fix(driver-historian-wonderware-client): resolve Low code-review findings (Driver.Historian.Wonderware.Client-003,004,006,008,010)

- Driver.Historian.Wonderware.Client-003: replaced the mixed Interlocked
  + healthLock counters with RecordOutcome that touches _totalQueries
  and exactly one of _totalSuccesses / _totalFailures under one
  acquisition.
- Driver.Historian.Wonderware.Client-004: InvokeAndClassifyAsync routes
  transport + sidecar classification through a single RecordOutcome
  call; the legacy ReclassifySuccessAsFailure two-step is gone.
- Driver.Historian.Wonderware.Client-006: removed the dead
  ReconnectInitialBackoff / ReconnectMaxBackoff options and added a
  doc <remarks> stating the channel performs a single in-place
  reconnect; retry/backoff stays with the caller.
- Driver.Historian.Wonderware.Client-008: the audit-suppression comment
  block now records advisory titles, why neither applies, and the
  revisit trigger.
- Driver.Historian.Wonderware.Client-010: reworded Dispose() to claim
  deadlock-safety and added a GetHealthSnapshot summary documenting the
  single-channel collapse + counter invariant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.Historian.Wonderware.Client/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-22 |
 | Commit reviewed | `76d35d1` |
 | Status | Reviewed |
-| Open findings | 5 |
+| Open findings | 0 |
 
 ## Checklist coverage
 
@@ -92,7 +92,7 @@ dead-lettered. Until then, document explicitly that this writer never produces
 | Severity | Low |
 | Category | Concurrency & thread safety |
 | Location | `WonderwareHistorianClient.cs:207`, `WonderwareHistorianClient.cs:132-150` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `_totalQueries` is mutated with `Interlocked.Increment` in `Invoke`, but
 read inside `GetHealthSnapshot` under `_healthLock`, and every other counter
@@ -106,7 +106,7 @@ and the counters are advisory, but the mixed model is a latent hazard.
 `_healthLock` block (a new `RecordQuery()` helper, or fold it into `RecordSuccess`/
 `RecordFailure`) so all six health fields share a single lock.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-23 — replaced the mixed `Interlocked.Increment(ref _totalQueries)` + `_healthLock`-protected outcome counters with a single `RecordOutcome(bool success, string? error)` helper that increments `_totalQueries` and exactly one of `_totalSuccesses` / `_totalFailures` under one `_healthLock` acquisition; `GetHealthSnapshot` documents the invariant that `TotalSuccesses + TotalFailures == TotalQueries` at every observed snapshot. Added the regression test `GetHealthSnapshot_ConcurrentCallsAndReads_CountersAreInternallyConsistent` that runs a polling reader concurrently with 50 calls and asserts the invariant never breaks (fails red against the previous code, passes green now).
 
 ### Driver.Historian.Wonderware.Client-004
 
@@ -115,7 +115,7 @@ and the counters are advisory, but the mixed model is a latent hazard.
 | Severity | Low |
 | Category | Concurrency & thread safety |
 | Location | `WonderwareHistorianClient.cs:203-267` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** A sidecar-reported failure is recorded in two non-atomic steps under
 separate lock acquisitions: `Invoke` calls `RecordSuccess()` (line 211) and then the
@@ -132,7 +132,7 @@ sidecar-level `Success` flag has been checked, or pass the reply success/error i
 single `RecordOutcome(bool transportOk, bool sidecarOk, string? error)` that updates all
 counters under one lock acquisition.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-23 — eliminated the `RecordSuccess` → `ReclassifySuccessAsFailure` undo dance. `InvokeAsync` now takes a `Func<TReply, (bool ok, string? error)>` evaluator, evaluates it once when the transport reply lands, and calls `RecordOutcome(bool success, string? error)` exactly once per call under a single `_healthLock` acquisition. A sidecar-reported failure is now classified as a failure on its first and only counter update — no transient "success then undo" state is observable. The read-side `InvokeAndClassifyAsync` wrapper preserves the prior `InvalidOperationException` throw on sidecar failure. Added regression test `GetHealthSnapshot_SidecarFailure_NeverInflatesSuccessCounter` pinning `TotalSuccesses=0`/`TotalFailures=1` after a sidecar-error call.
 
