs)
+                            : null,
+                        Enabled: d.Demote.Enabled ?? true)))]
                : [],
            Tags = dto.Tags is { Count: > 0 }
                ? [.. dto.Tags.Select(t => new AbLegacyTagDefinition(
@@ -209,6 +216,26 @@ public static class AbLegacyDriverFactoryExtensions
        ///     <c>null</c> at both levels = single attempt.
        /// </summary>
        public int? Retries { get; init; }
+
+        /// <summary>
+        ///     PR ablegacy-12 / #255 — optional per-device auto-demote knobs. <c>null</c>
+        ///     means "use the documented defaults" (<c>FailureThreshold=3</c>,
+        ///     <c>DemoteFor=30s</c>, <c>Enabled=true</c>) — the driver still demotes by
+        ///     default. Set <c>Enabled=false</c> in the JSON to opt out entirely.
+        /// </summary>
+        public AbLegacyDemoteDto? Demote { get; init; }
+    }
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — JSON DTO for the auto-demote knobs. Times are
+    ///     ms-suffixed for consistency with the rest of the driver config (TimeoutMs,
+    ///     IntervalMs).
+    /// </summary>
+    internal sealed class AbLegacyDemoteDto
+    {
+        public int? FailureThreshold { get; init; }
+        public int? DemoteForMs { get; init; }
+        public bool? Enabled { get; init; }
    }

    internal sealed class AbLegacyTagDto
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverOptions.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverOptions.cs
@@ -41,7 +41,39 @@ public sealed record AbLegacyDeviceOptions(
    AbLegacyPlcFamily PlcFamily = AbLegacyPlcFamily.Slc500,
    string? DeviceName = null,
    TimeSpan? Timeout = null,
-    int? Retries = null);
+    int? Retries = null,
+    AbLegacyDemoteOptions? Demote = null);
+
+/// <summary>
+///     PR ablegacy-12 / #255 — auto-demote knobs. After
+///     <see cref="FailureThreshold"/> consecutive read / probe failures the driver
+///     marks the device <c>Demoted</c> for <see cref="DemoteFor"/>; reads against
+///     a demoted device short-circuit with <c>BadCommunicationError</c> instead
+///     of dispatching through libplctag, so one slow PLC can't starve faster
+///     peers sharing the same driver. A successful probe clears the demotion
+///     early; a successful read just resets the consecutive-failure counter
+///     without leaving the demoted window.
+/// </summary>
+/// <param name="FailureThreshold">Consecutive read or probe failures that trip
+///   the demotion. Default <c>3</c>.</param>
+/// <param name="DemoteFor">Cool-down window before reads are dispatched again
+///   without a successful probe in between. Default <c>30s</c>.</param>
+/// <param name="Enabled">When <c>false</c> the failure tally still ticks but the
+///   driver never sets the demoted window — useful when an operator wants the
+///   diagnostic counters without the throttling behaviour.</param>
+public sealed record AbLegacyDemoteOptions(
+    int FailureThreshold = 3,
+    TimeSpan? DemoteFor = null,
+    bool Enabled = true)
+{
+    /// <summary>
+    ///     Effective demote window. Records can't have <c>TimeSpan</c> defaults
+    ///     because <c>TimeSpan.FromSeconds(30)</c> isn't a compile-time constant;
+    ///     callers that pass <c>null</c> get the documented 30-second default
+    ///     here.
+    /// </summary>
+    public TimeSpan EffectiveDemoteFor => DemoteFor ?? TimeSpan.FromSeconds(30);
+}

 /// <summary>
 ///     One PCCC-backed OPC UA variable. <c>Address</c> is the canonical PCCC file-address
--- a/src/ZB.MOM.WW.OtOpcUa.Server/HostStatusPublisher.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Server/HostStatusPublisher.cs
@@ -138,6 +138,11 @@ public sealed class HostStatusPublisher(
        HostState.Running => DriverHostState.Running,
        HostState.Stopped => DriverHostState.Stopped,
        HostState.Faulted => DriverHostState.Faulted,
+        // PR ablegacy-12 / #255 — Demoted is a driver-side back-off (skipped reads while
+        // we wait for a flaky host to recover). The Configuration enum doesn't have a
+        // dedicated value; surface it as Stopped so the Admin UI lights it up red-ish
+        // without the publisher needing a schema migration to differentiate.
