diff --git a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.i18n.yaml b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.i18n.yaml
index 16ecfcc400..a3dae54b9b 100644
--- a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.i18n.yaml
+++ b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.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 .agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md
-2026-07-16-explicit-turn-cancellation.md: 09631349226837d7f93112fe5e9c517f2fee099c
-2026-07-16-explicit-turn-cancellation.zh.md: baf039d49578895d53ffae4c6b88d29b3d1563b6
+2026-07-16-explicit-turn-cancellation.md: d59accf2dc3f894ffab226e5ddc9d2e746f4d93e
+2026-07-16-explicit-turn-cancellation.zh.md: 8fb0728eb2bbe8fa01dbefebe7e02aa8c836617a
diff --git a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md
index 0963134922..d59accf2dc 100644
--- a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md
+++ b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md
@@ -12,7 +12,7 @@ The [initiating Agent scope decision](2026-07-15-agent-initiator-scope.md) inten
 
 ## Decision
 
-Agent owns the runtime-only `AgentCancelCause` union `{ kind: 'user' } | { kind: 'parent' }`; `agent.cancel()` defaults to `user`. TypeScript enforces that vocabulary at this typed same-process boundary, with no runtime validator, fallback, or special compatibility contract for untyped callers. An active `TurnCancellation` copies the typed discriminant into a fresh frozen signal reason; idle cancellation has no holder to mutate and does not arm later work.
+Agent owns the runtime-only `AgentCancelCause` union `{ kind: 'user' } | { kind: 'parent' } | { kind: 'hook'; reason: string } | { kind: 'disposed' }`, and `agent.cancel()` requires one. TypeScript enforces that vocabulary at this typed same-process boundary, with no runtime validator, fallback, or special compatibility contract for untyped callers. The holder aborts with the caller's own object rather than a detached copy, so a transport reaching that signal can extend it: Node's fetch assigns a `stack` onto the reason it receives. The loop therefore copies the declared fields where the cause becomes durable data. Freezing the cause at `cancel()` is not an alternative — undici performs that assignment from strict-mode code, so a frozen reason makes `fetch` reject with `TypeError: Cannot add property stack` in place of the abort reason. Idle cancellation has no holder to mutate and does not arm later work.
 
 An interrupted live turn ends with the coarse durable `{ kind: 'aborted' }` outcome. The terminal event records what happened to the turn, while the runtime signal identifies who requested cancellation; it does not duplicate `user` or `parent` into replay. Session seed/load rejects legacy aborted records with a reason or any other extra field, so replay cannot reintroduce caller-owned cancellation detail. The process-local `agent/cancel-requested` notification is not durable; a future audit requirement uses a separate durable control-request event so a request and its eventual outcome remain distinct. Durable events contain no stack, signal, error object, free-form cancellation text, or backend-private detail.
 
@@ -22,7 +22,7 @@ The driver keeps only a cause-less pre-run marker for queued work cancelled befo
 
 The explicit event signatures pass a single payload object: agent-scoped events carry `agent` and `signal` in the payload with `next` last, and the remaining APIs keep `signal` immediately before a waterfall's final `next`. `PreStepContext` and `RequestFailureContext` are retired, with their fields folded into the `agent/pre-step` and `agent/request-error` payloads ([payload-object events](../../archived/architecture/2026-08-06-agent-event-payload-objects.md)). Pre-step entry, request configuration, request-error recovery, model generation, tool execution, approval, turn stopping, and subagent or workflow requests all receive the current signal. Hook bridges must also supply `RunHookOptions.signal`, so a turn cancellation reaches the bash executor's provider-managed termination and join boundary; the [native-containment decision](2026-08-28-subprocess-native-containment.md) owns the supported local scope, Job, and fallback mechanics. `SystemPrompt.assemble()` carries `signal?: AbortSignal` in `AssembleContext` because that object is an explicit request value that can also represent signal-less assembly outside a turn. Listeners may cooperate with the signal but must not retain it to control another turn.
 
-`ctx.agents` continues to carry only the initiating Agent. Ambient Agent presence does not imply liveness, a current turn, or cancellation authority. The cause reader is private to the loop and states the machine-private slot invariant (only `cancel()` aborts a turn controller, always with a canonical frozen cause) instead of re-validating the reason structurally; no public helper reads a cause off an arbitrary signal. Concurrent Agents isolate both their initiator identities and their turn signals; a child driver shadows the parent initiator while its parent request signal still travels through the subagent seam.
+`ctx.agents` continues to carry only the initiating Agent. Ambient Agent presence does not imply liveness, a current turn, or cancellation authority. The cause reader is private to the loop and states the machine-private slot invariant (only `cancel()` aborts a turn controller, always with a typed cause) instead of re-validating the reason structurally; no public helper reads a cause off an arbitrary signal. Concurrent Agents isolate both their initiator identities and their turn signals; a child driver shadows the parent initiator while its parent request signal still travels through the subagent seam.
 
