diff --git a/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.i18n.yaml b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.i18n.yaml
new file mode 100644
index 0000000000..612916a290
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.i18n.yaml
@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# 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/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md
+2026-08-10-subagent-empty-terminal-message-output.md: 693013f6810005ce02b08bd82f1f6a18511c40fb
+2026-08-10-subagent-empty-terminal-message-output.zh.md: 64d61af21f838ef3f515db8af116cbdd74e96179
diff --git a/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md
new file mode 100644
index 0000000000..693013f681
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md
@@ -0,0 +1,31 @@
+# Agent Note: One selection rule keeps subagent output past an empty terminal message
+
+Status: implemented
+
+English | [中文](2026-08-10-subagent-empty-terminal-message-output.zh.md)
+
+## Problem
+
+The agent loop appends an empty-content `assistant/message` when a `max-tokens` step assembled only tool-call blocks because `BlockAssembler.blocks()` drops truncated tool calls; the message records usage only. Three consumers selected the child's output independently and treated that usage record as output. The in-process driver's `readResult` and the continuable Activation's `subagent/end` capture selected the last `assistant/message` without filtering, while the SDK backend's observer let any `assistant/message` take precedence over accumulated text. In a multi-step turn cut off at max-tokens, the final empty message caused the real partial answer to be omitted from `SubagentResult.output`, the tool result, telemetry, and `subagent/end.lastAssistantMessage`. The in-process driver also lacked a streamed-text fallback, so a cancelled child whose only text existed in `assistant/chunk` events reported `[]`.
+
+## Decision
+
+`dsh-subagent` owns one canonical selection rule in `src/assistant-output.ts`: select the last non-empty assistant message; without one, select the accumulated `text-delta` stream; ignore empty-content messages. The incremental `AssistantOutputFold` implements the rule through `push(event)` for session-event transports, `pushText(text)` for chunk-only transports, and `collect()` for selection. `finalAssistantOutput(events)` applies it to a complete event suffix for the in-process `readResult` and Activation capture. The SDK backend folds notification events; the ACP backend exposes no complete assistant messages and folds raw chunk text. `SubagentResult.output` defines the result contract, and `subagent/end.lastAssistantMessage` uses the same rule. When a child produces neither form of output, the lifecycle field is absent rather than an empty array for both one-shot and continuable runs. A `max-tokens` or `aborted` result retains its actual stop reason.
+
+The foreground delegation tool uses the same selection. A non-`completed` result remains an `isError` tool result, but its message appends the child's partial text after the stop-reason headline so the parent model receives both the failure and available output.
+
+## Verification
+
+The keyless SDK backend test uses `FAKE_EMPTY_MESSAGE` to emit a usage-only terminal message. The `subagent-max-tokens-partial` ACP snapshot records a child that streams text and a tool call, ends at a tool-only max-tokens step with an empty usage message in its durable log, and returns the partial text through the parent's errored tool result. Unit coverage checks empty terminal messages, cancellation, message ordering, textless non-empty messages, and exclusion of tool-result content.
+
+## Alternatives considered
+
+**Fix each consumer in place without a shared helper.** Rejected: three independent selections had diverged, while observers of one run must agree on its output.
+
+**Stop the loop from appending the empty message.** Rejected: the message records usage and preserves the step in the durable log ("model-visible ⟺ logged"); changing session events to address output selection would affect every replay and projection consumer.
+
+**Treat empty-content messages as an error.** Rejected: the streamed text is the child's real partial answer, and the stop reason already tells the consumer the turn was cut short.
+
+## Consequences
+
+Multi-step children cut off at max-tokens report their earlier text; cancelled in-process children retain text streamed before the abort; one-shot and continuable `subagent/end` events agree with `SubagentResult.output`. A message whose content is non-empty but textless, such as reasoning-only content, is selected instead of streamed text because the rule tests content length rather than text presence. A non-empty message is also selected instead of text streamed after it: a child cancelled while streaming a later step reports its earlier complete message, while the stop reason records the truncation.
diff --git a/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.zh.md b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.zh.md
new file mode 100644
index 0000000000..64d61af21f
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.zh.md
@@ -0,0 +1,31 @@
+# Agent Note: 用同一条选取规则在空终止消息后保留子代理输出
+
+Status: implemented
+
+[English](2026-08-10-subagent-empty-terminal-message-output.md) | 中文
+
+## 问题
+
+当 `max-tokens` 步骤只组装了工具调用块时，agent loop（智能体循环）会追加一条空内容的 `assistant/message`，因为 `BlockAssembler.blocks()` 会丢弃被截断的工具调用；这条消息仅记录 usage。三个消费方独立选取子 agent 的输出，并把这条 usage 记录当成输出。进程内驱动的 `readResult` 与 continuable Activation 的 `subagent/end` capture 不加过滤地选取最后一条 `assistant/message`，SDK 后端的观察器则让任何 `assistant/message` 优先于累积的文本。在被 max-tokens 截断的多步轮次中，最后那条空消息导致 `SubagentResult.output`、工具结果、遥测与 `subagent/end.lastAssistantMessage` 都漏掉真实的部分回答。进程内驱动也没有流式文本兜底，因此被取消的子 agent 若其唯一文本只存在于 `assistant/chunk` 事件中，也会报告 `[]`。
+
+## 决策
+
+`dsh-subagent` 在 `src/assistant-output.ts` 中拥有唯一的规范选取规则：选取最后一条非空 assistant 消息；没有时选取累积的 `text-delta` 流；忽略空内容消息。增量的 `AssistantOutputFold` 通过 `push(event)` 处理会话事件传输，通过 `pushText(text)` 处理仅分片传输，并通过 `collect()` 完成选取。`finalAssistantOutput(events)` 把规则应用于完整的事件后缀，供进程内 `readResult` 与 Activation capture 使用。SDK 后端折叠通知事件；ACP 后端不暴露完整的 assistant 消息，而是折叠原始分片文本。`SubagentResult.output` 定义结果约定，`subagent/end.lastAssistantMessage` 使用同一规则。子 agent 不产生这两种输出中的任何一种时，一次性与 continuable 运行的生命周期字段都会缺省，而不是空数组。`max-tokens` 或 `aborted` 结果保留实际的终止原因。
+
+前台委派工具使用同一选取规则。非 `completed` 的结果仍是 `isError` 工具结果，但其消息会在终止原因标题之后附上子 agent 的部分文本，让父模型同时接收失败信息与已有输出。
+
+## 验证
+
+无密钥 SDK 后端测试使用 `FAKE_EMPTY_MESSAGE` 发出一条仅记录 usage 的终止消息。`subagent-max-tokens-partial` ACP 快照记录一个子 agent：它流式输出文本与一次工具调用，结束于仅含工具调用的 max-tokens 步骤，持久化日志中含一条空的 usage 消息，并通过父侧的错误工具结果返回部分文本。单元覆盖检查空终止消息、取消、消息顺序、不含文本的非空消息，以及排除工具结果内容。
+
+## 考虑过的替代方案
+
+**各消费方就地修复、不抽共享辅助函数。** 之所以否决：三处独立选取已发生分歧，而同一次运行的观察方必须对其输出达成一致。
+
+**让 loop 不再追加空消息。** 之所以否决：这条消息记录 usage，并在持久化日志中保留该步骤（"model-visible ⟺ logged"）；为处理输出选取而改动会话事件，会影响所有 replay 与 projection 消费方。
+
+**把空内容消息视为错误。** 之所以否决：流式文本才是子代理真实的部分回答，且终止原因已经告诉消费方轮次被截断。
+
+## 后果
+
+被 max-tokens 截断的多步子 agent 会报告其更早的文本；被取消的进程内子 agent 保留中止前已流式的文本；一次性与 continuable 的 `subagent/end` 事件同 `SubagentResult.output` 一致。内容非空但不含文本的消息（例如仅含 reasoning 的内容）仍然优先于流式文本，因为规则检查内容长度，而不是文本是否存在。非空消息同样优先于其后才流式出的文本：子 agent 在流式输出后续步骤时被取消，报告的是更早那条完整消息，终止原因则记录该截断。
diff --git a/docs/event-producer-consumer.i18n.yaml b/docs/event-producer-consumer.i18n.yaml
index b67ad96c9c..8dd7e6321a 100644
--- a/docs/event-producer-consumer.i18n.yaml
+++ b/docs/event-producer-consumer.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/event-producer-consumer.md
-event-producer-consumer.md: 4e68c0e5e6262ec7f31f4ed2d146ee18d3faa02a
-event-producer-consumer.zh.md: 1879e74f3e8c01143292b82a1745f4438f64e0ac
+event-producer-consumer.md: 19e2e660e58101b54054091b3d55b362d25d29dc
+event-producer-consumer.zh.md: d29cab8974d206b74b8869057b8efcf7542c1161
diff --git a/docs/event-producer-consumer.md b/docs/event-producer-consumer.md
index 4e68c0e5e6..19e2e660e5 100644
--- a/docs/event-producer-consumer.md
+++ b/docs/event-producer-consumer.md
@@ -37,10 +37,10 @@ This matrix shows which packages dispatch each harness-owned event and which pac
 | `settings/document-updated` | `emit` | [`packages/settings/settings/src/index.ts:170`](../packages/settings/settings/src/index.ts) | [`settings`](../packages/settings/settings) (`events.dispatch`) | `apiproxy` |
 | `settings/updated` | `emit` | [`packages/settings/settings/src/index.ts:157`](../packages/settings/settings/src/index.ts) | [`settings`](../packages/settings/settings) (`events.dispatch`) | [`settings`](../packages/settings/settings) |
 | `skills/change` | `emit` | [`packages/skill/skill/src/index.ts:297`](../packages/skill/skill/src/index.ts) | [`skill`](../packages/skill/skill) (`events.dispatch`) | - |
-| `subagent/end` | `emit` | [`packages/subagent/subagent/src/index.ts:164`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), `server`, [`subagent`](../packages/subagent/subagent) |
-| `subagent/provider-added` | `emit` | [`packages/subagent/subagent/src/index.ts:138`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`emit`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
-| `subagent/provider-removed` | `emit` | [`packages/subagent/subagent/src/index.ts:144`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
-| `subagent/start` | `emit` | [`packages/subagent/subagent/src/index.ts:155`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`subagent`](../packages/subagent/subagent) |
+| `subagent/end` | `emit` | [`packages/subagent/subagent/src/index.ts:165`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), `server`, [`subagent`](../packages/subagent/subagent) |
+| `subagent/provider-added` | `emit` | [`packages/subagent/subagent/src/index.ts:139`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`emit`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
+| `subagent/provider-removed` | `emit` | [`packages/subagent/subagent/src/index.ts:145`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
+| `subagent/start` | `emit` | [`packages/subagent/subagent/src/index.ts:156`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`subagent`](../packages/subagent/subagent) |
 | `system-prompt/assemble` | `waterfall` | [`packages/core/system-prompt/src/index.ts:31`](../packages/core/system-prompt/src/index.ts) | [`system-prompt`](../packages/core/system-prompt) (`waterfall`) | [`agent`](../packages/core/agent), [`system-prompt`](../packages/core/system-prompt) |
 | `system-prompt/change` | `emit` | [`packages/core/system-prompt/src/index.ts:37`](../packages/core/system-prompt/src/index.ts) | [`system-prompt`](../packages/core/system-prompt) (`emit`) | - |
 | `telemetry/record` | `waterfall` | [`packages/session/session-telemetry/src/index.ts:43`](../packages/session/session-telemetry/src/index.ts) | [`session-telemetry`](../packages/session/session-telemetry) (`waterfall`) | - |
diff --git a/docs/event-producer-consumer.zh.md b/docs/event-producer-consumer.zh.md
index 1879e74f3e..d29cab8974 100644
--- a/docs/event-producer-consumer.zh.md
+++ b/docs/event-producer-consumer.zh.md
@@ -39,10 +39,10 @@
 | `settings/document-updated` | `emit` | [`packages/settings/settings/src/index.ts:170`](../packages/settings/settings/src/index.ts) | [`settings`](../packages/settings/settings) (`events.dispatch`) | `apiproxy` |
 | `settings/updated` | `emit` | [`packages/settings/settings/src/index.ts:157`](../packages/settings/settings/src/index.ts) | [`settings`](../packages/settings/settings) (`events.dispatch`) | [`settings`](../packages/settings/settings) |
 | `skills/change` | `emit` | [`packages/skill/skill/src/index.ts:297`](../packages/skill/skill/src/index.ts) | [`skill`](../packages/skill/skill) (`events.dispatch`) | - |
-| `subagent/end` | `emit` | [`packages/subagent/subagent/src/index.ts:164`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), `server`, [`subagent`](../packages/subagent/subagent) |
-| `subagent/provider-added` | `emit` | [`packages/subagent/subagent/src/index.ts:138`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`emit`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
-| `subagent/provider-removed` | `emit` | [`packages/subagent/subagent/src/index.ts:144`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
-| `subagent/start` | `emit` | [`packages/subagent/subagent/src/index.ts:155`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`subagent`](../packages/subagent/subagent) |
+| `subagent/end` | `emit` | [`packages/subagent/subagent/src/index.ts:165`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), `server`, [`subagent`](../packages/subagent/subagent) |
+| `subagent/provider-added` | `emit` | [`packages/subagent/subagent/src/index.ts:139`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`emit`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
+| `subagent/provider-removed` | `emit` | [`packages/subagent/subagent/src/index.ts:145`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`subagent`](../packages/subagent/subagent), [`tool-subagent`](../packages/subagent/tool-subagent) |
+| `subagent/start` | `emit` | [`packages/subagent/subagent/src/index.ts:156`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`subagent`](../packages/subagent/subagent) |
 | `system-prompt/assemble` | `waterfall` | [`packages/core/system-prompt/src/index.ts:31`](../packages/core/system-prompt/src/index.ts) | [`system-prompt`](../packages/core/system-prompt) (`waterfall`) | [`agent`](../packages/core/agent), [`system-prompt`](../packages/core/system-prompt) |
 | `system-prompt/change` | `emit` | [`packages/core/system-prompt/src/index.ts:37`](../packages/core/system-prompt/src/index.ts) | [`system-prompt`](../packages/core/system-prompt) (`emit`) | - |
 | `telemetry/record` | `waterfall` | [`packages/session/session-telemetry/src/index.ts:43`](../packages/session/session-telemetry/src/index.ts) | [`session-telemetry`](../packages/session/session-telemetry) (`waterfall`) | - |
diff --git a/docs/subsystems/subagent.i18n.yaml b/docs/subsystems/subagent.i18n.yaml
index 9a3c08423f..86ecf90b02 100644
--- a/docs/subsystems/subagent.i18n.yaml
+++ b/docs/subsystems/subagent.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/subagent.md
-subagent.md: 220ecf493b2d3a84cede34c1cf72dbf0dcd80353
-subagent.zh.md: b0a0df5a778ed758421225e524c87cf8ef213c0b
+subagent.md: 42359215b2e21028e13734c2a4cd2488f86a07af
+subagent.zh.md: 0d1d4c42fa774867679ff248bfc99b0024b04257
diff --git a/docs/subsystems/subagent.md b/docs/subsystems/subagent.md
index 220ecf493b..42359215b2 100644
--- a/docs/subsystems/subagent.md
+++ b/docs/subsystems/subagent.md
@@ -293,7 +293,12 @@ The outcome of a one-shot run, resolved by `SubagentRun.result`. `structured` is
  * The terminal outcome of a subagent run, resolved by {@link SubagentRun.result}.
  */
 interface SubagentResult {
-  /** The child's final assistant output (the last assistant message's content). */
+  /**
+   * The child's final assistant output is the content of its last non-empty
+   * assistant message. Empty-content messages, including usage-only messages,
+   * are skipped. Without a non-empty message, the output is its accumulated
+   * assistant text stream, or `[]` when the child produced neither.
+   */
   readonly output: ContentBlock[]
   /**
    * The structured result after a requested `outputSchema` was successfully
@@ -614,7 +619,7 @@ async start(name: string, request: SubagentStartRequest): Promise<SubagentRun>
 
