diff --git a/docs/rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md b/docs/rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md
index c29fb4f83b..df6433dd26 100644
--- a/docs/rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md
+++ b/docs/rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md
@@ -613,7 +613,7 @@ Starting a run reads every top-level request field once before capability valida
 
 The driver first installs provider ownership. Only after that succeeds does it attach the request's abort listener and create one run-owner Cordis fiber under `parent.ctx`; an already-unloading provider therefore leaves neither a child nor an orphaned listener. Calling `runOwner.ctx.agents.create()` gives the child factory an explicit `ownerCtx` carrying the run-owner fiber and scope, while the registry's traced factory receiver preserves AgentLoop's injected dependency origin. Parent teardown, provider teardown, and manual run disposal all dispose this same run-owner node; moving it out of the active state synchronously prevents an unpublished setup from publishing afterward, while all three paths follow one quiescence promise. This structured ownership does not change the child's flat capability view.
 
-The provider's run separates acceptance from publication with `started: Promise<void>`, but the service does not expose that caller-owned handle directly. It captures `id`, `started`, `result`, and each method once, binds methods to the provider-owned run handle, and returns a frozen service-owned wrapper. Capturing `dispose` first also preserves a rollback capability if a later accessor or method check reveals a malformed handle.
+The provider's run separates acceptance from publication with `started: Promise<void>`, but the service does not expose that caller-owned handle directly. It captures `id`, `started`, `result`, and each method once, binds methods to the provider-owned run handle, and returns a frozen service-owned wrapper. Capturing `dispose` first also preserves a rollback capability if a later accessor or method check reveals a malformed handle. The wrapper installs its shared disposal promise before invoking the raw provider callback, so synchronous reentry through the returned wrapper and ordinary repeat calls join one provider disposal rather than slipping through a not-yet-assigned memo. If the raw disposer directly returns that same reentrant wrapper promise, the service rejects the cyclic provider contract instead of awaiting a promise that depends on itself forever.
 
 The wrapper's `result` promise captures `output`, optional `structured`, and `stopReason` once and resolves to one detached, deeply frozen lossless-JSON value shared by the caller and lifecycle telemetry. Malformed terminal data is an infrastructure fault; it rejects only after the service has started rollback of the provider attempt. The service observes the normalized result immediately, before waiting for readiness, so an early rejection is never temporarily unhandled.
 
@@ -658,24 +658,57 @@ SubagentService.start(...):
 Workflow worker bridge after receiving returnedRun:
   register the run so cancellation can reach pre-publication work
   attach result settlement handlers immediately and snapshot the outcome
-  if returnedRun.started fulfills:
-    send ChildStarted; then send the buffered or eventual outcome
-  else:
-    send ChildStartError and dispose the attempt
+  re-check terminal admission after provider start returns
+  if admission closed:
+    if the exact run remains registered: cancel once and dispose it
+    if worker-message admission remains open: send ChildStartError
+  else wait for returnedRun.started:
+    on fulfillment:
+      re-check terminal admission
+      if closed: apply the same identity-guarded refusal
+      else: send ChildStarted; then send the buffered or eventual outcome
+    on rejection:
+      if worker-message admission remains open: send ChildStartError
+      if the exact run remains registered: dispose it
 
-Before publishing the workflow's own result:
-  abort the shared child-request signal
-  call cancel("workflow settled") on every host-registered run
-  only then settle the workflow result
+Worker after choosing its result:
+  queue Result on the worker-to-host port
+  only then reap stray child handles
+
+Host at workflow Result receipt:
+  cancellationWasRequested = external cancellation is already in flight
+  atomically claim chosen =
+    if cancellationWasRequested and result is not cancelled:
+      cancelledResult
+    else:
+      result
+
+  if not cancellationWasRequested:
+    abort the shared child-request signal
+    call cancel("workflow settled") on every host-registered run
+
+  settle chosen
+
+Host at the first worker death signal:
+  close worker-message admission
+  claim death or preserve an earlier cancellation/Result/grace outcome
+  cancel and dispose every registered child
+  synthesize missing lifecycle ends
+
+Host at physical worker exit:
+  perform a final disposal-only sweep
+  do not repeat explicit child cancellation
 ```
 
-Every downstream protocol that announces a subagent must honor the same boundary. The workflow worker bridge therefore registers the returned run before waiting, observes and snapshots `result` immediately, sends `ChildStarted` only after `started` fulfills, and sends `ChildStartError` plus host-driven disposal when readiness rejects.
+Every downstream protocol that announces a subagent must honor the same boundary. The workflow worker bridge therefore registers the returned run before waiting, observes and snapshots `result` immediately, and sends `ChildStarted` only after `started` fulfills while admission remains open. A readiness rejection is refused and host-disposed; `ChildStartError` is sent while worker-message admission remains open, and an already-retired exact run is not cleaned twice. Provider `start()` is itself arbitrary code and may synchronously reenter workflow cancellation before its returned run reaches that registry. The bridge attaches both promise observers, re-checks terminal admission immediately after `start()` returns and again at readiness, and turns a closed boundary into identity-guarded cancellation, disposal, and refusal rather than late worker admission or lifecycle announcement. An arbitrary provider may still fulfill its own `started` promise after the workflow boundary; the bridge refuses and cleans up that attempt instead of claiming it can undo provider-side publication.
 
 Cancellation before readiness is a publication decision, not merely a flag for later result mapping. The in-process run synchronously deactivates its owner fiber. If cancellation lands before publication, the factory's liveness check prevents either creation edge. If it begins synchronously inside `session/created`, `agent/created`, or `agent/session-start`, the publication barrier lets the current notification phase unwind without revoking its world, the next liveness check prevents every later phase and driver start, and rollback pairs every creation edge that already began. In either case `started` rejects, no `subagent/start` or `subagent/end` is emitted, and the run result settles as `aborted`.
 
-Before the workflow's own result becomes observable, its host likewise drives both permitted cancellation channels: it aborts the shared request signal and calls each registered run's `cancel()`, including runs still waiting on readiness. Provider cancel callbacks are contained independently so one broken implementation cannot prevent peers from receiving cancellation or wedge the workflow result.
+Receipt of the worker's `Result` message is the workflow host's atomic first-wins boundary. The worker queues that message before its own settlement-reap `ChildCancel` messages, so same-port FIFO prevents an internal child callback from masquerading as earlier run cancellation. Each contender records its claim before its own callback fanout: external `cancel()` records its reason first, while Result receipt snapshots any earlier cancellation and claims the resulting terminal outcome before invoking settlement-cleanup provider code. A caller, signal, or dispose cancellation already in flight therefore overrides a non-cancelled worker report, while the report wins otherwise. Before exposing that chosen result, the host drives both permitted child-cancellation channels by aborting the shared request signal and calling every registered run's `cancel()`, including runs still waiting on readiness. Those calls are settlement-only cleanup, and the terminal claim makes a reentrant `WorkerRun.cancel()` a side-effect-free loser rather than merely repairing its result afterward. Host fanout and the worker's FIFO-later `ChildCancel` can both reach the explicit channel, so a per-call gate invokes each provider `cancel()` at most once; the seam does not require that callback to be idempotent. Explicit child cancel callbacks are contained independently so one throwing callback cannot starve peers or alter settlement.
 
-Together these rules prevent an early result rejection from going unhandled, ensure `workflow/agent-start` never names an unpublished child, and prevent a child from publishing after its workflow has ended.
+Unexpected worker death uses the same terminal-claim rule, but terminal ownership, message admission, and exit cleanup are separate state. The host snapshots whether external cancellation was already accepted, claims either `cancelled` or the death error, closes inbound worker messages, and only then reaps children and synthesizes missing lifecycle events. Closing admission is necessary because Node may emit `error`, deliver an already-queued `message`, and only then emit `exit`; without the logical barrier, that message could start a child or narrate after `workflow/end`. Provider code reentering cancellation during cleanup cannot rewrite a death-first error; conversely, a cancellation accepted before death remains the winner. If Result or grace already claimed the outcome, death preserves it while still reaping promptly. Physical exit then performs a final disposal-only sweep without repeating explicit provider cancellation. `handle.dispose()` claims its public promise before that traversal invokes cancellation or disposal callbacks, and every `disposeChild` path independently claims the call ID promise before invoking the wrapped child disposer. Public-first reentry returns the existing holder promise; worker-first reentry may begin holder disposal, whose child traversal joins the already-claimed call ID promise. This distinction is necessary because grace settlement precedes `worker.terminate()` and its exit event: suppressing a duplicate outcome, late message, or repeated cleanup request must not suppress disposal of survivors in the host registry.
+
+Together these rules prevent an early result rejection from going unhandled, ensure `workflow/agent-start` never names an unready child, and prevent the bridge from admitting or announcing a child after its workflow has ended.
 
 Parent teardown reaches `runOwner` by nesting; the provider and returned run handle reach the same node through their explicit disposers.