 Agent disposal requests the runtime-only `{ kind: 'disposed' }` interruption on the active holder. If cancellation already won the controller reason, the reason cannot be rewritten, so terminal classification first checks lifecycle state: disposed wins, then a supported `user` or `parent` cause becomes the coarse aborted outcome, and unrelated exceptions retain the existing error path. ACP cancellation maps to `user`; in-process spawn and fork propagation map to `parent`. Remote ACP subagents retain their existing wire protocol.
 
@@ -30,7 +30,7 @@ Cancellation remains cooperative. The loop checks interruption before and after
 
 ## Verification
 
-Contract tests verify the typed caller union, frozen detachment, default and first-wins behavior, the coarse Session JSON round trip and legacy-record rejection, ACP `user`, in-process subagent `parent`, and disposal precedence. Loop tests make cooperative listeners wait on the signal at pre-step, system-prompt assembly, request, model stream, request-error recovery, tool execution, and turn stopping; they assert one signal within a turn, a fresh signal across turns, and no cancellation authority during terminal publication or a blocked durability flush. A real hook bridge test cancels and reaps a blocked prompt hook before idle.
+Contract tests verify the typed caller union, field copying where the cause becomes durable data, default and first-wins behavior, the coarse Session JSON round trip and legacy-record rejection, ACP `user`, in-process subagent `parent`, and disposal precedence. Loop tests make cooperative listeners wait on the signal at pre-step, system-prompt assembly, request, model stream, request-error recovery, tool execution, and turn stopping; they assert one signal within a turn, a fresh signal across turns, and no cancellation authority during terminal publication or a blocked durability flush. A real hook bridge test cancels and reaps a blocked prompt hook before idle.
 
 Initiator-scope tests assert that every hook still observes the exact Agent and no ambient turn signal, concurrent Agents retain independent identities and signals, and a nested child driver shadows only identity. Race tests cover idle cancellation, pre-run cancellation, replacement submission from a `running` listener, repeated cancellation, and cancel-versus-dispose quiescence.
 
diff --git a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.zh.md b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.zh.md
index baf039d495..8fb0728eb2 100644
--- a/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.zh.md
+++ b/.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.zh.md
@@ -12,7 +12,7 @@ Status: implemented
 
 ## 决策
 
-Agent 拥有仅用于运行时的 `AgentCancelCause` 联合类型 `{ kind: 'user' } | { kind: 'parent' }`；`agent.cancel()` 默认使用 `user`。TypeScript 在这个类型化的同进程边界中强制执行该词汇，不提供运行时校验器、后备行为，也不为无类型调用方提供特殊兼容性约定。活跃的 `TurnCancellation` 会把类型化判别字段复制为一个全新且已冻结的 signal 原因；空闲状态下没有可修改的持有者，也不会让后续工作预先进入取消状态。
+Agent 拥有仅用于运行时的 `AgentCancelCause` 联合类型 `{ kind: 'user' } | { kind: 'parent' } | { kind: 'hook'; reason: string } | { kind: 'disposed' }`，`agent.cancel()` 必须显式传入其一。TypeScript 在这个类型化的同进程边界中强制执行该词汇，不提供运行时校验器、后备行为，也不为无类型调用方提供特殊兼容性约定。持有者中止时携带的是调用方自己的那个对象，而非一份分离的拷贝，因此能接触到该 signal 的传输层可以向其添加属性：Node 的 fetch 会给它收到的 reason 赋一个 `stack`。所以循环在 cause 成为持久数据的那一步拷贝已声明字段。在 `cancel()` 处冻结 cause 并非可选方案——undici 是在严格模式代码中执行该赋值的，被冻结的 reason 会让 `fetch` 以 `TypeError: Cannot add property stack` 取代中止原因而拒绝。空闲状态下没有可修改的持有者，也不会让后续工作预先进入取消状态。
 