 Types: [Agent](core.md) · [ContentBlock](llm-streaming.md) · [MessageId](llm-streaming.md) · [SessionId](core.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:169`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:170`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagent-events"></a>
 
@@ -640,7 +645,7 @@ A published child settled. Scope-filtered dispatch uses the same delegating pare
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:164`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:165`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-added--emit"></a>
 
@@ -657,7 +662,7 @@ A provider became resolvable in the registry.
 'subagent/provider-added'(provider: SubagentProvider): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:138`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:139`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-removed--emit"></a>
 
@@ -674,7 +679,7 @@ A provider left the registry. Accepted runs remain holder-owned.
 'subagent/provider-removed'(name: string): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:144`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:145`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentstart--emit"></a>
 
@@ -698,5 +703,5 @@ A provider established a published child. For in-process providers, `ctx.agents.
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:155`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:156`](../../packages/subagent/subagent/src/index.ts)
 <!-- END GENERATED cordis-surface -->
diff --git a/docs/subsystems/subagent.zh.md b/docs/subsystems/subagent.zh.md
index b0a0df5a77..0d1d4c42fa 100644
--- a/docs/subsystems/subagent.zh.md
+++ b/docs/subsystems/subagent.zh.md
@@ -293,7 +293,12 @@ type SubagentDescendantListEntry = SubagentListEntry & {
  * The terminal outcome of a subagent run, resolved by {@link SubagentRun.result}.
  */
 interface SubagentResult {
-  /** The child's final assistant output (the last assistant message's content). */
+  /**
+   * The child's final assistant output is the content of its last non-empty
+   * assistant message. Empty-content messages, including usage-only messages,
+   * are skipped. Without a non-empty message, the output is its accumulated
+   * assistant text stream, or `[]` when the child produced neither.
+   */
   readonly output: ContentBlock[]
   /**
    * The structured result after a requested `outputSchema` was successfully
@@ -616,7 +621,7 @@ async start(name: string, request: SubagentStartRequest): Promise<SubagentRun>
 
 Types: [Agent](core.md) · [ContentBlock](llm-streaming.md) · [MessageId](llm-streaming.md) · [SessionId](core.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:169`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:170`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagent-events"></a>
 
@@ -642,7 +647,7 @@ A published child settled. Scope-filtered dispatch uses the same delegating pare
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:164`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:165`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-added--emit"></a>
 
@@ -659,7 +664,7 @@ A provider became resolvable in the registry.
 'subagent/provider-added'(provider: SubagentProvider): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:138`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:139`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-removed--emit"></a>
 
@@ -676,7 +681,7 @@ A provider left the registry. Accepted runs remain holder-owned.
 'subagent/provider-removed'(name: string): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:144`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:145`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentstart--emit"></a>
 