 
diff --git a/packages/subagent/subagent/README.md b/packages/subagent/subagent/README.md
index 7e63f07376..e29706bd8b 100644
--- a/packages/subagent/subagent/README.md
+++ b/packages/subagent/subagent/README.md
@@ -21,7 +21,7 @@ Unlike the bash seam (one executor per context, second load throws), **multiple
 | `registerProvider(provider)` | Read and validate the name, capability object and four boolean flags, `inheritsParentContext`, and `start` callback exactly once, then register a frozen acceptance snapshot under the accepted name. Malformed fixed fields fail loud before registration; later caller mutation cannot change registry behavior or HMR cleanup, while `start` stays bound to the original provider receiver. Throws `SubagentError('DUPLICATE_PROVIDER')` on a name clash. Effect-scoped (HMR-safe); returns the disposer. |
 | `getProvider(name)` | Look up the frozen registry snapshot (`undefined` if absent). |
 | `list()` | Registered provider names (insertion order). |
-| `start(name, request)` | Resolve the provider (`NO_PROVIDER` if absent), read every caller field once into one acceptance snapshot, validate every requested START-TIME capability and scalar value before any child is created, and materialize prompt/schema/options/filter data through a single-pass lossless-JSON snapshot before delegating to `provider.start`. Acquire and memoize the provider run's disposer before reading the rest of its handle, then return a frozen service-owned wrapper whose fields are captured once, whose methods remain bound to the provider handle, and whose `result` is one detached, deeply frozen normalization shared by the caller and telemetry. Malformed handle access/binding starts rollback before the synchronous fault escapes; malformed terminal data rejects only after rollback reaches quiescence. Emit `subagent/start` only after `run.started` fulfills and the paired `subagent/end` after that started run settles; a pre-publication readiness rejection emits neither. |
+| `start(name, request)` | Resolve the provider (`NO_PROVIDER` if absent), read every caller field once into one acceptance snapshot, validate every requested START-TIME capability and scalar value before any child is created, and materialize prompt/schema/options/filter data through a single-pass lossless-JSON snapshot before delegating to `provider.start`. Acquire the provider run's disposer before reading the rest of its handle, then return a frozen service-owned wrapper whose fields are captured once, whose methods remain bound to the provider handle, and whose `result` is one detached, deeply frozen normalization shared by the caller and telemetry. The wrapper claims its shared disposal promise before invoking raw provider code, so synchronous reentry and ordinary repeats join one provider call; a raw disposer that directly returns that same reentrant wrapper promise is rejected as a cyclic provider contract instead of hanging forever. Malformed handle access/binding starts rollback before the synchronous fault escapes; malformed terminal data rejects only after rollback reaches quiescence. Emit `subagent/start` only after `run.started` fulfills and the paired `subagent/end` after that started run settles; a pre-publication readiness rejection emits neither. |
 
 ## Capabilities: two kinds, discovered two ways
 
diff --git a/packages/subagent/subagent/src/index.ts b/packages/subagent/subagent/src/index.ts
index b319a7f31d..74b7df171b 100644
--- a/packages/subagent/subagent/src/index.ts
+++ b/packages/subagent/subagent/src/index.ts
@@ -379,14 +379,30 @@ export class SubagentService extends Service {
     let disposal: Promise<void> | undefined
     const dispose = (): Promise<void> => {
       if (disposal === undefined) {
+        // Claim the shared transaction before invoking provider code: a raw
+        // disposer can synchronously reenter this wrapper through a reference
+        // retained by its caller, and both calls must join one provider call.
+        const claimed = Promise.withResolvers<undefined>()
+        disposal = claimed.promise
         try {
           // Invoke through the captured callable without reading its public
           // `bind`/`length`/`name` properties. Disposal is the recovery
           // capability itself; hostile function metadata must not prevent the
           // seam from exercising it when a later handle field is malformed.
-          disposal = Promise.resolve(Reflect.apply(inputDispose, acceptedRun, []))
+          const returned: unknown = Reflect.apply(inputDispose, acceptedRun, [])
+          // A raw disposer can reenter the service wrapper and directly return
+          // that same shared promise. Awaiting it here would make the promise
+          // depend on itself forever; reject the cyclic provider contract loud.
+          if (returned === claimed.promise) {
+            claimed.reject(new TypeError(`subagent provider "${name}" run dispose returned its own wrapper disposal promise`))
+            return disposal
+          }
+          void Promise.resolve(returned).then(
+            () => { claimed.resolve(undefined) },
+            (error: unknown) => { claimed.reject(error) },
+          )
         } catch (error: unknown) {
-          disposal = Promise.reject(error instanceof Error
+          claimed.reject(error instanceof Error
             ? error
             : new Error('subagent provider run dispose threw a non-Error value', { cause: error }))
         }
diff --git a/packages/subagent/subagent/tests/service.spec.ts b/packages/subagent/subagent/tests/service.spec.ts
index d1a531ca09..e9b66ce158 100644
--- a/packages/subagent/subagent/tests/service.spec.ts
+++ b/packages/subagent/subagent/tests/service.spec.ts
@@ -150,6 +150,57 @@ describe('SubagentService', () => {
     }
   })
 
+  it('claims wrapper disposal before a raw provider disposer can reenter it', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SubagentService)
+    const observed: { reentrant?: Promise<void> } = {}
+    const providerDispose = vi.fn(() => {
+      observed.reentrant = run.dispose()
+      return Promise.resolve()
+    })
+    ctx.subagents.registerProvider({
+      name: 'dispose-reentry',
+      capabilities: NO_CAPS,
+      inheritsParentContext: false,
+      start: () => ({
+        id: AgentId('dispose-reentry-child'),
+        started: Promise.resolve(),
+        result: Promise.resolve({ output: [], stopReason: 'completed' }),
+        cancel() {},
+        dispose: providerDispose,
+      }),
+    })
+    const run = ctx.subagents.start('dispose-reentry', baseRequest())
+
+    const disposal = run.dispose()
+
+    expect(observed.reentrant).toBe(disposal)
+    await disposal
+    expect(providerDispose).toHaveBeenCalledOnce()
+  })
+
+  it('rejects a raw disposer that directly returns its reentrant wrapper promise instead of hanging', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SubagentService)
+    const providerDispose = vi.fn(() => run.dispose())
+    ctx.subagents.registerProvider({
+      name: 'dispose-self-cycle',
+      capabilities: NO_CAPS,
+      inheritsParentContext: false,
+      start: () => ({
+        id: AgentId('dispose-self-cycle-child'),
+        started: Promise.resolve(),
+        result: Promise.resolve({ output: [], stopReason: 'completed' }),
+        cancel() {},
+        dispose: providerDispose,
+      }),
+    })
+    const run = ctx.subagents.start('dispose-self-cycle', baseRequest())
+
+    await expect(run.dispose()).rejects.toThrow('run dispose returned its own wrapper disposal promise')
+    expect(providerDispose).toHaveBeenCalledOnce()
+  })
+
   it.each([
     { label: 'a non-string name', patch: { name: 42 }, message: 'name must be a string' },
     { label: 'null capabilities', patch: { capabilities: null }, message: 'capabilities must be an object' },
diff --git a/packages/workflow/workflow-workerthread/README.md b/packages/workflow/workflow-workerthread/README.md
index 1879e60e7e..409ab57132 100644
--- a/packages/workflow/workflow-workerthread/README.md
+++ b/packages/workflow/workflow-workerthread/README.md
@@ -25,7 +25,7 @@ What the seam guarantees regardless, because benign scripts hit these constantly
 
 Inside the worker, `runWorkerSession` builds the execution core (hooks, combinators, concurrency semaphore, caps, fatal-error discipline) over a **child port**. `agent()` sends `child-start`, and the host starts the child through the holder-bound `SubagentService` handle captured synchronously by `start()`, with parent attribution, the shared per-run abort signal, and `outputSchema`/`model` pass-through. This capture is part of the seam's holder-owned lifetime: unloading the engine removes `ctx.workflows` for new calls but does not invalidate an already returned run whose worker starts another child afterward.
 
-The host observes `run.result` immediately but buffers its snapshotted wire projection until `run.started` fulfills. It then replies `child-started` with the child id before forwarding settlement, so `workflow/agent-start` always names a ready child and precedes its end. A readiness rejection replies `child-start-error`, emits no workflow agent pair, and makes the host dispose the attempt because the worker never received a handle; the worker classifies it as fatal `AGENT_START` unless cancellation already owns the run. If readiness fulfills, an infrastructure result rejection crosses as `child-failed`/`AGENT_RESULT` regardless of whether that rejection settled before or after readiness. Child disposal acknowledgements complete the RPC.
+The host observes `run.result` immediately but buffers its snapshotted wire projection until `run.started` fulfills. Provider `start()` is arbitrary code and can synchronously reenter workflow cancellation before its returned run reaches the host registry, so the host registers the run, attaches both promise observers, and re-checks admission after `start()` returns and again at readiness. A closed boundary never admits or announces the run to the worker: while the exact run remains registered, the host invokes explicit cancel once and disposes it; `child-start-error` is sent only while worker-message admission remains open. If the run was already retired, the identity guard sends no cleanup through the deleted call ID. An ordinary readiness rejection sends `child-start-error` while possible and disposes the provider attempt without adding an explicit cancellation. Otherwise the host replies `child-started` with the child id before forwarding settlement, so `workflow/agent-start` always names a ready child and precedes its end. The worker classifies a start error as fatal `AGENT_START` unless cancellation already owns the run. If readiness fulfills, an infrastructure result rejection crosses as `child-failed`/`AGENT_RESULT` regardless of whether that rejection settled before or after readiness. Child disposal acknowledgements complete the RPC.
 
 Observer narration (`phase`/`log`/`agent-start`/`agent-end`) crosses as messages and re-emits as the seam's `workflow/*` events. A **ready→go handshake** gates the body: a cancellation racing worker boot arrives before `go`, so a run cancelled before start never executes the body at all.
 
@@ -35,9 +35,15 @@ Values LEAVING the script (hook options/schemas, the script's return) are materi
 
 ## Cancellation, death, disposal
 
-Per-run limits: a concurrency semaphore (`maxConcurrentAgents`), a total-`agent()` cap (`maxTotalAgents`), and a per-call item cap (`maxItemsPerCall`), all config. `cancel()` posts the cancel to the worker (its hooks start throwing `CANCELLED`; the script dies at its next await) and cancels every host-side child NOW on **both seam channels** — the shared request signal aborts AND each registered child's explicit `cancel()` is called host-side, because the seam leaves a provider free to honor either channel and a worker wedged in a synchronous spin could not relay its own per-child cancel RPCs (those later land as idempotent no-ops). Each provider-owned cancel callback is exception-contained independently, so a broken child cannot prevent peer cancellation or workflow settlement. The caller's optional start-signal callback is retained by exact identity only while the run is live and removed at the first settlement or teardown, so a long-lived signal cannot retain completed `WorkerRun` instances. The grace then arms: a run still unsettled `disposeGraceMs` later force-settles `cancelled` and the worker is **terminated**. A cancellation that lands before the body runs (the ready→go handshake) reports `cancelled` without executing anything; a worker `result` racing an in-flight host cancellation reports `cancelled` too (first-wins settlement — the seam-visible result had not settled when cancellation was requested); post-cancel `phase`/`log` narration is suppressed host-side, while cancelled children still deliver their paired `agent-end`.