+        HostState.Demoted => DriverHostState.Stopped,
        _ => DriverHostState.Unknown,
    };
 }
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests/AbLegacyAutoDemoteTests.cs
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests/AbLegacyAutoDemoteTests.cs
@@ -0,0 +1,102 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests;
+
+/// <summary>
+///     PR ablegacy-12 / #255 — wire-level smoke for auto-demote on comm failure.
+///     Runs only when ab_server is reachable. Two devices: one healthy (the live
+///     ab_server slc500 simulator), one pointed at <c>127.0.0.1:1</c> which
+///     refuses every connection. After three consecutive failures the faulty
+///     device's reads must short-circuit with <c>BadCommunicationError</c>
+///     while the healthy device keeps returning <c>Good</c> — the whole point
+///     of the feature: one slow / unreachable PLC sharing the driver thread
+///     can't starve faster peers.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Build-only by default — the assertion that demotion latency is
+///         bounded depends on the ab_server simulator timing out on the faulty
+///         port within the per-device timeout. We pin the faulty endpoint at
+///         <c>127.0.0.1:1</c> (the bogus-port standard) which RST's the
+///         connection immediately on most stacks; environments that whitelist
+///         outbound to localhost:1 will see different timing but still trip
+///         the threshold within the test budget.
+///     </para>
+///     <para>
+///         The Docker fixture extension (<c>slc500-faulty</c>) noted in the PR
+///         plan is a documentation-only placeholder for now — implementing a
+///         refusing-proxy container is non-trivial and the localhost:1 trick
+///         covers the same surface deterministically.
+///     </para>
+/// </remarks>
+[Collection(AbLegacyServerCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Simulator", "ab_server-PCCC")]
+public sealed class AbLegacyAutoDemoteTests(AbLegacyServerFixture sim)
+{
+    [AbLegacyFact]
+    public async Task Two_devices_one_unreachable_does_not_starve_healthy_reads()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+
+        var healthy = $"ab://{sim.Host}:{sim.Port}/{sim.CipPath}";
+        // 127.0.0.1:1 is the bogus-port standard — typical Linux/Windows TCP
+        // stacks RST immediately. The driver still reports it as a comm
+        // failure (libplctag wraps the failure as a transient throw).
+        var faulty = "ab://127.0.0.1:1/1,0";
+
+        await using var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices =
+            [
+                new AbLegacyDeviceOptions(healthy, AbLegacyPlcFamily.Slc500,
+                    Timeout: TimeSpan.FromSeconds(5)),
+                new AbLegacyDeviceOptions(faulty, AbLegacyPlcFamily.Slc500,
+                    // Snappy timeout so the test budget stays short.
+                    Timeout: TimeSpan.FromMilliseconds(500),
+                    Demote: new AbLegacyDemoteOptions(
+                        FailureThreshold: 3,
+                        DemoteFor: TimeSpan.FromSeconds(30))),
+            ],
+            Tags =
+            [
+                new AbLegacyTagDefinition("Healthy", healthy, "N7:0", AbLegacyDataType.Int),
+                new AbLegacyTagDefinition("Faulty", faulty, "N7:0", AbLegacyDataType.Int),
+            ],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, driverInstanceId: "ablegacy-auto-demote-it");
+
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        // Trip the demote on the faulty device.
+        for (var i = 0; i < 3; i++)
+        {
+            await drv.ReadAsync(["Faulty"], TestContext.Current.CancellationToken);
+        }
+
+        // Healthy host MUST keep returning Good even though the sibling is demoted.
+        var healthyResult = await drv.ReadAsync(["Healthy"], TestContext.Current.CancellationToken);
+        healthyResult[0].StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+
+        // Faulty host now short-circuits without waiting on libplctag's timeout.
+        var sw = System.Diagnostics.Stopwatch.StartNew();
+        var faultyResult = await drv.ReadAsync(["Faulty"], TestContext.Current.CancellationToken);
+        sw.Stop();
+        faultyResult[0].StatusCode.ShouldBe(AbLegacyStatusMapper.BadCommunicationError);
+        // Short-circuit should be ~1 ms; pad generously for CI noise. The pre-PR-12
+        // path would have waited the full 500 ms timeout.
+        sw.ElapsedMilliseconds.ShouldBeLessThan(200);
+
+        // Counter access via the public diagnostic short-circuit path — the
+        // internal Snapshot() seam isn't visible from this assembly.
