fix(WRK-22,WRK-24,WRK-25,WRK-27,IPC-26): worker write-seam hardening

WRK-22/IPC-26: tombstone a WriteAsync/WriteBatchAsync cancelled while
waiting for the write lock (PendingFrame.Claimed under _gate; DequeueNext
skips cancelled, claims the frame it returns) so a cancelled write never
reaches the wire unless already claimed mid-write (documented residual).

WRK-25: add WriteBatchAsync; RunEventDrainLoopAsync submits the drained
event batch through it, so a burst of N events costs one flush not N.
IPC-30 oversized-event structured fault preserved via FindOversizedEvent.

WRK-24: reject a below-1024 negotiated frame maximum at the handshake
(MinNegotiableFrameBytes, matching GatewayOptionsValidator floor).

WRK-27: alarm poll advertises StaCallInProgress on the heartbeat snapshot
so the watchdog suppresses to the ceiling, not the grace.

Docs (WorkerFrameProtocol.md, MxAccessWorkerInstanceDesign.md) and the
2026-07-12 remediation registers/change-log updated in the same commit.

archreview/2026-07-12/remediation/20-worker.md

@@ -17,12 +17,12 @@ members, no positional records). The worker builds and tests only on the Windows
 | ID | Sev | Tier | Eff | Dep | Status | Title |
 |----|-----|------|-----|-----|--------|-------|
 | WRK-21 | Medium | P0 | M | IPC-23 (same defect, fix owned here); WRK-28 (same lines) | Done | DrainEvents bound is count-based only; an oversized reply still kills the session and loses the drained events |
-| WRK-22 | Low | — | S | IPC-26 (same defect, fix owned here) | Not started | Cancelled `WriteAsync` leaves its frame queued; it is still written later |
+| WRK-22 | Low | — | S | IPC-26 (same defect, fix owned here) | Done | Cancelled `WriteAsync` leaves its frame queued; it is still written later |
 | WRK-23 | Low | — | S | WRK-21 (rejection path becomes backstop-only) | Done | Rejected frames consume sequence numbers, producing wire gaps |
-| WRK-24 | Low | — | S | — | Not started | `AdoptNegotiatedMaxMessageBytes` has no lower-bound sanity check |
-| WRK-25 | Low | P2 | S | WRK-22 (both touch enqueue/dequeue) | Not started | WRK-12 flush coalescing never engages on the event hot path |
+| WRK-24 | Low | — | S | — | Done | `AdoptNegotiatedMaxMessageBytes` has no lower-bound sanity check |
+| WRK-25 | Low | P2 | S | WRK-22 (both touch enqueue/dequeue) | Done | WRK-12 flush coalescing never engages on the event hot path |
 | WRK-26 | Low | P1 | S | WRK-23 (soft — sequence prose); discharges IPC-29 | Done | Write-priority and overflow doc drift from the WRK-07 change |
-| WRK-27 | Low | — | S | — | Not started | Alarm poll bypasses the watchdog's in-flight suppression (15 s vs 75 s) |
+| WRK-27 | Low | — | S | — | Done | Alarm poll bypasses the watchdog's in-flight suppression (15 s vs 75 s) |
 | WRK-28 | Low | — | S | WRK-21 (land in the same commit cluster) | Done | 10,000 drain cap is a duplicated magic constant with a comment-only sync contract |
 
 ---

archreview/2026-07-12/remediation/30-contracts-ipc.md

@@ -15,7 +15,7 @@ All `path:line` citations were re-verified against the working tree at `4f5371f`
 | IPC-23 | Medium | P0 | S¹ | WRK-21 | In progress — mechanics landed with WRK-21; proto-comment/doc wave pending | DrainEvents bound is count-based only; byte-heavy queue still builds a session-killing reply frame (contract requirements here; fix mechanics in WRK-21) |
 | IPC-24 | Medium | P0 | S | — | Not started | CI's unconditional Java churn-revert masks real generated-code drift for message-level proto changes |
 | IPC-25 | Medium | P0 | M | — | Not started | Committed Go/Python worker bindings are stale at HEAD; no guard covers them |
-| IPC-26 | Low | P2 | S¹ | WRK-22 | Not started | Cancelled write leaves a ghost frame that is still written (contract requirement here; fix mechanics in WRK-22) |
+| IPC-26 | Low | P2 | S¹ | WRK-22 | Done (mechanics landed in WRK-22) | Cancelled write leaves a ghost frame that is still written (contract requirement here; fix mechanics in WRK-22) |
 | IPC-27 | Low | P2 | S | — | Not started | Descriptor freshness test blind to enums, enum values, services/methods, and the Galaxy contract |
 | IPC-28 | Low | — | S | — | Done | `docs/Grpc.md` omits the `CommandTooLarge` → `ResourceExhausted` mapping |
 | IPC-29 | Low | — | S | — | Done (discharged by WRK-26) | Worker writer priority scheduling and write-time sequence stamping undocumented in the frame-protocol doc |

docs/MxAccessWorkerInstanceDesign.md

@@ -747,6 +747,23 @@ heartbeat fields until dedicated thresholds own those warnings. The worker
 reports stale STA activity, but the gateway owns the final kill decision
 through its existing heartbeat and worker lifecycle policy.
 
+The alarm poll runs outside the command dispatcher — `RunAlarmPollLoopAsync`
+invokes `PollOnce` directly on the STA rather than through
+`StaCommandDispatcher`, so it does not inflate `PendingCommandCount` or perturb
+dispatch ordering for real gateway commands. Because it is not a dispatched
+command it has no `CurrentCommandCorrelationId`, so a healthy-but-slow poll (a
+large `GetXmlCurrentAlarms2` against a busy provider) blocking the STA past
+`HeartbeatGrace` would otherwise fault a healthy session at 15 s while a
+dispatched command gets the 75 s ceiling. To close that asymmetry (WRK-27) the
+poll advertises itself on the heartbeat snapshot's `StaCallInProgress` flag —
+set on the STA thread for exactly the span of the COM call — and the watchdog
+suppression honors that flag alongside `CurrentCommandCorrelationId`. The poll
+therefore receives the same grace-to-ceiling treatment as a dispatched command:
+suppressed up to `HeartbeatStuckCeiling`, faulted past it (a poll that blocks
+the STA more than 75 s without pumping *should* fault — that is the ceiling's
+contract). The flag is named generically so any future non-dispatcher STA work
+reuses it.
+
 The in-flight-command suppression itself is bounded by
 `WorkerPipeSessionOptions.HeartbeatStuckCeiling` (default 75 seconds = 5 ×
 `HeartbeatGrace`). The motivating case for the suppression is a legitimately

docs/WorkerFrameProtocol.md

@@ -29,6 +29,17 @@ default. A `max_frame_bytes` of 0 (an older gateway that never set the field)
 means "use the worker's built-in default". This keeps both ends framing to the
 same limit rather than depending on matched compile-time constants.
 