+Cancellation is bounded and host-driven. Per-run limits are a concurrency semaphore (`maxConcurrentAgents`), a total-`agent()` cap (`maxTotalAgents`), and a per-call item cap (`maxItemsPerCall`), all config. `cancel()` first records its reason, then posts to the worker (its hooks start throwing `CANCELLED`; the script dies at its next await) and cancels every host-side child NOW on **both seam channels**: the shared request signal aborts and each registered child's explicit `cancel()` runs host-side. The seam leaves a provider free to honor either channel, and a worker wedged in a synchronous spin could not relay its own per-child cancel RPCs. A host-side per-call gate turns the worker's later explicit-cancel relay into a no-op, because the seam does not require `SubagentRun.cancel()` to be idempotent. Each explicit child `cancel()` callback is exception-contained independently, post-cancel `phase`/`log` narration is suppressed host-side, and cancelled children still deliver paired `agent-end` events. The caller's optional start-signal callback is retained by exact identity only while the run is live and removed at the first settlement or teardown.
 
-A worker that dies unexpectedly (an OOM, a script reaching `process.exit` through the documented vm escape) settles the run `stopReason: 'error'` with the exit diagnostics — or `'cancelled'` when a cancel was in flight — and the host-side child registry is what winds every surviving child down. `dispose()` = cancel + immediate host-driven disposal of every registered child (a wedged worker can relay no dispose RPC, so child teardown overlaps the grace instead of starting after it; the worker's own dispose RPCs join the same per-child disposal) + bounded wait (result, then child-registry quiescence, capped by the grace) + unconditional `worker.terminate()`: the thread never outlives its run. Before an ordinary run settlement becomes observable, the host cancels every stray child on both channels too—even a fire-and-forget run still waiting on `started`, for which the worker has no handle yet—and `dispose()` then waits for their disposal (bounded by the grace) before returning. `agent-start`/`agent-end` pairing is host-guaranteed the same way: forwarded starts live in a ledger, worker-reported ends pair them on the graceful paths, and the termination paths (grace force-settle, worker death) synthesize the missing ends (outcome `cancelled`) before the run settles — a start still in flight across the force-settle can surface after `workflow/end`, immediately paired the same way.
+Terminal arbitration is first-wins at explicit host-side claim points. A cancellation before ready→go reports `cancelled` without executing the body. For a later race, the worker queues Result before its settlement-reap `ChildCancel` messages; external `cancel()` records its reason before its fanout, while Result receipt snapshots any earlier cancellation and records the terminal outcome before settlement-cleanup fanout. Same-port FIFO and those claim points mean earlier caller/signal/dispose cancellation overrides a non-cancelled report, while an arrived report cannot be rewritten by a cleanup callback. Once Result has won, a losing reentrant `cancel()` has no state, message, child-fanout, or grace-timer effect. If no earlier terminal source settles the run, the grace callback claims `cancelled`, synthesizes missing lifecycle ends, settles the result, and terminates the worker after `disposeGraceMs`.
+
+Worker death separates outcome ownership, message admission, and resource cleanup. An unexpected OOM, `error`, message failure, or premature exit claims `stopReason: 'error'` with diagnostics—or preserves an external cancellation already in flight—before reaping children or synthesizing observer events. Reentrant provider cancellation therefore cannot turn a death-first error into cancellation. The first death signal also closes worker-message admission because Node may deliver a queued `message` between `error` and `exit`; late protocol data cannot start a child, emit narration, or compete with the outcome. If Result or grace already claimed the outcome, death preserves it while still reaping promptly. The eventual `exit` then performs a final disposal-only sweep, joining any in-flight disposal without repeating explicit child cancellation. This separation lets grace settlement become observable before `worker.terminate()` reports exit without leaking the host-side registry.
+
+Disposal is the holder's bounded resource guarantee: cancel, begin host-driven disposal of every registered child immediately, wait for result plus child-registry quiescence up to the same grace, and unconditionally terminate the worker. `handle.dispose()` claims its public promise before that traversal invokes cancellation or disposal callbacks. Independently, every `disposeChild` path claims the call ID's promise before invoking the wrapped child disposer. Public-first reentry therefore returns the existing holder promise; worker-first reentry may begin holder disposal, whose child traversal joins the already-claimed call ID promise. Neither order can start a second provider disposal. A wedged worker can relay no dispose RPC, so host-driven teardown overlaps the grace; any later worker RPC joins the same per-child disposal. Before ordinary settlement becomes observable, the host also cancels every stray on both channels, including a fire-and-forget run still waiting on readiness. That work is settlement-only cleanup after the terminal claim, so provider reentry cannot rewrite the chosen result; `dispose()` then waits for its completion within the bound.
+
+Lifecycle pairing is host-guaranteed independently of outcome arbitration. Forwarded starts live in a ledger and worker-reported ends pair them on graceful paths. When death or grace is the terminal source, the host synthesizes missing ends with outcome `cancelled` before `workflow/end`. If Result settled first, later death cleanup may synthesize a survivor's end afterward; a start already crossing force-settlement may likewise surface after `workflow/end`. The same ledger still pairs every forwarded start exactly once.
 
 **Engine-specific limitations**: worker startup is paid per run; on a termination path `agentsStarted` reports the HOST-observed count (accepted `child-start`s — calls still queued worker-side for a concurrency slot are unknowable then); and a returned promise or thenable resolves per JavaScript semantics BEFORE materialization — that is what makes an un-awaited `return agent('x')` work — with the value-boundary guard applying to the resolution.
 
diff --git a/packages/workflow/workflow-workerthread/src/host.ts b/packages/workflow/workflow-workerthread/src/host.ts
index 8c839fbccb..7e87c14fae 100644
--- a/packages/workflow/workflow-workerthread/src/host.ts
+++ b/packages/workflow/workflow-workerthread/src/host.ts
@@ -7,19 +7,31 @@
  * run.
  *
  * The run's `result` promise settles exactly once, from whichever of these
- * lands first: the worker's `result` message (a host-side cancellation in
- * flight overrides a non-cancelled report — the seam-visible result had not
- * settled when cancellation was requested), an unexpected worker death
- * (`error`/`messageerror`/premature `exit` → `stopReason: 'error'`, or
- * `'cancelled'` when a cancel was in flight), or the post-cancel grace timer
- * (a script that never settles is force-settled `cancelled` and its worker
- * terminated — the real kill an in-process engine could not perform).
+ * lands first: receipt of the worker's `result` message, an unexpected worker
+ * death (`error`/`messageerror`/premature `exit` → `stopReason: 'error'`, or
+ * `'cancelled'` when a cancel was in flight), or the post-cancel grace timer (a
+ * script that never settles is force-settled `cancelled` and its worker
+ * terminated — the real kill an in-process engine could not perform). At
+ * `result` receipt the host snapshots whether caller/signal/dispose
+ * cancellation is already in flight: an earlier cancellation overrides a
+ * non-cancelled report; otherwise the report wins before settlement-only child
+ * cleanup invokes arbitrary provider callbacks. Worker death uses the same
+ * boundary: it claims `error` (or a previously requested `cancelled`) before
+ * reaping children, so cleanup callbacks cannot rewrite the outcome. That
+ * first signal also closes inbound message admission: Node may emit `error`,
+ * then deliver queued messages, then emit `exit`, but those late messages may
+ * neither create work nor narrate after settlement. If Result or grace already
+ * owns the outcome, death preserves it while still cleaning resources; the
+ * eventual exit performs a final disposal-only sweep without repeating child
+ * cancellation.
  *
  * Children live in a host-side registry (callId → run) as soon as the provider
  * accepts them, so cancellation reaches even a pre-publication attempt. Both
  * explicit run cancellation and the shared request signal are driven when the
  * workflow is cancelled OR normally settles, so a fire-and-forget child cannot
- * survive merely by honoring only one channel. The host observes `result`
+ * survive merely by honoring only one channel. A per-call gate invokes each
+ * explicit provider `cancel()` at most once even though host fanout and the
+ * worker's later relay can both request it. The host observes `result`
  * immediately but acknowledges the child to the worker only after `started`
  * fulfills; readiness failure is a start error and the host disposes the
  * attempt because the worker never received a handle. The
@@ -31,10 +43,12 @@
  * paths share ONE disposal per child (memoized by callId; the seam's
  * dispose() is idempotent anyway, the memo keeps the bookkeeping and the
  * containment warn single). Lifecycle pairing is host-guaranteed the same
- * way: every forwarded `agent-start` lives in a ledger, and a start the
- * dead or terminated worker never paired is closed by a synthesized
- * `agent-end` (outcome `'cancelled'`) before the run settles. On a
- * termination path `agentsStarted` reports the
+ * way: every forwarded `agent-start` lives in a ledger, and a start the dead
+ * or terminated worker never paired is closed exactly once by a synthesized
+ * `agent-end` (outcome `'cancelled'`). When death or grace is the terminal
+ * source, already-known pairs close before the run settles; cleanup after an
+ * earlier Result can close a survivor afterward. On a termination path
+ * `agentsStarted` reports the
  * HOST-observed count (accepted `child-start` messages) — `agent()` calls
  * still queued worker-side for a concurrency slot are unknowable then; the
  * worker's own count rides the result message on every graceful path.