 ### Driver.Historian.Wonderware.Client-005
 
@@ -167,7 +167,7 @@ the reader.
 | Severity | Low |
 | Category | Error handling & resilience |
 | Location | `Internal/PipeChannel.cs:96-107`, `WonderwareHistorianClientOptions.cs:11-12` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `PipeChannel.InvokeAsync` retries exactly once on transport failure and
 otherwise propagates. The options expose `ReconnectInitialBackoff` and
@@ -182,7 +182,7 @@ or the options are dead config that misleads operators.
 path, or remove the two unused option fields and their XML docs and state plainly that
 retry/backoff is owned by the caller (the alarm drain worker / history router).
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-23 — removed the dead `ReconnectInitialBackoff`/`ReconnectMaxBackoff` fields (and their `Effective*` accessors) from `WonderwareHistorianClientOptions` and added a `<remarks>` block stating that retry/backoff is owned by the caller (the alarm drain worker and the read-side history router) and that the channel itself performs exactly one in-place reconnect with no delay. Confirmed no consumer referenced the removed fields (only `code-reviews/` references remain). Solution-level build clean — Server picks up the new options shape without change.
 
 ### Driver.Historian.Wonderware.Client-007
 
@@ -218,7 +218,7 @@ deserializing.
 | Severity | Low |
 | Category | Security |
 | Location | `ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.csproj:29-32` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** The csproj suppresses two NuGet audit advisories
 (`GHSA-37gx-xxp4-5rgx`, `GHSA-w3x6-4m5h-cxqf`) for the `MessagePack` 2.5.187 dependency
@@ -232,7 +232,7 @@ advisory title, why it does not apply to this module usage, and a revisit trigge
 follow-up to upgrade `MessagePack` once a patched version is available so the suppressions
 can be dropped.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-23 — the suppression block in the csproj (already added under finding 007) records each advisory title (GHSA-37gx-xxp4-5rgx unsafe-dynamic-codegen, GHSA-w3x6-4m5h-cxqf typeless-resolver gadget chain), why neither applies to this module (default `StandardResolver` only, no `TypelessContractlessStandardResolver` / `DynamicUnion` / `DynamicGenericResolver`, plus the 64 KiB per-sample ValueBytes cap in `DeserializeSampleValue` from finding 007), and the revisit trigger ("Revisit once MessagePack 3.x is available and drop these suppressions at that time"). All three pieces the recommendation asked for are present; the single comment block above both `NuGetAuditSuppress` entries was confirmed to satisfy the audit-trail gap.
 
 ### Driver.Historian.Wonderware.Client-009
 
@@ -272,7 +272,7 @@ silent `[Key]` drift between the two duplicated contract sets is caught at build
 | Severity | Low |
 | Category | Documentation & comments |
 | Location | `WonderwareHistorianClient.cs:355-361`, `WonderwareHistorianClient.cs:132-150` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** Two doc/behaviour mismatches.
 (1) The `Dispose()` XML comment asserts the underlying channel async cleanup is
@@ -291,4 +291,4 @@ node concept. The collapse is reasonable but undocumented.
 short remark on `GetHealthSnapshot` explaining that the single-channel client maps both
 connection flags to one transport and does not track per-node health.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-23 — (1) reworded the `Dispose()` XML comment to drop the "non-blocking" claim and instead state that the bridge is **deadlock-safe** because the cleanup never awaits a captured `SynchronizationContext` nor takes any lock the caller could hold, while acknowledging that `NamedPipeClientStream` teardown can block briefly on OS handle release. (2) Added a full `<summary>` + `<remarks>` block to `GetHealthSnapshot` explaining the single-channel collapse — both `ProcessConnectionOpen` and `EventConnectionOpen` report the same channel state, and `ActiveProcessNode`/`ActiveEventNode`/`Nodes` are intentionally null/empty because the client has no per-node telemetry. The remarks also pin the finding-003/004 invariant `TotalSuccesses + TotalFailures == TotalQueries`.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client/WonderwareHistorianClient.cs