+The worker accepts a negotiated value in the closed range [1024, 256 MiB]
+(`MinNegotiableFrameBytes` .. `MaxNegotiableFrameBytes`); 0 keeps the default.
+A value outside that range is rejected at the handshake with a fault frame
+rather than adopted, because a nonsensical maximum — a gateway bug or a
+foreign/old peer — would otherwise leave a session that handshakes cleanly and
+then fails every subsequent frame with per-frame size errors, the worst
+diagnostic shape for an operator. The 1024-byte floor matches the gateway's own
+`GatewayOptionsValidator.MinimumMaxMessageBytes`, so the worker never rejects a
+value the gateway's validator accepts as legal configuration, and 1024 still
+guarantees hellos, heartbeats, acks, and faults fit.
+
 Every worker-to-gateway frame must serialize within this limit, control replies
 included, so reply builders truncate to fit rather than emit a frame the writer
 will reject. `WorkerPipeSession` pre-sizes a `DrainEvents` reply below the
@@ -112,20 +123,29 @@ runs after the whole batch, and only then does every successfully-written
 frame's completion resolve — so a caller's `WriteAsync` still does not
 complete until its bytes are both written *and* flushed, but a batch that
 happened to contain several queued frames pays one flush instead of one per
-frame. In practice this coalescing currently engages only when multiple
-frames are queued at the moment a lock-holder starts draining. The event
-drain loop (`WorkerPipeSession.RunEventDrainLoopAsync`) awaits each drained
-event's `WriteAsync` individually before writing the next, so today at most
-one event frame is queued per drain pass and each event still costs its own
-flush; a dedicated batch write entry point that submits a whole drained
-event batch under one lock acquisition is designed but not yet landed, so a
-burst of N events currently costs N flushes on the event hot path, not one.
+frame. The event drain loop (`WorkerPipeSession.RunEventDrainLoopAsync`)
+submits a whole drained event batch through `WriteBatchAsync`, which enqueues
+every frame under one `_gate` acquisition, takes the write lock once, and
+drains them together, so a burst of N events costs one flush rather than N —
+the coalescing the batch machinery was built for now engages on the event hot
+path, not only when independent producers happen to queue behind a blocked
+write. Intra-batch order is preserved (FIFO enqueue under one lock), and a
+concurrently queued control frame is still drained ahead of the batch. A
+per-frame rejection inside a batch (for example one oversized event) surfaces
+from the batch's awaited completions as that frame's
+`WorkerFrameProtocolException`; the remaining completions are still observed
+so none faults unobserved.
 
-Cancellation semantics for a `WriteAsync` call that is still waiting for the
-write lock when its token fires are not yet defined at this layer — pending
-a fix that will tombstone the queued frame so a cancelled call is guaranteed
-never to reach the wire. Until that lands, a cancelled caller may still see
-its frame written by whichever caller next holds the lock.
+Cancellation of a `WriteAsync`/`WriteBatchAsync` call that is still waiting
+for the write lock when its token fires tombstones the queued frame: the
+cancelled caller marks its frame under `_gate`, and the draining lock-holder's
+`DequeueNext` skips any tombstoned frame, so a cancelled call is guaranteed
+never to reach the wire — *unless* a lock-holder has already claimed the frame
+to write it. Claiming and cancelling are interlocked under `_gate`, so exactly
+one wins; a frame already claimed is mid-write and can no longer be recalled,
+so the caller observes `OperationCanceledException` while that one frame still
+reaches the wire. That residual window is by design: blocking the canceller
+behind the very write it is abandoning would defeat the point of cancellation.
 
 ## Verification
 

src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerFrameProtocolTests.cs

@@ -481,6 +481,192 @@ public sealed class WorkerFrameProtocolTests
         Assert.Equal(WorkerFrameProtocolErrorCode.InvalidConfiguration, exception.ErrorCode);
     }
 