@@ -132,6 +146,10 @@ export class WorkerRun implements WorkflowRun {
   readonly result: Promise<WorkflowResult>
   private settleResolve!: (result: WorkflowResult) => void
   private settled = false
+  /** A Result/death/grace outcome atomically won before teardown callbacks. */
+  private terminalClaimed = false
+  /** The first death signal closes worker-message admission and owns failure-time cleanup. */
+  private workerDeathObserved = false
   private cancelReason: string | undefined
   private graceTimer: NodeJS.Timeout | undefined
   private readonly worker: Worker
@@ -143,6 +161,8 @@ export class WorkerRun implements WorkflowRun {
   private readonly children = new Map<number, SubagentRun>()
   /** In-flight child disposals by callId — the memo that gives every path (worker RPC, dispose(), reap) ONE shared disposal per child. */
   private readonly childDisposals = new Map<number, Promise<void>>()
+  /** callIds whose explicit provider cancel callback has already been invoked. */
+  private readonly childCancellations = new Set<number>()
   /** Started-but-not-ended agents by seq — the pairing ledger the HOST guarantees (see {@link endAgent}). */
   private readonly liveAgents = new Map<number, WorkflowAgentInfo>()
   private readonly quiescenceWaiters: (() => void)[] = []
@@ -172,12 +192,12 @@ export class WorkerRun implements WorkflowRun {
     const { entry, options } = resolveWorkerSpawn(init)
     this.worker = new Worker(entry, options)
     this.worker.on('message', (message: WorkerToHostMessage) => { this.onMessage(message) })
-    this.worker.on('error', (error) => { this.onWorkerDeath(`workflow worker failed: ${renderThrown(error)}`) })
+    this.worker.on('error', (error) => { this.onWorkerDeath(`workflow worker failed: ${renderThrown(error)}`, false) })
     /* v8 ignore next -- messageerror: not constructible from the engine's own protocol (every payload is JSON data) */
-    this.worker.on('messageerror', (error) => { this.onWorkerDeath(`workflow worker message failed to deserialize: ${renderThrown(error)}`) })
+    this.worker.on('messageerror', (error) => { this.onWorkerDeath(`workflow worker message failed to deserialize: ${renderThrown(error)}`, false) })
     this.worker.on('exit', (code) => {
       this.workerGone = true
-      this.onWorkerDeath(`workflow worker exited before the run settled (exit code ${code})`)
+      this.onWorkerDeath(`workflow worker exited before the run settled (exit code ${code})`, true)
     })
     if (signal?.aborted) {
       this.cancel('workflow start signal already aborted')
@@ -205,18 +225,24 @@ export class WorkerRun implements WorkflowRun {
    * @param reason - human-readable cause (default `'workflow cancelled'`).
    */
   cancel(reason?: string): void {
-    // A settled run has nothing left to cancel: without this guard the
+    // A settled run has nothing left to cancel, and a terminal source claimed
+    // before its cleanup callbacks must exclude cancellation reentered by one
+    // of those callbacks. Without the settled guard the
     // ordinary consumer path (await result, then dispose -> cancel) would arm
     // a grace timer nothing ever clears, pinning the run and its Worker
     // closure until the grace expires - a bounded leak per completed run.
-    if (this.settled || this.cancelReason !== undefined) return
+    if (this.settled || this.terminalClaimed || this.cancelReason !== undefined) return
     this.cancelReason = reason ?? 'workflow cancelled'
     this.post(HostToWorkerType.Cancel, { reason: this.cancelReason })
     // The explicit channel is driven host-side, not left to the worker: a
     // provider honoring only run.cancel() must not wait on a wedged worker's
-    // ChildCancel relay (those later RPCs land as idempotent no-ops).
+    // ChildCancel relay (the per-call cancellation gate makes those later
+    // RPCs no-ops without imposing idempotence on the provider).
     this.cancelChildren(this.cancelReason)
     this.graceTimer = setTimeout(() => {
+      // Cancellation already owns the race through cancelReason; close the
+      // terminal boundary explicitly before observer teardown callbacks.
+      this.terminalClaimed = true
       // The worker may no longer speak (it is about to be terminated): pair
       // every stranded start before the run settles, so ends precede
       // workflow/end.
@@ -244,7 +270,13 @@ export class WorkerRun implements WorkflowRun {
    * @returns resolves when the run's resources are released or abandoned.
    */
   dispose(): Promise<void> {
-    this.disposed ??= (async () => {
+    if (this.disposed !== undefined) return this.disposed
+    // Claim the public transaction BEFORE its body invokes child/provider
+    // disposal. A raw provider callback can reenter handle.dispose(); it must
+    // join this promise rather than start a second traversal.
+    const claimed = Promise.withResolvers<undefined>()
+    this.disposed = claimed.promise
+    void (async () => {
       this.detachInputSignal()
       this.cancel('workflow disposed')
       for (const [callId, run] of [...this.children]) void this.disposeChild(callId, run)
@@ -257,13 +289,17 @@ export class WorkerRun implements WorkflowRun {
       ])
       await this.worker.terminate()
       this.reapChildren('workflow disposed')
-    })()
+    })().then(
+      () => { claimed.resolve(undefined) },
+      /* v8 ignore next -- result/quiescence never reject and Worker.terminate is the only external promise */
+      (error: unknown) => { claimed.reject(error) },
+    )
     return this.disposed
   }
 
   /** Post one message to the worker (payload looked up from the tag's map entry), tolerating a thread that is already gone. */
   private post<T extends HostToWorkerType>(type: T, payload: HostToWorkerPayloads[T]): void {
-    if (this.workerGone) return
+    if (this.workerGone || this.workerDeathObserved) return
     try {
       this.worker.postMessage({ type, ...payload })
     } catch (error: unknown) {
@@ -276,6 +312,11 @@ export class WorkerRun implements WorkflowRun {
   }
 
   private onMessage(message: WorkerToHostMessage): void {
+    // Node may emit `error`, then deliver an already-queued `message`, then
+    // emit `exit`. The first death signal is the host's logical delivery
+    // barrier: nothing arriving afterward may create a child, narrate after
+    // workflow/end, or compete with the chosen outcome.
+    if (this.workerDeathObserved) return
     switch (message.type) {
       case WorkerToHostType.Ready:
         this.post(HostToWorkerType.Go, {})
@@ -308,7 +349,7 @@ export class WorkerRun implements WorkflowRun {
       case WorkerToHostType.ChildCancel:
         {
           const run = this.children.get(message.callId)
-          if (run !== undefined) this.cancelChild(run, message.reason)
+          if (run !== undefined) this.cancelChild(message.callId, run, message.reason)
         }
         break
       case WorkerToHostType.ChildDispose:
@@ -323,12 +364,27 @@ export class WorkerRun implements WorkflowRun {
     }
   }
 
-  private onChildStart(callId: number, request: ChildStartRequest): void {
+  /** Why a child may no longer cross the provider readiness boundary. */
+  private childAdmissionFailure(): { reason: string; rendered: string } | undefined {
     if (this.cancelReason !== undefined) {
-      // The worker's start raced our cancel: refuse — a child must never
-      // start on an already-aborted signal (a provider subscribing only to
-      // future abort events would never observe it).
-      this.post(HostToWorkerType.ChildStartError, { callId, rendered: `workflow run cancelled: ${this.cancelReason}` })
+      return { reason: this.cancelReason, rendered: `workflow run cancelled: ${this.cancelReason}` }
+    }
+    if (this.workerDeathObserved) {
+      return { reason: 'workflow worker gone', rendered: 'workflow worker is no longer available' }
+    }
+    if (this.terminalClaimed) {
+      return { reason: 'workflow settled', rendered: 'workflow run already settled' }
+    }
+    return undefined
+  }
+
+  private onChildStart(callId: number, request: ChildStartRequest): void {
+    const initialFailure = this.childAdmissionFailure()
+    if (initialFailure !== undefined) {
+      // Refuse after a terminal boundary: a child must never start on an
+      // already-aborted signal (a provider subscribing only to future abort
+      // events would never observe it).
+      this.post(HostToWorkerType.ChildStartError, { callId, rendered: initialFailure.rendered })
       return
     }
     this.hostStarted += 1
@@ -382,22 +438,54 @@ export class WorkerRun implements WorkflowRun {
       },
     )
 
-    // The provider owns the publication boundary. Only acknowledge the child
-    // after it is real, then flush any result that settled unusually early. A
-    // readiness rejection is a START failure, not AGENT_RESULT: the worker
-    // never receives a handle, so the host must also dispose the registered
-    // attempt. A concurrent host disposal may already have removed it; the
-    // identity guard preserves the one-disposal memo in that race.
+    // The provider owns the publication boundary. Observe both promises before
+    // invoking cancellation/disposal below: provider.start() itself is
+    // arbitrary code and may have reentered handle.cancel() before the returned
+    // run reached our registry. Exactly one branch answers this ChildStart.
+    let startReplySent = false
+    const refusePublication = (failure: { reason: string; rendered: string }): void => {
+      startReplySent = true
+      this.post(HostToWorkerType.ChildStartError, { callId, rendered: failure.rendered })
+      // A prior dispose/death can finish and remove this run while readiness
+      // is still pending. In that case teardown already owned cancellation and
+      // disposal; touching the retired callId would repeat cancel and orphan a
+      // fresh gate entry after finishChild deleted it.
+      if (this.children.get(callId) !== run) return
+      this.cancelChild(callId, run, failure.reason)
+      void this.disposeChild(callId, run)
+    }
+
+    // Only acknowledge the child after it is real, then flush any result that
+    // settled unusually early. Re-check admission at that exact boundary: a
+    // cancellation while readiness was pending is a refusal, not a late
+    // publication into a terminal workflow. A readiness rejection is a START
+    // failure, not AGENT_RESULT; the worker never receives a handle, so the
+    // host disposes the registered attempt. Identity guards preserve the one
+    // disposal memo against concurrent host teardown.
     void run.started.then(
       () => {
+        if (startReplySent) return
+        const failure = this.childAdmissionFailure()
+        if (failure !== undefined) {
+          refusePublication(failure)
+          return
+        }
+        startReplySent = true
         this.post(HostToWorkerType.ChildStarted, { callId, childId })
         void forwardResult.then((forward) => { forward() })
       },
       (error: unknown) => {
+        if (startReplySent) return
+        startReplySent = true
         this.post(HostToWorkerType.ChildStartError, { callId, rendered: renderThrown(error) })
         if (this.children.get(callId) === run) void this.disposeChild(callId, run)
       },
     )
+
+    // Close the synchronous hole around provider.start(): cancel()/dispose()
+    // can run before the returned run is visible to their children loop.