@@ -72,8 +72,9 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
             MaxValues = (int)Math.Min(maxValuesPerNode, int.MaxValue),
             CorrelationId = Guid.NewGuid().ToString("N"),
         };
-        var reply = await Invoke<ReadRawRequest, ReadRawReply>(MessageKind.ReadRawRequest, MessageKind.ReadRawReply, req, cancellationToken).ConfigureAwait(false);
+        var reply = await InvokeAndClassifyAsync<ReadRawRequest, ReadRawReply>(
-        ThrowIfFailed(reply.Success, reply.Error, "ReadRaw");
+            MessageKind.ReadRawRequest, MessageKind.ReadRawReply, req,
+            r => (r.Success, r.Error), "ReadRaw", cancellationToken).ConfigureAwait(false);
         return new HistoryReadResult(ToSnapshots(reply.Samples), ContinuationPoint: null);
     }
 
@@ -90,8 +91,9 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
             AggregateColumn = MapAggregate(aggregate),
             CorrelationId = Guid.NewGuid().ToString("N"),
         };
-        var reply = await Invoke<ReadProcessedRequest, ReadProcessedReply>(MessageKind.ReadProcessedRequest, MessageKind.ReadProcessedReply, req, cancellationToken).ConfigureAwait(false);
+        var reply = await InvokeAndClassifyAsync<ReadProcessedRequest, ReadProcessedReply>(
-        ThrowIfFailed(reply.Success, reply.Error, "ReadProcessed");
+            MessageKind.ReadProcessedRequest, MessageKind.ReadProcessedReply, req,
+            r => (r.Success, r.Error), "ReadProcessed", cancellationToken).ConfigureAwait(false);
         return new HistoryReadResult(ToAggregateSnapshots(reply.Buckets), ContinuationPoint: null);
     }
 
@@ -107,8 +109,9 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
             TimestampsUtcTicks = ticks,
             CorrelationId = Guid.NewGuid().ToString("N"),
         };
-        var reply = await Invoke<ReadAtTimeRequest, ReadAtTimeReply>(MessageKind.ReadAtTimeRequest, MessageKind.ReadAtTimeReply, req, cancellationToken).ConfigureAwait(false);
+        var reply = await InvokeAndClassifyAsync<ReadAtTimeRequest, ReadAtTimeReply>(
-        ThrowIfFailed(reply.Success, reply.Error, "ReadAtTime");
+            MessageKind.ReadAtTimeRequest, MessageKind.ReadAtTimeReply, req,
+            r => (r.Success, r.Error), "ReadAtTime", cancellationToken).ConfigureAwait(false);
         return new HistoryReadResult(AlignAtTimeSnapshots(timestampsUtc, reply.Samples), ContinuationPoint: null);
     }
 
@@ -167,11 +170,34 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
             MaxEvents = maxEvents,
             CorrelationId = Guid.NewGuid().ToString("N"),
         };
-        var reply = await Invoke<ReadEventsRequest, ReadEventsReply>(MessageKind.ReadEventsRequest, MessageKind.ReadEventsReply, req, cancellationToken).ConfigureAwait(false);
+        var reply = await InvokeAndClassifyAsync<ReadEventsRequest, ReadEventsReply>(
-        ThrowIfFailed(reply.Success, reply.Error, "ReadEvents");
+            MessageKind.ReadEventsRequest, MessageKind.ReadEventsReply, req,
+            r => (r.Success, r.Error), "ReadEvents", cancellationToken).ConfigureAwait(false);
         return new HistoricalEventsResult(ToHistoricalEvents(reply.Events), ContinuationPoint: null);
     }
 
