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.

.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-2026-07-25-subagent-policy-inheritance.md: bc5eb5b17ce34872db50b4cf848f6a1784ede0fc
-2026-07-25-subagent-policy-inheritance.zh.md: 3498b1b01987c44156b922fe0dcdd6223495fed6
+2026-07-25-subagent-policy-inheritance.md: 42196f9b23b9bf31a1f441e9db900493f08e8094
+2026-07-25-subagent-policy-inheritance.zh.md: 3ad2bd3dde913850c0b7726a8b650d4f7f75ceda

.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md

@@ -12,8 +12,8 @@ Session policy overrides are per-session log folds: the effective sandbox mode i
 
 The shared in-process driver (`startInProcessRun` in `packages/subagent/subagent-inprocess`) snapshots the parent's policy overrides at delegation and stamps them onto the child as ordinary log events inside the child's FIRST turn:
 
-- **Read at creation, write at first `agent/prompt-submit`.** The driver installs a one-shot child-scoped `agent/prompt-submit` listener during the creation transaction's setup window. Prompt-submit runs after `turn/start` and before prompt assembly, so the stamped events are turn-enclosed (durable — a bare between-turn event is crash-tail garbage on reload) and visible to the child's very first request (an inherited `'never'` reaches the child's first system prompt). This is the same anchoring the ACP bridge uses for idle preset switches.
-- **Only the override chain is copied, through the canonical write paths.** `SandboxPolicyService.inheritOverride(parent, child)` and `ApprovalService.inheritOverride(parent, child)` each fold the parent's FULL live log (not the fork seed), append via `setSandboxMode`/`setApprovalPolicy` only when the parent has an override the child does not already fold to, and never copy the deployment default — an unswitched parent stamps nothing, so a resumed child keeps following the LIVE default. The driver consumes both services opportunistically (`ctx.get`, type-only imports): compositions without them delegate policy-free, unchanged.
+- **Capture synchronously at delegation, stamp at first `agent/prompt-submit`.** The driver reads `overrideOf(parent.session)` for both knobs 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, not the child — and installs a one-shot child-scoped `agent/prompt-submit` listener during the creation transaction's setup window, PREPENDED so a veto-capable listener (a denying UserPromptSubmit hook) cannot close the first turn without the stamp. Prompt-submit runs after `turn/start` and before prompt assembly, so the stamped events are turn-enclosed (durable — a bare between-turn event is crash-tail garbage on reload) and visible to the child's very first request (an inherited `'never'` reaches the child's first system prompt). This is the same anchoring the ACP bridge uses for idle preset switches.
+- **Only the override chain is copied, through the canonical write paths.** `overrideOf(session)` is the fold alone — never the deployment/configured default — so an unswitched parent stamps nothing and a resumed child keeps following the LIVE default; `stampOverride(child, value)` appends via `setSandboxMode`/`setApprovalPolicy` unless the child already folds to the value. The driver consumes both services opportunistically (`ctx.get`, type-only imports): compositions without them delegate policy-free, unchanged.
 - **Fork stale-seed precedence falls out of log order.** The stamped event lands after any switch the seed carried, so the existing last-event-wins fold resolves the child's mode with no new precedence machinery; an equal seed-carried override is deduplicated instead of re-stamped.
 - **Nesting composes by construction.** A grandchild's stamp folds its parent-the-child's log, which already contains the child's stamped (or self-switched) override — the chain collapses one level per delegation, at any depth. One-shot `allowed-once` escalation grants never enter any log, so they can never leak down the chain.
 
@@ -31,7 +31,7 @@ A confined child that hits the wall gets the ordinary denial marker; an escalati
 