 正在运行的轮次被中断后，以粗粒度的持久化结果 `{ kind: 'aborted' }` 结束。终态事件记录轮次发生了什么，运行时 signal 标识谁请求了取消；回放不会重复保存 `user` 或 `parent`。会话 seed/load 会拒绝携带取消原因或任何其他额外字段的旧式中止记录，因此回放无法重新引入由调用方持有的取消细节。仅限进程内的 `agent/cancel-requested` 通知不会持久化；未来若有审计需求，应使用独立的持久化控制请求事件，让请求与最终结果保持为两项事实。持久化事件不包含调用栈、signal、错误对象、自由文本取消原因或后端私有细节。
 
@@ -22,7 +22,7 @@ AgentLoop 为每个待启动轮次私有地持有一个 `TurnCancellation`。它
 
 显式事件签名传递单个 payload 对象：agent 作用域事件在 payload 中携带 `agent` 和 `signal`，`next` 位于最后；其余 API 保持 `signal` 紧邻 waterfall（瀑布式事件）的最终 `next` 之前。`PreStepContext` 与 `RequestFailureContext` 已退役，其字段并入 `agent/pre-step` 与 `agent/request-error` 的 payload（[payload-object 事件](../../archived/architecture/2026-08-06-agent-event-payload-objects.md)）。进入 pre-step 时、请求配置、请求错误恢复、模型生成、工具执行、审批、轮次停止以及 subagent 或工作流请求都会收到当前 signal。钩子桥接器也必须提供 `RunHookOptions.signal`，使轮次取消能够到达 Bash 执行器由提供方管理的终止与等待边界；[原生 containment 决策](2026-08-28-subprocess-native-containment.zh.md)负责受支持本地路径上的 scope、Job 与 fallback 机制。`SystemPrompt.assemble()` 在 `AssembleContext` 中携带 `signal?: AbortSignal`，因为该对象是显式请求值，也可表示轮次之外不携带 signal 的组装。监听器可以配合该 signal 取消，但不得保留它来控制其他轮次。
 
-`ctx.agents` 仍只携带发起 Agent。环境中的 Agent 并不代表存活、当前轮次或取消权限。cause 读取器是 loop 私有的，它直接陈述机器私有的 slot 不变量（只有 `cancel()` 会中止轮次控制器，且总是携带规范的冻结 cause），而不是对 reason 做结构化再校验；不存在从任意 signal 读取 cause 的公开辅助函数。并发 Agent 会同时隔离各自的发起方身份和轮次 signal；子驱动会遮蔽父发起方，而父请求 signal 仍通过 subagent seam 传递。
+`ctx.agents` 仍只携带发起 Agent。环境中的 Agent 并不代表存活、当前轮次或取消权限。cause 读取器是 loop 私有的，它直接陈述机器私有的 slot 不变量（只有 `cancel()` 会中止轮次控制器，且总是携带类型化的 cause），而不是对 reason 做结构化再校验；不存在从任意 signal 读取 cause 的公开辅助函数。并发 Agent 会同时隔离各自的发起方身份和轮次 signal；子驱动会遮蔽父发起方，而父请求 signal 仍通过 subagent seam 传递。
 
 Agent dispose（资源释放）会在活跃持有者上请求仅用于运行时的 `{ kind: 'disposed' }` 中断。若取消已经先占用控制器的中断原因，该原因便无法改写，因此终态分类会先检查生命周期状态：资源释放结果优先，之后受支持的 `user` 或 `parent` 取消原因形成粗粒度的中止结果，其他异常保留现有错误路径。ACP（Agent Client Protocol）取消映射为 `user`；进程内 spawn 和 fork 的传播映射为 `parent`。远程 ACP subagent 保持现有协议。
 
@@ -30,7 +30,7 @@ Agent dispose（资源释放）会在活跃持有者上请求仅用于运行时
 
 ## 验证
 
-约定测试验证类型化调用方联合类型、冻结且与调用方分离、默认行为与首次请求优先行为、粗粒度的会话 JSON 往返与旧式记录拒绝、ACP `user`、进程内 subagent `parent` 以及 dispose 优先级。AgentLoop 测试让协作式监听器在 pre-step、系统提示词组装、请求、模型流、请求错误恢复、工具执行和轮次停止处等待 signal；并断言同一轮次使用一个 signal，不同轮次使用全新的 signal，终态发布期间和持久化刷新受阻期间不存在取消权限。真实钩子桥接器测试会在报告空闲状态前取消并回收受阻的提示词钩子。
+约定测试验证类型化调用方联合类型、在 cause 成为持久数据处拷贝字段、默认行为与首次请求优先行为、粗粒度的会话 JSON 往返与旧式记录拒绝、ACP `user`、进程内 subagent `parent` 以及 dispose 优先级。AgentLoop 测试让协作式监听器在 pre-step、系统提示词组装、请求、模型流、请求错误恢复、工具执行和轮次停止处等待 signal；并断言同一轮次使用一个 signal，不同轮次使用全新的 signal，终态发布期间和持久化刷新受阻期间不存在取消权限。真实钩子桥接器测试会在报告空闲状态前取消并回收受阻的提示词钩子。
 