@@ -700,5 +705,5 @@ A provider established a published child. For in-process providers, `ctx.agents.
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:155`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:156`](../../packages/subagent/subagent/src/index.ts)
 <!-- END GENERATED cordis-surface -->
diff --git a/examples/acp-agent/tests/acp.snapshot.ts b/examples/acp-agent/tests/acp.snapshot.ts
index 69b7b7336d..a05bce3269 100644
--- a/examples/acp-agent/tests/acp.snapshot.ts
+++ b/examples/acp-agent/tests/acp.snapshot.ts
@@ -341,6 +341,10 @@ const SCENARIOS: Scenario[] = [
   // Windows bash process-tree kill is deferred with the Bash execution domain.
   { name: 'cancel-tool-calls', hasModelTurn: true, recorded: false, overridden: true, posixOnly: true },
   { name: 'subagent-spawn', hasModelTurn: true, recorded: true },
+  // Keyless authored scenario: the child ends at max-tokens with an empty
+  // usage-only assistant/message after earlier text and a tool call. The
+  // parent's tool result must retain that assistant output and stop reason.
+  { name: 'subagent-max-tokens-partial', hasModelTurn: true, recorded: false },
   { name: 'subagent-multi', hasModelTurn: true, recorded: true },
   { name: 'subagent-fork', hasModelTurn: true, recorded: true },
   { name: 'subagent-mixed', hasModelTurn: true, recorded: true },
diff --git a/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/input.json b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/input.json
new file mode 100644
index 0000000000..640bf92f7f
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/input.json
@@ -0,0 +1,14 @@
+{
+  "steps": [
+    {
+      "op": "initialize"
+    },
+    {
+      "op": "newSession"
+    },
+    {
+      "op": "prompt",
+      "text": "Use the subagent tool exactly once to delegate this subtask: \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\" After the subagent returns, reply with the single word PARENT_DONE and stop."
+    }
+  ]
+}
diff --git a/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.1.jsonl b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.1.jsonl
new file mode 100644
index 0000000000..d1f5447275
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.1.jsonl
@@ -0,0 +1,31 @@
+{"type":"session","version":0,"id":"22222222-2222-4222-8222-222222222222","createdAt":2,"cwd":"{{cwd}}","parentSession":"11111111-1111-4111-8111-111111111111","origin":"subagent","delegationDepth":1}
+{"type":"approval/policy","seq":0,"time":1786373921809,"data":{"policy":"never","source":"delegation"}}
+{"type":"agent/inbox/spliced","seq":1,"time":1786373921809,"data":{"target":"next-turn","start":0,"inserted":[{"content":[{"type":"text","text":"Write the words 'partial one', call todo_write once, then keep going until you are cut off."}],"source":{"kind":"user"},"role":"user","id":"dbf0670a-79cc-4e2c-a298-c4d804e6fe61"}]}}
+{"type":"turn/start","seq":2,"time":1786373921810,"data":{"turn":1}}
+{"type":"agent/inbox/spliced","seq":3,"time":1786373921810,"data":{"target":"next-turn","start":0,"removedCount":1,"inserted":[]}}
+{"type":"subagent/descriptor","seq":4,"time":1786373921831,"data":{"version":2,"mode":"one-shot","provider":"spawn","label":"Truncated child"}}
+{"type":"step/start","seq":5,"time":1786373921836,"data":{"turn":1,"step":1}}
+{"type":"user/message","seq":6,"time":1786348800142,"data":{"content":[{"type":"text","text":"Write the words 'partial one', call todo_write once, then keep going until you are cut off."}],"source":{"kind":"user"},"role":"user","id":"dbf0670a-79cc-4e2c-a298-c4d804e6fe61"},"surfaceOp":"append"}
+{"type":"user/message","seq":7,"time":1786373921837,"data":{"content":[{"type":"text","text":"Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nCurrent DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations.\n\nApproval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`).\n\nYou are a delegated subagent: your permission scope was fixed when you were started and cannot be widened from inside this session — operations that require approval are rejected automatically. When the task needs access beyond that scope, do not retry the denied operation; state the limitation in your reply so the delegating agent can handle it."}],"source":{"kind":"plugin","plugin":"@deepseek-ai/dsh-system-prompt","form":"snapshot","sections":[{"name":"sandbox:policy","text":"Current DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations."},{"name":"approval:policy","text":"Approval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."},{"name":"subagent:delegation","text":"You are a delegated subagent: your permission scope was fixed when you were started and cannot be widened from inside this session — operations that require approval are rejected automatically. When the task needs access beyond that scope, do not retry the denied operation; state the limitation in your reply so the delegating agent can handle it."}]},"role":"user","id":"885ea744-63dd-4198-95be-267b9db94a57"},"surfaceOp":"append"}
+{"type":"session/title","seq":8,"time":1786373921837,"data":{"title":"Write the words 'partial one',","messageSeqs":[6],"source":{"kind":"fallback"}}}
+{"type":"request/header","seq":9,"time":1786348800142,"data":{"header":{"config":{"provider":"deepseek-official","model":"deepseek-v4-flash"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
+{"type":"request/context","seq":10,"time":1786348800142,"data":{"provider":"deepseek-official","model":"deepseek-v4-flash"}}
+{"type":"assistant/chunk","seq":11,"time":0,"data":{"turn":1,"step":1,"chunk":{"type":"block-start","index":0,"blockType":"text"}}}
+{"type":"assistant/chunk","seq":12,"time":1786348800146,"data":{"turn":1,"step":1,"chunk":{"type":"block-end","index":0,"block":{"type":"text","text":"partial one"}}}}
+{"type":"assistant/chunk","seq":13,"time":1786348800146,"data":{"turn":1,"step":1,"chunk":{"type":"block-start","index":1,"blockType":"tool-call"}}}
+{"type":"assistant/chunk","seq":14,"time":1786348800146,"data":{"turn":1,"step":1,"chunk":{"type":"block-end","index":1,"block":{"type":"tool-call","id":"call_child_1","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"keep going\", \"status\": \"in_progress\"}]}"}}}}
+{"type":"assistant/chunk","seq":15,"time":1786348800146,"data":{"turn":1,"step":1,"chunk":{"type":"usage","usage":{"inputTokens":20,"outputTokens":9}}}}
+{"type":"assistant/chunk","seq":16,"time":1786348800146,"data":{"turn":1,"step":1,"chunk":{"type":"finish","reason":{"kind":"tool-calls"}}}}
+{"type":"assistant/message","seq":17,"time":1786348800146,"data":{"turn":1,"step":1,"message":{"role":"assistant","content":[{"type":"text","text":"partial one"},{"type":"tool-call","id":"call_child_1","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"keep going\", \"status\": \"in_progress\"}]}"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"5e4d07b2-6ce2-4ab6-8be0-fbdf2d3af138"},"usage":{"inputTokens":20,"outputTokens":9}},"sourceEventSeqs":[11,12,13,14,15,16],"surfaceOp":"append"}
+{"type":"tool/call","seq":18,"time":1786348800146,"data":{"turn":1,"step":1,"callId":"call_child_1","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"keep going\", \"status\": \"in_progress\"}]}"}}
+{"type":"todo/write","seq":19,"time":1786348800150,"data":{"todos":[{"content":"keep going","status":"in_progress"}]}}
+{"type":"tool/result","seq":20,"time":1786348800151,"data":{"turn":1,"step":1,"message":{"source":{"kind":"tool","callId":"call_child_1"},"content":[{"type":"tool-result","toolCallId":"call_child_1","content":[{"type":"text","text":"Updated todo list: 0 pending, 1 in progress, 0 completed."}],"isError":false}],"role":"user","id":"67efbbf3-ca1e-4d23-8f19-940cb391ff1e"}},"sourceEventSeqs":[18],"surfaceOp":"append"}
+{"type":"step/end","seq":21,"time":1786348800151,"data":{"turn":1,"step":1}}
+{"type":"step/start","seq":22,"time":1786348800156,"data":{"turn":1,"step":2}}
+{"type":"assistant/chunk","seq":23,"time":1786348800160,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":0,"blockType":"tool-call"}}}
+{"type":"assistant/chunk","seq":24,"time":1786348800160,"data":{"turn":1,"step":2,"chunk":{"type":"block-end","index":0,"block":{"type":"tool-call","id":"call_child_2","name":"todo_write","arguments":"{\"todos\": [{\"content\": \"keep going\", \"status\": \"completed\"}]}"}}}}
+{"type":"assistant/chunk","seq":25,"time":1786348800160,"data":{"turn":1,"step":2,"chunk":{"type":"usage","usage":{"inputTokens":30,"outputTokens":4}}}}
+{"type":"assistant/chunk","seq":26,"time":1786348800160,"data":{"turn":1,"step":2,"chunk":{"type":"finish","reason":{"kind":"max-tokens"}}}}
+{"type":"assistant/message","seq":27,"time":1786348800160,"data":{"turn":1,"step":2,"message":{"role":"assistant","content":[],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"bb92e4ec-f260-4415-9782-b71147ea378d"},"usage":{"inputTokens":30,"outputTokens":4}},"sourceEventSeqs":[23,24,25,26],"surfaceOp":"append"}
+{"type":"step/end","seq":28,"time":1786348800160,"data":{"turn":1,"step":2}}
+{"type":"turn/end","seq":29,"time":1786348800160,"data":{"turn":1,"reason":{"kind":"max-tokens"}}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.jsonl b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.jsonl
new file mode 100644
index 0000000000..f07b01589d
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/session.jsonl
@@ -0,0 +1,26 @@
+{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1,"cwd":"{{cwd}}","delegationDepth":0}
+{"type":"agent/inbox/spliced","seq":0,"time":1786348800078,"data":{"target":"next-turn","start":0,"inserted":[{"content":[{"type":"text","text":"Use the subagent tool exactly once to delegate this subtask: \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\" After the subagent returns, reply with the single word PARENT_DONE and stop."}],"source":{"kind":"user"},"role":"user","id":"8787ce07-4f1f-4368-bf58-18e30484ed44"}]}}
+{"type":"turn/start","seq":1,"time":1786348800079,"data":{"turn":1}}
+{"type":"agent/inbox/spliced","seq":2,"time":1786348800079,"data":{"target":"next-turn","start":0,"removedCount":1,"inserted":[]}}
+{"type":"step/start","seq":3,"time":1786348800114,"data":{"turn":1,"step":1}}
+{"type":"user/message","seq":4,"time":1786348800114,"data":{"content":[{"type":"text","text":"Use the subagent tool exactly once to delegate this subtask: \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\" After the subagent returns, reply with the single word PARENT_DONE and stop."}],"source":{"kind":"user"},"role":"user","id":"8787ce07-4f1f-4368-bf58-18e30484ed44"},"surfaceOp":"append"}
+{"type":"user/message","seq":5,"time":1786348800114,"data":{"content":[{"type":"text","text":"Current runtime context. This snapshot supersedes earlier runtime-context snapshots.\n\nCurrent DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations.\n\nApproval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."}],"source":{"kind":"plugin","plugin":"@deepseek-ai/dsh-system-prompt","form":"snapshot","sections":[{"name":"sandbox:policy","text":"Current DSH file policy: danger-full-access. The DSH file sandbox does not restrict file modifications by available operations."},{"name":"approval:policy","text":"Approval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`)."}]},"role":"user","id":"32a8b2ce-f1f9-411b-940d-c80f772561ac"},"surfaceOp":"append"}
+{"type":"session/title","seq":6,"time":1786348800114,"data":{"title":"Use the subagent tool exactly","messageSeqs":[4],"source":{"kind":"fallback"}}}
+{"type":"request/header","seq":7,"time":1786348800115,"data":{"header":{"config":{"provider":"deepseek-official","model":"deepseek-v4-flash"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
+{"type":"request/context","seq":8,"time":1786348800115,"data":{"provider":"deepseek-official","model":"deepseek-v4-flash"}}
+{"type":"assistant/chunk","seq":9,"time":1786348800120,"data":{"turn":1,"step":1,"chunk":{"type":"block-start","index":0,"blockType":"tool-call"}}}
+{"type":"assistant/chunk","seq":10,"time":1786348800120,"data":{"turn":1,"step":1,"chunk":{"type":"block-end","index":0,"block":{"type":"tool-call","id":"call_parent_1","name":"subagent","arguments":"{\"description\": \"Truncated child\", \"prompt\": \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\"}"}}}}
+{"type":"assistant/chunk","seq":11,"time":1786348800120,"data":{"turn":1,"step":1,"chunk":{"type":"usage","usage":{"inputTokens":10,"outputTokens":5}}}}
+{"type":"assistant/chunk","seq":12,"time":1786348800120,"data":{"turn":1,"step":1,"chunk":{"type":"finish","reason":{"kind":"tool-calls"}}}}
+{"type":"assistant/message","seq":13,"time":1786348800120,"data":{"turn":1,"step":1,"message":{"role":"assistant","content":[{"type":"tool-call","id":"call_parent_1","name":"subagent","arguments":"{\"description\": \"Truncated child\", \"prompt\": \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\"}"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"f4269cd2-9132-4b68-8f9b-ff3a40321bc9"},"usage":{"inputTokens":10,"outputTokens":5}},"sourceEventSeqs":[9,10,11,12],"surfaceOp":"append"}
+{"type":"tool/call","seq":14,"time":1786348800121,"data":{"turn":1,"step":1,"callId":"call_parent_1","name":"subagent","arguments":"{\"description\": \"Truncated child\", \"prompt\": \"Write the words 'partial one', call todo_write once, then keep going until you are cut off.\"}"}}
+{"type":"tool/result","seq":15,"time":1786348800163,"data":{"turn":1,"step":1,"message":{"source":{"kind":"tool","callId":"call_parent_1"},"content":[{"type":"tool-result","toolCallId":"call_parent_1","content":[{"type":"text","text":"Error: subagent run hit its token limit before finishing\nPartial output before the run ended:\npartial one"}],"isError":true}],"role":"user","id":"5dd34050-a533-4f1b-99ee-5fc62c6a4502"}},"sourceEventSeqs":[14],"surfaceOp":"append"}
+{"type":"step/end","seq":16,"time":1786348800163,"data":{"turn":1,"step":1}}
+{"type":"step/start","seq":17,"time":1786348800169,"data":{"turn":1,"step":2}}
+{"type":"assistant/chunk","seq":18,"time":1786348800173,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":0,"blockType":"text"}}}
+{"type":"assistant/chunk","seq":19,"time":1786348800173,"data":{"turn":1,"step":2,"chunk":{"type":"block-end","index":0,"block":{"type":"text","text":"PARENT_DONE"}}}}
+{"type":"assistant/chunk","seq":20,"time":1786348800173,"data":{"turn":1,"step":2,"chunk":{"type":"usage","usage":{"inputTokens":12,"outputTokens":2}}}}
+{"type":"assistant/chunk","seq":21,"time":1786348800173,"data":{"turn":1,"step":2,"chunk":{"type":"finish","reason":{"kind":"stop"}}}}
+{"type":"assistant/message","seq":22,"time":1786348800173,"data":{"turn":1,"step":2,"message":{"role":"assistant","content":[{"type":"text","text":"PARENT_DONE"}],"source":{"kind":"model","provider":"deepseek-official","model":"deepseek-v4-flash"},"id":"fb14560d-1d98-4b18-8736-b079de400315"},"usage":{"inputTokens":12,"outputTokens":2}},"sourceEventSeqs":[18,19,20,21],"surfaceOp":"append"}
+{"type":"step/end","seq":23,"time":1786348800173,"data":{"turn":1,"step":2}}
+{"type":"turn/end","seq":24,"time":1786348800173,"data":{"turn":1,"reason":{"kind":"completed"}}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/stdout.expected.jsonl b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/stdout.expected.jsonl
new file mode 100644
index 0000000000..a460e019d4
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/subagent-max-tokens-partial/stdout.expected.jsonl
@@ -0,0 +1,4 @@
+{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":1,"agentInfo":{"name":"deepseek-harness-acp","version":"0.0.1"},"agentCapabilities":{"promptCapabilities":{"image":false,"audio":false,"embeddedContext":false}},"authMethods":[]}}
+{"jsonrpc":"2.0","id":2,"result":{"sessionId":"{{sessionId}}"}}
+{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_message_chunk","content":{"type":"text","text":"PARENT_DONE"}}}}
+{"jsonrpc":"2.0","id":3,"result":{"stopReason":"end_turn"}}
diff --git a/packages/scaffold/client/tests/fake-runtime.ts b/packages/scaffold/client/tests/fake-runtime.ts
index 85d5253765..4fb2b6017a 100644
--- a/packages/scaffold/client/tests/fake-runtime.ts
+++ b/packages/scaffold/client/tests/fake-runtime.ts
@@ -26,6 +26,8 @@
  *   array; `FAKE_MESSAGE_WITHOUT_DATA`: assistant/message with no data
  *   member; `FAKE_MALFORMED_REASON`: `session.finished` reason is a bare
  *   string (wire-validation probes).
+ * - `FAKE_EMPTY_MESSAGE`: the turn streams a text chunk, then records an empty
+ *   assistant/message for a usage-only max-tokens step.
  * - `FAKE_HANG_INIT`: never answer `initialize` (mid-handshake cancel probe).
  * - `FAKE_INIT_READY` + `FAKE_INIT_GO`: touch the READY file when `initialize`
  *   arrives, then poll for the GO file before answering (deterministic
@@ -117,7 +119,9 @@ function runTurn(sessionId: string): void {
     message: {
       id: `fake-assistant-${seq}`,
       role: 'assistant',
-      content: [{ type: 'text', text }],
+      // Model the usage-only message recorded after a max-tokens step that
+      // assembled no output blocks.
+      content: env.FAKE_EMPTY_MESSAGE !== undefined ? [] : [{ type: 'text', text }],
       source: { kind: 'model', provider: 'fake', model: 'fake' },
     },
   })
diff --git a/packages/scaffold/protocol/README.i18n.yaml b/packages/scaffold/protocol/README.i18n.yaml
index f038434410..541155d37c 100644
--- a/packages/scaffold/protocol/README.i18n.yaml
+++ b/packages/scaffold/protocol/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/scaffold/protocol/README.md
-README.md: 88a48957d0d44cec9f776d31eab7d25bd353de5f
-README.zh.md: 6618d8838a00f945c79d7ec24b1e7491df08a3f1
+README.md: 082a890454f900aec51df123669f28814d39d601
+README.zh.md: d9b8460e51b5313f4c3a8ac66471e8cd39142430
diff --git a/packages/scaffold/protocol/README.md b/packages/scaffold/protocol/README.md
index 88a48957d0..082a890454 100644
--- a/packages/scaffold/protocol/README.md
+++ b/packages/scaffold/protocol/README.md
@@ -22,7 +22,7 @@ The shared wire protocol for the DeepSeek Harness SDK runtime: one newline-delim
 | server→client | `subagent.started` | `SubagentStartedNotification` |
 | server→client | `subagent.finished` | `SubagentFinishedNotification` (in-process runs only) |
 