 ## Consequences
 
-- A parent's tightened sandbox mode and `'never'` approval stance now bind spawn children, fork children (regardless of seed timing), and grandchildren; the delegation bypass is closed at every depth. Pinned by the real-wall suite in `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` (a scripted-model child hitting the real `dsh-fs-sandbox` fence through the real `write` tool, asserted on disk state and denial markers) and the `inheritOverride` contract tests in the two service suites.
+- A parent's tightened sandbox mode and `'never'` approval stance now bind spawn children, fork children (regardless of seed timing), and grandchildren; the delegation bypass is closed at every depth. Pinned by the real-wall suite in `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` (a scripted-model child hitting the real `dsh-fs-sandbox` fence through the real `write` tool, asserted on disk state and denial markers — including the delegation-vs-late-switch race and a veto-capable prompt-submit listener) and the `overrideOf`/`stampOverride` contract tests in the two service suites.
 - The stamped override is the child's own durable record: resume replays it like any switch, and the child may later be switched independently without the driver re-stamping over it (one-shot listener + fold dedup).
 - Accepted limits: a parent switch made while a child is already running does not propagate (snapshot semantics); a child hard-killed before its first `turn/end` loses the stamp on resume (worthless-resume corner, recorded above); out-of-process backends (`subagent-acp`, subprocess children) inherit nothing here — their policy belongs to the child harness's own deployment, the sandbox Agent Note's deferred phase.
 - `dsh-subagent-inprocess` now declares `dsh-sandbox-policy` and `dsh-user-approval` as peers for the `ctx.get` typing; both remain runtime-optional.

.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.zh.md

@@ -12,8 +12,8 @@ Status: implemented
 
 共享的进程内驱动器（`packages/subagent/subagent-inprocess` 中的 `startInProcessRun`）在委派时快照父级的策略覆盖项，并在子 agent 的第一个轮次内把它们作为普通日志事件盖章写入子会话：
 
-- **创建时读取，首个 `agent/prompt-submit` 时写入。**驱动器在创建事务的 setup 窗口内安装一个一次性的、限定子 agent 作用域的 `agent/prompt-submit` 监听器。prompt-submit 阶段在 `turn/start` 之后、提示词组装之前运行，因此盖章事件被包围在轮次内（具备持久性：轮次之间的裸事件在重新加载时只是崩溃残留的尾部垃圾），并且对子 agent 的第一次请求可见（继承来的 `'never'` 能进入子 agent 的第一份系统提示词）。ACP（Agent Client Protocol）桥接器处理空闲时预设切换所用的正是同一种锚定方式。
-- **只复制覆盖链，且全部走规范写入路径。**`SandboxPolicyService.inheritOverride(parent, child)` 与 `ApprovalService.inheritOverride(parent, child)` 各自折叠父级的完整实时日志（而非 fork 种子），只在父级持有子 agent 尚未折叠出的覆盖项时才通过 `setSandboxMode`/`setApprovalPolicy` 追加，并且从不复制部署默认值：未切换过的父级不盖任何章，因此恢复后的子 agent 继续跟随实时默认值。驱动器以可选方式消费这两个服务（`ctx.get`，仅类型导入）：未挂载它们的组合照旧进行无策略委派，行为不变。
+- **委派时同步捕获，首个 `agent/prompt-submit` 时盖章。**驱动器在自己的第一个 await 之前就为两个策略旋钮读取 `overrideOf(parent.session)`——委派时刻即快照点，因此与异步的子 agent 创建过程赛跑的父级切换属于父级的未来，而非子 agent——并在创建事务的 setup 窗口内安装一个一次性的、限定子 agent 作用域的 `agent/prompt-submit` 监听器，且采用前置安装，使得具备否决能力的监听器（会作出拒绝的 UserPromptSubmit 钩子）无法在未盖章的情况下结束第一个轮次。prompt-submit 阶段在 `turn/start` 之后、提示词组装之前运行，因此盖章事件被包围在轮次内（具备持久性：轮次之间的裸事件在重新加载时只是崩溃残留的尾部垃圾），并且对子 agent 的第一次请求可见（继承来的 `'never'` 能进入子 agent 的第一份系统提示词）。ACP（Agent Client Protocol）桥接器处理空闲时预设切换所用的正是同一种锚定方式。
+- **只复制覆盖链，且全部走规范写入路径。**`overrideOf(session)` 只是折叠本身——从不包含部署／配置默认值——因此未切换过的父级不盖任何章，恢复后的子 agent 继续跟随实时默认值；`stampOverride(child, value)` 通过 `setSandboxMode`/`setApprovalPolicy` 追加，除非子 agent 已折叠出该值。驱动器以可选方式消费这两个服务（`ctx.get`，仅类型导入）：未挂载它们的组合照旧进行无策略委派，行为不变。
 - **fork 陈旧种子的优先级由日志顺序自然得出。**盖章事件落在种子携带的任何切换之后，因此既有的「最后一个事件生效」折叠即可解析出子 agent 的模式，无需新增优先级机制；种子已携带相同覆盖项时会去重，而不会重复盖章。
 - **嵌套按构造即可组合。**孙代 agent 盖章时折叠的是其父级（即上一层的子 agent）的日志，而该日志已经包含这个子 agent 被盖章（或自行切换）的覆盖项：这条链在每层委派处收拢一级，任意深度均成立。一次性的 `allowed-once` 升级授权从不进入任何日志，因此永远不可能沿链向下泄漏。
 
