subagent: capture overrides at delegation; stamp ahead of prompt vetoes

Review fixes (ds-review-bot on #623):

- Capture-at-delegation: the driver now reads overrideOf(parent.session)
  for both knobs synchronously before its first await, and the prompt-submit
  listener stamps those captured values — a parent switch racing the child's
  asynchronous creation belongs to the parent's future, not the child. The
  inheritOverride(parent, child) service method is split into its two halves
  (overrideOf / stampOverride) accordingly.
- Veto safety: the one-shot prompt-submit listener registers with
  prepend: true, so a veto-capable listener (a denying UserPromptSubmit
  hook) cannot close the child's first turn without the durable stamp.

Both regressions are pinned red-first in inheritance.spec.ts: the
delegation-vs-late-switch race (delegate tool flips the caller wider while
the creation transaction is pending) and a blocking prompt-submit listener
(stamp survives a promptless first turn). Service contract tests renamed to
the split API; READMEs and the bilingual Agent Note updated.

packages/subagent/subagent-inprocess/README.md

@@ -16,7 +16,7 @@ The driver follows this sequence:
 
 The child gets the parent's working-directory/session lineage and inherits the parent model unless `request.agentOptions` overrides it. It gets a fresh flat registration scope: parent ownership does not import parent tool restrictions or establish an authority subset.
 
-The child also inherits the parent's session POLICY overrides: a one-shot `agent/prompt-submit` listener installed during setup stamps the parent's effective `sandbox/mode` and `approval/policy` overrides onto the child through `ctx.sandboxPolicy.inheritOverride` / `ctx.approval.inheritOverride` (both consumed opportunistically — compositions without them delegate policy-free). Anchoring inside the child's first turn keeps the stamp turn-enclosed (durable) and ahead of the first request, and its log position after any fork-seed switch lets the ordinary last-event-wins fold resolve stale-seed timing; only the override chain is copied, so an unswitched parent stamps nothing and the child follows the live deployment default. Nesting composes: each stamp folds the delegating session's already-stamped log ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+The child also inherits the parent's session POLICY overrides. The driver captures `ctx.sandboxPolicy.overrideOf(parent.session)` and `ctx.approval.overrideOf(parent.session)` synchronously before its first await — the delegation moment is the snapshot point, so a parent switch racing the asynchronous child creation belongs to the parent's future — and a one-shot PREPENDED `agent/prompt-submit` listener stamps the captured values through `stampOverride` (both services consumed opportunistically — compositions without them delegate policy-free). Anchoring inside the child's first turn keeps the stamp turn-enclosed (durable) and ahead of the first request; prepending puts it before veto-capable listeners, so a denying UserPromptSubmit hook cannot close the first turn without the stamp; its log position after any fork-seed switch lets the ordinary last-event-wins fold resolve stale-seed timing. Only the override chain is copied, so an unswitched parent stamps nothing and the child follows the live deployment default. Nesting composes: each capture folds the delegating session's already-stamped log ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
 
 ## Cancellation and ownership
 

packages/subagent/subagent-inprocess/src/index.ts

@@ -100,6 +100,17 @@ export async function startInProcessRun(
     subagentDepth: childDepth,
   }
 