-`HarnessSdkRequestMap` and `HarnessSdkNotificationMap` index these by method name. `SessionPromptResult.messageId` identifies the queued `UserMessage`; it does not identify a later assistant message, turn ending, or prompt result. Clients combine the open-ended `session.event` stream with agent-wide `session.status` according to their own activity ownership. `InitializeParams.maxTokens` is an optional positive safe integer that caps each conversation-model output for SDK-created agents and their in-process descendants; omission allows the selected adapter's exact-model default to apply, or otherwise preserves provider behavior. The notification payload types depend on `SessionEvent` (`dsh-session`), `ContentBlock` (`dsh-llm`), and `SubagentStopReason` (`dsh-subagent`) — the protocol streams full session-log envelopes, so the session vocabulary is part of the wire contract. `serverInfo.name` stays the wire-stable `deepseek-harness-sdk-runtime`.
+`HarnessSdkRequestMap` and `HarnessSdkNotificationMap` index these by method name. `SessionPromptResult.messageId` identifies the queued `UserMessage`; it does not identify a later assistant message, turn ending, or prompt result. Clients combine the open-ended `session.event` stream with agent-wide `session.status` according to their own activity ownership. `SubagentFinishedNotification.lastAssistantMessage` contains the child's last non-empty assistant message or, when no such message exists, its accumulated assistant text; the field is absent when the child produced neither. `InitializeParams.maxTokens` is an optional positive safe integer that caps each conversation-model output for SDK-created agents and their in-process descendants; omission allows the selected adapter's exact-model default to apply, or otherwise preserves provider behavior. The notification payload types depend on `SessionEvent` (`dsh-session`), `ContentBlock` (`dsh-llm`), and `SubagentStopReason` (`dsh-subagent`) — the protocol streams full session-log envelopes, so the session vocabulary is part of the wire contract. `serverInfo.name` stays the wire-stable `deepseek-harness-sdk-runtime`.
 
 ## Model Experience
 
diff --git a/packages/scaffold/protocol/README.zh.md b/packages/scaffold/protocol/README.zh.md
index 6618d8838a..d9b8460e51 100644
--- a/packages/scaffold/protocol/README.zh.md
+++ b/packages/scaffold/protocol/README.zh.md
@@ -22,7 +22,7 @@ DeepSeek Harness SDK 运行时的共享协议格式（wire format）：一个按
 | server→client | `subagent.started` | `SubagentStartedNotification` |
 | server→client | `subagent.finished` | `SubagentFinishedNotification`（仅进程内运行） |
 
-`HarnessSdkRequestMap` 与 `HarnessSdkNotificationMap` 按方法名索引这些类型。`SessionPromptResult.messageId` 标识已排队的 `UserMessage`；它不标识后续的助手消息、轮次结束或提示词结果。客户端根据自己对活动区间的所有权，组合持续开放的 `session.event` 流与 agent 级的 `session.status`。`InitializeParams.maxTokens` 是可选的正的安全整数，用于限制 SDK 创建的 agent 及其进程内后代的每次对话模型输出；省略时会应用所选适配器的确切模型默认值，否则提供方行为保持不变。通知载荷类型依赖 `SessionEvent`（`dsh-session`）、`ContentBlock`（`dsh-llm`）与 `SubagentStopReason`（`dsh-subagent`）——协议以完整会话日志封套进行流式传输，因此会话词汇是协议格式约定的一部分。`serverInfo.name` 的协议值固定为 `deepseek-harness-sdk-runtime`。
+`HarnessSdkRequestMap` 与 `HarnessSdkNotificationMap` 按方法名索引这些类型。`SessionPromptResult.messageId` 标识已排队的 `UserMessage`；它不标识后续的助手消息、轮次结束或提示词结果。客户端根据自己对活动区间的所有权，组合持续开放的 `session.event` 流与 agent 级的 `session.status`。`SubagentFinishedNotification.lastAssistantMessage` 包含子 agent 最后一条非空 assistant 消息；若不存在这类消息，则包含其累积的 assistant 文本；子 agent 两种输出均未产生时，该字段缺省。`InitializeParams.maxTokens` 是可选的正的安全整数，用于限制 SDK 创建的 agent 及其进程内后代的每次对话模型输出；省略时会应用所选适配器的确切模型默认值，否则提供方行为保持不变。通知载荷类型依赖 `SessionEvent`（`dsh-session`）、`ContentBlock`（`dsh-llm`）与 `SubagentStopReason`（`dsh-subagent`）——协议以完整会话日志封套进行流式传输，因此会话词汇是协议格式约定的一部分。`serverInfo.name` 的协议值固定为 `deepseek-harness-sdk-runtime`。
 
 ## 模型体验
 
diff --git a/packages/scaffold/protocol/src/types.ts b/packages/scaffold/protocol/src/types.ts
index dc8e11587f..16af2a76ac 100644
--- a/packages/scaffold/protocol/src/types.ts
+++ b/packages/scaffold/protocol/src/types.ts
@@ -85,7 +85,7 @@ export interface SubagentFinishedNotification {
   status: SdkRunStatus
   /** The provider-reported stop reason. */
   stopReason: SubagentStopReason
-  /** The child's final assistant message, when it produced one. */
+  /** The child's selected assistant output; absent when the child produced none. */
   lastAssistantMessage?: ContentBlock[]
 }
 
diff --git a/packages/scaffold/server/tests/built-scope-carrier.e2e.ts b/packages/scaffold/server/tests/built-scope-carrier.e2e.ts
index b3519110b5..a51c88ddb3 100644
--- a/packages/scaffold/server/tests/built-scope-carrier.e2e.ts
+++ b/packages/scaffold/server/tests/built-scope-carrier.e2e.ts
@@ -106,6 +106,8 @@ describe.skipIf(!existsSync(jsonrpcBundle))('dsh-jsonrpc BUILT scope carrier', (
     })
 
     expect(stderr).not.toContain('listener threw')
+    // A result without output omits lastAssistantMessage from the wire; it
+    // never sends `[]`.
     expect(JSON.parse(stdout) as unknown).toEqual([{
       method: 'subagent.finished',
       params: {
@@ -115,7 +117,6 @@ describe.skipIf(!existsSync(jsonrpcBundle))('dsh-jsonrpc BUILT scope carrier', (
         childSessionId: 'built-child',
         status: 'ok',
         stopReason: 'completed',
-        lastAssistantMessage: [],
       },
     }])
   })
diff --git a/packages/scaffold/server/tests/server.spec.ts b/packages/scaffold/server/tests/server.spec.ts
index 6cd1c6ed99..256e7bfe42 100644
--- a/packages/scaffold/server/tests/server.spec.ts
+++ b/packages/scaffold/server/tests/server.spec.ts
@@ -736,6 +736,8 @@ describe('HarnessSdkServer', () => {
         stopReason: 'error',
       })
 