+    /// <summary>
+    /// Verifies a negotiated frame maximum below the worker floor is rejected as
+    /// <c>InvalidConfiguration</c> (WRK-24), and that exactly the floor is adopted. The floor matches
+    /// the gateway's own <c>GatewayOptionsValidator.MinimumMaxMessageBytes</c>, so the worker never
+    /// rejects a value the gateway's validator accepts, yet a nonsensical tiny value faults at the
+    /// handshake instead of leaving a session that fails every later frame.
+    /// </summary>
+    [Fact]
+    public void AdoptNegotiatedMaxMessageBytes_BelowFloor_ThrowsInvalidConfiguration()
+    {
+        WorkerFrameProtocolOptions belowFloor = CreateOptions();
+        WorkerFrameProtocolException exception = Assert.Throws<WorkerFrameProtocolException>(
+            () => belowFloor.AdoptNegotiatedMaxMessageBytes(512));
+        Assert.Equal(WorkerFrameProtocolErrorCode.InvalidConfiguration, exception.ErrorCode);
+
+        // Boundary: exactly the floor is accepted.
+        WorkerFrameProtocolOptions atFloor = CreateOptions();
+        atFloor.AdoptNegotiatedMaxMessageBytes((uint)WorkerFrameProtocolOptions.MinNegotiableFrameBytes);
+        Assert.Equal(WorkerFrameProtocolOptions.MinNegotiableFrameBytes, atFloor.MaxMessageBytes);
+    }
+
+    /// <summary>
+    /// WRK-22 / IPC-26. A <c>WriteAsync</c> cancelled while it waits for the write lock must never
+    /// have its frame written by the next lock-holder. Writer A holds the lock mid-write (blocked in
+    /// the stream); an event write is queued and then cancelled; when A is released and a later
+    /// control frame drains, the wire carries A's frame and the control frame only — the cancelled
+    /// event envelope is tombstoned and skipped.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteAsync_CancelledWhileWaitingForLock_FrameIsNeverWritten()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using GatedWriteStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        // Writer A occupies the writer and blocks inside the stream, holding the write lock.
+        Task firstWrite = writer.WriteAsync(CreateGatewayHelloEnvelope(), WorkerFrameWritePriority.Control);
+        await AwaitWithTimeoutAsync(stream.FirstWriteStarted);
+
+        // Queue an event write with its own CTS while the lock is held, then cancel it.
+        using CancellationTokenSource cts = new();
+        Task cancelledWrite = writer.WriteAsync(CreateEventEnvelope(), WorkerFrameWritePriority.Event, cts.Token);
+        await Task.Delay(50);
+        cts.Cancel();
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await cancelledWrite);
+
+        // Release A, then drive a fresh control write.
+        stream.ReleaseFirstWrite();
+        await AwaitWithTimeoutAsync(firstWrite);
+        await writer.WriteAsync(CreateGatewayHelloEnvelope(), WorkerFrameWritePriority.Control);
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        WorkerEnvelope frame1 = await reader.ReadAsync();
+        WorkerEnvelope frame2 = await reader.ReadAsync();
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, frame1.BodyCase);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, frame2.BodyCase);
+        // The cancelled event never reached the wire — no third frame, and sequences stay contiguous.
+        Assert.Equal(stream.Length, stream.Position);
+        Assert.Equal(1UL, frame1.Sequence);
+        Assert.Equal(2UL, frame2.Sequence);
+    }
+
+    /// <summary>
+    /// WRK-22 / IPC-26, the review's shutdown scenario. A cancelled event frame queued before a
+    /// shutdown-ack control frame must not trail the ack on the wire: the tombstone rule plus the
+    /// control-before-event scheduler keeps the ack the last frame written.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteAsync_CancelledEventFrame_DoesNotTrailShutdownAck()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using GatedWriteStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        Task firstWrite = writer.WriteAsync(CreateGatewayHelloEnvelope(), WorkerFrameWritePriority.Control);
+        await AwaitWithTimeoutAsync(stream.FirstWriteStarted);
+
+        using CancellationTokenSource cts = new();
+        Task cancelledEvent = writer.WriteAsync(CreateEventEnvelope(), WorkerFrameWritePriority.Event, cts.Token);
+        await Task.Delay(50);
+        cts.Cancel();
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await cancelledEvent);
+
+        // The shutdown ack (a control frame) is queued behind the still-blocked first write.
+        Task ackWrite = writer.WriteAsync(CreateShutdownAckEnvelope(), WorkerFrameWritePriority.Control);
+        await Task.Delay(50);
+
+        stream.ReleaseFirstWrite();
+        await AwaitWithTimeoutAsync(Task.WhenAll(firstWrite, ackWrite));
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        WorkerEnvelope frame1 = await reader.ReadAsync();
+        WorkerEnvelope frame2 = await reader.ReadAsync();
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, frame1.BodyCase);
+        // The ack is the last frame — the cancelled event did not trail it.
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerShutdownAck, frame2.BodyCase);
+        Assert.Equal(stream.Length, stream.Position);
+    }
+
+    /// <summary>
+    /// WRK-25. The batch entry point enqueues a whole event burst under one lock acquisition and
+    /// drains it together, so N events cost exactly one flush and reach the wire in batch order with
+    /// monotonic sequences — the coalescing WRK-12 shipped, now on the event hot path.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteBatchAsync_FlushesOnceAndPreservesOrder()
+    {
+        const int count = 8;
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using FlushCountingStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        WorkerEnvelope[] batch = new WorkerEnvelope[count];
+        for (int index = 0; index < count; index++)
+        {
+            batch[index] = CreateEventEnvelope(workerSequence: (ulong)(100 + index));
+        }
+
+        await writer.WriteBatchAsync(batch, WorkerFrameWritePriority.Event);
+
+        // The whole batch was queued before the single lock wait, so it drained in one pass => one flush.
+        Assert.Equal(1, stream.FlushCount);
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        for (int index = 0; index < count; index++)
+        {
+            WorkerEnvelope frame = await reader.ReadAsync();
+            Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerEvent, frame.BodyCase);
+            // Wire order matches batch order.
+            Assert.Equal((ulong)(100 + index), frame.WorkerEvent.Event.WorkerSequence);
+            // Write-time stamped sequence is monotonic 1..count.
+            Assert.Equal((ulong)(index + 1), frame.Sequence);
+        }
+    }
+
+    /// <summary>
+    /// WRK-25. Control-before-event still holds mid-batch: a control frame queued while a batch is
+    /// draining jumps ahead of the batch's remaining events.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task WriteBatchAsync_ControlFrameQueuedDuringBatch_JumpsRemainingEvents()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using GatedWriteStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        WorkerEnvelope[] batch = new[]
+        {
+            CreateEventEnvelope(),
+            CreateEventEnvelope(),
+            CreateEventEnvelope(),
+        };
+
+        // The batch takes the lock and blocks writing its first event frame inside the stream.
+        Task batchWrite = writer.WriteBatchAsync(batch, WorkerFrameWritePriority.Event);
+        await AwaitWithTimeoutAsync(stream.FirstWriteStarted);
+
+        // A control frame queued mid-drain must jump the batch's remaining events.
+        Task controlWrite = writer.WriteAsync(CreateGatewayHelloEnvelope(), WorkerFrameWritePriority.Control);
+        await Task.Delay(50);
+
+        stream.ReleaseFirstWrite();
+        await AwaitWithTimeoutAsync(Task.WhenAll(batchWrite, controlWrite));
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        WorkerEnvelope f1 = await reader.ReadAsync();
+        WorkerEnvelope f2 = await reader.ReadAsync();
+        WorkerEnvelope f3 = await reader.ReadAsync();
+        WorkerEnvelope f4 = await reader.ReadAsync();
+
+        // First event was already writing when the control frame queued; the control frame then jumps
+        // ahead of the two remaining events.
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerEvent, f1.BodyCase);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, f2.BodyCase);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerEvent, f3.BodyCase);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerEvent, f4.BodyCase);
+    }
+
     private static WorkerEnvelope CreateGatewayHelloEnvelope(ulong sequence = 1)
     {
         return new WorkerEnvelope
@@ -522,6 +708,47 @@ public sealed class WorkerFrameProtocolTests
         };
     }
 
+    private static WorkerEnvelope CreateEventEnvelope(ulong workerSequence)
+    {
+        WorkerEnvelope envelope = CreateEventEnvelope();
+        envelope.WorkerEvent.Event.WorkerSequence = workerSequence;
+        return envelope;
+    }
+
+    private static WorkerEnvelope CreateShutdownAckEnvelope()
+    {
+        return new WorkerEnvelope
+        {
+            ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+            SessionId = SessionId,
+            WorkerShutdownAck = new WorkerShutdownAck
+            {
+                Status = new ProtocolStatus
+                {
+                    Code = ProtocolStatusCode.Ok,
+                    Message = "OK",
+                },
+            },
+        };
+    }
+
+    // A MemoryStream that counts FlushAsync calls without gating any write, so a batch write can be
+    // asserted to flush exactly once.
+    private sealed class FlushCountingStream : MemoryStream
+    {
+        private int _flushCount;
+
+        /// <summary>Gets the number of <see cref="FlushAsync"/> calls observed so far.</summary>
+        public int FlushCount => Volatile.Read(ref _flushCount);
+
+        /// <inheritdoc />
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            Interlocked.Increment(ref _flushCount);
+            return base.FlushAsync(cancellationToken);
+        }
+    }
+
     // A MemoryStream whose first WriteAsync blocks until released, so a test can queue additional frames
     // behind an in-progress write and observe the writer's priority ordering.
     private sealed class GatedWriteStream : MemoryStream

src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerPipeSessionTests.cs

@@ -1112,6 +1112,184 @@ public sealed class WorkerPipeSessionTests
         await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
     }
 