+        var demoteCountRef = $"_Diagnostics/{faulty}/DemoteCount";
+        var lastDemotedRef = $"_Diagnostics/{faulty}/LastDemotedUtc";
+        var diag = await drv.ReadAsync(
+            [demoteCountRef, lastDemotedRef], TestContext.Current.CancellationToken);
+        ((long)diag[0].Value!).ShouldBeGreaterThan(0);
+        ((string)diag[1].Value!).Length.ShouldBeGreaterThan(0);
+    }
+}
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests/Docker/docker-compose.yml
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.IntegrationTests/Docker/docker-compose.yml
@@ -72,3 +72,30 @@ services:
      "--tag=F8[120]",
      "--tag=B3[10]"
    ]
+
+  # PR ablegacy-12 / #255 — faulty-PLC fixture for the auto-demote contract.
+  # FIXTURE-TIER FOLLOW-UP: implementing a refusing-proxy container that
+  # round-trips libplctag's CIP framing far enough to trigger comm failures
+  # (vs. just RST'ing the TCP handshake) is non-trivial — the integration
+  # test currently uses 127.0.0.1:1 (the bogus-port standard) which RST's
+  # immediately on most TCP stacks. That gets us deterministic comm-failure
+  # coverage without standing up a second container; if the localhost:1
+  # trick stops working on a future test runner (e.g. a sandbox that
+  # blocks port 1) re-enable this stub:
+  #
+  # slc500-faulty:
+  #   profiles: ["slc500-faulty"]
+  #   image: otopcua-ab-server:libplctag-release
+  #   build:
+  #     context: ../../ZB.MOM.WW.OtOpcUa.Driver.AbCip.IntegrationTests/Docker
+  #     dockerfile: Dockerfile
+  #   container_name: otopcua-ab-server-slc500-faulty
+  #   restart: "no"
+  #   ports:
+  #     - "44819:44819"
+  #   # Hostile entrypoint: bind the port but exit immediately so subsequent
+  #   # connection attempts get RST'd. Future iteration: a libplctag-aware
+  #   # proxy that accepts the CIP open and then drops the wire halfway
+  #   # through, exercising the read-timeout path rather than the
+  #   # connection-refused path.
+  #   entrypoint: ["sh", "-c", "exit 1"]
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAutoDemoteTests.cs
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAutoDemoteTests.cs
@@ -0,0 +1,380 @@
+using System.Text.Json;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     PR ablegacy-12 / #255 — auto-demote on consecutive comm failure. After
+///     <c>FailureThreshold</c> consecutive read or probe failures the driver
+///     marks the device <c>Demoted</c> for <c>DemoteFor</c>; subsequent reads
+///     short-circuit with <c>BadCommunicationError</c> without invoking
+///     libplctag, so one slow PLC sharing the driver thread can't starve faster
+///     peers. Probe success clears the demote early; read success resets the
+///     consecutive-failure tally without leaving the demote window.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyAutoDemoteTests
+{
+    private const string Host = "ab://10.0.0.5/1,0";
+    private const string SecondHost = "ab://10.0.0.6/1,0";
+
+    /// <summary>
+    ///     Disable the probe by default — every test wants deterministic
+    ///     control over the failure tally without a background loop racing
+    ///     against the read path.
+    /// </summary>
+    private static AbLegacyDriverOptions BaseOptions(
+        AbLegacyDemoteOptions? demote = null,
+        IReadOnlyList<AbLegacyDeviceOptions>? devices = null,
+        IReadOnlyList<AbLegacyTagDefinition>? tags = null) => new()
+        {
+            Devices = devices ?? [new AbLegacyDeviceOptions(Host, AbLegacyPlcFamily.Slc500, Demote: demote)],
+            Tags = tags ?? [new AbLegacyTagDefinition("X", Host, "N7:0", AbLegacyDataType.Int)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        };
+
+    private static (AbLegacyDriver drv, FakeAbLegacyTagFactory factory) NewDriver(
+        AbLegacyDemoteOptions? demote = null,
+        IReadOnlyList<AbLegacyDeviceOptions>? devices = null,
+        IReadOnlyList<AbLegacyTagDefinition>? tags = null)
+    {
+        var factory = new FakeAbLegacyTagFactory();
+        var drv = new AbLegacyDriver(BaseOptions(demote, devices, tags), "drv-demote", factory);
+        return (drv, factory);
+    }
+
+    private static FakeAbLegacyTag SeedFailingTag(FakeAbLegacyTagFactory factory)
+    {
+        // Cause every read to throw — exception-driven failures count as
+        // BadCommunicationError per RecordError(commFailure:true).