+    const reentrantFailure = this.childAdmissionFailure()
+    if (reentrantFailure !== undefined) refusePublication(reentrantFailure)
   }
 
   private onChildDispose(callId: number): void {
@@ -427,17 +515,26 @@ export class WorkerRun implements WorkflowRun {
   private disposeChild(callId: number, run: SubagentRun): Promise<void> {
     let disposal = this.childDisposals.get(callId)
     if (disposal === undefined) {
+      // Claim before run.dispose() invokes provider code. Reentrant holder
+      // disposal then joins this exact child transaction instead of entering
+      // the provider wrapper twice before either memo is installed.
+      const claimed = Promise.withResolvers<undefined>()
+      disposal = claimed.promise
+      this.childDisposals.set(callId, disposal)
       // The seam promises a Promise, but invoke inside an async boundary so a
       // contract-violating synchronous throw is contained exactly like a
       // rejected disposal and cannot break host quiescence.
-      disposal = (async () => { await run.dispose() })().then(
-        () => { this.finishChild(callId) },
+      void (async () => { await run.dispose() })().then(
+        () => {
+          this.finishChild(callId)
+          claimed.resolve(undefined)
+        },
         (error: unknown) => {
           this.ctx.logger.warn(`workflow-workerthread: child dispose failed: ${renderThrown(error)}`)
           this.finishChild(callId)
+          claimed.resolve(undefined)
         },
       )
-      this.childDisposals.set(callId, disposal)
     }
     return disposal
   }
@@ -446,6 +543,7 @@ export class WorkerRun implements WorkflowRun {
   private finishChild(callId: number): void {
     this.children.delete(callId)
     this.childDisposals.delete(callId)
+    this.childCancellations.delete(callId)
     if (this.children.size === 0) {
       for (const waiter of this.quiescenceWaiters.splice(0)) waiter()
     }
@@ -469,11 +567,16 @@ export class WorkerRun implements WorkflowRun {
   /** Drive both cancellation channels for every child already accepted by the host. */
   private cancelChildren(reason: string): void {
     this.controller.abort(reason)
-    for (const run of this.children.values()) this.cancelChild(run, reason)
+    for (const [callId, run] of this.children) this.cancelChild(callId, run, reason)
   }
 
-  /** Contain one provider-owned cancel callback so every peer still receives cancellation. */
-  private cancelChild(run: SubagentRun, reason?: string): void {
+  /** Invoke one provider-owned cancel callback at most once and contain its exception. */
+  private cancelChild(callId: number, run: SubagentRun, reason?: string): void {
+    // Host fanout and the worker's FIFO-later ChildCancel relay are two paths
+    // to the same provider callback. The seam does not require cancel() to be
+    // idempotent, so claim the callId before invoking arbitrary provider code.
+    if (this.childCancellations.has(callId)) return
+    this.childCancellations.add(callId)
     try {
       run.cancel(reason)
     } catch (error: unknown) {
@@ -482,12 +585,30 @@ export class WorkerRun implements WorkflowRun {
   }
 
   private onResult(result: WorkflowResult): void {
+    // The owned worker session sends one Result. Keep a late duplicate or a
+    // Result queued behind another terminal source completely side-effect-free.
+    if (this.terminalClaimed) return
+    // First-wins is decided when the Result message reaches the host. If no
+    // external cancellation was already in flight, this result won. Reaping a
+    // stray child below may synchronously reenter cancel() through provider
+    // callbacks, but that internal post-result cleanup must not retroactively
+    // rewrite the worker result that arrived first.
+    const cancellationWasRequested = this.cancelReason !== undefined
+    // Claim before either settlement-cleanup cancellation channel invokes
+    // provider code. A provider callback can reenter cancel() synchronously or
+    // from a queued microtask; once Result won, that losing cancellation must
+    // have no state, message, child-fanout, or grace-timer side effects.
+    this.terminalClaimed = true
     // The worker cancels handles it already received, but a fire-and-forget
     // child may still be waiting on readiness and therefore have no worker
     // handle. Drive BOTH provider-permitted channels from the host before the
     // workflow becomes externally settled.
-    if (this.cancelReason === undefined) this.cancelChildren('workflow settled')
-    if (this.cancelReason !== undefined && result.stopReason !== 'cancelled') {
+    if (!cancellationWasRequested) {
+      this.cancelChildren('workflow settled')
+      this.settleResult(result)
+      return
+    }
+    if (result.stopReason !== 'cancelled') {
       // The script settled while our cancel was crossing the thread boundary
       // — the seam-visible result had NOT settled when cancellation was
       // requested, so report cancelled (the vm drive()'s post-settle check,
@@ -498,21 +619,39 @@ export class WorkerRun implements WorkflowRun {
     this.settleResult(result)
   }
 
-  /** An unexpected worker death (or the expected exit after termination). */
-  private onWorkerDeath(message: string): void {
-    // Whatever the worker left behind must not leak — abort + dispose it all.
-    if (this.children.size > 0) this.reapChildren('workflow worker gone')
-    // The thread is gone: no more worker-authored agent-ends can arrive —
-    // pair every stranded start (a start that crossed between the grace
-    // force-settle and this exit included) before the run settles.
-    this.endStrandedAgents()
-    // settleResult no-ops on an already-settled run (the expected exit after
-    // a dispose's terminate lands here too).
-    if (this.cancelReason !== undefined) {
-      this.settleResult(this.cancelledResult(this.hostStarted))
-      return
+  /** Process an error/messageerror/exit signal; `exit` also performs the final disposal sweep. */
+  private onWorkerDeath(message: string, isExit: boolean): void {
+    if (!this.workerDeathObserved) {
+      // Close message admission BEFORE cleanup callbacks: Node can deliver a
+      // message queued before the crash after its `error` event. Treating the
+      // first death signal as a logical barrier prevents that late message
+      // from creating work or narrating after workflow/end.
+      this.workerDeathObserved = true
+      const outcomeWasClaimed = this.terminalClaimed
+      const cancellationWasRequested = this.cancelReason !== undefined
+      // When death is itself the terminal source, claim BEFORE child reap or
+      // synthesized observer callbacks. Either can reenter cancel(); a death
+      // that arrived first remains an error, while a cancellation already
+      // accepted before death remains cancelled. If Result/grace already won,
+      // preserve it while still performing prompt failure-time cleanup.
+      if (!outcomeWasClaimed) this.terminalClaimed = true
+      if (this.children.size > 0) this.reapChildren('workflow worker gone')
+      this.endStrandedAgents()
+      if (!outcomeWasClaimed) {
+        if (cancellationWasRequested) {
+          this.settleResult(this.cancelledResult(this.hostStarted))
+        } else {
+          this.settleResult({ value: null, stopReason: 'error', error: message, agentsStarted: this.hostStarted })
+        }
+      }
     }
-    this.settleResult({ value: null, stopReason: 'error', error: message, agentsStarted: this.hostStarted })
+    if (!isExit) return
+    // `error` is not Node's physical delivery barrier: a queued message may
+    // precede `exit`. Admission is already closed, so this final sweep only
+    // joins/starts disposal for registry survivors; it deliberately does not
+    // repeat explicit provider cancellation.
+    for (const [callId, run] of [...this.children]) void this.disposeChild(callId, run)
+    this.endStrandedAgents()
   }
 
   /**
@@ -531,11 +670,14 @@ export class WorkerRun implements WorkflowRun {
   /**
    * Synthesize the missing `agent-end` for every started-but-unpaired agent,
    * outcome `'cancelled'`: the reap cancels every child, and a real
-   * settlement racing the force-settle loses to the cancellation — the same
-   * first-wins override {@link onResult} applies to the run's own result.
+   * settlement racing the force-settle loses to that already-started external
+   * cancellation. The atomic terminal boundaries in {@link onResult} and
+   * {@link onWorkerDeath} deliberately exclude teardown callbacks as contenders.
    * Called where the worker can no longer speak (the grace force-settle,
-   * worker death), BEFORE settleResult, so the paired ends reach observers
-   * before `workflow/end`.
+   * worker death, physical exit). When grace/death is the terminal source it
+   * runs before settleResult, so already-known pairs precede `workflow/end`;
+   * after an earlier Result, exit cleanup may close a survivor afterward.
+   * The ledger preserves exactly-once pairing in both orders.
    */
   private endStrandedAgents(): void {
     for (const info of [...this.liveAgents.values()]) {
@@ -563,7 +705,11 @@ export class WorkerRun implements WorkflowRun {
 
   /** First settle wins; disarms the grace timer and releases the caller signal. */
   private settleResult(result: WorkflowResult): void {
+    // Every current terminal source claims ownership before calling here; keep
+    // the fallback local so a future caller cannot resolve twice.
+    /* v8 ignore next -- defensive fallback outside the claimed state machine */
     if (this.settled) return
+    this.terminalClaimed = true
     this.settled = true
     this.detachInputSignal()
     clearTimeout(this.graceTimer)
diff --git a/packages/workflow/workflow-workerthread/src/runtime.ts b/packages/workflow/workflow-workerthread/src/runtime.ts
index 2add6d537c..9b324c1118 100644
--- a/packages/workflow/workflow-workerthread/src/runtime.ts
+++ b/packages/workflow/workflow-workerthread/src/runtime.ts
@@ -83,7 +83,9 @@ function defaultLabel(prompt: string): string {
 /**
  * One live script execution inside the worker. Constructed per run by the
  * session; `drive()` is called exactly once and NEVER rejects — every failure
- * becomes a {@link WorkflowResult} with a non-`completed` stop reason.