@@ -31,7 +31,7 @@ Status: implemented
 
 ## 后果
 
-- 父级收紧后的沙箱模式与 `'never'` 审批立场现在会约束 spawn 子 agent、fork 子 agent（无论种子时机如何）与孙代 agent；委派旁路在每一层深度都已封死。该行为由 `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` 中的真实围栏测试套件钉住（脚本化模型驱动的子 agent 通过真实 `write` 工具撞上真实的 `dsh-fs-sandbox` 围栏，按落盘状态与拒绝标记断言），并由两个服务各自测试套件中的 `inheritOverride` 契约测试钉住。
+- 父级收紧后的沙箱模式与 `'never'` 审批立场现在会约束 spawn 子 agent、fork 子 agent（无论种子时机如何）与孙代 agent；委派旁路在每一层深度都已封死。该行为由 `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` 中的真实围栏测试套件钉住（脚本化模型驱动的子 agent 通过真实 `write` 工具撞上真实的 `dsh-fs-sandbox` 围栏，按落盘状态与拒绝标记断言——其中包括委派与延迟切换之间的竞态用例，以及一个具备否决能力的 prompt-submit 监听器用例），并由两个服务各自测试套件中的 `overrideOf`/`stampOverride` 契约测试钉住。
 - 盖章写入的覆盖项是子 agent 自己的持久记录：恢复时它像任何一次切换一样被回放；子 agent 之后仍可被独立切换，驱动器不会重新盖章覆盖它（一次性监听器加折叠去重）。
 - 已接受的限制：子 agent 已在运行时父级再做的切换不会传播（快照语义）；子 agent 在第一个 `turn/end` 前被强制杀死后，恢复时会丢失盖章（恢复无价值的边角场景，上文已记录）；进程外后端（`subagent-acp`、子进程形态的子 agent）在这里不继承任何内容：它们的策略归子 harness 自身的部署所有，属于沙箱 Agent Note 中延后的阶段。
 - `dsh-subagent-inprocess` 现在将 `dsh-sandbox-policy` 与 `dsh-user-approval` 声明为对等依赖（peer dependency），以支撑 `ctx.get` 的类型；两者在运行时仍然可选。

packages/sandbox/sandbox-policy/README.md

@@ -17,7 +17,7 @@ Two families enforce the same mode vocabulary: the sandboxed bash executor (`@de
 - `ctx.sandboxPolicy.defaultMode` / `ctx.sandboxPolicy.workspaceRoot` — the deployment default and fallback root used by `resolve()`.
 - `effectiveSandboxMode(events)` — the pure fold of a session's `sandbox/mode` events (the last switch wins, or `undefined`), used inside `resolve()`.
 - `setSandboxMode(session, mode)` — THE write path for a per-session override: appends exactly one `sandbox/mode` event. The switch IS its event; nothing mutates the mode out of band.
-- `ctx.sandboxPolicy.inheritOverride(parent, child)` — the delegation-inheritance step: stamps the parent session's effective override (never the deployment default) onto a child session through `setSandboxMode`, skipping a child that already folds to it. The in-process subagent driver calls it inside the child's first turn so a delegating parent's tightened mode binds its children ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+- `ctx.sandboxPolicy.overrideOf(session)` / `ctx.sandboxPolicy.stampOverride(child, mode)` — the two halves of delegation inheritance: the fold alone (never the deployment default), and the write of a captured override through `setSandboxMode`, skipping a child that already folds to it. The in-process subagent driver captures at delegation and stamps inside the child's first turn so a delegating parent's tightened mode binds its children ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
 - `SANDBOX_MODES` — every mode, for option advertisement and runtime validation.
 