+      // A result without output omits lastAssistantMessage from the wire; it
+      // never sends `[]`.
       expect(transport.notifications).toContainEqual({
         method: 'subagent.finished',
         params: {
@@ -745,7 +747,6 @@ describe('HarnessSdkServer', () => {
           childSessionId: 'fallback-child-session',
           status: 'ok',
           stopReason: 'max-tokens',
-          lastAssistantMessage: [],
         },
       })
       expect(transport.notifications).toContainEqual({
diff --git a/packages/subagent/subagent-acp/src/run.ts b/packages/subagent/subagent-acp/src/run.ts
index 7c82bfe9fb..38329244ba 100644
--- a/packages/subagent/subagent-acp/src/run.ts
+++ b/packages/subagent/subagent-acp/src/run.ts
@@ -24,6 +24,7 @@ import {
 } from '@agentclientprotocol/sdk'
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import { SessionId } from '@deepseek-ai/dsh-session'
+import { AssistantOutputFold } from '@deepseek-ai/dsh-subagent'
 import type { SubagentResult, SubagentRun, SubagentStartRequest, SubagentStopReason } from '@deepseek-ai/dsh-subagent'
 import type { SubprocessHandle, SubprocessSpawnSpec } from '@deepseek-ai/dsh-subprocess'
 
@@ -232,8 +233,9 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
   let processDisposal: Promise<void> | undefined
   const disposeProcess = (): Promise<void> => (processDisposal ??= disposeAcpChild(child, spec.disposeEofGraceMs))
 
-  // Accumulate the child's streamed assistant text — the SubagentResult output.
-  const output: string[] = []
+  // ACP exposes no complete assistant messages, so the shared fold selects its
+  // accumulated assistant text.
+  const fold = new AssistantOutputFold()
   // Shared mutable state keeps cancellation visible across async closures.
   const flags = { cancelled: false }
 
@@ -241,7 +243,7 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
     sessionUpdate(params: SessionNotification): Promise<void> {
       const update = params.update
       if (update.sessionUpdate === 'agent_message_chunk') {
-        output.push(acpContentText(update.content))
+        fold.pushText(acpContentText(update.content))
       }
       // Other updates (thoughts, tool calls, plans) are consumed but not
       // surfaced — the subagent returns only its final answer.
@@ -284,13 +286,8 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
   const onAbort = (): void => { requestCancel() }
   request.signal.addEventListener('abort', onAbort, { once: true })
 
-  // The accumulated child text as harness ContentBlocks (empty array when the
-  // child streamed nothing). Read at every return so a partial answer survives
-  // a later cancel/error.
-  const collectOutput = (): ContentBlock[] => {
-    const text = output.join('')
-    return text.length > 0 ? [{ type: 'text', text }] : []
-  }
+  // Read at every return so a partial answer survives a later cancel/error.
+  const collectOutput = (): ContentBlock[] => fold.collect() ?? []
 
   // Establish the remote session before publishing a handle. Any failure owns
   // the still-private process and therefore reaps it before rejecting.
diff --git a/packages/subagent/subagent-dsh-sdk/README.i18n.yaml b/packages/subagent/subagent-dsh-sdk/README.i18n.yaml
index b070f9f5d6..0d7e60cc46 100644
--- a/packages/subagent/subagent-dsh-sdk/README.i18n.yaml
+++ b/packages/subagent/subagent-dsh-sdk/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/subagent/subagent-dsh-sdk/README.md
-README.md: 0bbcfa105ecf024a2492d39d3bf8d28956110050
-README.zh.md: 8c8551f85951aa8475ab2ce95771e4d54e0ed89a
+README.md: 493bb187d45c7654958cfb3dbbe1dee6bb21b368
+README.zh.md: 2e1d9b1e602f2180d20d43fe8c358163ec4ec024
diff --git a/packages/subagent/subagent-dsh-sdk/README.md b/packages/subagent/subagent-dsh-sdk/README.md
index 0bbcfa105e..493bb187d4 100644
--- a/packages/subagent/subagent-dsh-sdk/README.md
+++ b/packages/subagent/subagent-dsh-sdk/README.md
@@ -10,7 +10,7 @@ The SDK provider runs each subagent as a complete DeepSeek Harness runtime in a
 
 The working directory resolves exactly like the ACP backend, through the seam's shared out-of-process helpers ([`dsh-subagent`](../subagent/README.md)): the configured `cwd` override when set (validated once at load), else the delegating parent session's cwd — never the server process's own cwd. The resolved path becomes the child process cwd and the workspace cwd of its SDK session.
 
-The returned run id is minted in the parent namespace; the child runtime's session id exists only inside the child process. After publication the provider owns one SDK activity and reads the child's answer from its session events: the last complete `assistant/message`, or the `text-delta` stream accumulated before the activity was cut short — a partial answer survives cancel and error paths.
+The returned run id is minted in the parent namespace; the child runtime's session id exists only inside the child process. After publication the provider owns one SDK activity and reads the child's answer from its session events: the last complete non-empty `assistant/message` (an empty-content message that records usage is skipped), or the accumulated `text-delta` stream when no such message exists. Partial output remains available after cancellation or an error.
 
 `dispose()` is idempotent: it settles the result locally as `aborted` (there is no wire-level prompt cancel), then closes the runtime — a bounded protocol `shutdown` request followed by the shared stdin-EOF → SIGTERM → SIGKILL ladder to actual exit.
 
diff --git a/packages/subagent/subagent-dsh-sdk/README.zh.md b/packages/subagent/subagent-dsh-sdk/README.zh.md
index 8c8551f859..2e1d9b1e60 100644
--- a/packages/subagent/subagent-dsh-sdk/README.zh.md
+++ b/packages/subagent/subagent-dsh-sdk/README.zh.md
@@ -10,7 +10,7 @@ SDK 提供方会在全新的子进程中把每个 subagent 作为完整的 DeepS
 
 工作目录的解析与 ACP 后端完全一致，并使用 seam 共享的进程外辅助工具（[`dsh-subagent`](../subagent/README.md)）：设置了 `cwd` 覆盖值时使用该值（加载时校验一次），否则使用发起委派的父会话 cwd，绝不使用服务器进程自身的 cwd。解析出的路径同时成为子进程 cwd 和其 SDK 会话的工作区 cwd。
 
-返回的 run id 在父级命名空间中生成；子运行时的会话 id 只存在于子进程内部。发布后，提供方拥有一段 SDK 活动，并从子会话事件中读取答案：最后一条完整的 `assistant/message`，或该活动中断前已经累积的 `text-delta` 流；部分答案在取消和错误路径上都得以保留。
+返回的 run id 在父级命名空间中生成；子运行时的会话 id 只存在于子进程内部。发布后，提供方拥有一段 SDK 活动，并从子会话事件中读取答案：最后一条完整且非空的 `assistant/message`（记录 usage 的空内容消息会被跳过）；若没有这类消息，则取累积的 `text-delta` 流。取消或发生错误后，部分输出仍然可用。
 
 `dispose()`（资源释放）是幂等的：先在本地把结果确定为 `aborted`（协议层面没有提示词取消机制），再关闭运行时，即先发出一次有界的协议 `shutdown` 请求，随后通过共享的 stdin-EOF → SIGTERM → SIGKILL 阶梯使进程实际退出。
 
diff --git a/packages/subagent/subagent-dsh-sdk/src/run.ts b/packages/subagent/subagent-dsh-sdk/src/run.ts
index b8a01ea383..194ce3badf 100644
--- a/packages/subagent/subagent-dsh-sdk/src/run.ts
+++ b/packages/subagent/subagent-dsh-sdk/src/run.ts
@@ -16,7 +16,7 @@ import { DeepSeekHarness, type HarnessNotification } from '@deepseek-ai/dsh-sdk-
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import { SessionId, type SessionEvent, type TurnEndReason } from '@deepseek-ai/dsh-session'
 import type { SubagentResult, SubagentRun, SubagentStartRequest, SubagentStopReason } from '@deepseek-ai/dsh-subagent'
-import { settleRunResult, subprocessRunHandle } from '@deepseek-ai/dsh-subagent'
+import { AssistantOutputFold, settleRunResult, subprocessRunHandle } from '@deepseek-ai/dsh-subagent'
 import { scrubbedParentEnv } from '@deepseek-ai/dsh-subprocess'
 
 /** Resolved spawn spec for an SDK runtime child process (no defaults — see Config). */
@@ -163,24 +163,14 @@ export async function startSdkRun(request: SubagentStartRequest, spec: SdkRunSpe
   }
 
   const childSessionId = `session-${randomUUID().replaceAll('-', '')}`
-  // The child's final answer: the last complete assistant message when one
-  // exists, else the text streamed so far (a partial answer surviving cancel).
-  let lastMessage: ContentBlock[] | undefined
-  const partial: string[] = []
+  // The child's final answer under the seam's canonical selection rule
+  // (`AssistantOutputFold`); a partial answer survives cancel and error paths.
+  const fold = new AssistantOutputFold()
   const observe = (notification: HarnessNotification): void => {
     if (notification.method !== 'session.event' || notification.params.sessionId !== childSessionId) return
-    const event = notification.params.event as SessionEvent
-    if (event.type === 'assistant/chunk' && event.data.chunk.type === 'text-delta') {
-      partial.push(event.data.chunk.text)
-    } else if (event.type === 'assistant/message') {
-      lastMessage = event.data.message.content
-    }
-  }
-  const collectOutput = (): ContentBlock[] => {
-    if (lastMessage !== undefined) return lastMessage
-    const text = partial.join('')
-    return text.length > 0 ? [{ type: 'text', text }] : []
+    fold.push(notification.params.event as SessionEvent)
   }
+  const collectOutput = (): ContentBlock[] => fold.collect() ?? []
 
   // Race the child turn against local cancellation; the shared settlement
   // flattens failures under the seam's never-reject contract.
diff --git a/packages/subagent/subagent-dsh-sdk/tests/subagent-dsh-sdk.spec.ts b/packages/subagent/subagent-dsh-sdk/tests/subagent-dsh-sdk.spec.ts
index 26043f6a8d..1f518bde8d 100644
--- a/packages/subagent/subagent-dsh-sdk/tests/subagent-dsh-sdk.spec.ts
+++ b/packages/subagent/subagent-dsh-sdk/tests/subagent-dsh-sdk.spec.ts
@@ -176,6 +176,20 @@ describe('dsh-subagent-dsh-sdk provider', () => {
     await ctx.fiber.dispose()
   })
 