+ * becomes a {@link WorkflowResult} with a non-`completed` stop reason. After
+ * the session publishes that result it calls {@link reapAfterResult} exactly
+ * once to cancel any dropped child work without racing terminal publication.
  */
 export class WorkflowExecution {
   /** 1-based count of `agent()` calls started (the `agentsStarted` result field). */
@@ -171,7 +173,8 @@ export class WorkflowExecution {
    * worker. Idempotent; the first reason wins.
    * @param reason - human-readable cause, carried on the CANCELLED error and
    * into child cancel RPCs. Required: every caller (the session's cancel
-   * message, drive()'s settle-reap) has a concrete reason.
+   * message and its post-result {@link reapAfterResult} call) has a concrete
+   * reason.
    */
   cancel(reason: string): void {
     if (this.cancelReason !== undefined) return
@@ -185,8 +188,9 @@ export class WorkflowExecution {
    * Run the script to settlement. Resolves — never rejects — with the run's
    * {@link WorkflowResult}: the materialized return value on `completed`, the
    * failure message on `error`, and `cancelled` when the script died of
-   * cancellation. After settlement, any stray children a script fired without
-   * awaiting are cancelled (their `agent()` wrappers dispose them via RPC).
+   * cancellation. This method only chooses the result; the session must publish
+   * it and then call {@link reapAfterResult}, so the terminal message precedes
+   * settlement-only child cancellation on the worker-to-host FIFO channel.
    * @returns the settled outcome — this promise NEVER rejects (the seam's
    * `result`-never-rejects contract); every failure maps to a variant.
    */
@@ -214,15 +218,19 @@ export class WorkflowExecution {
       // cannot throw — drive() resolving is the `result` never-rejects seam
       // contract.
       return { value: null, stopReason: 'error', error: renderThrown(error), agentsStarted: this.started }
-    } finally {
-      // Reap strays: a script that fired agent() calls without awaiting them
-      // leaves live children behind after settlement — cancel them all. (The
-      // per-call wrappers dispose each child; the contain() consumer keeps
-      // their rejections from going unhandled.)
-      if (this.cancelReason === undefined) this.cancel('workflow settled')
     }
   }
 
+  /**
+   * Reap strays only after the caller publishes the chosen terminal result.
+   * Aborting the controller synchronously sends child-cancel RPCs, so calling
+   * this before publication would let a provider callback reenter host
+   * cancellation and misclassify a result the script had already chosen.
+   */
+  reapAfterResult(): void {
+    if (this.cancelReason === undefined) this.cancel('workflow settled')
+  }
+
   /**
    * Attach a no-op rejection consumer WITHOUT changing what the caller
    * receives: if the script drops the promise (no await), cancellation cannot
diff --git a/packages/workflow/workflow-workerthread/src/session.ts b/packages/workflow/workflow-workerthread/src/session.ts
index b159892021..f6b14f0fca 100644
--- a/packages/workflow/workflow-workerthread/src/session.ts
+++ b/packages/workflow/workflow-workerthread/src/session.ts
@@ -14,6 +14,11 @@
  * A `cancel` arriving instead of `go` still releases the gate: `drive()`
  * sees the cancelled state and settles without running the body.
  *
+ * Terminal ordering is Result first, settlement cleanup second. The session
+ * queues the Result message before asking the execution to reap stray children;
+ * MessagePort FIFO therefore lets the host atomically claim the result before a
+ * cleanup ChildCancel can invoke arbitrary provider code.
+ *
  * @module @deepseek-ai/dsh-workflow-workerthread/session
  */
 
@@ -208,5 +213,12 @@ export async function runWorkerSession(port: MessagePort, init: WorkerInit): Pro
   post(WorkerToHostType.Ready, {})
   await gate.promise
   const result = await execution.drive()
-  post(WorkerToHostType.Result, { result })
+  try {
+    // This post is the worker's terminal claim. Queue it BEFORE aborting stray
+    // children: MessagePort FIFO then guarantees the host claims Result before
+    // any settlement-only ChildCancel can invoke arbitrary provider callbacks.
+    post(WorkerToHostType.Result, { result })
+  } finally {
+    execution.reapAfterResult()
+  }
 }
diff --git a/packages/workflow/workflow-workerthread/tests/session.spec.ts b/packages/workflow/workflow-workerthread/tests/session.spec.ts
index 00051ad56a..e2dab8948f 100644
--- a/packages/workflow/workflow-workerthread/tests/session.spec.ts
+++ b/packages/workflow/workflow-workerthread/tests/session.spec.ts
@@ -281,6 +281,36 @@ describe('runWorkerSession over an in-process MessageChannel', () => {
     }
   })
 
+  it('queues Result before settlement-only cancellation of a ready stray', async () => {
+    const host = fakeHost({ manual: true })
+    const session = runWorkerSession(host.port, init(`
+      agent('ready stray')
+      return await agent('gate')
+    `))
+    await vi.waitFor(() => { expect(host.ofType(WorkerToHostType.ChildStart)).toHaveLength(2) })
+    const starts = host.ofType(WorkerToHostType.ChildStart)
+    const stray = starts.find(message => message.request.prompt === 'ready stray')!
+    const gate = starts.find(message => message.request.prompt === 'gate')!
+    host.send({ type: HostToWorkerType.ChildStarted, callId: stray.callId, childId: 'stray-child' })
+    host.send({ type: HostToWorkerType.ChildStarted, callId: gate.callId, childId: 'gate-child' })
+    host.send({ type: HostToWorkerType.ChildSettled, callId: gate.callId, result: text('gate completed') })
+
+    const result = await host.result()
+    await session
+    await vi.waitFor(() => {
+      expect(host.ofType(WorkerToHostType.ChildCancel).map(message => message.callId)).toContain(stray.callId)
+    })
+
+    expect(result).toMatchObject({ value: 'gate completed', stopReason: 'completed', agentsStarted: 2 })
+    const resultIndex = host.messages.findIndex(message => message.type === WorkerToHostType.Result)
+    const strayCancelIndex = host.messages.findIndex(message =>
+      message.type === WorkerToHostType.ChildCancel && message.callId === stray.callId)
+    expect(resultIndex).toBeGreaterThanOrEqual(0)
+    expect(strayCancelIndex).toBeGreaterThan(resultIndex)
+    host.send({ type: HostToWorkerType.ChildSettled, callId: stray.callId, result: { output: [], stopReason: 'aborted' } })
+    host.close()
+  })
+
   it('an unparseable body settles an error result instead of dying without one (host pre-parse skew guard)', async () => {
     const host = fakeHost()
     await runWorkerSession(host.port, init('return ((('))
diff --git a/packages/workflow/workflow-workerthread/tests/workflow-workerthread.spec.ts b/packages/workflow/workflow-workerthread/tests/workflow-workerthread.spec.ts
index 641b26518d..9e37e0bbed 100644
--- a/packages/workflow/workflow-workerthread/tests/workflow-workerthread.spec.ts
+++ b/packages/workflow/workflow-workerthread/tests/workflow-workerthread.spec.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it, vi } from 'vitest'
 import { fileURLToPath } from 'node:url'
+import type { Worker } from 'node:worker_threads'
 import { Context } from 'cordis'
 import Loader from '@cordisjs/plugin-loader'
 import { AgentId } from '@deepseek-ai/dsh-agent'
@@ -8,7 +9,7 @@ import SubagentService from '@deepseek-ai/dsh-subagent'
 import type { SubagentCapabilities, SubagentProvider, SubagentResult, SubagentRun, SubagentStartRequest } from '@deepseek-ai/dsh-subagent'
 import type { WorkflowMeta, WorkflowResult, WorkflowResultInfo, WorkflowRunInfo } from '@deepseek-ai/dsh-workflow'
 import * as workerEngineModule from '../src/index.ts'
-import WorkerWorkflowEngine, { HostToWorkerType, type Config } from '../src/index.ts'
+import WorkerWorkflowEngine, { HostToWorkerType, WorkerToHostType, type Config } from '../src/index.ts'
 
 /** A minimal parent stand-in: the engine only threads it through to the provider. */
 function fakeParent(): Agent {
@@ -74,6 +75,8 @@ class StubProvider implements SubagentProvider {
     private readonly reply?: (request: SubagentStartRequest, index: number) => SubagentResult,
     private readonly disposeDelayMs = 0,
     private readonly deferStart = false,
+    private readonly onCancel?: (reason: string | undefined, index: number) => void,
+    private readonly onSignalAbort?: (reason: unknown, index: number) => void,
   ) {}
 
   start(request: SubagentStartRequest): SubagentRun {
@@ -91,7 +94,10 @@ class StubProvider implements SubagentProvider {
     }
     this.runs.push(controlled)
     const index = this.runs.length - 1
-    request.signal?.addEventListener('abort', () => { terminal.resolve({ output: [], stopReason: 'aborted' }) }, { once: true })
+    request.signal?.addEventListener('abort', () => {
+      this.onSignalAbort?.(request.signal?.reason, index)
+      terminal.resolve({ output: [], stopReason: 'aborted' })
+    }, { once: true })
     if (!this.deferStart) readiness.resolve(undefined)
     if (this.reply) {
       const reply = this.reply
@@ -103,6 +109,7 @@ class StubProvider implements SubagentProvider {
       result: terminal.promise,
       cancel: (reason?: string) => {
         controlled.cancelled = reason ?? 'cancelled'
+        this.onCancel?.(reason, index)
         terminal.resolve({ output: [], stopReason: 'aborted' })
       },
       dispose: () => {
@@ -133,6 +140,8 @@ interface SetupOptions {
   manual?: boolean
   disposeDelayMs?: number
   deferStart?: boolean
+  onChildCancel?: (reason: string | undefined, index: number) => void
+  onChildSignalAbort?: (reason: unknown, index: number) => void
 }
 
 async function setup(options?: SetupOptions) {
@@ -143,6 +152,8 @@ async function setup(options?: SetupOptions) {
     options?.manual ? undefined : options?.reply ?? (() => text('stub reply')),
     options?.disposeDelayMs ?? 0,
     options?.deferStart ?? false,
+    options?.onChildCancel,
+    options?.onChildSignalAbort,
   )
   ctx.subagents.registerProvider(provider)
   // A fixed concurrency ceiling: the auto-resolved default is machine-derived
@@ -821,6 +832,153 @@ describe('dsh-workflow-workerthread', () => {
       expect(provider.runs[0]!.disposeCalls).toBe(1)
     })
 
+    it('post-result child cleanup cannot reentrantly rewrite a completed workflow as cancelled', async () => {
+      let cancelCallbacks = 0
+      let signalCallbacks = 0
+      const { ctx, parent, provider } = await setup({
+        manual: true,
+        deferStart: true,
+        onChildCancel: () => {
+          cancelCallbacks += 1
+          // The first callback is host cleanup for the already-arrived Result.