 The optional `./invariant` companion rejects a forged durable `sandbox/mode` event whose value falls outside that closed vocabulary; Session and its companion own the surrounding storage and turn-enclosure rules.

packages/sandbox/sandbox-policy/src/index.ts

@@ -106,21 +106,32 @@ export class SandboxPolicyService extends Service {
   }
 
   /**
-   * Stamp the parent's sandbox-mode OVERRIDE onto a child session through the
-   * canonical write path — the delegation-inheritance step: a child agent runs
+   * A session's sandbox-mode OVERRIDE — the fold alone, never the deployment
+   * default. The read half of delegation inheritance: the subagent driver
+   * captures this synchronously at delegation, so a parent switch racing the
+   * child's asynchronous creation belongs to the parent's future, not to the
+   * child ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+   * @param session - the session whose override chain to fold.
+   * @returns the last switched mode, or `undefined` for a never-switched session.
+   */
+  overrideOf(session: Session): SandboxMode | undefined {
+    return effectiveSandboxMode(session.events)
+  }
+
+  /**
+   * Stamp a captured override onto a child session through the canonical
+   * write path — the write half of delegation inheritance: a child agent runs
    * under the policy its delegating parent was switched to, not under the
-   * (possibly wider) deployment default. Only the override chain is copied: an
-   * unswitched parent stamps nothing, so the child keeps following the LIVE
-   * deployment default. A child whose log (e.g. a fork seed) already folds to
-   * the inherited mode is left untouched. Callers must append inside an open
-   * child turn — a bare between-turn event is crash-tail garbage on reload.
-   * @param parent - the delegating session whose effective override is read.
+   * (possibly wider) deployment default. A child whose log (e.g. a fork seed)
+   * already folds to the mode is left untouched. Callers must append inside
+   * an open child turn — a bare between-turn event is crash-tail garbage on
+   * reload.
    * @param child - the child session the override is appended to.
+   * @param mode - the captured {@link overrideOf} value to stamp.
    */
-  inheritOverride(parent: Session, child: Session): void {
-    const inherited = effectiveSandboxMode(parent.events)
-    if (inherited === undefined || effectiveSandboxMode(child.events) === inherited) return
-    setSandboxMode(child, inherited)
+  stampOverride(child: Session, mode: SandboxMode): void {
+    if (effectiveSandboxMode(child.events) === mode) return
+    setSandboxMode(child, mode)
   }
 }
 

packages/sandbox/sandbox-policy/tests/policy.spec.ts

@@ -143,45 +143,40 @@ describe('the sandbox/mode session kit', () => {
   })
 })
 
-describe('inheritOverride (parent → child stamping)', () => {
+describe('delegation inheritance (overrideOf + stampOverride)', () => {
   const modeEvents = (session: Session) => session.events.filter(e => e.type === 'sandbox/mode')
 
-  it('stamps the parent LAST override onto the child through the canonical write path', async () => {
-    const ctx = await mounted()
+  it('overrideOf folds to the LAST override and never falls back to the deployment default', async () => {
+    const ctx = await mounted({ mode: 'workspace-write' })
     const parent = session('sess-inherit-parent')
-    const child = session('sess-inherit-child')
     setSandboxMode(parent, 'workspace-write')
     setSandboxMode(parent, 'read-only')
 
-    ctx.sandboxPolicy.inheritOverride(parent, child)
-
-    const stamped = modeEvents(child)
-    expect(stamped).toHaveLength(1)
-    expect(stamped[0]?.data).toEqual({ mode: 'read-only' })
+    expect(ctx.sandboxPolicy.overrideOf(parent)).toBe('read-only')
+    // undefined, NOT the deployment default — a child stamped with the
+    // default would stop following the LIVE default across resumes.
+    expect(ctx.sandboxPolicy.overrideOf(session('sess-inherit-unswitched'))).toBeUndefined()
   })
 