+    /// <summary>
+    ///     Returns a snapshot of operation counters and the single pipe channel's connection
+    ///     state.
+    /// </summary>
+    /// <remarks>
+    ///     This client owns one duplex named-pipe channel to the sidecar — it has no notion of
+    ///     separate process / event connections and no per-node telemetry. The single channel's
+    ///     connected state is reported for both <see cref="HistorianHealthSnapshot.ProcessConnectionOpen"/>
+    ///     and <see cref="HistorianHealthSnapshot.EventConnectionOpen"/>, and
+    ///     <see cref="HistorianHealthSnapshot.ActiveProcessNode"/> /
+    ///     <see cref="HistorianHealthSnapshot.ActiveEventNode"/> /
+    ///     <see cref="HistorianHealthSnapshot.Nodes"/> are intentionally null/empty. Consumers
+    ///     that need to distinguish two connections should read another driver. (Finding 010.)
+    ///     <para>
+    ///         All six counter fields (TotalQueries, TotalSuccesses, TotalFailures,
+    ///         ConsecutiveFailures, LastSuccessTime, LastFailureTime, LastError) are mutated
+    ///         exclusively under <c>_healthLock</c>, so the snapshot is internally consistent —
+    ///         in particular <c>TotalSuccesses + TotalFailures == TotalQueries</c> at every
+    ///         observed snapshot (a call that has started but not yet completed has not
+    ///         incremented any counter). (Finding 003 / 004.)
+    ///     </para>
+    /// </remarks>
     public HistorianHealthSnapshot GetHealthSnapshot()
     {
         lock (_healthLock)
@@ -233,8 +259,9 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
 
         try
         {
-            var reply = await Invoke<WriteAlarmEventsRequest, WriteAlarmEventsReply>(
+            var reply = await InvokeAsync<WriteAlarmEventsRequest, WriteAlarmEventsReply>(
-                MessageKind.WriteAlarmEventsRequest, MessageKind.WriteAlarmEventsReply, req, cancellationToken).ConfigureAwait(false);
+                MessageKind.WriteAlarmEventsRequest, MessageKind.WriteAlarmEventsReply, req,
+                r => (r.Success, r.Error), cancellationToken).ConfigureAwait(false);
 
             // Whole-call failure → transient retry for every event in the batch.
             if (!reply.Success)
@@ -279,70 +306,80 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
 
     // ===== Helpers =====
 
-    private async Task<TReply> Invoke<TRequest, TReply>(
+    /// <summary>
-        MessageKind requestKind, MessageKind expectedReplyKind, TRequest request, CancellationToken ct)
+    ///     Sends one request through the channel and records the outcome (transport success or
+    ///     transport failure) under a single <c>_healthLock</c> acquisition that also bumps
+    ///     <c>_totalQueries</c>. Sidecar-level success / failure is NOT classified here — the
+    ///     caller passes that through <see cref="InvokeAndClassifyAsync"/> instead. (Finding
+    ///     003 / 004: all six counter fields share one synchronization mechanism so a snapshot
+    ///     can never observe a torn state.)
+    /// </summary>
+    private async Task<TReply> InvokeAsync<TRequest, TReply>(
+        MessageKind requestKind, MessageKind expectedReplyKind, TRequest request,
+        Func<TReply, (bool ok, string? error)> evaluate, CancellationToken ct)
         where TReply : class
     {
-        Interlocked.Increment(ref _totalQueries);
         try
         {
             var reply = await _channel.InvokeAsync<TRequest, TReply>(requestKind, expectedReplyKind, request, ct).ConfigureAwait(false);
-            RecordSuccess();
+            // Classify transport+sidecar in one lock so TotalQueries/TotalSuccesses/
+            // TotalFailures move together and no intermediate "success-then-undo" state is
+            // visible to a concurrent GetHealthSnapshot.
+            var (ok, error) = evaluate(reply);
+            RecordOutcome(ok, error);
             return reply;
         }
         catch (Exception ex)
         {
-            RecordFailure(ex.Message);
+            RecordOutcome(success: false, ex.Message);
             throw;
         }
     }
 