+          // Reentering cancel() here is later than that message and must not
+          // retroactively win the result race. Its nested child cancel is
+          // intentionally ignored to keep the adversarial callback finite.
+          if (cancelCallbacks === 1) handle.cancel('reentrant child cleanup')
+        },
+        onChildSignalAbort: () => {
+          signalCallbacks += 1
+          handle.cancel('reentrant signal cleanup')
+        },
+      })
+      const handle = ctx.workflows.start({
+        ...scripted(`
+          agent('readiness-pending stray')
+          return 'completed first'
+        `),
+        parent,
+      })
+
+      const result = await handle.result
+
+      expect(result).toMatchObject({ value: 'completed first', stopReason: 'completed', agentsStarted: 1 })
+      expect(signalCallbacks).toBe(1)
+      expect(cancelCallbacks).toBe(1)
+      // Readiness crossing after Result is a terminal-admission refusal: no
+      // ChildStarted/lifecycle publication, and host-owned disposal begins.
+      provider.runs[0]!.publish()
+      await waitFor(() => { expect(provider.runs[0]!.disposed).toBe(true) }, 1000)
+      expect(cancelCallbacks).toBe(1)
+      await handle.dispose()
+      await ctx.fiber.dispose()
+    })
+
+    it('late readiness after completed disposal cannot cancel or dispose the retired child twice', async () => {
+      let explicitCancels = 0
+      const lifecycle: string[] = []
+      const { ctx, parent, provider } = await setup({
+        manual: true,
+        deferStart: true,
+        onChildCancel: () => { explicitCancels += 1 },
+      })
+      ctx.on('workflow/agent-start', () => { lifecycle.push('start') })
+      ctx.on('workflow/agent-end', () => { lifecycle.push('end') })
+      const handle = ctx.workflows.start({
+        ...scripted("agent('retired readiness')\nreturn 'done'"),
+        parent,
+      })
+
+      await expect(handle.result).resolves.toMatchObject({ stopReason: 'completed' })
+      expect(explicitCancels).toBe(1)
+      await handle.dispose()
+      expect(provider.runs[0]!.disposed).toBe(true)
+      expect(provider.runs[0]!.disposeCalls).toBe(1)
+
+      // The Promise may still fulfill after its run left every host ledger.
+      // Refusal replies once but must not recreate the deleted cancel gate.
+      provider.runs[0]!.publish()
+      await Promise.resolve()
+      await Promise.resolve()
+      expect(explicitCancels).toBe(1)
+      expect(provider.runs[0]!.disposeCalls).toBe(1)
+      expect(lifecycle).toEqual([])
+      await ctx.fiber.dispose()
+    })
+
+    it.each([
+      ['synchronous', (cancel: () => void) => { cancel() }],
+      ['microtask', (cancel: () => void) => { queueMicrotask(cancel) }],
+    ])('a ready stray %s cleanup callback cannot beat the earlier worker result claim', async (_mode, reenter) => {
+      let reentered = false
+      const explicitCancels = new Map<number, number>()
+      const { ctx, parent, provider } = await setup({
+        manual: true,
+        onChildCancel: (_reason, index) => {
+          explicitCancels.set(index, (explicitCancels.get(index) ?? 0) + 1)
+          if (index !== 0 || reentered) return
+          reentered = true
+          reenter(() => { handle.cancel('reentered from child cleanup') })
+        },
+      })
+      const handle = ctx.workflows.start({
+        ...scripted(`
+          agent('ready stray')
+          return await agent('gate')
+        `),
+        parent,
+      })
+      const cancelChildSpy = vi.spyOn(handle as unknown as {
+        cancelChild(callId: number, run: SubagentRun, reason?: string): void
+      }, 'cancelChild')
+      await waitFor(() => { expect(provider.runs).toHaveLength(2) })
+      provider.runs[1]!.settle(text('gate completed'))
+
+      const result = await handle.result
+      await Promise.resolve()
+
+      expect(result).toMatchObject({ value: 'gate completed', stopReason: 'completed', agentsStarted: 2 })
+      expect(reentered).toBe(true)
+      // The host claim and worker's FIFO-later ChildCancel both reach the
+      // routing gate, but the provider callback is not an idempotent seam:
+      // invoke it exactly once for this callId.
+      await waitFor(() => {
+        expect(cancelChildSpy.mock.calls.filter(([callId]) => callId === 1)).toHaveLength(2)
+      }, 1000)
+      expect(explicitCancels.get(0)).toBe(1)
+      cancelChildSpy.mockRestore()
+      await handle.dispose()
+      await ctx.fiber.dispose()
+    })
+
+    it('a duplicate Result after the terminal claim cannot repeat cleanup or rewrite the outcome', async () => {
+      let explicitCancels = 0
+      const { ctx, parent, provider } = await setup({
+        manual: true,
+        onChildCancel: (_reason, index) => { if (index === 0) explicitCancels += 1 },
+      })
+      const handle = ctx.workflows.start({
+        ...scripted("agent('stray')\nawait new Promise(() => {})"),
+        parent,
+      })
+      await waitFor(() => { expect(provider.runs).toHaveLength(1) })
+      const worker = (handle as unknown as { worker: Worker }).worker
+
+      worker.emit('message', {
+        type: WorkerToHostType.Result,
+        result: { value: 'first', stopReason: 'completed', agentsStarted: 1 },
+      })
+      worker.emit('message', {
+        type: WorkerToHostType.Result,
+        result: { value: 'late', stopReason: 'completed', agentsStarted: 1 },
+      })
+
+      await expect(handle.result).resolves.toMatchObject({ value: 'first', stopReason: 'completed' })
+      expect(explicitCancels).toBe(1)
+      await handle.dispose()
+      expect(explicitCancels).toBe(1)
+      await ctx.fiber.dispose()
+    })
+
     it('contains a throwing child cancel and still settles after cancelling peer strays', async () => {
       const ctx = new Context()
       await ctx.plugin(SubagentService)
@@ -918,6 +1076,198 @@ describe('dsh-workflow-workerthread', () => {
       await handle.dispose()
     }, 15_000)
 
+    it.each(['fulfills', 'rejects'] as const)('provider.start() reentrant cancellation refuses the run when readiness later %s', async (readinessOutcome) => {
+      const ctx = new Context()
+      await ctx.plugin(SubagentService)
+      const readiness = Promise.withResolvers<undefined>()
+      let starts = 0
+      let explicitCancels = 0
+      let disposals = 0
+      let sawAbortedSignal = false
+      const lifecycle: string[] = []
+      const provider: SubagentProvider = {
+        name: 'start-reentry',
+        capabilities: { outputSchema: true, depthLimit: true, toolFilter: true, persona: false },
+        inheritsParentContext: false,
+        start: (request) => {
+          starts += 1
+          // This arbitrary provider callback runs before onChildStart can put
+          // the returned run in its registry. Cancellation must be rechecked
+          // after return instead of trusting the pre-start admission check.
+          handle.cancel('provider start reentered cancellation')
+          sawAbortedSignal = request.signal?.aborted === true
+          return {
+            id: AgentId('start-reentry-child'),
+            started: readiness.promise,
+            result: new Promise(() => { /* refusal owns teardown */ }),
+            // Deliberately honors only the explicit channel. It must still be
+            // reached promptly even though the first host fanout saw no run.
+            cancel: () => { explicitCancels += 1 },
+            dispose: () => {
+              disposals += 1
+              return Promise.resolve()
+            },
+          }
+        },
+      }
+      ctx.subagents.registerProvider(provider)
+      await ctx.plugin(WorkerWorkflowEngine, {
+        provider: 'start-reentry',
+        maxConcurrentAgents: 2,
+        disposeGraceMs: 30_000,
+      })
+      ctx.on('workflow/agent-start', () => { lifecycle.push('start') })
+      ctx.on('workflow/agent-end', () => { lifecycle.push('end') })
+      const handle = ctx.workflows.start({
+        ...scripted("await agent('reentrant provider')\nreturn 'unreachable'"),
+        parent: fakeParent(),
+      })
+
+      await waitFor(() => { expect(starts).toBe(1) })
+      // Either later readiness settlement must not answer the already-refused
+      // start again or emit a workflow lifecycle pair.