+        factory.Customise = p => new FakeAbLegacyTag(p)
+        {
+            ThrowOnRead = true,
+            Exception = new TimeoutException("simulated comm failure"),
+        };
+        // Return value is the prototype so a caller that wants to flip the
+        // failure off later can do so via factory.Tags["N7:0"].
+        return null!;
+    }
+
+    [Fact]
+    public async Task Three_consecutive_failures_demote_the_device()
+    {
+        var (drv, factory) = NewDriver();
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.DemotedUntilUtc.ShouldNotBeNull();
+        var snap = drv.DiagnosticTags.Snapshot(Host);
+        snap.DemoteCount.ShouldBe(1);
+        snap.LastDemotedUtc.ShouldNotBeNull();
+        drv.GetHostStatuses().Single().State.ShouldBe(HostState.Demoted);
+    }
+
+    [Fact]
+    public async Task Reads_while_demoted_short_circuit_without_invoking_libplctag()
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyDemoteOptions(FailureThreshold: 3, DemoteFor: TimeSpan.FromMinutes(5)));
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Trip the demotion.
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        var readsBeforeDemote = factory.Tags["N7:0"].ReadCount;
+
+        // Subsequent reads MUST NOT call into libplctag — the short-circuit
+        // returns BadCommunicationError before EnsureTagRuntimeAsync.
+        var result = await drv.ReadAsync(["X"], CancellationToken.None);
+        result[0].StatusCode.ShouldBe(AbLegacyStatusMapper.BadCommunicationError);
+        factory.Tags["N7:0"].ReadCount.ShouldBe(readsBeforeDemote);
+
+        var result2 = await drv.ReadAsync(["X"], CancellationToken.None);
+        result2[0].StatusCode.ShouldBe(AbLegacyStatusMapper.BadCommunicationError);
+        factory.Tags["N7:0"].ReadCount.ShouldBe(readsBeforeDemote);
+    }
+
+    [Fact]
+    public async Task After_DemoteFor_expires_next_read_dispatches_through()
+    {
+        // Tiny window so the cool-down expires within the test.
+        var (drv, factory) = NewDriver(
+            new AbLegacyDemoteOptions(FailureThreshold: 2, DemoteFor: TimeSpan.FromMilliseconds(50)));
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Trip with two failures.
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.DemotedUntilUtc.ShouldNotBeNull();
+        var readsBeforeWait = factory.Tags["N7:0"].ReadCount;
+
+        // Flip the fake to succeed and wait past the demote window.
+        factory.Tags["N7:0"].ThrowOnRead = false;
+        factory.Tags["N7:0"].Value = 42;
+        factory.Tags["N7:0"].Status = 0;
+        await Task.Delay(TimeSpan.FromMilliseconds(120));
+
+        var result = await drv.ReadAsync(["X"], CancellationToken.None);
+        result[0].StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        result[0].Value.ShouldBe(42);
+        // The window expiry path dispatched through to libplctag.
+        factory.Tags["N7:0"].ReadCount.ShouldBeGreaterThan(readsBeforeWait);
+    }
+
+    [Fact]
+    public async Task Successful_read_resets_consecutive_failure_counter()
+    {
+        var (drv, factory) = NewDriver();
+        // Initial state — every read fails.
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.ConsecutiveFailures.ShouldBe(2);
+
+        // One successful read — flip the existing fake.
+        factory.Tags["N7:0"].ThrowOnRead = false;
+        factory.Tags["N7:0"].Value = 99;
+        factory.Tags["N7:0"].Status = 0;
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        state.ConsecutiveFailures.ShouldBe(0);
+        state.DemotedUntilUtc.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task Failure_success_failure_does_not_demote_at_threshold_three()
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyDemoteOptions(FailureThreshold: 3));
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // 2 failures.
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        // 1 success — counter resets.
+        factory.Tags["N7:0"].ThrowOnRead = false;
+        factory.Tags["N7:0"].Status = 0;
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        // 2 more failures — should still be below the threshold.
+        factory.Tags["N7:0"].ThrowOnRead = true;
+        factory.Tags["N7:0"].Exception = new TimeoutException("flap");
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.DemotedUntilUtc.ShouldBeNull();
+        drv.DiagnosticTags.Snapshot(Host).DemoteCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task DemoteCount_and_LastDemotedUtc_surface_via_diagnostic_short_circuit()
+    {
+        var (drv, factory) = NewDriver();
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        // Read the synthetic _Diagnostics counters.