 发起方作用域测试断言所有钩子仍观察到同一个 Agent 且没有环境中的轮次 signal，并发 Agent 保持独立的身份与 signal，嵌套子驱动只遮蔽身份。竞态测试覆盖空闲状态取消、运行前取消、从 `running` 监听器提交替代提示词、重复取消以及取消与 dispose 竞争下的完全停稳。
 
diff --git a/docs/architecture.i18n.yaml b/docs/architecture.i18n.yaml
index bb5e63f987..a31bcf99da 100644
--- a/docs/architecture.i18n.yaml
+++ b/docs/architecture.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 docs/architecture.md
-architecture.md: b6acbd0906610469b96b2bdcc22410440ea237ff
-architecture.zh.md: ea2df78fbd4a90a6a671f4c917980e97ef02d8f8
+architecture.md: 514bd52cfa8f50fe8cf73c391094d34942b3cca3
+architecture.zh.md: 848fa1b3810403e4a3906ac491c8552052e4ee4e
diff --git a/docs/architecture.md b/docs/architecture.md
index b6acbd0906..514bd52cfa 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -112,7 +112,7 @@ One inbox feeds the driver; injected context waits for a waking message. AgentLo
 
 `agent/pre-step` decides the accepted input. Listeners may rewrite or reject claimed messages; a rejected or empty first claim closes a durable turn without a step. An enter decision may set `startsRequestSeries`: the loop logs a fresh `request/header` (reason `series`, or `change` with `startsSeries: true` when the envelope also changed). Wrapping listeners preserve that declaration with `{ ...decision, messages }`. After assembly and `step/start`, `agent/request` and `prepareCall()` resolve the actual route before the system prompt and accepted users are committed; cancellation during either async phase commits neither. The prepared call capability governs prompt admission, not the preceding `request/context`. Every attempt synchronously reconciles the same rendered assembly, appends users only on the first attempt, logs header/context as needed, and derives and freezes the request before streaming the bound call. Retries do not repeat assembly or `agent/pre-step`. Surface replacements and image-offload decisions after attachment start a new request series, including during the first resumed pre-step; unchanged resume continues the series. The first admitted step reserves the system head before user messages even for an empty prompt (no wire message). The prompt travels only as `system/message` history: an empty rendering clears all active system nodes, leaving no old prompt model-visible; capable routes can append non-empty updates after the cached prefix; incapable routes and new request series consolidate non-empty prompt text at the first system node, with logged empty replacements for non-empty later system nodes ([decision](../.agents/notes/implemented/architecture/2026-09-02-system-prompt-as-surface-node.md); [decision rule](../packages/core/agent-loop/README.md#understand-the-implementation)).
 
-The loop sends immutable requests while keeping cancellation live. It reuses message-freeze evidence only for identities it has fully frozen; [agent-loop](../packages/core/agent-loop/README.md) owns the request construction rules.
+The loop sends immutable requests while keeping cancellation live. It reuses message-freeze evidence only for identities it has fully frozen; [agent-loop](../packages/core/agent-loop/README.md) owns the request construction and cancellation-cause rules.
 
 Details: the [sequence diagram](agent-lifecycle.md), the [tool pipeline](tool-execution-pipeline.md), and [cancellation and error recovery](subsystems/core.md#the-agent-handle).
 
diff --git a/docs/architecture.zh.md b/docs/architecture.zh.md
index ea2df78fbd..848fa1b381 100644
--- a/docs/architecture.zh.md
+++ b/docs/architecture.zh.md
@@ -116,7 +116,7 @@ turn/end
 