+  it('keeps streamed text when the terminal message is an empty usage-only step', async () => {
+    // The child streams its answer, then emits an empty-content
+    // assistant/message (the harness loop appends one to host usage on a
+    // max-tokens step that assembled no text blocks). The empty message is
+    // not assistant output and must not erase the streamed answer.
+    const ctx = await setup({ FAKE_EMPTY_MESSAGE: '1', FAKE_REASON_KIND: 'max-tokens' })
+    const run = await ctx.subagents.start('dsh-sdk', request())
+    const result = await run.result
+    expect(result.stopReason).toBe('max-tokens')
+    expect(text(result.output)).toBe('hello from fake runtime')
+    await run.dispose()
+    await ctx.fiber.dispose()
+  })
+
   it('reports a settled-without-turn child as an error', async () => {
     const ctx = await setup({ FAKE_REASON_KIND: 'none', FAKE_STATUS: 'error' })
     const run = await ctx.subagents.start('dsh-sdk', request())
diff --git a/packages/subagent/subagent-inprocess/README.i18n.yaml b/packages/subagent/subagent-inprocess/README.i18n.yaml
index 78106c0d6d..2514a7d420 100644
--- a/packages/subagent/subagent-inprocess/README.i18n.yaml
+++ b/packages/subagent/subagent-inprocess/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/subagent/subagent-inprocess/README.md
-README.md: 209f1e9526ff4a01af6f4c96955068de4b2b06c0
-README.zh.md: 8623be4bc1ab39aa7718de204dd0507843b0ab14
+README.md: 69def8bf8f41e3685d017ac4b003b26a37f064ef
+README.zh.md: bf5e7cb5cc8517ee7020695ef10e3b58d613541b
diff --git a/packages/subagent/subagent-inprocess/README.md b/packages/subagent/subagent-inprocess/README.md
index 209f1e9526..69def8bf8f 100644
--- a/packages/subagent/subagent-inprocess/README.md
+++ b/packages/subagent/subagent-inprocess/README.md
@@ -14,7 +14,7 @@ The driver follows this sequence:
 2. Call `parent.ctx.agents.create` directly, passing the required request signal into the factory's creation transaction.
 3. During that transaction's unpublished setup window, install the requested persona, tool restriction, and structured-output runtime.
 4. Publish the child, retain the returned `AgentHandle`, and drive one task with `child.followup(prompt)` followed by `child.whenIdle()`.
-5. Read the child's own last assistant message and final durable turn reason from the complete owned child run, excluding any fork seed.
+5. Read the child's own output — its last non-empty assistant message (an empty-content message that records usage is skipped), or its accumulated assistant text when no such message exists — and the final durable turn reason from the complete owned child run, excluding any fork seed.
 
 The child gets the parent's working-directory/session lineage and inherits the parent provider, model, and output-token cap unless `request.agentOptions` overrides them. It gets a fresh flat registration scope: parent ownership does not import parent tool restrictions or establish an authority subset.
 
diff --git a/packages/subagent/subagent-inprocess/README.zh.md b/packages/subagent/subagent-inprocess/README.zh.md
index 8623be4bc1..bf5e7cb5cc 100644
--- a/packages/subagent/subagent-inprocess/README.zh.md
+++ b/packages/subagent/subagent-inprocess/README.zh.md
@@ -14,7 +14,7 @@
 2. 直接调用 `parent.ctx.agents.create`，把必需的请求信号传入工厂的创建事务。
 3. 在该事务未发布的设置窗口中，安装请求的 persona、工具限制和结构化输出运行时。
 4. 发布子 agent，保留返回的 `AgentHandle`，并通过先调用 `child.followup(prompt)`、再调用 `child.whenIdle()` 来驱动一项任务。
-5. 从完整的自有子运行中读取子 agent 自身最后一条 assistant 消息和最终持久化的轮次原因，并排除任何 fork 初始内容。
+5. 从完整的自有子运行中读取子 agent 自身的输出——最后一条非空 assistant 消息（记录 usage 的空内容消息会被跳过），若没有这类消息则取其累积的 assistant 文本——以及最终持久化的轮次原因，并排除任何 fork 初始内容。
 
 子 agent 会获得父 agent 的工作目录／会话谱系；除非 `request.agentOptions` 覆盖，否则还会继承父 agent 的提供方、模型和输出 token 上限。它获得全新的扁平注册作用域：父级所有权不会导入父 agent 的工具限制，也不会建立权限子集。
 
diff --git a/packages/subagent/subagent-inprocess/src/index.ts b/packages/subagent/subagent-inprocess/src/index.ts
index ac32fe6b66..b0ab7425a5 100644
--- a/packages/subagent/subagent-inprocess/src/index.ts
+++ b/packages/subagent/subagent-inprocess/src/index.ts
@@ -22,6 +22,7 @@ import {
   assertSubagentMaxDepth,
   captureDelegatedPolicyOverrides,
   childSessionMeta,
+  finalAssistantOutput,
   resolveChildAgentOptions,
   resolveChildDepth,
 } from '@deepseek-ai/dsh-subagent'
@@ -206,9 +207,9 @@ function readResult(
   structured?: { captured?: { value: unknown } | undefined },
 ): SubagentResult {
   const own = child.session.events.slice(boundary)
-  const lastMessage = own.findLast((event): event is SessionEvent<'assistant/message'> => event.type === 'assistant/message')
   const lastEnd = findLastMessageTurnEnd(own)
-  const output: ContentBlock[] = lastMessage?.data.message.content ?? []
+  // The seam's canonical selection rule; a partial answer survives cancel and truncation.
+  const output: ContentBlock[] = finalAssistantOutput(own) ?? []
   const recorded = toStopReason(lastEnd?.data.reason)
   // Disposal can tear the owner down before the loop records its ordinary
   // `aborted` end, yielding `disposed` instead.
diff --git a/packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts b/packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts
index c490bc7d3b..4b9041ebf9 100644
--- a/packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts
+++ b/packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts
@@ -1,4 +1,4 @@
-import { createUserMessage } from '@deepseek-ai/dsh-llm'
+import { CallId, createUserMessage } from '@deepseek-ai/dsh-llm'
 import { describe, expect, it } from 'vitest'
 import { Context } from '@deepseek-ai/cordis'
 import { type Agent, type AgentOptions } from '@deepseek-ai/dsh-agent'
@@ -10,7 +10,8 @@ import * as SessionInvariant from '@deepseek-ai/dsh-session/invariant'
 import * as AgentInvariant from '@deepseek-ai/dsh-agent/invariant'
 import * as AgentLoopInvariant from '@deepseek-ai/dsh-agent-loop/invariant'
 import SubagentService, { snapshotSubagentDescriptor } from '@deepseek-ai/dsh-subagent'
-import { maxTokensResponse, MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
+import { defineContentToolFixture } from '@deepseek-ai/dsh-tools'
+import { maxTokensResponse, MockAdapter, textResponse, toolCallResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
 import { startInProcessRun } from '../src/index.ts'
 
 type Script = ConstructorParameters<typeof MockAdapter>[0]
@@ -155,6 +156,31 @@ describe('startInProcessRun', () => {
     await run.dispose()
   })
 
+  it('keeps earlier streamed text when the final step appends an empty usage-only message', async () => {
+    // A tool-only max-tokens step records an empty assistant/message for
+    // usage. The result retains the preceding assistant output.
+    const { ctx, parent } = await setup([
+      toolCallResponse('t1', 'noop', {}, 'partial one'),
+      [
+        { type: 'block-start', index: 0, blockType: 'tool-call' },
+        { type: 'tool-call-delta', index: 0, id: CallId('t2'), name: 'noop', argumentsDelta: '{}' },
+        { type: 'block-end', index: 0, block: { type: 'tool-call', id: CallId('t2'), name: 'noop', arguments: '{}' } },
+        { type: 'usage', usage: { inputTokens: 20, outputTokens: 5 } },
+        { type: 'finish', reason: { kind: 'max-tokens' } },
+      ],
+    ])
+    const disposeNoop = ctx.tools.register(defineContentToolFixture({
+      name: 'noop', description: 'probe', parameters: {},
+      execute() { return Promise.resolve([{ type: 'text', text: 'noop result' }]) },
+    }))
+    const run = await startInProcessRun(request(parent), {})
+    const result = await run.result
+    expect(result.stopReason).toBe('max-tokens')
+    expect(text(result.output)).toBe('partial one')
+    await run.dispose()
+    disposeNoop()
+  })
+
   it('seeds a forked child but reads only the child-owned output', async () => {
     const { ctx, parent } = await setup([textResponse('parent answer'), textResponse('child answer')])
     parent.followup(createUserMessage({ content: [{ type: 'text', text: 'parent question' }], source: { kind: 'user' } }))
@@ -278,7 +304,12 @@ describe('startInProcessRun', () => {
     const signalled = await startInProcessRun(request(parent, controller.signal), {})
     await new Promise(resolve => setTimeout(resolve, 30))
     controller.abort('stop child')
-    await expect(signalled.result).resolves.toMatchObject({ stopReason: 'aborted' })
+    // No step completed a message, so the text streamed before the abort is
+    // the cancelled run's output.
+    await expect(signalled.result).resolves.toEqual({
+      output: [{ type: 'text', text: 'partial' }],
+      stopReason: 'aborted',
+    })
     expect(adapter.requests[0]?.signal?.reason).toEqual({ kind: 'parent' })
     const child = parent.ctx.agents.get(signalled.id)
     const turnEnd = child?.session.events.findLast(event => event.type === 'turn/end')
diff --git a/packages/subagent/subagent/README.i18n.yaml b/packages/subagent/subagent/README.i18n.yaml
index bbcdac2d00..c70bcdf58b 100644
--- a/packages/subagent/subagent/README.i18n.yaml
+++ b/packages/subagent/subagent/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/subagent/subagent/README.md
-README.md: 42a10adeccb8e299e25ff0a5e0a918ef09b79617
-README.zh.md: 34e2ed6c1ca23df9b3158f3caea10cd19bafa841
+README.md: 28f649ef54bbf88feda24a9ce197c2c366f8349b
+README.zh.md: 595c5e5e7fffc367f2e3fd8142b779dc22fb3b79
diff --git a/packages/subagent/subagent/README.md b/packages/subagent/subagent/README.md
index 42a10adecc..28f649ef54 100644
--- a/packages/subagent/subagent/README.md
+++ b/packages/subagent/subagent/README.md
@@ -64,7 +64,7 @@ Both in-process delegation paths fix the child's permission scope at the delegat
 
 `provider.start(request): Promise<SubagentRun>` is the ownership-transfer boundary; the delegation tool also uses it inside its one-shot Task-backed background path. Before fulfillment, the provider owns setup and must cancel, roll back, and quiesce unpublished resources on every failure. After fulfillment, the caller owns the run and must call `dispose()` on every path; remaining prompt and turn work belongs to `SubagentRun.result`.
 
-`SubagentRun.result` resolves to `{ output, structured?, stopReason }`. Child-level failures resolve with a non-`completed` reason; only an infrastructure fault that the seam cannot represent may reject. `dispose()` is idempotent, cancels remaining work, and waits for both result settlement and child-resource quiescence. A result rejection remains on `result`; `dispose()` rejects only for an independent resource-release failure.
+`SubagentRun.result` resolves to `{ output, structured?, stopReason }`. Child-level failures resolve with a non-`completed` reason; only an infrastructure fault that the seam cannot represent may reject. `dispose()` is idempotent, cancels remaining work, and waits for both result settlement and child-resource quiescence. A result rejection remains on `result`; `dispose()` rejects only for an independent resource-release failure. `output` and the `subagent/end` event's `lastAssistantMessage` use the exported `AssistantOutputFold`/`finalAssistantOutput` helpers to select the child's last non-empty assistant message, or its accumulated assistant text when no such message exists. `output` is `[]` and the event field is absent when the child produced neither ([`SubagentResult.output`](../../../docs/subsystems/subagent.md#the-terminal-result-subagentresult) owns the result contract).
 