-  it('appends NOTHING when the parent never switched (the deployment default must stay live)', async () => {
-    const ctx = await mounted({ mode: 'workspace-write' })
-    const parent = session('sess-inherit-default-parent')
-    const child = session('sess-inherit-default-child')
+  it('stampOverride appends the captured mode through the canonical write path', async () => {
+    const ctx = await mounted()
+    const child = session('sess-inherit-child')
 
-    ctx.sandboxPolicy.inheritOverride(parent, child)
+    ctx.sandboxPolicy.stampOverride(child, 'read-only')
 
-    // No event — a resumed child keeps following whatever the deployment
-    // default is THEN, instead of a frozen copy of today's default.
-    expect(child.events).toHaveLength(0)
+    const stamped = modeEvents(child)
+    expect(stamped).toHaveLength(1)
+    expect(stamped[0]?.data).toEqual({ mode: 'read-only' })
   })
 
-  it('skips the append when the child already folds to the inherited mode (fork-seed dedup)', async () => {
+  it('stampOverride skips a child already folding to the mode (fork-seed dedup)', async () => {
     const ctx = await mounted()
-    const parent = session('sess-inherit-dedup-parent')
     const child = session('sess-inherit-dedup-child')
-    setSandboxMode(parent, 'read-only')
     // A fork seed can already carry the parent's switch; stamping again would
     // append a redundant event on every delegation.
     setSandboxMode(child, 'read-only')
 
-    ctx.sandboxPolicy.inheritOverride(parent, child)
+    ctx.sandboxPolicy.stampOverride(child, 'read-only')
 
     expect(modeEvents(child)).toHaveLength(1)
   })

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 = []

packages/ui/user-approval/README.md

@@ -6,7 +6,7 @@ Each request must belong to an open agent turn. The service appends a paired `ap
 
 Answerers are `approval/request` waterfall listeners. Return an outcome to answer for an owned agent or call `next()` to delegate. Agent-scoped listeners receive only that agent's requests; compose one terminal answerer per deployment because sibling listener order is not a policy priority mechanism. The ACP bridge is the shipped human answerer.
 
-`ApprovalPolicy` is `'ask'` or `'never'`. The effective value is the last `approval/policy` event, falling back to config; `setApprovalPolicy()` is the write path. `'never'` rejects before interactive dispatch and is the only policy stated in the prompt. Switches produce at most one coalesced notice, attributed to the user when the override follows the last `request/header` and to operator/config otherwise. `ctx.approval.inheritOverride(parent, child)` stamps a parent session's override (never the configured default) onto a child session through that write path — the in-process subagent driver calls it inside the child's first turn so a `'never'` parent cannot mint prompting children ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+`ApprovalPolicy` is `'ask'` or `'never'`. The effective value is the last `approval/policy` event, falling back to config; `setApprovalPolicy()` is the write path. `'never'` rejects before interactive dispatch and is the only policy stated in the prompt. Switches produce at most one coalesced notice, attributed to the user when the override follows the last `request/header` and to operator/config otherwise. `ctx.approval.overrideOf(session)` / `ctx.approval.stampOverride(child, policy)` are the two halves of delegation inheritance — the fold alone (never the configured default), and the write of a captured override through that write path; the in-process subagent driver captures at delegation and stamps inside the child's first turn so a `'never'` parent cannot mint prompting children ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
 
 The tools pipeline routes `ask` decisions through this seam and fails closed when it is absent; the sandboxed bash tool also uses it for escalated retries. The ACP bridge is the shipped human answerer for calls it owns. Audit events remain log-only, so the model sees only the asking consumer's result. See the [approval-seam Agent Note](../../../.agents/notes/implemented/feature/2026-07-06-approval-seam.md) and [sandbox Agent Note](../../../.agents/notes/implemented/feature/2026-07-06-sandbox.md).
 