 `agent/pre-step` 决定接纳的输入。监听器可以改写或拒绝已领取消息；首次领取被拒绝或为空时，关闭不含步骤的持久轮次。enter 决策可设置 `startsRequestSeries`：循环记录新的 `request/header`（原因为 `series`，或在封装同时变化时为携带 `startsSeries: true` 的 `change`）。包装监听器通过 `{ ...decision, messages }` 保留该声明。组装与 `step/start` 之后，`agent/request` 和 `prepareCall()` 先解析实际路由，再提交系统提示词与已接纳用户消息；在任一异步阶段取消都不会提交这两者。提示词准入依据已准备调用的能力，而非先前的 `request/context`。每次尝试同步协调同一份已渲染组装结果、仅在首次尝试追加用户消息、按需记录 header/context、派生并冻结请求，再通过绑定调用发起流式请求。重试不重复组装或 `agent/pre-step`。附接后的 surface 替换和图片省略决定开启新请求序列，包括恢复后的首次 pre-step 中发生的替换；未变化的恢复延续序列。首次接纳的步骤在用户消息之前预留系统头节点，即使提示词为空（不产生协议消息）。提示词仅通过 `system/message` 历史传递：空渲染文本清除所有生效的系统节点，模型不再看到旧提示词；具备能力的路由可在缓存前缀之后追加非空更新；不具备能力的路由与新请求序列将非空提示词文本归并到首个系统节点，并为非空的后续系统节点记录空内容替换（[决策](../.agents/notes/implemented/architecture/2026-09-02-system-prompt-as-surface-node.zh.md)；[决策规则](../packages/core/agent-loop/README.zh.md#understand-the-implementation)）。
 
-循环发送不可变请求，同时保留实时取消能力。只有已由该循环完整冻结的消息对象身份才能复用冻结证明；[agent-loop](../packages/core/agent-loop/README.zh.md)拥有请求构造规则。
+循环发送不可变请求，同时保留实时取消能力。只有已由该循环完整冻结的消息对象身份才能复用冻结证明；[agent-loop](../packages/core/agent-loop/README.zh.md)拥有请求构造与取消原因记录规则。
 
 详情见[时序图](agent-lifecycle.zh.md)、[工具流水线](tool-execution-pipeline.zh.md)和[取消与错误恢复](subsystems/core.zh.md#the-agent-handle)。
 
diff --git a/docs/subsystems/core.i18n.yaml b/docs/subsystems/core.i18n.yaml
index e1fd460b35..3c4fdb398a 100644
--- a/docs/subsystems/core.i18n.yaml
+++ b/docs/subsystems/core.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 docs/subsystems/core.md
-core.md: bba024e8ee4cb89ceac4698bf66f060c7f74dcd6
-core.zh.md: 2e04d5a9a078f3411b2a0d338835ec7d71296f10
+core.md: a1e876dcfcd31dfb9fd4840d0d0e217bb71e43bb
+core.zh.md: a63e38cf6381d97656f2350cc16502030772b1a7
diff --git a/docs/subsystems/core.md b/docs/subsystems/core.md
index bba024e8ee..a1e876dcfc 100644
--- a/docs/subsystems/core.md
+++ b/docs/subsystems/core.md
@@ -297,7 +297,7 @@ type AgentCancelCause =
   | { readonly kind: 'disposed' }
 ```
 
-The cause is a TypeScript-enforced same-process input. An active cancellation holder copies it into the runtime-only `AbortSignal.reason`; a signal grants cooperating listeners no classification authority. Durable `turn/end` records the outcome as `{ kind: 'aborted', reason: TurnEndCancelCause }`, so the cancel cause lands in the terminal result.
+The cause is a TypeScript-enforced same-process input. An active cancellation holder exposes that same object as the runtime-only `AbortSignal.reason`; a signal grants cooperating listeners no classification authority. Durable `turn/end` records the outcome as `{ kind: 'aborted', reason: TurnEndCancelCause }`, so the cancel cause lands in the terminal result.
 
 The [event taxonomy](../architecture.md#events) owns the `agent/*` lifecycle, checkpoint, and waterfall contracts. Turn and step boundaries are durable session events rather than agent emits.
 
diff --git a/docs/subsystems/core.zh.md b/docs/subsystems/core.zh.md
index 2e04d5a9a0..a63e38cf63 100644
--- a/docs/subsystems/core.zh.md
+++ b/docs/subsystems/core.zh.md
@@ -301,7 +301,7 @@ type AgentCancelCause =
   | { readonly kind: 'disposed' }
 ```
 
-cause 是由 TypeScript 强制约束的同进程输入。活跃的取消持有者会将它复制到仅运行时的 `AbortSignal.reason`；signal 不授予协作监听器任何分类权限。持久 `turn/end` 以 `{ kind: 'aborted', reason: TurnEndCancelCause }` 记录结果，取消原因随终态结果一起持久化。
+cause 是由 TypeScript 强制约束的同进程输入。活跃的取消持有者把同一个对象暴露为仅运行时的 `AbortSignal.reason`；signal 不授予协作监听器任何分类权限。持久 `turn/end` 以 `{ kind: 'aborted', reason: TurnEndCancelCause }` 记录结果，取消原因随终态结果一起持久化。
 