+    /// <summary>
+    ///     WRK-27. An STA call outside the command dispatcher (the alarm poll) advertises itself on
+    ///     the heartbeat snapshot's <c>StaCallInProgress</c> flag, and the watchdog grants it the same
+    ///     grace-to-ceiling suppression as a dispatched command: stale STA activity within the ceiling
+    ///     does not fault while the flag is set, but stale activity beyond the ceiling faults anyway.
+    ///     This closes the 15 s-vs-75 s asymmetry between polls and commands.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Watchdog_StaCallInProgress_SuppressedUntilCeiling()
+    {
+        using CancellationTokenSource cancellation = new(TimeSpan.FromSeconds(10));
+
+        // Phase 1 — within the ceiling: stale beyond grace, empty correlation id, StaCallInProgress
+        // set. The default 75 s ceiling is far beyond the 5 s staleness, so the watchdog must suppress.
+        using (PipePair pipePair = await PipePair.CreateAsync(cancellation.Token))
+        {
+            FakeRuntimeSession runtime = new();
+            runtime.SetSnapshot(new WorkerRuntimeHeartbeatSnapshot(
+                DateTimeOffset.UtcNow - TimeSpan.FromSeconds(5),
+                pendingCommandCount: 0,
+                outboundEventQueueDepth: 0,
+                lastEventSequence: 0,
+                currentCommandCorrelationId: string.Empty,
+                staCallInProgress: true));
+            WorkerPipeSession session = CreatePipeSession(
+                pipePair.WorkerStream,
+                runtime,
+                new WorkerPipeSessionOptions
+                {
+                    HeartbeatInterval = TimeSpan.FromMilliseconds(20),
+                    HeartbeatGrace = TimeSpan.FromMilliseconds(50),
+                });
+            Task runTask = session.RunAsync(cancellation.Token);
+            await CompleteGatewayHandshakeAsync(pipePair, cancellation.Token);
+
+            const int framesToInspect = 6;
+            int heartbeatsObserved = 0;
+            for (int index = 0; index < framesToInspect; index++)
+            {
+                WorkerEnvelope envelope = await pipePair.GatewayReader.ReadAsync(cancellation.Token);
+                Assert.NotEqual(WorkerEnvelope.BodyOneofCase.WorkerFault, envelope.BodyCase);
+                if (envelope.BodyCase == WorkerEnvelope.BodyOneofCase.WorkerHeartbeat)
+                {
+                    heartbeatsObserved++;
+                }
+            }
+
+            Assert.True(
+                heartbeatsObserved >= 2,
+                $"Expected multiple heartbeats during the in-progress STA-call window; observed {heartbeatsObserved}.");
+
+            await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
+        }
+
+        // Phase 2 — beyond the ceiling: same StaCallInProgress flag, but staleness (5 s) exceeds the
+        // 200 ms ceiling, so the watchdog must fire even with the poll in progress.
+        using (PipePair pipePair = await PipePair.CreateAsync(cancellation.Token))
+        {
+            FakeRuntimeSession runtime = new();
+            runtime.SetSnapshot(new WorkerRuntimeHeartbeatSnapshot(
+                DateTimeOffset.UtcNow - TimeSpan.FromSeconds(5),
+                pendingCommandCount: 0,
+                outboundEventQueueDepth: 0,
+                lastEventSequence: 0,
+                currentCommandCorrelationId: string.Empty,
+                staCallInProgress: true));
+            WorkerPipeSession session = CreatePipeSession(
+                pipePair.WorkerStream,
+                runtime,
+                new WorkerPipeSessionOptions
+                {
+                    HeartbeatInterval = TimeSpan.FromMilliseconds(20),
+                    HeartbeatGrace = TimeSpan.FromMilliseconds(50),
+                    HeartbeatStuckCeiling = TimeSpan.FromMilliseconds(200),
+                });
+            Task runTask = session.RunAsync(cancellation.Token);
+            await CompleteGatewayHandshakeAsync(pipePair, cancellation.Token);
+
+            WorkerEnvelope fault = await ReadUntilAsync(
+                pipePair.GatewayReader,
+                WorkerEnvelope.BodyOneofCase.WorkerFault,
+                cancellation.Token);
+
+            Assert.Equal(WorkerFaultCategory.StaHung, fault.WorkerFault.Category);
+
+            await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
+        }
+    }
+
+    /// <summary>
+    ///     WRK-25. The event drain loop submits a whole drained batch through the writer's batch entry
+    ///     point, so a burst of 128 events costs one flush, not 128 — the assertion the WRK-12
+    ///     tracking claim needed to actually hold on the event hot path.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task EventBurst_DrainLoopCoalescesFlushes()
+    {
+        using CancellationTokenSource cancellation = new(TimeSpan.FromSeconds(15));
+        using PipePair pipePair = await PipePair.CreateAsync(cancellation.Token);
+        FakeRuntimeSession runtime = new();
+        // A far-off heartbeat interval keeps heartbeat flushes out of the measurement window.
+        FlushCountingPassthroughStream countingStream = new(pipePair.WorkerStream);
+        WorkerPipeSession session = CreatePipeSession(
+            countingStream,
+            runtime,
+            new WorkerPipeSessionOptions
+            {
+                HeartbeatInterval = TimeSpan.FromMinutes(5),
+                HeartbeatGrace = TimeSpan.FromSeconds(30),
+            });
+        Task runTask = session.RunAsync(cancellation.Token);
+        await CompleteGatewayHandshakeAsync(pipePair, cancellation.Token);
+
+        // Let the idle drain loop settle (no events yet → no flushes) and record the baseline.
+        await Task.Delay(100, cancellation.Token);
+        int baselineFlushes = countingStream.FlushCount;
+
+        // Enqueue a full 128-event batch atomically so the drain loop sees it as one batch.
+        const int burst = 128;
+        List<WorkerEvent> batch = new(burst);
+        for (int index = 0; index < burst; index++)
+        {
+            batch.Add(CreateWorkerEvent(sequence: (ulong)(index + 1)));
+        }
+
+        runtime.EnqueueEvents(batch);
+
+        // Drain all 128 events off the gateway side.
+        for (int index = 0; index < burst; index++)
+        {
+            await ReadUntilAsync(
+                pipePair.GatewayReader,
+                WorkerEnvelope.BodyOneofCase.WorkerEvent,
+                cancellation.Token);
+        }
+
+        // The whole burst cost exactly one additional flush.
+        Assert.Equal(1, countingStream.FlushCount - baselineFlushes);
+
+        await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
+    }
+
+    /// <summary>
+    ///     WRK-24. A GatewayHello negotiating a frame maximum below the worker floor faults at the
+    ///     handshake with a fault frame rather than being adopted — mirroring the above-ceiling
+    ///     handshake behavior — so a nonsensical tiny value never leaves a session that fails every
+    ///     later frame. No message loop is entered.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Handshake_GatewayHelloWithTinyMaxFrameBytes_FaultsAtHandshake()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        using MemoryStream inbound = new();
+        await new WorkerFrameWriter(inbound, options)
+            .WriteAsync(CreateGatewayHelloEnvelope(maxFrameBytes: 512));
+        inbound.Position = 0;
+        using MemoryStream outbound = new();
+        WorkerPipeSession session = CreateSession(inbound, outbound, options);
+        bool initialized = false;
+
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await session.CompleteStartupHandshakeAsync(
+                    _ =>
+                    {
+                        initialized = true;
+                        return Task.CompletedTask;
+                    }));
+
+        Assert.False(initialized);
+        Assert.Equal(WorkerFrameProtocolErrorCode.InvalidConfiguration, exception.ErrorCode);
+        WorkerEnvelope fault = Assert.Single(ReadWrittenFrames(outbound, options));
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerFault, fault.BodyCase);
+    }
+
     /// <summary>
     ///     Regression test: a long in-flight STA command that keeps pumping
     ///     must NOT self-fault as <c>StaHung</c>, and its reply must still be
@@ -1976,6 +2154,78 @@ public sealed class WorkerPipeSessionTests
         }
     }
 