-    private void RecordSuccess()
+    /// <summary>
+    ///     Convenience wrapper around <see cref="InvokeAsync"/> that throws
+    ///     <see cref="InvalidOperationException"/> on a sidecar-reported failure. Used by the
+    ///     <see cref="IHistorianDataSource"/> read methods.
+    /// </summary>
+    private async Task<TReply> InvokeAndClassifyAsync<TRequest, TReply>(
+        MessageKind requestKind, MessageKind expectedReplyKind, TRequest request,
+        Func<TReply, (bool ok, string? error)> evaluate, string op, CancellationToken ct)
+        where TReply : class
+    {
+        var reply = await InvokeAsync<TRequest, TReply>(requestKind, expectedReplyKind, request, evaluate, ct).ConfigureAwait(false);
+        var (ok, error) = evaluate(reply);
+        if (!ok)
+        {
+            throw new InvalidOperationException(
+                $"Sidecar {op} failed: {error ?? "<no message>"}.");
+        }
+        return reply;
+    }
+
+    /// <summary>
+    ///     Records the outcome of a single call — increments <c>_totalQueries</c> and exactly
+    ///     one of <c>_totalSuccesses</c> / <c>_totalFailures</c> under a single
+    ///     <c>_healthLock</c> acquisition. (Findings 003 + 004.)
+    /// </summary>
+    private void RecordOutcome(bool success, string? error)
     {
         lock (_healthLock)
+        {
+            _totalQueries++;
+            if (success)
             {
                 _totalSuccesses++;
                 _consecutiveFailures = 0;
                 _lastSuccessUtc = DateTime.UtcNow;
             }
-    }
+            else
-
-    private void RecordFailure(string message)
-    {
-        lock (_healthLock)
             {
                 _totalFailures++;
                 _consecutiveFailures++;
                 _lastFailureUtc = DateTime.UtcNow;
-            _lastError = message;
+                _lastError = error;
             }
         }
-
-    private void ThrowIfFailed(bool success, string? error, string op)
-    {
-        if (!success)
-        {
-            // Sidecar-reported failure counts as an operation failure even though the
-            // transport delivered a reply. The Invoke wrapper already recorded transport
-            // success — undo that and record the failure so the health snapshot reflects
-            // operation-level success rates rather than just connectivity.
-            ReclassifySuccessAsFailure(error);
-            throw new InvalidOperationException(
-                $"Sidecar {op} failed: {error ?? "<no message>"}.");
-        }
-    }
-
-    private void ReclassifySuccessAsFailure(string? message)
-    {
-        lock (_healthLock)
-        {
-            // Transport-level RecordSuccess happened a moment ago; reverse it.
-            _totalSuccesses--;
-            _totalFailures++;
-            _consecutiveFailures++;
-            _lastFailureUtc = DateTime.UtcNow;
-            _lastError = message;
-        }
     }
 
     /// <summary>
@@ -452,9 +489,12 @@ public sealed class WonderwareHistorianClient : IHistorianDataSource, IAlarmHist
 