 [事件分类](../architecture.zh.md#events)负责 `agent/*` 生命周期、检查点与 waterfall（瀑布式事件）约定。轮次和步骤边界是持久会话事件，而不是 agent emit。
 
diff --git a/packages/core/agent-loop/README.i18n.yaml b/packages/core/agent-loop/README.i18n.yaml
index 8a503e803f..bfbfe913e9 100644
--- a/packages/core/agent-loop/README.i18n.yaml
+++ b/packages/core/agent-loop/README.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 packages/core/agent-loop/README.md
-README.md: 7c3073aefb90fa2ec74d521d6c9d97aff5f74ace
-README.zh.md: 44621a0b02eadfc574f6f6b924f64cfeb4141cd3
+README.md: f74182e468c8c844be71f762c6f0dd266cf799f3
+README.zh.md: e55d481dc61ae52b7d08ff09ffb0595ce085fd76
diff --git a/packages/core/agent-loop/README.md b/packages/core/agent-loop/README.md
index 7c3073aefb..f74182e468 100644
--- a/packages/core/agent-loop/README.md
+++ b/packages/core/agent-loop/README.md
@@ -122,6 +122,8 @@ Prompt admission uses the actual `prepareCall()` result, not the preceding `requ
 
 ### Failure and cancellation
 
+`turn/end` declares `TurnEndCancelCause`; cancellation records a fresh `AgentCancelCause` there, retaining the caller's `kind` and the hook's `reason` text. The live `AbortSignal.reason` remains the caller's object, which a transport may extend — Node's fetch assigns a `stack` onto it — so the copy keeps that trace out of the log and keeps the ending appendable.
+
 Final adapter selection, dispatch, and iteration failures arrive as terminal finishes and enter `agent/request-error`; a handling listener returns `{ kind: 'retry' }` without calling `next()`, while an unhandled failure is terminal. Middleware, result-processing, tool, and other extension failures remain thrown and close the turn directly — plugin failure ends the turn, not the loop. Undispatched model tool calls after cancellation receive synthetic `tool/call` plus `ABORTED_BEFORE_DISPATCH` result pairs. The [explicit-cancellation decision](../../../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md) owns the signal lifecycle.
 
 </details>
diff --git a/packages/core/agent-loop/README.zh.md b/packages/core/agent-loop/README.zh.md
index 44621a0b02..e55d481dc6 100644
--- a/packages/core/agent-loop/README.zh.md
+++ b/packages/core/agent-loop/README.zh.md
@@ -122,6 +122,8 @@ const handle = await ctx.agents.create({
 
 ### 失败与取消
 
+`turn/end` 声明的类型是 `TurnEndCancelCause`；取消时，循环在其中记录一份新的 `AgentCancelCause`，保留调用方的 `kind` 和 hook 的 `reason` 文本。实时 `AbortSignal.reason` 仍是调用方的那个对象，传输层可能向其添加属性——Node 的 fetch 会给它赋一个 `stack`——因此这份拷贝既让该调用栈不进入日志，也让结束事件保持可追加。
+
 最终适配器选择、分发与迭代失败以终止结束的形式到达并进入 `agent/request-error`；处理该失败的监听器返回 `{ kind: 'retry' }` 且不调用 `next()`，未被处理的失败则是终态。Middleware、结果处理、工具及其他扩展失败仍会抛出并直接关闭轮次——插件失败结束的是轮次，不是循环。取消后未分发的模型工具调用会收到合成的 `tool/call` 加 `ABORTED_BEFORE_DISPATCH` 结果对。[显式取消决策](../../../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.zh.md)拥有信号生命周期。
 
 </details>
diff --git a/packages/core/agent-loop/src/agent.ts b/packages/core/agent-loop/src/agent.ts
index 4ea7593551..98114f53cd 100644
--- a/packages/core/agent-loop/src/agent.ts
+++ b/packages/core/agent-loop/src/agent.ts
@@ -23,7 +23,7 @@ import {
   errorChain,
   markAgentLoopRequest,
 } from '@deepseek-ai/dsh-llm'
-import { deepFreeze } from '@deepseek-ai/dsh-util-values'
+import { assertNever, deepFreeze } from '@deepseek-ai/dsh-util-values'
 import type { Scope } from '@deepseek-ai/dsh-scope'
 import { createScope } from '@deepseek-ai/dsh-scope'
 import type { EpochHeader, RequestContext, Session, SessionId, TurnEndReason, UserMessage } from '@deepseek-ai/dsh-session'
@@ -68,6 +68,31 @@ function requestProposal(header: EpochHeader): LlmCallConfig {
   return proposal
 }
 