 A local run publishes an ordinary child agent/session before `start()` fulfills, returns that shared session id as `SubagentRun.id`, exposes the exact child as `SubagentRun.localAgent`, records `request.parent.session.id` in the child's `parentSession` header, and appends the resolved descriptor inside its initial turn. Remote providers instead mint a parent-scoped lifecycle id and return `localAgent: undefined`; without a local child session, their one-shot runs are not part of trace-backed enumeration.
 
diff --git a/packages/subagent/subagent/README.zh.md b/packages/subagent/subagent/README.zh.md
index 34e2ed6c1c..595c5e5e7f 100644
--- a/packages/subagent/subagent/README.zh.md
+++ b/packages/subagent/subagent/README.zh.md
@@ -64,7 +64,7 @@ subagent seam 允许一个 agent（智能体）通过具名提供方把工作委
 
 `provider.start(request): Promise<SubagentRun>` 是所有权转移边界；委派工具也会在其由 Task 支撑的一次性后台路径中使用它。兑现前，提供方拥有设置过程，并且每次失败时都必须取消、回滚并使未发布资源完全停稳。兑现后，调用方拥有该运行，并且必须在每条路径上调用 `dispose()`；剩余提示词和轮次工作属于 `SubagentRun.result`。
 
-`SubagentRun.result` 兑现为 `{ output, structured?, stopReason }`。子 agent 级失败会以非 `completed` 原因兑现；只有 seam 无法表示的基础设施故障才可以拒绝。`dispose()` 是幂等的，会取消剩余工作，并等待结果结算以及子 agent 资源完全停稳。`result` 的 rejection 仍归 `result` 通道；只有独立的资源释放失败会使 `dispose()` 拒绝。
+`SubagentRun.result` 兑现为 `{ output, structured?, stopReason }`。子 agent 级失败会以非 `completed` 原因兑现；只有 seam 无法表示的基础设施故障才可以拒绝。`dispose()` 是幂等的，会取消剩余工作，并等待结果结算以及子 agent 资源完全停稳。`result` 的 rejection 仍归 `result` 通道；只有独立的资源释放失败会使 `dispose()` 拒绝。`output` 与 `subagent/end` 事件的 `lastAssistantMessage` 使用导出的 `AssistantOutputFold`／`finalAssistantOutput` 辅助函数选取子 agent 最后一条非空 assistant 消息；若没有这类消息，则选取其累积的 assistant 文本。子 agent 两种输出均未产生时，`output` 为 `[]`，该事件字段缺省（结果约定归 [`SubagentResult.output`](../../../docs/subsystems/subagent.md#the-terminal-result-subagentresult) 所有）。
 
 本地运行会在 `start()` 兑现前发布普通的子 agent／会话，把该共享会话 id 作为 `SubagentRun.id` 返回，以 `SubagentRun.localAgent` 公开准确的子 agent，把 `request.parent.session.id` 记录到子 agent 的 `parentSession` header，并在其初始轮次内追加已解析的描述符。远程提供方则生成 parent 作用域的生命周期 id，并返回 `localAgent: undefined`；由于没有本地 child 会话，其一次性运行不会进入基于追踪的枚举结果。
 
diff --git a/packages/subagent/subagent/src/assistant-output.ts b/packages/subagent/subagent/src/assistant-output.ts
new file mode 100644
index 0000000000..6701327cfd
--- /dev/null
+++ b/packages/subagent/subagent/src/assistant-output.ts
@@ -0,0 +1,74 @@
+/**
+ * Canonical selection of a child's final assistant output. Backend run results
+ * and `subagent/end.lastAssistantMessage` apply the same rule: select the last
+ * non-empty assistant message. An empty-content message records usage only
+ * when the loop appends it after a max-tokens step with no executable blocks,
+ * so it does not replace earlier output. If no non-empty message exists,
+ * select the accumulated assistant text. Selection is independent of the
+ * run's stop reason.
+ *
+ * @module @deepseek-ai/dsh-subagent/assistant-output
+ */
+
+import type { ContentBlock } from '@deepseek-ai/dsh-llm'
+import type { SessionEvent } from '@deepseek-ai/dsh-session'
+
+/**
+ * Incremental fold of the selection rule, for backends that observe a child's
+ * output as it streams: session-event backends {@link push} each event, and
+ * transports without session events (ACP content chunks) {@link pushText} raw
+ * text into the same streamed fallback.
+ */
+export class AssistantOutputFold {
+  private message: ContentBlock[] | undefined
+  private partial: string[] = []
+
+  /**
+   * Fold one session event: a non-empty assistant message becomes the
+   * candidate final answer, and a `text-delta` chunk extends the streamed
+   * fallback; every other event contributes nothing.
+   * @param event - the next observed session event.
+   */
+  push(event: SessionEvent): void {
+    if (event.type === 'assistant/message') {
+      const content = event.data.message.content
+      if (content.length > 0) this.message = content
+    } else if (event.type === 'assistant/chunk' && event.data.chunk.type === 'text-delta') {
+      this.pushText(event.data.chunk.text)
+    }
+  }
+
+  /**
+   * Extend the streamed fallback with text observed outside session events.
+   * @param text - the next streamed text piece (an empty piece is a no-op).
+   */
+  pushText(text: string): void {
+    if (text.length > 0) this.partial.push(text)
+  }
+
+  /**
+   * Select the final output folded so far.
+   * @returns the last non-empty assistant message, else the accumulated
+   *   streamed text, or `undefined` when the child produced neither.
+   */
+  collect(): ContentBlock[] | undefined {
+    if (this.message !== undefined) return this.message
+    const text = this.partial.join('')
+    return text.length > 0 ? [{ type: 'text', text }] : undefined
+  }
+}
+
+/**
+ * Apply the selection rule to one complete child-owned event suffix.
+ * @param events - the child-owned events (after any seed or epoch boundary).
+ * @returns the selected output, or `undefined` when the child produced none.
+ */
+export function finalAssistantOutput(events: readonly SessionEvent[]): ContentBlock[] | undefined {
+  // TODO: this folds the complete suffix once per run/epoch settlement. If a
+  // long continuable epoch ever profiles hot here, scan backward with early
+  // exit for the last non-empty message and fold text deltas only on the
+  // no-message fallback.
+  const fold = new AssistantOutputFold()
+  for (const event of events) fold.push(event)
+  return fold.collect()
+}
diff --git a/packages/subagent/subagent/src/index.ts b/packages/subagent/subagent/src/index.ts
index 916ce431c5..36acb8de28 100644
--- a/packages/subagent/subagent/src/index.ts
+++ b/packages/subagent/subagent/src/index.ts
@@ -69,6 +69,7 @@ import { snapshotSubagentDescriptor } from './descriptor.ts'
 import { subagentIdentityProjectionDefinition, subagentTimingProjectionDefinition } from './projection.ts'
 
 export * from './out-of-process.ts'
+export { AssistantOutputFold, finalAssistantOutput } from './assistant-output.ts'
 export { SubagentRunId } from './types.ts'
 export type {
   ContinuableCreateRequest,
diff --git a/packages/subagent/subagent/src/lifecycle.ts b/packages/subagent/subagent/src/lifecycle.ts
index eff0534d9f..3482a00c0b 100644
--- a/packages/subagent/subagent/src/lifecycle.ts
+++ b/packages/subagent/subagent/src/lifecycle.ts
@@ -20,6 +20,7 @@ import type { Agent } from '@deepseek-ai/dsh-agent'
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import { findLastMessageTurnEnd } from '@deepseek-ai/dsh-session'
 import type { SessionEvent, SessionId } from '@deepseek-ai/dsh-session'
+import { finalAssistantOutput } from './assistant-output.ts'
 import { SubagentRunId } from './types.ts'
 import type { SubagentResult, SubagentRun, SubagentRunEndInfo, SubagentRunInfo } from './types.ts'
 
@@ -128,7 +129,8 @@ export function observeRun(
       emit('subagent/end', {
         ...identity,
         stopReason: result.stopReason,
-        lastAssistantMessage: result.output,
+        // Omit the field when no output exists, matching continuable epochs.
+        ...result.output.length === 0 ? {} : { lastAssistantMessage: result.output },
       }, parent)
     },
     () => {
@@ -173,7 +175,7 @@ export function createActivationObserver(
     },
     capture: (child: Agent): void => {
       const own = child.session.events.slice(boundary)
-      const output = lastAssistantOutput(own)
+      const output = finalAssistantOutput(own)
       captured = {
         stopReason: epochStopReason(own),
         ...output === undefined ? {} : { output },
@@ -220,19 +222,6 @@ function epochStopReason(events: readonly SessionEvent[]): SubagentResult['stopR
   }
 }
 
-/**
- * The child's last assistant message content, for one Activation's terminal
- * lifecycle edge. Absent when no assistant message reached the log.
- * @param events - this epoch's own event suffix.
- * @returns its final assistant content, or `undefined` when it produced none.
- */
-function lastAssistantOutput(events: readonly SessionEvent[]): ContentBlock[] | undefined {
-  const message = events.findLast(
-    (event): event is SessionEvent<'assistant/message'> => event.type === 'assistant/message',
-  )
-  return message?.data.message.content
-}
-
 /** Render any listener-thrown value without letting coercion escape containment. */
 function renderThrown(value: unknown): string {
   try {
diff --git a/packages/subagent/subagent/src/types.ts b/packages/subagent/subagent/src/types.ts
index de2fd37115..63a890176e 100644
--- a/packages/subagent/subagent/src/types.ts
+++ b/packages/subagent/subagent/src/types.ts
@@ -64,7 +64,11 @@ export interface SubagentRunEndInfo {
   readonly local: boolean
   /** The terminal stop reason. */
   readonly stopReason: SubagentResult['stopReason']
-  /** The child's final assistant output, absent on infrastructure rejection. */
+  /**
+   * The child's final assistant output, selected by the same rule as
+   * {@link SubagentResult.output}; absent on infrastructure rejection or when
+   * the child produced none.
+   */
   readonly lastAssistantMessage?: ContentBlock[]
 }
 