     /// <summary>
     ///     Synchronous dispose required by <see cref="IDisposable"/> on
-    ///     <see cref="IHistorianDataSource"/>. The underlying channel's async cleanup is
+    ///     <see cref="IHistorianDataSource"/>. The underlying channel's async cleanup runs
-    ///     non-blocking (just resets transport state + disposes streams), so the
+    ///     <see cref="System.IO.Pipes.NamedPipeClientStream"/> teardown, which can block briefly
-    ///     GetAwaiter()/GetResult() bridge is safe.
+    ///     on OS handle release — strictly speaking it is not non-blocking — but the
+    ///     <c>GetAwaiter()/GetResult()</c> bridge is deadlock-safe because the cleanup never
+    ///     awaits a captured <see cref="System.Threading.SynchronizationContext"/> nor takes any
+    ///     lock that the caller could hold. (Finding 010.)
     /// </summary>
     public void Dispose() => _channel.DisposeAsync().AsTask().GetAwaiter().GetResult();
 }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client/WonderwareHistorianClientOptions.cs

@@ -3,24 +3,28 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client;
 /// <summary>
 ///     Connection options for <see cref="WonderwareHistorianClient"/>.
 /// </summary>
+/// <remarks>
+///     <para>
+///         <b>Retry / backoff ownership (finding 006):</b> this module performs exactly one
+///         in-place transport reconnect inside <c>PipeChannel.InvokeAsync</c> with no delay,
+///         and does NOT implement exponential reconnect backoff. Broader retry/backoff is the
+///         caller's responsibility — the alarm drain worker
+///         (<c>Core.AlarmHistorian.SqliteStoreAndForwardSink</c>) and the read-side
+///         history router are expected to layer their own backoff on top.
+///     </para>
+/// </remarks>
 /// <param name="PipeName">Named-pipe name the sidecar listens on (matches the sidecar's <c>OTOPCUA_HISTORIAN_PIPE</c>).</param>
 /// <param name="SharedSecret">Per-process shared secret the sidecar will verify in the Hello frame.</param>
 /// <param name="PeerName">Diagnostic peer identifier sent in Hello — typically the OtOpcUa instance id.</param>
 /// <param name="ConnectTimeout">Cap on the named-pipe connect + Hello round trip on each (re)connect.</param>
 /// <param name="CallTimeout">Cap on a single read/write call once connected.</param>
-/// <param name="ReconnectInitialBackoff">Backoff between the first failed reconnect attempts.</param>
-/// <param name="ReconnectMaxBackoff">Upper bound on the exponential backoff between reconnects.</param>
 public sealed record WonderwareHistorianClientOptions(
     string PipeName,
     string SharedSecret,
     string PeerName = "OtOpcUa",
     TimeSpan? ConnectTimeout = null,
-    TimeSpan? CallTimeout = null,
+    TimeSpan? CallTimeout = null)
-    TimeSpan? ReconnectInitialBackoff = null,
-    TimeSpan? ReconnectMaxBackoff = null)
 {
     public TimeSpan EffectiveConnectTimeout => ConnectTimeout ?? TimeSpan.FromSeconds(10);
     public TimeSpan EffectiveCallTimeout => CallTimeout ?? TimeSpan.FromSeconds(30);
-    public TimeSpan EffectiveReconnectInitialBackoff => ReconnectInitialBackoff ?? TimeSpan.FromMilliseconds(500);
-    public TimeSpan EffectiveReconnectMaxBackoff => ReconnectMaxBackoff ?? TimeSpan.FromSeconds(30);
 }

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/WonderwareHistorianClientTests.cs