+/**
+ * Read the cause `cancel()` passed when aborting a loop-owned signal, copying
+ * only the fields `turn/end` records. The live reason stays the caller's
+ * object, and Node's fetch assigns a `stack` onto it that `Session.append`
+ * would either log or reject as data JSON cannot hold.
+ * @param signal - a turn or maintenance signal this loop owns.
+ * @returns the copied cause, or undefined while the signal is still live.
+ */
+function abortedCancelCause(signal: AbortSignal): AgentCancelCause | undefined {
+  if (!signal.aborted) return undefined
+  // `cancel()` is the only aborter of the signals this loop owns.
+  const cause = signal.reason as AgentCancelCause
+  switch (cause.kind) {
+    case 'user':
+    case 'parent':
+    case 'disposed':
+      return { kind: cause.kind }
+    case 'hook':
+      return { kind: 'hook', reason: cause.reason }
+    /* v8 ignore next -- cancel accepts the closed AgentCancelCause union */
+    default:
+      return assertNever(cause)
+  }
+}
+
 /** Drives one session through turn and step boundaries. */
 export class ReactLoopAgent implements Agent {
   readonly inbox: ReactLoopInbox
@@ -170,7 +195,7 @@ export class ReactLoopAgent implements Agent {
         return await job(maintenance.abort.signal)
       } finally {
         this.setPhase({ kind: 'idle', lastTurn: maintenance.lastTurn })
-        const cause = maintenance.abort.signal.reason as AgentCancelCause | undefined
+        const cause = abortedCancelCause(maintenance.abort.signal)
         if (cause?.kind !== 'disposed' && maintenance.wakeRequested && this.inbox.hasPending) this.wakeDriver()
         done.resolve()
       }
@@ -190,7 +215,7 @@ export class ReactLoopAgent implements Agent {
       // Maintenance and aborted drivers cannot deliver the wake: latch it for
       // replay at convergence. Live drivers claim queued work themselves;
       // disposal never latches, so teardown waits on no model turn.
-      const reason = this.phase.abort.signal.reason as AgentCancelCause | undefined
+      const reason = abortedCancelCause(this.phase.abort.signal)
       if (reason?.kind !== 'disposed' && (this.phase.kind === 'maintenance' || wakeAfterAbort)) {
         this.phase.wakeRequested = true
       }
@@ -321,8 +346,10 @@ export class ReactLoopAgent implements Agent {
         target = 'next-step'
       }
     } catch (error: unknown) {
-      if (signal.aborted) {
-        turnEnds = { kind: 'aborted', reason: signal.reason as AgentCancelCause }
+      // A cause is present exactly while the signal is aborted.
+      const cause = abortedCancelCause(signal)
+      if (cause !== undefined) {
+        turnEnds = { kind: 'aborted', reason: cause }
         throw error
       }
       // Every failure is structured: an `LlmError` keeps its facts, anything
diff --git a/packages/core/agent-loop/tests/cancel.spec.ts b/packages/core/agent-loop/tests/cancel.spec.ts
index 872e983639..add362835b 100644
--- a/packages/core/agent-loop/tests/cancel.spec.ts
+++ b/packages/core/agent-loop/tests/cancel.spec.ts
@@ -3,7 +3,8 @@ import { ToolCallId, createUserMessage, expandAssistantStream } from '@deepseek-
  * Tests for the queue-aware `Agent.cancel()` primitive. The default clears
  * queued and steering work, while `keepInbox` preserves pending input for a
  * later wake after the active turn reaches quiescence. The suite
- * covers every landing window plus signal reset and `whenIdle()` quiescence.
+ * covers every landing window plus signal reset, `whenIdle()` quiescence, and
+ * the cause each cancelled turn records.
  * @module dsh-agent-loop/tests/cancel
  */
 