+        var demoteCountRef = $"{AbLegacyDiagnosticTags.DiagnosticsFolderPrefix}{Host}/DemoteCount";
+        var lastDemotedRef = $"{AbLegacyDiagnosticTags.DiagnosticsFolderPrefix}{Host}/LastDemotedUtc";
+        var counts = await drv.ReadAsync([demoteCountRef, lastDemotedRef], CancellationToken.None);
+
+        counts[0].StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        counts[0].Value.ShouldBe(1L);
+        counts[1].StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        counts[1].Value.ShouldBeOfType<string>();
+        ((string)counts[1].Value!).Length.ShouldBeGreaterThan(0); // ISO-8601 stamp
+    }
+
+    [Fact]
+    public async Task Demote_disabled_never_short_circuits_reads()
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyDemoteOptions(FailureThreshold: 1, Enabled: false));
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // 5 failures — would normally trip a single-fail threshold, but Enabled=false.
+        for (var i = 0; i < 5; i++) await drv.ReadAsync(["X"], CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.DemotedUntilUtc.ShouldBeNull();
+        var snap = drv.DiagnosticTags.Snapshot(Host);
+        snap.DemoteCount.ShouldBe(0);
+        // Failures still get recorded as comm errors though — the diagnostic
+        // surface is honest about what happened, just no auto-throttle.
+        snap.CommFailures.ShouldBe(5);
+        // libplctag was invoked every time — that's the whole point of opting out.
+        factory.Tags["N7:0"].ReadCount.ShouldBe(5);
+    }
+
+    [Fact]
+    public async Task Reinit_preserves_DemoteCount_but_clears_active_demotion()
+    {
+        var (drv, factory) = NewDriver();
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        drv.DiagnosticTags.Snapshot(Host).DemoteCount.ShouldBe(1);
+        drv.GetDeviceState(Host)!.DemotedUntilUtc.ShouldNotBeNull();
+
+        await drv.ReinitializeAsync("{}", CancellationToken.None);
+
+        // Active demotion cleared (the device is freshly tracked); cumulative count survives.
+        drv.GetDeviceState(Host)!.DemotedUntilUtc.ShouldBeNull();
+        drv.GetDeviceState(Host)!.ConsecutiveFailures.ShouldBe(0);
+        drv.DiagnosticTags.Snapshot(Host).DemoteCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Disposing_driver_after_demotion_does_not_throw()
+    {
+        var (drv, factory) = NewDriver();
+        SeedFailingTag(factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        await drv.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task Demote_options_dto_round_trips_through_factory_extensions()
+    {
+        const string json = """
+            {
+              "Devices": [
+                {
+                  "HostAddress": "ab://10.0.0.5/1,0",
+                  "PlcFamily": "Slc500",
+                  "Demote": {
+                    "FailureThreshold": 5,
+                    "DemoteForMs": 60000,
+                    "Enabled": true
+                  }
+                }
+              ],
+              "Probe": { "Enabled": false },
+              "Tags": [
+                { "Name": "X", "DeviceHostAddress": "ab://10.0.0.5/1,0", "Address": "N7:0", "DataType": "Int" }
+              ]
+            }
+            """;
+
+        var drv = AbLegacyDriverFactoryExtensions.CreateInstance("drv-demote-roundtrip", json);
+        await drv.InitializeAsync(json, CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        state.Options.Demote.ShouldNotBeNull();
+        state.Options.Demote!.FailureThreshold.ShouldBe(5);
+        state.Options.Demote.EffectiveDemoteFor.ShouldBe(TimeSpan.FromMinutes(1));
+        state.Options.Demote.Enabled.ShouldBeTrue();
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Two_devices_one_faulty_does_not_starve_the_healthy_one()
+    {
+        // Mixed factory — one host's tag throws, the other's reads cleanly.
+        var factory = new FakeAbLegacyTagFactory();
+        factory.Customise = p =>
+        {
+            // Identify by the Gateway portion of the create params.
+            var fail = p.Gateway == "10.0.0.6";
+            return new FakeAbLegacyTag(p)
+            {
+                ThrowOnRead = fail,
+                Exception = fail ? new TimeoutException("faulty") : null,
+                Value = 42,
+                Status = 0,
+            };
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices =
+            [
+                new AbLegacyDeviceOptions(Host, AbLegacyPlcFamily.Slc500),
+                new AbLegacyDeviceOptions(SecondHost, AbLegacyPlcFamily.Slc500),
+            ],
+            Tags =
+            [
+                new AbLegacyTagDefinition("Healthy", Host, "N7:0", AbLegacyDataType.Int),
+                new AbLegacyTagDefinition("Faulty", SecondHost, "N7:0", AbLegacyDataType.Int),
+            ],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-mix", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Trip the faulty side.