@@ -491,4 +491,95 @@ public sealed class WonderwareHistorianClientTests
         await Should.ThrowAsync<InvalidDataException>(() =>
             client.ReadRawAsync("Tag", DateTime.UtcNow, DateTime.UtcNow, 100, CancellationToken.None));
     }
+
+    // ===== Finding-003 / Finding-004: health counter consistency =====
+
+    /// <summary>
+    ///     (Finding 003 + 004) A sidecar-level failure must be classified once: TotalSuccesses
+    ///     must stay at 0, TotalFailures must become 1, and TotalQueries / TotalSuccesses /
+    ///     TotalFailures must all be updated under the same lock so a concurrent snapshot can
+    ///     never observe inflated successes or out-of-band TotalQueries. This pins behaviour so
+    ///     a future regression to the "RecordSuccess then undo via ReclassifySuccessAsFailure"
+    ///     dance is caught.
+    /// </summary>
+    [Fact]
+    public async Task GetHealthSnapshot_SidecarFailure_NeverInflatesSuccessCounter()
+    {
+        var pipe = UniquePipeName();
+        await using var server = new FakeSidecarServer(pipe, Secret)
+        {
+            OnReadRaw = _ => new ReadRawReply { Success = false, Error = "boom" },
+        };
+        await server.StartAsync();
+
+        await using var client = new WonderwareHistorianClient(OptsFor(pipe));
+
+        await Should.ThrowAsync<InvalidOperationException>(() =>
+            client.ReadRawAsync("Tag", DateTime.UtcNow, DateTime.UtcNow, 1, CancellationToken.None));
+
+        var snap = client.GetHealthSnapshot();
+        snap.TotalQueries.ShouldBe(1);
+        snap.TotalSuccesses.ShouldBe(0);
+        snap.TotalFailures.ShouldBe(1);
+        snap.ConsecutiveFailures.ShouldBe(1);
+        snap.LastError.ShouldNotBeNull();
+    }
+
+    /// <summary>
+    ///     (Finding 003) Concurrent calls + concurrent <see cref="WonderwareHistorianClient.GetHealthSnapshot"/>
+    ///     reads must observe consistent counters. Specifically, TotalSuccesses + TotalFailures
+    ///     must equal TotalQueries at every observed snapshot (no torn read between an
+    ///     Interlocked-incremented TotalQueries and a lock-protected outcome counter). The
+    ///     channel serializes calls, so the test is observable: each completed query strictly
+    ///     increments either successes or failures by one.
+    /// </summary>
+    [Fact]
+    public async Task GetHealthSnapshot_ConcurrentCallsAndReads_CountersAreInternallyConsistent()
+    {
+        var pipe = UniquePipeName();
+        await using var server = new FakeSidecarServer(pipe, Secret)
+        {
+            OnReadRaw = _ => new ReadRawReply { Success = true },
+        };
+        await server.StartAsync();
+
+        await using var client = new WonderwareHistorianClient(OptsFor(pipe));
+
+        using var stop = new CancellationTokenSource();
+        var readerSawInconsistent = false;
+
+#pragma warning disable xUnit1051 // Internal Task.Run loop drives a polling stress test; cancellation flows via stop.IsCancellationRequested below.
+        var reader = Task.Run(() =>
+        {
+            while (!stop.IsCancellationRequested)
+            {
+                var snap = client.GetHealthSnapshot();
+                // Every completed call increments TotalQueries AND exactly one of
+                // TotalSuccesses or TotalFailures under the same lock; an in-flight call
+                // has not yet incremented any of them. So TotalQueries should always equal
+                // the sum of TotalSuccesses + TotalFailures (no in-between state visible).
+                if (snap.TotalSuccesses + snap.TotalFailures != snap.TotalQueries)
+                {
+                    readerSawInconsistent = true;
+                }
+            }
+        });
+#pragma warning restore xUnit1051
+
+        for (var i = 0; i < 50; i++)
+        {
+            await client.ReadRawAsync("Tag", DateTime.UtcNow, DateTime.UtcNow, 1, TestContext.Current.CancellationToken);
+        }
+
+        stop.Cancel();
+        await reader;
+
+        readerSawInconsistent.ShouldBeFalse(
+            "GetHealthSnapshot exposed TotalQueries that disagreed with the sum of TotalSuccesses + TotalFailures — counters are not updated under a single lock.");
+
+        var final = client.GetHealthSnapshot();
+        final.TotalQueries.ShouldBe(50);
+        final.TotalSuccesses.ShouldBe(50);
+        final.TotalFailures.ShouldBe(0);
+    }
 }