+      if (readinessOutcome === 'fulfills') readiness.resolve(undefined)
+      else readiness.reject(new Error('late readiness rejection after refusal'))
+      let result: WorkflowResult | undefined
+      void handle.result.then((value) => { result = value })
+      await waitFor(() => {
+        expect(explicitCancels).toBe(1)
+        expect(disposals).toBe(1)
+        expect(result?.stopReason).toBe('cancelled')
+      }, 1000)
+      expect(sawAbortedSignal).toBe(true)
+      expect(lifecycle).toEqual([])
+      await handle.dispose()
+      expect(explicitCancels).toBe(1)
+      expect(disposals).toBe(1)
+      await ctx.fiber.dispose()
+    })
+
+    it('claims workflow and child disposal before a raw provider disposer reenters handle.dispose()', async () => {
+      const ctx = new Context()
+      await ctx.plugin(SubagentService)
+      const terminal = Promise.withResolvers<SubagentResult>()
+      const observed: { reentrant?: Promise<void> } = {}
+      let starts = 0
+      let rawDisposeCalls = 0
+      const provider: SubagentProvider = {
+        name: 'dispose-reentry',
+        capabilities: { outputSchema: true, depthLimit: true, toolFilter: true, persona: false },
+        inheritsParentContext: false,
+        start: () => {
+          starts += 1
+          return {
+            id: AgentId('dispose-reentry-child'),
+            started: Promise.resolve(),
+            result: terminal.promise,
+            cancel: () => { terminal.resolve({ output: [], stopReason: 'aborted' }) },
+            dispose: () => {
+              rawDisposeCalls += 1
+              observed.reentrant = handle.dispose()
+              return Promise.resolve()
+            },
+          }
+        },
+      }
+      ctx.subagents.registerProvider(provider)
+      await ctx.plugin(WorkerWorkflowEngine, { provider: 'dispose-reentry', maxConcurrentAgents: 2 })
+      const handle = ctx.workflows.start({
+        ...scripted("await agent('live child')\nreturn 'unreachable'"),
+        parent: fakeParent(),
+      })
+      await waitFor(() => { expect(starts).toBe(1) })
+
+      const disposal = handle.dispose()
+
+      expect(observed.reentrant).toBe(disposal)
+      await disposal
+      expect(rawDisposeCalls).toBe(1)
+      await expect(handle.result).resolves.toMatchObject({ stopReason: 'cancelled' })
+      await ctx.fiber.dispose()
+    })
+
+    it('claims worker-originated child disposal before its raw disposer reenters holder disposal', async () => {
+      const ctx = new Context()
+      await ctx.plugin(SubagentService)
+      const terminal = Promise.withResolvers<SubagentResult>()
+      const observed: { reentrant?: Promise<void> } = {}
+      let starts = 0
+      let rawDisposeCalls = 0
+      const provider: SubagentProvider = {
+        name: 'child-dispose-reentry',
+        capabilities: { outputSchema: true, depthLimit: true, toolFilter: true, persona: false },
+        inheritsParentContext: false,
+        start: () => {
+          starts += 1
+          return {
+            id: AgentId('child-dispose-reentry-child'),
+            started: Promise.resolve(),
+            result: terminal.promise,
+            cancel: () => { terminal.resolve({ output: [], stopReason: 'aborted' }) },
+            dispose: () => {
+              rawDisposeCalls += 1
+              // This begins holder disposal from the worker's ChildDispose
+              // callback, before any public handle.dispose() call exists.
+              observed.reentrant = handle.dispose()
+              return Promise.resolve()
+            },
+          }
+        },
+      }
+      ctx.subagents.registerProvider(provider)
+      await ctx.plugin(WorkerWorkflowEngine, { provider: 'child-dispose-reentry', maxConcurrentAgents: 2 })
+      const handle = ctx.workflows.start({
+        ...scripted("return await agent('settling child')"),
+        parent: fakeParent(),
+      })
+      const finishChildSpy = vi.spyOn(handle as unknown as {
+        finishChild(callId: number): void
+      }, 'finishChild')
+      await waitFor(() => { expect(starts).toBe(1) })
+
+      terminal.resolve({ output: [{ type: 'text', text: 'done' }], stopReason: 'completed' })
+
+      await waitFor(() => { expect(observed.reentrant).toBeDefined() }, 1000)
+      await observed.reentrant
+      expect(rawDisposeCalls).toBe(1)
+      expect(finishChildSpy.mock.calls.filter(([callId]) => callId === 1)).toHaveLength(1)
+      finishChildSpy.mockRestore()
+      await expect(handle.result).resolves.toMatchObject({ stopReason: 'cancelled' })
+      await ctx.fiber.dispose()
+    })
+
+    it('a grace-terminated worker reaps its child on exit without waiting for consumer dispose()', async () => {
+      const { ctx, parent, provider } = await setup({
+        manual: true,
+        config: { provider: 'stub', maxConcurrentAgents: 2, disposeGraceMs: 100 },
+      })
+      const handle = ctx.workflows.start({
+        // Let child-start cross, then make the worker unable to process its
+        // Cancel message. Grace settles the result and terminates the thread;
+        // that exit must independently own the host registry's disposal pass.
+        ...scripted(`
+          agent('survives until exit reap')
+          for (let i = 0; i < 20; i++) await null
+          const end = Date.now() + 1500
+          while (Date.now() < end) {}
+          return 'unreachable'
+        `),
+        parent,
+      })
+      await waitFor(() => { expect(provider.runs).toHaveLength(1) })
+
+      handle.cancel('force termination')
+      const result = await handle.result
+
+      expect(result.stopReason).toBe('cancelled')
+      // Deliberately assert before handle.dispose(): host-owned worker exit,
+      // not consumer courtesy, is responsible for this resource guarantee.
+      await waitFor(() => { expect(provider.runs[0]!.disposed).toBe(true) }, 1000)
+      expect(provider.runs[0]!.disposeCalls).toBe(1)
+      await handle.dispose()
+      await ctx.fiber.dispose()
+    }, 15_000)
+
     it('dispose() on a wedged worker host-drives child disposal inside the grace: it returns with the children DISPOSED, not with their teardown still in flight', async () => {
       const { ctx, parent, provider } = await setup({
         manual: true,
@@ -1045,23 +1395,71 @@ describe('dsh-workflow-workerthread', () => {
   })
 
   describe('worker death', () => {
+    it('the first death signal closes admission to messages Node delivers before exit', async () => {
+      const { ctx, parent, provider } = await setup({ manual: true })
+      const phases: string[] = []
+      ctx.on('workflow/phase', (_info, title) => { phases.push(title) })
+      const handle = ctx.workflows.start({
+        ...scripted('await new Promise(() => {})'),
+        parent,
+      })
+      const worker = (handle as unknown as { worker: Worker }).worker
+
+      // Node may physically emit error -> queued message -> exit. Reproduce
+      // that ordering deterministically at the Worker event boundary: the
+      // late protocol data must not create work, narrate, or rewrite error.
+      worker.emit('error', new Error('synthetic error-before-message'))
+      worker.emit('message', { type: WorkerToHostType.Phase, title: 'late phase' })
+      worker.emit('message', {
+        type: WorkerToHostType.ChildStart,
+        callId: 999,
+        request: { prompt: 'late child' },
+      })
+      worker.emit('message', {
+        type: WorkerToHostType.Result,
+        result: { value: 'late', stopReason: 'completed', agentsStarted: 1 },
+      })
+
+      const result = await handle.result
+      expect(result.stopReason).toBe('error')
+      expect(result.error).toContain('synthetic error-before-message')
+      expect(provider.runs).toHaveLength(0)
+      expect(phases).toEqual([])
+      await handle.dispose()
+      await ctx.fiber.dispose()
+    })
+
     it('a worker that exits before settling reports an error result and reaps its children', async () => {
       const ctx = new Context()
       await ctx.plugin(SubagentService)
       // The child's dispose() REJECTS on top of the worker death: the reap
       // must contain it (warn, not crash) while still emptying the registry.
       const cancelled: string[] = []
+      const signalAborts: unknown[] = []
       const provider: SubagentProvider = {
         name: 'doomed',
         capabilities: { outputSchema: true, depthLimit: true, toolFilter: true, persona: false },
         inheritsParentContext: false,
-        start: () => ({
-          id: AgentId('doomed-child'),
-          started: Promise.resolve(),
-          result: new Promise(() => { /* never settles; the reap is the teardown */ }),
-          cancel: (reason?: string) => { cancelled.push(reason ?? 'cancelled') },
-          dispose: () => Promise.reject(new Error('dispose exploded during reap')),
-        }),
+        start: (request) => {
+          request.signal?.addEventListener('abort', () => {
+            signalAborts.push(request.signal?.reason)
+            // The death claim precedes the shared-signal fanout. This
+            // synchronous callback cannot turn death into cancellation.
+            handle.cancel('reentered from worker-death signal cleanup')
+          }, { once: true })
+          return {
+            id: AgentId('doomed-child'),
+            started: Promise.resolve(),
+            result: new Promise(() => { /* never settles; the reap is the teardown */ }),
+            cancel: (reason?: string) => {
+              cancelled.push(reason ?? 'cancelled')
+              // Exercise the later microtask case too: terminal ownership
+              // remains closed after the death callback returns.
+              queueMicrotask(() => { handle.cancel('reentered from worker-death child cleanup') })
+            },
+            dispose: () => Promise.reject(new Error('dispose exploded during reap')),
+          }
+        },
       }
       ctx.subagents.registerProvider(provider)
       await ctx.plugin(WorkerWorkflowEngine, { provider: 'doomed', maxConcurrentAgents: 2 })
@@ -1089,7 +1487,12 @@ describe('dsh-workflow-workerthread', () => {
       expect(runEnds).toEqual([{ stopReason: 'error', error: result.error, agentsStarted: 1 }])
       // Result already settled — this is the reap's promptness, not a
       // cold-start race; tight explicit bound (see the helper's doc comment).
-      await waitFor(() => { expect(cancelled.length).toBe(1) }, 1000)
+      await waitFor(() => {
+        expect(signalAborts).toEqual(['workflow worker gone'])
+        expect(cancelled).toEqual(['workflow worker gone'])
+      }, 1000)
+      await Promise.resolve()
+      expect(result.stopReason).toBe('error')
       await handle.dispose()
     }, 15_000)