+    // Wraps the worker side of the pipe and counts FlushAsync calls so a test can assert the event
+    // drain loop coalesces a burst into a single flush. Delegates every other operation to the inner
+    // stream; does not own the inner stream's lifetime (PipePair disposes it).
+    private sealed class FlushCountingPassthroughStream : Stream
+    {
+        private readonly Stream inner;
+        private int flushCount;
+
+        /// <summary>Initializes the passthrough over the given inner stream.</summary>
+        /// <param name="inner">The stream to delegate to.</param>
+        public FlushCountingPassthroughStream(Stream inner)
+        {
+            this.inner = inner;
+        }
+
+        /// <summary>Gets the number of <see cref="FlushAsync"/> calls observed so far.</summary>
+        public int FlushCount => Volatile.Read(ref flushCount);
+
+        /// <inheritdoc />
+        public override bool CanRead => inner.CanRead;
+
+        /// <inheritdoc />
+        public override bool CanSeek => inner.CanSeek;
+
+        /// <inheritdoc />
+        public override bool CanWrite => inner.CanWrite;
+
+        /// <inheritdoc />
+        public override long Length => inner.Length;
+
+        /// <inheritdoc />
+        public override long Position
+        {
+            get => inner.Position;
+            set => inner.Position = value;
+        }
+
+        /// <inheritdoc />
+        public override void Flush()
+        {
+            Interlocked.Increment(ref flushCount);
+            inner.Flush();
+        }
+
+        /// <inheritdoc />
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            Interlocked.Increment(ref flushCount);
+            return inner.FlushAsync(cancellationToken);
+        }
+
+        /// <inheritdoc />
+        public override int Read(byte[] buffer, int offset, int count) => inner.Read(buffer, offset, count);
+
+        /// <inheritdoc />
+        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            => inner.ReadAsync(buffer, offset, count, cancellationToken);
+
+        /// <inheritdoc />
+        public override long Seek(long offset, SeekOrigin origin) => inner.Seek(offset, origin);
+
+        /// <inheritdoc />
+        public override void SetLength(long value) => inner.SetLength(value);
+
+        /// <inheritdoc />
+        public override void Write(byte[] buffer, int offset, int count) => inner.Write(buffer, offset, count);
+
+        /// <inheritdoc />
+        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            => inner.WriteAsync(buffer, offset, count, cancellationToken);
+    }
+
     private sealed class PipePair : IDisposable
     {
         private readonly NamedPipeServerStream gatewayStream;

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessStaSessionTests.cs

@@ -438,6 +438,53 @@ public sealed class MxAccessStaSessionTests
         Assert.Contains("alarm poll failed", fault.DiagnosticMessage, StringComparison.OrdinalIgnoreCase);
     }
 
+    /// <summary>
+    /// WRK-27. While the alarm poll's PollOnce is executing on the STA, a heartbeat captured mid-poll
+    /// must report <see cref="WorkerRuntimeHeartbeatSnapshot.StaCallInProgress"/> so the watchdog
+    /// grants the poll the same grace-to-ceiling suppression as a dispatched command; once the poll
+    /// returns the flag clears. PollOnce is blocked on a gate so the heartbeat can be captured while
+    /// the STA call is genuinely in flight.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task CaptureHeartbeat_DuringAlarmPoll_ReportsStaCallInProgress()
+    {
+        FakeAlarmCommandHandler handler = new() { BlockPoll = true };
+        FakeMxAccessComObjectFactory factory = new();
+        FakeMxAccessEventSink eventSink = new();
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(
+            runtime,
+            factory,
+            eventSink,
+            new MxAccessEventQueue(),
+            (_eq, _affinity, _comFactory) => handler);
+
+        await session.StartAsync("session-1", workerProcessId: 1);
+
+        // Wait until PollOnce is blocked mid-call on the STA thread.
+        Assert.True(
+            handler.WaitForPollEntered(TimeSpan.FromSeconds(5)),
+            "Expected the alarm poll to start within 5 seconds.");
+
+        // Captured mid-poll, the heartbeat advertises the in-progress STA call.
+        Assert.True(session.CaptureHeartbeat().StaCallInProgress);
+
+        // Release the poll and stop blocking; the flag clears once the poll returns.
+        handler.BlockPoll = false;
+        handler.ReleasePoll();
+
+        using CancellationTokenSource timeout = new(TimeSpan.FromSeconds(5));
+        while (session.CaptureHeartbeat().StaCallInProgress && !timeout.IsCancellationRequested)
+        {
+            await Task.Delay(25, CancellationToken.None);
+        }
+
+        Assert.False(
+            session.CaptureHeartbeat().StaCallInProgress,
+            "Expected StaCallInProgress to clear once the alarm poll returned.");
+    }
+
     /// <summary>
     /// The STA-affinity guard throws when an
     /// IMxAccessAlarmConsumer call is attempted off the thread that created
@@ -472,6 +519,8 @@ public sealed class MxAccessStaSessionTests
     private sealed class FakeAlarmCommandHandler : IAlarmCommandHandler
     {
         private readonly object gate = new object();
+        private readonly ManualResetEventSlim pollEntered = new(false);
+        private readonly ManualResetEventSlim releasePoll = new(false);
         private int pollCount;
         private int? lastPollThreadId;
 
@@ -484,6 +533,17 @@ public sealed class MxAccessStaSessionTests
         /// <summary>Exception thrown by PollOnce; null to succeed.</summary>
         public Exception? PollException { get; set; }
 
+        /// <summary>When set, <see cref="PollOnce"/> blocks until <see cref="ReleasePoll"/> is called.</summary>
+        public bool BlockPoll { get; set; }
+
+        /// <summary>Waits until a blocking <see cref="PollOnce"/> has entered and is blocked.</summary>
+        /// <param name="timeout">Maximum time to wait.</param>
+        /// <returns>True if a poll entered within the timeout.</returns>
+        public bool WaitForPollEntered(TimeSpan timeout) => pollEntered.Wait(timeout);
+
+        /// <summary>Releases a <see cref="PollOnce"/> blocked on the gate.</summary>
+        public void ReleasePoll() => releasePoll.Set();
+
         /// <summary>Gets the count of PollOnce calls.</summary>
         public int PollCount
         {
@@ -533,6 +593,12 @@ public sealed class MxAccessStaSessionTests
                 lastPollThreadId = Thread.CurrentThread.ManagedThreadId;
             }
 
+            if (BlockPoll)
+            {
+                pollEntered.Set();
+                releasePoll.Wait(TimeSpan.FromSeconds(10));
+            }
+
             if (PollException is not null)
             {
                 throw PollException;
@@ -540,6 +606,10 @@ public sealed class MxAccessStaSessionTests
         }
 