+        for (var i = 0; i < 3; i++)
+            await drv.ReadAsync(["Faulty"], CancellationToken.None);
+
+        // Healthy host MUST keep returning Good even though the sibling is demoted.
+        var healthyResult = await drv.ReadAsync(["Healthy"], CancellationToken.None);
+        healthyResult[0].StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        healthyResult[0].Value.ShouldBe(42);
+
+        // Reads against the faulty host short-circuit.
+        var faultyResult = await drv.ReadAsync(["Faulty"], CancellationToken.None);
+        faultyResult[0].StatusCode.ShouldBe(AbLegacyStatusMapper.BadCommunicationError);
+
+        drv.GetDeviceState(Host)!.DemotedUntilUtc.ShouldBeNull();
+        drv.GetDeviceState(SecondHost)!.DemotedUntilUtc.ShouldNotBeNull();
+    }
+
+    [Fact]
+    public async Task BadNodeIdUnknown_does_not_count_toward_demote_tally()
+    {
+        // -14 maps to BadNodeIdUnknown — terminal, not a comm failure.
+        var (drv, factory) = NewDriver();
+        factory.Customise = p => new FakeAbLegacyTag(p) { Status = -14 };
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        for (var i = 0; i < 5; i++)
+            await drv.ReadAsync(["X"], CancellationToken.None);
+
+        var state = drv.GetDeviceState(Host).ShouldNotBeNull();
+        // Five terminal failures shouldn't trip the demote threshold — they're
+        // a config / decoder mismatch, not a sign of a flapping link.
+        state.DemotedUntilUtc.ShouldBeNull();
+        drv.DiagnosticTags.Snapshot(Host).DemoteCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void HostState_enum_has_Demoted_value()
+    {
+        // Belt-and-braces: the abstraction surface must carry the new value
+        // for downstream consumers (HostStatusPublisher, Admin UI, …) to
+        // see and route it.
+        Enum.IsDefined(typeof(HostState), HostState.Demoted).ShouldBeTrue();
+        ((int)HostState.Demoted).ShouldBeGreaterThan((int)HostState.Faulted);
+    }
+}
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDiagnosticsTests.cs
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDiagnosticsTests.cs
@@ -173,7 +173,9 @@ public sealed class AbLegacyDiagnosticsTests
        var diagVars = builder.Variables
            .Where(v => v.Info.FullName.StartsWith(AbLegacyDiagnosticTags.DiagnosticsFolderPrefix))
            .ToList();
-        diagVars.Count.ShouldBe(14); // 7 names × 2 devices
+        // PR ablegacy-12 / #255 — DemoteCount + LastDemotedUtc bring the canonical
+        // count to 9 names per device (was 7 in PR ablegacy-10).
+        diagVars.Count.ShouldBe(AbLegacyDiagnosticTags.DiagnosticTagNames.Count * 2);
        diagVars.ShouldAllBe(v => v.Info.SecurityClass == SecurityClassification.ViewOnly);
    }

--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs
@@ -84,7 +84,14 @@ internal class FakeAbLegacyTag : IAbLegacyTagRuntime

 internal sealed class FakeAbLegacyTagFactory : IAbLegacyTagFactory
 {
-    public Dictionary<string, FakeAbLegacyTag> Tags { get; } = new(StringComparer.OrdinalIgnoreCase);
+    // PR ablegacy-12 / #255 — switched from plain Dictionary to ConcurrentDictionary so
+    // the read path (test thread) and the probe loop (background Task) can both call
+    // Create without corrupting the dict. Pre-PR-12 the race existed but only tipped
+    // a few percent of test runs into KeyNotFoundException; PR-12's added
+    // Interlocked.Exchange writes shifted timing enough to make it deterministic-flaky
+    // (~60%).
+    public System.Collections.Concurrent.ConcurrentDictionary<string, FakeAbLegacyTag> Tags { get; } =
+        new(StringComparer.OrdinalIgnoreCase);
    public Func<AbLegacyTagCreateParams, FakeAbLegacyTag>? Customise { get; set; }

    public IAbLegacyTagRuntime Create(AbLegacyTagCreateParams p)