+  // Policy inheritance, read half: capture the parent's sandbox/approval
+  // OVERRIDES synchronously, before the first await — the delegation moment
+  // is the semantic snapshot point, and a parent switch racing the child's
+  // asynchronous creation must belong to the parent's future, not the child.
+  // Both services are consumed opportunistically — without them, delegation
+  // stays policy-free.
+  const sandboxPolicy = parent.ctx.get('sandboxPolicy')
+  const approval = parent.ctx.get('approval')
+  const inheritedMode = sandboxPolicy?.overrideOf(parent.session)
+  const inheritedPolicy = approval?.overrideOf(parent.session)
+
   let structured: StructuredAttachment | undefined
   const setup = (childCtx: Context): void => {
     if (request.persona !== undefined) {
@@ -109,20 +120,23 @@ export async function startInProcessRun(
     if (request.outputSchema !== undefined) {
       structured = attachStructuredRuntime(childCtx, request.outputSchema)
     }
-    // Policy inheritance: stamp the parent's sandbox/approval OVERRIDES onto
-    // the child once, anchored inside the child's FIRST turn (prompt-submit
-    // runs after turn/start, before prompt assembly) — a bare between-turn
-    // append would be crash-tail garbage on reload, and stamping here also
-    // orders the override after any stale switch a fork seed carried, so the
-    // ordinary last-event-wins fold resolves it. One-shot: later turns must
-    // not re-stamp over a switch the child made itself. Both services are
-    // consumed opportunistically — without them, delegation stays policy-free.
-    const disposeInherit = childCtx.on('agent/prompt-submit', (childAgent, _content, _source, _signal, next) => {
-      disposeInherit()
-      parent.ctx.get('sandboxPolicy')?.inheritOverride(parent.session, childAgent.session)
-      parent.ctx.get('approval')?.inheritOverride(parent.session, childAgent.session)
-      return next()
-    })
+    // Write half: stamp the captured overrides once, anchored inside the
+    // child's FIRST turn (prompt-submit runs after turn/start, before prompt
+    // assembly) — a bare between-turn append would be crash-tail garbage on
+    // reload, and stamping here also orders the override after any stale
+    // switch a fork seed carried, so the ordinary last-event-wins fold
+    // resolves it. PREPENDED so a veto-capable listener (a denying
+    // UserPromptSubmit hook) cannot close the first turn without the stamp —
+    // the stamp must be durable even for a blocked first prompt. One-shot:
+    // later turns must not re-stamp over a switch the child made itself.
+    if (inheritedMode !== undefined || inheritedPolicy !== undefined) {
+      const disposeInherit = childCtx.on('agent/prompt-submit', (childAgent, _content, _source, _signal, next) => {
+        disposeInherit()
+        if (inheritedMode !== undefined) sandboxPolicy?.stampOverride(childAgent.session, inheritedMode)
+        if (inheritedPolicy !== undefined) approval?.stampOverride(childAgent.session, inheritedPolicy)
+        return next()
+      }, { prepend: true })
+    }
   }
 
   const flags = { cancelled: false }

packages/subagent/subagent-inprocess/tests/inheritance.spec.ts

@@ -102,9 +102,11 @@ async function setupBare(script: Script) {
  * "user switched while idle, model delegates in the very next turn" fork
  * timing constructible (the post-seed switch lives in the still-open turn).
  * `fork: true` seeds the child with the caller's completed-turn prefix,
- * mirroring the fork provider's slice.
+ * mirroring the fork provider's slice. `raceSwitch` flips the CALLER's mode
+ * synchronously after `startInProcessRun`'s synchronous prologue but before
+ * its creation transaction resolves — the delegation-vs-late-switch race.
  */
-function registerDelegate(ctx: Context, captured: Agent[]): void {
+function registerDelegate(ctx: Context, captured: Agent[], raceSwitch?: 'danger-full-access'): void {
   ctx.tools.register(defineTool({
     name: 'delegate',
     description: 'delegate a task to an in-process child (test scaffold)',
@@ -115,10 +117,15 @@ function registerDelegate(ctx: Context, captured: Agent[]): void {
       const events = caller.session.events
       const lastEnd = events.findLast(e => e.type === 'turn/end')
       const seed = lastEnd === undefined ? [] : events.slice(0, lastEnd.seq + 1)
-      const run = await startInProcessRun(
+      const starting = startInProcessRun(
         { prompt: [{ type: 'text', text: 'delegated task' }], parent: caller, signal: exec.signal },
         args.fork === true && seed.length > 0 ? { seed } : {},
       )
+      // The caller's turn is still open, so this switch is legal — and it lands
+      // while the child's creation transaction is pending, strictly before the
+      // child's first prompt-submit could ever run.
+      if (raceSwitch !== undefined) setSandboxMode(caller.session, raceSwitch)
+      const run = await starting
       captured.push(run.localAgent as Agent)
       const result = await run.result
       await run.dispose()
@@ -264,6 +271,38 @@ describe('sandbox-mode inheritance against the real fs fence', () => {
     expect(overrideEvents(child).sandbox).toBe(1)
   })
 