         /// <inheritdoc />
-        public void Dispose() { }
+        public void Dispose()
+        {
+            pollEntered.Dispose();
+            releasePoll.Dispose();
+        }
     }
 }

src/ZB.MOM.WW.MxGateway.Worker.Tests/TestSupport/FakeRuntimeSession.cs

@@ -372,6 +372,23 @@ internal sealed class FakeRuntimeSession : IWorkerRuntimeSession
         }
     }
 
+    /// <summary>
+    ///     Enqueues a batch of worker events atomically under one lock so the drain loop cannot
+    ///     observe a partial batch. Lets a test assert the drain loop coalesces a whole batch into one
+    ///     flush (WRK-25) without racing a mid-enqueue drain that would split the batch.
+    /// </summary>
+    /// <param name="workerEvents">The events to enqueue in order.</param>
+    public void EnqueueEvents(IEnumerable<WorkerEvent> workerEvents)
+    {
+        lock (gate)
+        {
+            foreach (WorkerEvent workerEvent in workerEvents)
+            {
+                events.Enqueue(workerEvent);
+            }
+        }
+    }
+
     /// <inheritdoc />
     public void Dispose()
     {

src/ZB.MOM.WW.MxGateway.Worker/Ipc/WorkerFrameProtocolOptions.cs

@@ -18,6 +18,17 @@ public sealed class WorkerFrameProtocolOptions
     /// </summary>
     public const int MaxNegotiableFrameBytes = 256 * 1024 * 1024;
 
+    /// <summary>
+    /// Lower floor the worker will accept for a gateway-negotiated frame maximum
+    /// (<c>GatewayHello.max_frame_bytes</c>). Matches the gateway's own
+    /// <c>GatewayOptionsValidator.MinimumMaxMessageBytes</c> validation floor so the worker never
+    /// rejects a value the gateway's own validator accepts as legal configuration, yet a nonsensical
+    /// tiny value (a gateway bug or a foreign/old peer) is rejected at the handshake rather than
+    /// leaving a session that handshakes cleanly and then fails every subsequent frame with
+    /// per-frame size errors. 1024 bytes still guarantees hellos, heartbeats, acks, and faults fit.
+    /// </summary>
+    public const int MinNegotiableFrameBytes = 1024;
+
     /// <summary>Initializes a new instance of the WorkerFrameProtocolOptions class from WorkerOptions.</summary>
     /// <param name="options">Worker initialization options.</param>
     public WorkerFrameProtocolOptions(WorkerOptions options)
@@ -118,7 +129,9 @@ public sealed class WorkerFrameProtocolOptions
     /// <summary>
     /// Adopts the gateway-negotiated frame maximum conveyed in <c>GatewayHello.max_frame_bytes</c>.
     /// A value of 0 (an older gateway that never set the field) is ignored and the
-    /// constructor default is kept. A value above <see cref="MaxNegotiableFrameBytes"/> is rejected.
+    /// constructor default is kept. A value outside the accepted range
+    /// [<see cref="MinNegotiableFrameBytes"/>, <see cref="MaxNegotiableFrameBytes"/>] is rejected so a
+    /// nonsensical negotiated value faults at the handshake rather than mid-session.
     /// </summary>
     /// <param name="negotiatedMaxFrameBytes">The gateway-negotiated maximum, or 0 for "keep default".</param>
     internal void AdoptNegotiatedMaxMessageBytes(uint negotiatedMaxFrameBytes)
@@ -128,6 +141,14 @@ public sealed class WorkerFrameProtocolOptions
             return;
         }
 
+        if (negotiatedMaxFrameBytes < MinNegotiableFrameBytes)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidConfiguration,
+                $"GatewayHello negotiated frame maximum {negotiatedMaxFrameBytes} is below the worker floor "
+                + $"of {MinNegotiableFrameBytes} bytes.");
+        }
+
         if (negotiatedMaxFrameBytes > MaxNegotiableFrameBytes)
         {
             throw new WorkerFrameProtocolException(

src/ZB.MOM.WW.MxGateway.Worker/Ipc/WorkerFrameWriter.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Runtime.ExceptionServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Google.Protobuf;
@@ -33,6 +34,16 @@ public sealed class WorkerFrameWriter
 
         /// <summary>Gets the completion source signaled once the frame has been written or has failed.</summary>
         public TaskCompletionSource<bool> Completion { get; }
+
+        /// <summary>
+        /// Set to <c>true</c> by <see cref="DequeueNext"/> — under <c>_gate</c> — at the instant the
+        /// draining lock-holder takes ownership of this frame to write it. A cancelled caller
+        /// tombstones its frame only while it is still unclaimed, so a claim and a cancel can never
+        /// both win: the flag is the interlock between the two. A frame already claimed is mid-write
+        /// and can no longer be recalled (see <see cref="WriteAsync(WorkerEnvelope, WorkerFrameWritePriority, CancellationToken)"/>).
+        /// Mutated only under <c>_gate</c>.
+        /// </summary>
+        public bool Claimed;
     }
 