packages/ui/user-approval/src/index.ts

@@ -327,21 +327,31 @@ export class ApprovalService extends Service {
   }
 
   /**
-   * Stamp the parent's approval-policy OVERRIDE onto a child session through
-   * the canonical write path — the delegation-inheritance step: a `'never'`
+   * A session's approval-policy OVERRIDE — the fold alone, never the
+   * configured default. The read half of delegation inheritance: the subagent
+   * driver captures this synchronously at delegation, so a parent switch
+   * racing the child's asynchronous creation belongs to the parent's future,
+   * not to the child ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+   * @param session - the session whose override chain to fold.
+   * @returns the last switched policy, or `undefined` for a never-switched session.
+   */
+  overrideOf(session: Session): ApprovalPolicy | undefined {
+    return effectiveApprovalPolicy(session.events)
+  }
+
+  /**
+   * Stamp a captured override onto a child session through the canonical
+   * write path — the write half of delegation inheritance: a `'never'`
    * (headless/CI) parent must not mint children that fall back to a prompting
-   * default. Only the override chain is copied: an unswitched parent stamps
-   * nothing, so the child keeps following the LIVE configured default. A
-   * child whose log (e.g. a fork seed) already folds to the inherited policy
+   * default. A child whose log (e.g. a fork seed) already folds to the policy
    * is left untouched. Callers must append inside an open child turn — a bare
    * between-turn event is crash-tail garbage on reload.
-   * @param parent - the delegating session whose effective override is read.
    * @param child - the child session the override is appended to.
+   * @param policy - the captured {@link overrideOf} value to stamp.
    */
-  inheritOverride(parent: Session, child: Session): void {
-    const inherited = effectiveApprovalPolicy(parent.events)
-    if (inherited === undefined || effectiveApprovalPolicy(child.events) === inherited) return
-    setApprovalPolicy(child, inherited)
+  stampOverride(child: Session, policy: ApprovalPolicy): void {
+    if (effectiveApprovalPolicy(child.events) === policy) return
+    setApprovalPolicy(child, policy)
   }
 
   /**

packages/ui/user-approval/tests/approval.spec.ts

@@ -577,44 +577,39 @@ describe('approval policy (the approval/policy fold)', () => {
   })
 })
 
-describe('inheritOverride (parent → child stamping)', () => {
+describe('delegation inheritance (overrideOf + stampOverride)', () => {
   const policyEvents = (session: Session) => session.events.filter(e => e.type === 'approval/policy')
 
   function bareSession(id: string): Session {
     return new Session(SessionId(id))
   }
 
-  it('stamps the parent LAST override onto the child through the canonical write path', async () => {
+  it('overrideOf folds to the LAST override and never falls back to the configured default', async () => {
     const ctx = await mounted()
     const parent = bareSession('sess-appr-inherit-parent')
-    const child = bareSession('sess-appr-inherit-child')
     setApprovalPolicy(parent, 'never')
 
-    ctx.approval.inheritOverride(parent, child)
-
-    const stamped = policyEvents(child)
-    expect(stamped).toHaveLength(1)
-    expect(stamped[0]?.data).toEqual({ policy: 'never' })
+    expect(ctx.approval.overrideOf(parent)).toBe('never')
+    expect(ctx.approval.overrideOf(bareSession('sess-appr-unswitched'))).toBeUndefined()
   })
 
-  it('appends NOTHING when the parent never switched (the configured default must stay live)', async () => {
+  it('stampOverride appends the captured policy through the canonical write path', async () => {
     const ctx = await mounted()
-    const parent = bareSession('sess-appr-default-parent')
-    const child = bareSession('sess-appr-default-child')
+    const child = bareSession('sess-appr-inherit-child')
 
-    ctx.approval.inheritOverride(parent, child)
+    ctx.approval.stampOverride(child, 'never')
 
-    expect(child.events).toHaveLength(0)
+    const stamped = policyEvents(child)
+    expect(stamped).toHaveLength(1)
+    expect(stamped[0]?.data).toEqual({ policy: 'never' })
   })
 
-  it('skips the append when the child already folds to the inherited policy (fork-seed dedup)', async () => {
+  it('stampOverride skips a child already folding to the policy (fork-seed dedup)', async () => {
     const ctx = await mounted()
-    const parent = bareSession('sess-appr-dedup-parent')
     const child = bareSession('sess-appr-dedup-child')
-    setApprovalPolicy(parent, 'never')
     setApprovalPolicy(child, 'never')
 
-    ctx.approval.inheritOverride(parent, child)
+    ctx.approval.stampOverride(child, 'never')
 
     expect(policyEvents(child)).toHaveLength(1)
   })