+  it('inherits the mode AT delegation, not a parent switch racing child creation', async () => {
+    const script: Script = []
+    const captured: Agent[] = []
+    const { ctx, parent } = await setupWalled(script)
+    // The delegate scaffold flips the parent to danger-full-access AFTER
+    // startInProcessRun's synchronous prologue, while the child's creation
+    // transaction is still pending — the value at delegation is read-only.
+    registerDelegate(ctx, captured, 'danger-full-access')
+    const blocked = join(workspace, 'race-blocked.txt')
+    script.push(
+      () => {
+        setSandboxMode(parent.session, 'read-only')
+        return textResponse('staged')
+      },
+      toolCallResponse('d-race', 'delegate', { fork: false }),
+      toolCallResponse('c-write', 'write', { file_path: blocked, content: 'escaped' }),
+      textResponse('race child done'),
+      textResponse('turn two done'),
+    )
+    parent.send([{ type: 'text', text: 'stage' }])
+    await parent.whenIdle()
+    parent.send([{ type: 'text', text: 'delegate' }])
+    await parent.whenIdle()
+
+    const child = captured[0] as Agent
+    // The child runs under the snapshot taken at delegation — the racing
+    // wider switch belongs to the parent's own future, not to the child.
+    await expect(readFile(blocked, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' })
+    expect(toolResultTexts(child).join('\n')).toContain(READ_ONLY_DENIAL)
+    expect(ctx.sandboxPolicy.resolve({ session: child.session }).mode).toBe('read-only')
+  })
+
   it('a GRANDCHILD inherits through the chain (child delegates again)', async () => {
     const script: Script = []
     const captured: Agent[] = []
@@ -297,6 +336,43 @@ describe('sandbox-mode inheritance against the real fs fence', () => {
   })
 })
 
+describe('inheritance survives prompt vetoes', () => {
+  it('stamps the child even when an earlier-registered prompt-submit listener vetoes without next()', async () => {
+    const script: Script = []
+    const { ctx, parent } = await setupWalled(script)
+    // A veto-capable listener registered BEFORE the child exists — the
+    // Claude/Codex UserPromptSubmit hook shape: it blocks the child's prompt
+    // and never delegates. Inheritance must still run for the first turn.
+    ctx.on('agent/prompt-submit', (agent, _content, _source, _signal, next) => {
+      if (agent.session.header.parentSession !== undefined) {
+        return Promise.resolve({ kind: 'block' as const, reason: 'vetoed by test hook' })
+      }
+      return next()
+    })
+    script.push(
+      () => {
+        setSandboxMode(parent.session, 'read-only')
+        return textResponse('staged')
+      },
+      // No child model entries: the blocked prompt closes a zero-step turn.
+    )
+    parent.send([{ type: 'text', text: 'stage' }])
+    await parent.whenIdle()
+
+    const run = await startInProcessRun(spawnRequest(parent), {})
+    await run.result
+    const child = run.localAgent as Agent
+
+    // The veto closed the first turn promptless, but the stamp is inside that
+    // turn regardless — a later resume must not fall back to the deployment
+    // default just because the first prompt was blocked.
+    expect(overrideEvents(child)).toEqual({ sandbox: 1, approval: 0 })
+    expect(ctx.sandboxPolicy.resolve({ session: child.session }).mode).toBe('read-only')
+
+    await run.dispose()
+  })
+})
+
 describe('inheritance guards (must hold before AND after the fix)', () => {
   it('a child of an unswitched parent runs under the live deployment default, with ZERO stamped events', async () => {
     const script: Script = []