     private readonly WorkerFrameProtocolOptions _options;
@@ -75,6 +86,13 @@ public sealed class WorkerFrameWriter
     /// <param name="priority">Scheduling priority; control frames are written ahead of event frames.</param>
     /// <param name="cancellationToken">Token to cancel waiting for the write lock.</param>
     /// <returns>A task that completes when the frame has been written and flushed.</returns>
+    /// <remarks>
+    /// Cancellation contract (WRK-22): if the token fires while this call is waiting for the write
+    /// lock, the frame is tombstoned so it is never written — unless a draining lock-holder has
+    /// already claimed it, in which case the frame may still reach the wire even though this call
+    /// observes <see cref="OperationCanceledException"/>. That residual window is by design: blocking
+    /// the canceller behind the very write it is abandoning would defeat the point of cancellation.
+    /// </remarks>
     public async Task WriteAsync(
         WorkerEnvelope envelope,
         WorkerFrameWritePriority priority,
@@ -101,7 +119,19 @@ public sealed class WorkerFrameWriter
         // Contend for the single writer: whoever wins drains every currently-queued frame in priority
         // order, so this frame is written by this call or by a concurrent caller that got the lock
         // first. Either way it completes via its own TaskCompletionSource.
-        await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            // Tombstone the queued frame so DequeueNext skips it — but only if a draining lock-holder
+            // has not already claimed it. If it is claimed it is mid-write and cannot be recalled; the
+            // caller still observes cancellation while the frame reaches the wire (documented above).
+            TombstoneIfUnclaimed(frame, cancellationToken);
+            throw;
+        }
+
         try
         {
             await DrainQueuedFramesAsync().ConfigureAwait(false);
@@ -114,6 +144,123 @@ public sealed class WorkerFrameWriter
         await frame.Completion.Task.ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// Queues a whole batch of envelopes at one priority under a single lock acquisition and drains
+    /// it, so a burst of frames — the event drain loop's hot path — pays one flush for the batch
+    /// rather than one per frame (WRK-25, realizing the WRK-12 coalescing on the path it was built
+    /// for). Intra-batch order is preserved because the enqueue is atomic under <c>_gate</c> and each
+    /// class queue is FIFO; the control-before-event guarantee still holds because any concurrently
+    /// queued control frame is drained ahead of this batch by <see cref="DequeueNext"/>. Every frame's
+    /// "written and flushed before completion" contract is unchanged.
+    /// </summary>
+    /// <param name="envelopes">Envelopes to write, in order.</param>
+    /// <param name="priority">Scheduling priority for the whole batch.</param>
+    /// <param name="cancellationToken">Token to cancel waiting for the write lock.</param>
+    /// <returns>A task that completes when every frame in the batch has been written and flushed.</returns>
+    /// <remarks>
+    /// A per-frame rejection inside the batch (for example one oversized event) surfaces from the
+    /// awaited completions as its <see cref="WorkerFrameProtocolException"/>; the remaining frames are
+    /// still observed so none faults unobserved. Cancellation while waiting for the lock tombstones
+    /// every still-unclaimed frame in the batch, per the WRK-22 contract on
+    /// <see cref="WriteAsync(WorkerEnvelope, WorkerFrameWritePriority, CancellationToken)"/>.
+    /// </remarks>
+    public async Task WriteBatchAsync(
+        IReadOnlyList<WorkerEnvelope> envelopes,
+        WorkerFrameWritePriority priority,
+        CancellationToken cancellationToken = default)
+    {
+        if (envelopes is null)
+        {
+            throw new ArgumentNullException(nameof(envelopes));
+        }
+
+        if (envelopes.Count == 0)
+        {
+            return;
+        }
+
+        PendingFrame[] frames = new PendingFrame[envelopes.Count];
+        for (int index = 0; index < envelopes.Count; index++)
+        {
+            WorkerEnvelope envelope = envelopes[index]
+                ?? throw new ArgumentException("Batch envelopes must not contain null.", nameof(envelopes));
+            frames[index] = new PendingFrame(envelope);
+        }
+
+        lock (_gate)
+        {
+            Queue<PendingFrame> queue = priority == WorkerFrameWritePriority.Event ? _eventFrames : _controlFrames;
+            foreach (PendingFrame frame in frames)
+            {
+                queue.Enqueue(frame);
+            }
+        }
+
+        try
+        {
+            await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            TombstoneUnclaimed(frames, cancellationToken);
+            throw;
+        }
+
+        try
+        {
+            await DrainQueuedFramesAsync().ConfigureAwait(false);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+
+        // Await every completion so no per-frame rejection faults unobserved, but surface the first
+        // failure (in batch order) to the caller — the drain loop maps it back to the offending event.
+        Exception? firstFailure = null;
+        foreach (PendingFrame frame in frames)
+        {
+            try
+            {
+                await frame.Completion.Task.ConfigureAwait(false);
+            }
+            catch (Exception exception)
+            {
+                firstFailure ??= exception;
+            }
+        }
+
+        if (firstFailure is not null)
+        {
+            ExceptionDispatchInfo.Capture(firstFailure).Throw();
+        }
+    }
+
+    private void TombstoneIfUnclaimed(PendingFrame frame, CancellationToken cancellationToken)
+    {
+        lock (_gate)
+        {
+            if (!frame.Claimed)
+            {
+                frame.Completion.TrySetCanceled(cancellationToken);
+            }
+        }
+    }
+
+    private void TombstoneUnclaimed(PendingFrame[] frames, CancellationToken cancellationToken)
+    {
+        lock (_gate)
+        {
+            foreach (PendingFrame frame in frames)
+            {
+                if (!frame.Claimed)
+                {
+                    frame.Completion.TrySetCanceled(cancellationToken);
+                }
+            }
+        }
+    }
+
     // Runs only under _writeLock. Drains control frames before event frames, stamping and writing each.
     // The stream write itself is not cancellable: a frame is written atomically or fails, never left
     // half-written on the pipe because a caller gave up waiting.
@@ -198,18 +345,36 @@ public sealed class WorkerFrameWriter
             or WorkerFrameProtocolErrorCode.SessionMismatch;
     }
 
+    // Returns the next frame to write, control frames first, skipping any frame a cancelled caller
+    // tombstoned while it waited for the lock (WRK-22). The frame actually returned is marked Claimed
+    // under _gate in the same critical section that checks the tombstone, so a claim and a concurrent
+    // cancel are mutually exclusive: whichever acquires _gate first wins.
     private PendingFrame? DequeueNext()
     {
         lock (_gate)
         {
-            if (_controlFrames.Count > 0)
+            while (_controlFrames.Count > 0)
             {
-                return _controlFrames.Dequeue();
+                PendingFrame frame = _controlFrames.Dequeue();
+                if (frame.Completion.Task.IsCanceled)
+                {
+                    continue;
+                }
+
+                frame.Claimed = true;
+                return frame;
             }
 
-            if (_eventFrames.Count > 0)
+            while (_eventFrames.Count > 0)
             {
-                return _eventFrames.Dequeue();
+                PendingFrame frame = _eventFrames.Dequeue();
+                if (frame.Completion.Task.IsCanceled)
+                {
+                    continue;
+                }
+
+                frame.Claimed = true;
+                return frame;
             }
 
             return null;

src/ZB.MOM.WW.MxGateway.Worker/Ipc/WorkerPipeSession.cs

@@ -371,27 +371,57 @@ public sealed class WorkerPipeSession
                 continue;
             }
 
-            foreach (WorkerEvent workerEvent in events)
+            // Submit the whole drained batch through the writer's batch entry point under one lock
+            // acquisition so the burst pays a single flush instead of one per event (WRK-25). Events
+            // are the low-priority frame class: the writer holds them behind any pending control frame
+            // (reply, fault, heartbeat, shutdown ack) so those are not delayed behind an event backlog,
+            // and intra-batch order is preserved.
+            WorkerEnvelope[] envelopes = new WorkerEnvelope[events.Count];
+            for (int index = 0; index < events.Count; index++)
             {
-                // Events are the low-priority frame class: the writer holds them behind any pending
-                // control frame (reply, fault, heartbeat, shutdown ack) so those are not delayed
-                // behind an event backlog.
-                try
-                {
-                    await _writer
-                        .WriteAsync(CreateEnvelope(workerEvent), WorkerFrameWritePriority.Event, cancellationToken)
-                        .ConfigureAwait(false);
-                }
-                catch (WorkerFrameProtocolException exception)
-                    when (exception.ErrorCode == WorkerFrameProtocolErrorCode.MessageTooLarge)
-                {
-                    await FaultOnOversizedEventAsync(workerEvent, exception, cancellationToken)
-                        .ConfigureAwait(false);
-                }
+                envelopes[index] = CreateEnvelope(events[index]);
+            }
+
+            try
+            {
+                await _writer
+                    .WriteBatchAsync(envelopes, WorkerFrameWritePriority.Event, cancellationToken)
+                    .ConfigureAwait(false);
+            }
+            catch (WorkerFrameProtocolException exception)
+                when (exception.ErrorCode == WorkerFrameProtocolErrorCode.MessageTooLarge)
+            {
+                // A single oversized event surfaces from the batch's awaited completions; the death is
+                // still IPC-30's structured, event-naming fault. Map the rejection back to the first
+                // event in batch order whose envelope overshoots the negotiated maximum — the same
+                // frame the writer rejected first.
+                await FaultOnOversizedEventAsync(
+                        FindOversizedEvent(events, envelopes),
+                        exception,
+                        cancellationToken)
+                    .ConfigureAwait(false);
             }
         }
     }
 