@@ -13,7 +14,7 @@ import LlmRuntime from '@deepseek-ai/dsh-llm'
 import SessionStore, { Session, SessionId, SessionLogOffset, TurnEndReason } from '@deepseek-ai/dsh-session'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRuntime, { defineContentToolFixture, TOOL_ABORTED_BEFORE_DISPATCH } from '@deepseek-ai/dsh-tools'
-import AgentRegistry, { type Agent } from '@deepseek-ai/dsh-agent'
+import AgentRegistry, { type Agent, type AgentCancelCause } from '@deepseek-ai/dsh-agent'
 import AgentLoop from '@deepseek-ai/dsh-agent-loop'
 import SessionProjectionRegistry from '@deepseek-ai/dsh-session-projection'
 import { MockAdapter, textResponse, toolCallResponse } from './mock-adapter.ts'
@@ -57,6 +58,92 @@ function userTexts(agent: Agent): string[] {
 }
 
 describe('Agent.cancel()', () => {
+  /**
+   * Cancel one turn, let `mutate` alter the caller's cause the way a transport
+   * does once it observes the abort, then report every recorded turn ending.
+   */
+  async function endingsAfterMutatingCancel(
+    cause: AgentCancelCause,
+    mutate: (cause: AgentCancelCause) => void,
+  ): Promise<{ endings: TurnEndReason[]; errors: unknown[]; requests: number }> {
+    const adapter = new MockAdapter([textResponse('next turn')])
+    const ctx = await harness(adapter)
+    try {
+      const agent = await ctx.agentLoop.create(SessionId('cancel-mutated-cause'), { provider: 'mock', model: 'mock' })
+      const errors: unknown[] = []
+      ctx.on('agent/error', ({ error }) => { errors.push(error) })
+      ctx.on('agent/request', ({ signal, turn }, next) => {
+        if (turn === 1) {
+          signal.addEventListener('abort', () => { mutate(cause) }, { once: true })
+          agent.cancel(cause)
+        }
+        return next()
+      })
+
+      send(agent, 'cancel this turn')
+      await agent.whenIdle()
+      // A second turn proves the cancelled one closed: an unclosed turn would
+      // make the next `turn/start` fail the session's turn-bracket invariant.
+      send(agent, 'continue')
+      await agent.whenIdle()
+
+      return {
+        endings: agent.session.snapshotEvents()
+          .flatMap(event => event.type === 'turn/end' ? [event.data.reason] : []),
+        errors,
+        requests: adapter.requests.length,
+      }
+    } finally {
+      await ctx.fiber.dispose()
+    }
+  }
+
+  /**
+   * Node's fetch assigns an enumerable `stack` string onto the abort reason it
+   * receives (`node:internal/deps/undici`), which a plain assignment reproduces.
+   */
+  function assignTransportStack(cause: AgentCancelCause): void {
+    Object.defineProperty(cause, 'stack', {
+      value: 'Error\n    at node:internal/deps/undici/undici',
+      enumerable: true,
+      writable: true,
+      configurable: true,
+    })
+  }
+
+  /** Add an own property JSON discards, the descriptor `Error.captureStackTrace` produces. */
+  function captureNonEnumerableStack(cause: AgentCancelCause): void {
+    Error.captureStackTrace(cause)
+  }
+
+  it.each<AgentCancelCause>([
+    { kind: 'user' },
+    { kind: 'parent' },
+    { kind: 'hook', reason: 'policy stopped this turn' },
+    { kind: 'disposed' },
+  ])('records $kind cancellation without the stack a transport assigned to the abort reason', async (cause) => {
+    const expectedCause = { ...cause }
+
+    const { endings, errors, requests } = await endingsAfterMutatingCancel(cause, assignTransportStack)
+
+    expect(Object.getOwnPropertyDescriptor(cause, 'stack')?.enumerable).toBe(true)
+    expect(errors).toEqual([])
+    expect(endings).toEqual([{ kind: 'aborted', reason: expectedCause }, { kind: 'completed' }])
+    expect(requests).toBe(1)
+  })
+
+  it('closes a cancelled turn whose abort reason acquired an own property JSON discards', async () => {
+    const cause: AgentCancelCause = { kind: 'user' }
+
+    // `Session.append` rejects a payload JSON cannot hold rather than logging a
+    // lossy ending, so the caller's object must not reach the log by reference.
+    const { endings, errors } = await endingsAfterMutatingCancel(cause, captureNonEnumerableStack)
+
+    expect(Object.getOwnPropertyDescriptor(cause, 'stack')?.enumerable).toBe(false)
+    expect(errors).toEqual([])
+    expect(endings).toEqual([{ kind: 'aborted', reason: { kind: 'user' } }, { kind: 'completed' }])
+  })
+
   it('cancel() on an idle agent with nothing queued is a no-op; the next prompt runs (F2 leak guard)', async () => {
     const adapter = new MockAdapter([textResponse('reply')])
     const ctx = await harness(adapter)