+    private WorkerEvent FindOversizedEvent(
+        IReadOnlyList<WorkerEvent> events,
+        WorkerEnvelope[] envelopes)
+    {
+        for (int index = 0; index < envelopes.Length; index++)
+        {
+            if (envelopes[index].CalculateSize() > _options.MaxMessageBytes)
+            {
+                return events[index];
+            }
+        }
+
+        // Unreachable in practice: WriteBatchAsync surfaced MessageTooLarge, so at least one envelope
+        // exceeded the negotiated maximum. Fall back to the first event so the fault still names a
+        // concrete event rather than throwing a second, less useful exception from the fault path.
+        return events[0];
+    }
+
     /// <summary>
     ///     Ends the session on an event that cannot be framed, but deliberately and diagnosably
     ///     (IPC-30). An event above the negotiated frame maximum is undeliverable end to end — the
@@ -1085,16 +1115,16 @@ public sealed class WorkerPipeSession
             return;
         }
 
-        if (!string.IsNullOrEmpty(snapshot.CurrentCommandCorrelationId)
+        if ((!string.IsNullOrEmpty(snapshot.CurrentCommandCorrelationId) || snapshot.StaCallInProgress)
             && staleFor <= _sessionOptions.HeartbeatStuckCeiling)
         {
-            // A command is in flight and we are still within the defensive
-            // suppression ceiling — the STA is busy executing it, not
-            // hung. The next MarkActivity() in StaRuntime.ProcessQueuedCommands
-            // will refresh LastActivityUtc once the command returns, at which
-            // point this branch stops being taken. The heartbeat already
-            // surfaces the in-flight correlation id so the gateway can apply
-            // its own per-command timeout if it considers the command too slow.
+            // A command is in flight, or an STA call outside the dispatcher (the alarm poll, WRK-27) is
+            // executing, and we are still within the defensive suppression ceiling — the STA is busy
+            // doing that work, not hung. The next MarkActivity() in StaRuntime.ProcessQueuedCommands
+            // will refresh LastActivityUtc once the work returns, at which point this branch stops
+            // being taken. The heartbeat already surfaces the in-flight correlation id so the gateway
+            // can apply its own per-command timeout if it considers the command too slow; a poll that
+            // blocks the STA past the ceiling still faults, which is the ceiling's contract.
             return;
         }
 

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessStaSession.cs

@@ -24,6 +24,14 @@ public sealed class MxAccessStaSession : IWorkerRuntimeSession
     private CancellationTokenSource? alarmPollCts;
     private Task? alarmPollTask;
     private int? alarmConsumerThreadId;
+
+    // True on the STA thread exactly around the alarm PollOnce COM call. The alarm poll runs outside
+    // the StaCommandDispatcher (so it does not inflate PendingCommandCount or perturb command dispatch
+    // ordering), which means CaptureHeartbeat would otherwise see no in-flight activity during a long
+    // poll and the watchdog would fault the session at the 15 s grace instead of the 75 s ceiling
+    // granted to dispatched commands. Surfacing the poll on the heartbeat closes that asymmetry
+    // (WRK-27). Volatile: written on the STA thread, read on the heartbeat thread.
+    private volatile bool staAlarmPollInProgress;
     private bool disposed;
 
     /// <summary>
@@ -247,8 +255,20 @@ public sealed class MxAccessStaSession : IWorkerRuntimeSession
                     await staRuntime.InvokeAsync(
                         () =>
                         {
-                            EnsureOnAlarmConsumerThread();
-                            handler.PollOnce();
+                            // Advertise the poll to the watchdog for exactly the span of the COM call
+                            // (WRK-27): set on the STA thread immediately before the affinity check and
+                            // PollOnce, clear in the finally so a heartbeat captured mid-poll reports
+                            // StaCallInProgress and one captured after does not.
+                            staAlarmPollInProgress = true;
+                            try
+                            {
+                                EnsureOnAlarmConsumerThread();
+                                handler.PollOnce();
+                            }
+                            finally
+                            {
+                                staAlarmPollInProgress = false;
+                            }
                         },
                         cancellationToken).ConfigureAwait(false);
                 }
@@ -377,7 +397,8 @@ public sealed class MxAccessStaSession : IWorkerRuntimeSession
             pendingCommandCount,
             (uint)eventQueue.Count,
             eventQueue.LastEventSequence,
-            currentCommandCorrelationId);
+            currentCommandCorrelationId,
+            staAlarmPollInProgress);
     }
 
     /// <inheritdoc />

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/WorkerRuntimeHeartbeatSnapshot.cs

@@ -10,18 +10,26 @@ public sealed class WorkerRuntimeHeartbeatSnapshot
     /// <param name="outboundEventQueueDepth">Current depth of the worker event queue.</param>
     /// <param name="lastEventSequence">Sequence number of the most recent event.</param>
     /// <param name="currentCommandCorrelationId">Correlation ID of the in-flight command.</param>
+    /// <param name="staCallInProgress">
+    /// True while an STA call outside the command dispatcher is executing (currently the alarm poll,
+    /// WRK-27). The watchdog treats this like an in-flight command: it suppresses the stale-STA fault
+    /// up to the stuck ceiling instead of the shorter grace, so a healthy-but-slow poll does not fault
+    /// a healthy session. Named generically so any future non-dispatcher STA work reuses it.
+    /// </param>
     public WorkerRuntimeHeartbeatSnapshot(
         DateTimeOffset lastStaActivityUtc,
         uint pendingCommandCount,
         uint outboundEventQueueDepth,
         ulong lastEventSequence,
-        string currentCommandCorrelationId)
+        string currentCommandCorrelationId,
+        bool staCallInProgress = false)
     {
         LastStaActivityUtc = lastStaActivityUtc;
         PendingCommandCount = pendingCommandCount;
         OutboundEventQueueDepth = outboundEventQueueDepth;
         LastEventSequence = lastEventSequence;
         CurrentCommandCorrelationId = currentCommandCorrelationId ?? string.Empty;
+        StaCallInProgress = staCallInProgress;
     }
 
     /// <summary>Gets the last STA activity timestamp in UTC.</summary>
@@ -38,4 +46,11 @@ public sealed class WorkerRuntimeHeartbeatSnapshot
 
     /// <summary>Gets the correlation ID of the in-flight command.</summary>
     public string CurrentCommandCorrelationId { get; }
+
+    /// <summary>
+    /// Gets a value indicating whether an STA call outside the command dispatcher (the alarm poll) is
+    /// executing. When true the watchdog grants the poll the same grace-to-ceiling suppression as a
+    /// dispatched command (WRK-27).
+    /// </summary>
+    public bool StaCallInProgress { get; }
 }