@@ -213,7 +217,12 @@ export type SubagentStopReason = SubagentStopReasonMap[keyof SubagentStopReasonM
  * The terminal outcome of a subagent run, resolved by {@link SubagentRun.result}.
  */
 export interface SubagentResult {
-  /** The child's final assistant output (the last assistant message's content). */
+  /**
+   * The child's final assistant output is the content of its last non-empty
+   * assistant message. Empty-content messages, including usage-only messages,
+   * are skipped. Without a non-empty message, the output is its accumulated
+   * assistant text stream, or `[]` when the child produced neither.
+   */
   readonly output: ContentBlock[]
   /**
    * The structured result after a requested `outputSchema` was successfully
diff --git a/packages/subagent/subagent/tests/assistant-output.spec.ts b/packages/subagent/subagent/tests/assistant-output.spec.ts
new file mode 100644
index 0000000000..3d510a94e8
--- /dev/null
+++ b/packages/subagent/subagent/tests/assistant-output.spec.ts
@@ -0,0 +1,92 @@
+import { describe, expect, it } from 'vitest'
+import type { ContentBlock } from '@deepseek-ai/dsh-llm'
+import type { SessionEvent } from '@deepseek-ai/dsh-session'
+import { AssistantOutputFold, finalAssistantOutput } from '../src/assistant-output.ts'
+
+function message(content: ContentBlock[]): SessionEvent {
+  return { type: 'assistant/message', data: { message: { content } } } as SessionEvent
+}
+
+function textDelta(text: string): SessionEvent {
+  return { type: 'assistant/chunk', data: { chunk: { type: 'text-delta', text } } } as SessionEvent
+}
+
+function reasoningDelta(text: string): SessionEvent {
+  return { type: 'assistant/chunk', data: { chunk: { type: 'reasoning-delta', text } } } as SessionEvent
+}
+
+function toolResult(text: string): SessionEvent {
+  return {
+    type: 'tool/result',
+    data: {
+      message: {
+        content: [{
+          type: 'tool-result',
+          toolCallId: 'call-1',
+          content: [{ type: 'text', text }],
+          isError: false,
+        }],
+      },
+    },
+  } as SessionEvent
+}
+
+describe('finalAssistantOutput', () => {
+  it('selects the last non-empty message past a later empty usage-only message', () => {
+    const events = [
+      message([{ type: 'text', text: 'step one' }]),
+      message([{ type: 'text', text: 'step two' }]),
+      message([]),
+    ]
+    expect(finalAssistantOutput(events)).toEqual([{ type: 'text', text: 'step two' }])
+  })
+
+  it('prefers a non-empty message over text streamed before and after it', () => {
+    const events = [
+      textDelta('earlier partial'),
+      message([{ type: 'text', text: 'complete answer' }]),
+      textDelta('later partial'),
+      message([]),
+    ]
+    expect(finalAssistantOutput(events)).toEqual([{ type: 'text', text: 'complete answer' }])
+  })
+
+  it('treats textless assistant content as a non-empty message', () => {
+    const content: ContentBlock[] = [{ type: 'reasoning', text: 'complete reasoning' }]
+    expect(finalAssistantOutput([
+      textDelta('streamed text'),
+      message(content),
+      textDelta('later partial'),
+    ])).toEqual(content)
+  })
+
+  it('falls back to text deltas without including reasoning or tool-result content', () => {
+    const events = [
+      reasoningDelta('thinking'),
+      textDelta('partial '),
+      toolResult('tool output'),
+      textDelta('answer'),
+      message([]),
+    ]
+    expect(finalAssistantOutput(events)).toEqual([{ type: 'text', text: 'partial answer' }])
+  })
+
+  it('returns undefined when the child produced neither messages nor text', () => {
+    expect(finalAssistantOutput([])).toBeUndefined()
+    expect(finalAssistantOutput([reasoningDelta('thinking'), message([])])).toBeUndefined()
+  })
+})
+
+describe('AssistantOutputFold', () => {
+  it('folds raw text pieces into the same streamed fallback (ACP chunk transport)', () => {
+    const fold = new AssistantOutputFold()
+    fold.pushText('partial ')
+    fold.pushText('')
+    fold.pushText('answer')
+    expect(fold.collect()).toEqual([{ type: 'text', text: 'partial answer' }])
+  })
+
+  it('collects undefined until any output is folded', () => {
+    expect(new AssistantOutputFold().collect()).toBeUndefined()
+  })
+})
diff --git a/packages/subagent/subagent/tests/continuation.spec.ts b/packages/subagent/subagent/tests/continuation.spec.ts
index c91abcf1c0..50bd71b9cc 100644
--- a/packages/subagent/subagent/tests/continuation.spec.ts
+++ b/packages/subagent/subagent/tests/continuation.spec.ts
@@ -12,10 +12,10 @@ import JsonlSessionPersistence from '@deepseek-ai/dsh-session-persistence-jsonl'
 import * as SubagentSpawn from '@deepseek-ai/dsh-subagent-spawn'
 import * as SubagentFork from '@deepseek-ai/dsh-subagent-fork'
 import type { GenerateOptions, MessageId, StreamChunk } from '@deepseek-ai/dsh-llm'
-import { createUserMessage, LlmAdapter } from '@deepseek-ai/dsh-llm'
+import { CallId, createUserMessage, LlmAdapter } from '@deepseek-ai/dsh-llm'
 import { defineTool } from '@deepseek-ai/dsh-tools'
 import InvariantService from '@deepseek-ai/dsh-invariants'
-import { MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
+import { MockAdapter, textResponse, toolCallResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
 import SubagentService, {
   SubagentError,
   SUBAGENT_DESCRIPTOR_VERSION,
@@ -1200,6 +1200,44 @@ describe('continuable review regressions', () => {
     expect(ends[1]!.lastAssistantMessage).toEqual([{ type: 'text', text: 'second answer' }])
   })
 
+  it('keeps the epoch\'s earlier text past a final empty usage-only message', async () => {
+    // A tool-only max-tokens step records an empty assistant/message for
+    // usage. The terminal event retains the previous assistant content,
+    // including its tool call but not the intervening tool result.
+    const { ctx, parent } = await setup([
+      toolCallResponse('t1', 'noop', {}, 'partial one'),
+      [
+        { type: 'block-start', index: 0, blockType: 'tool-call' },
+        { type: 'tool-call-delta', index: 0, id: CallId('t2'), name: 'noop', argumentsDelta: '{}' },
+        { type: 'block-end', index: 0, block: { type: 'tool-call', id: CallId('t2'), name: 'noop', arguments: '{}' } },
+        { type: 'usage', usage: { inputTokens: 20, outputTokens: 5 } },
+        { type: 'finish', reason: { kind: 'max-tokens' } },
+      ],
+    ])
+    ctx.tools.register(defineTool({
+      name: 'noop',
+      description: 'does nothing',
+      parameters: {},
+      output: {
+        schema: { type: 'object', additionalProperties: false, properties: {} },
+        render: () => [{ type: 'text', text: 'noop' }],
+      },
+      execute: () => Promise.resolve({}),
+    }))
+    const ends: SubagentRunEndInfo[] = []
+    ctx.on('subagent/end', (info) => { ends.push(info) })
+
+    const started = await ctx.subagents.startContinuable(startSpec(parent))
+    await waitNoActivation(ctx, started.childId)
+
+    await vi.waitFor(() => { expect(ends).toHaveLength(1) })
+    expect(ends[0]!.stopReason).toBe('max-tokens')
+    expect(ends[0]!.lastAssistantMessage).toEqual([
+      { type: 'text', text: 'partial one' },
+      { type: 'tool-call', id: 't1', name: 'noop', arguments: '{}' },
+    ])
+  })
+
   it('reports a resumed epoch that opened no turn without the previous answer', async () => {
     const { ctx, parent } = await setup([textResponse('first answer')])
     const started = await ctx.subagents.startContinuable(startSpec(parent))
diff --git a/packages/subagent/subagent/tests/service.spec.ts b/packages/subagent/subagent/tests/service.spec.ts
index 5106bc3427..42c778627a 100644
--- a/packages/subagent/subagent/tests/service.spec.ts
+++ b/packages/subagent/subagent/tests/service.spec.ts
@@ -15,6 +15,7 @@ import SubagentService, {
   type SubagentProvider,
   type SubagentResult,
   type SubagentRun,
+  type SubagentRunEndInfo,
   type SubagentStartRequest,
 } from '@deepseek-ai/dsh-subagent'
 import { SessionId, type SessionEvent } from '@deepseek-ai/dsh-session'
@@ -263,6 +264,17 @@ describe('SubagentService', () => {
       stopReason: 'completed',
     }))
 
+    // The lifecycle event omits lastAssistantMessage when output is empty,
+    // matching the continuable epoch event.
+    const silent = new StubProvider('silent', NO_CAPS, { output: [], stopReason: 'completed' })
+    subagents.registerProvider(silent)
+    const silentRun = await subagents.start('silent', baseRequest())
+    await silentRun.result
+    await Promise.resolve()
+    const silentEnd = ended.mock.calls.map(call => call[0] as SubagentRunEndInfo).find(info => info.provider === 'silent')
+    expect(silentEnd).toBeDefined()
+    expect('lastAssistantMessage' in silentEnd!).toBe(false)
+
     const failure = Promise.withResolvers<SubagentResult>()
     subagents.registerProvider({
       name: 'infra',
diff --git a/packages/subagent/tool-subagent/README.i18n.yaml b/packages/subagent/tool-subagent/README.i18n.yaml
index e5f0c43f61..8c41c6413a 100644
--- a/packages/subagent/tool-subagent/README.i18n.yaml
+++ b/packages/subagent/tool-subagent/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/subagent/tool-subagent/README.md
-README.md: 6ec313b3b97f0ffa7488025d4314b1c6231a6f6a
-README.zh.md: 1fd88363b3ade9d57c194580f81295eed139ac50
+README.md: ac3ec0563cce9128608ca31860b034a103dc1a3a
+README.zh.md: d64831d7cf64800ad3307ce6cb7f294500a0a6f0
diff --git a/packages/subagent/tool-subagent/README.md b/packages/subagent/tool-subagent/README.md
index 6ec313b3b9..ac3ec0563c 100644
--- a/packages/subagent/tool-subagent/README.md
+++ b/packages/subagent/tool-subagent/README.md
@@ -8,7 +8,7 @@ The model-facing delegation tool over one configured `ctx.subagents` provider. C
 
 Each plugin instance binds one `provider` to one `toolName`; the model receives no provider selector. Load another distinctly named instance to expose another transport. The tool registers only while its provider exists, avoiding sibling load-order and provider-reload dependencies. Its description follows `provider.inheritsParentContext`: fresh children require standalone prompts, while forked children already see completed parent turns.
 
-A foreground call passes the execution signal through startup and execution, awaits `run.result`, and always awaits `run.dispose()` before returning. Only `completed` returns the canonical `{ kind: 'foreground', runId, output: JsonValue[] }`, rendered as the same final text; abort, refusal, token limit, and other failures become errored tool results without partial output. If result collection and disposal both reject, the errored result preserves both diagnostics.
+A foreground call passes the execution signal through startup and execution, awaits `run.result`, and always awaits `run.dispose()` before returning. Only `completed` returns the canonical `{ kind: 'foreground', runId, output: JsonValue[] }`, rendered as the same final text; abort, refusal, token limit, and other failures become errored tool results whose message appends the child's preserved partial text (the `SubagentResult.output` selection) after the stop-reason headline, so a truncated answer is never reported as success yet never silently lost. If result collection and disposal both reject, the errored result preserves both diagnostics.
 
 With `run_in_background: true`, `backgroundMode` selects the route. `one-shot` registers a plain parent-owned Task and returns canonical `{ kind: 'background', taskId }`, rendered as `started background subagent task <id>`, even when the provider supports continuable children; generic task tools own its later status, collection, cancellation, and notices. `continuable` requires a provider with the `prepareContinuable` capability, calls `ctx.subagents.startContinuable()`, and returns `{ kind: 'continuable', subagentId }`, rendered as `started subagent <childId>`. The continuable route resolves at inbox acceptance: the child owns its own turns from there, so this call neither waits for nor collects a result, and the child does not report back — its transcript by that id is the source of its output, and the optional global `send_message` tool sends it more work. Starting continuable work does not require `send_message` to be loaded. See the [background subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md), the [continuable subagents Agent Note](../../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md), and the [merged-service Agent Note](../../../.agents/notes/implemented/simplification/2026-07-26-merge-subagent-control-service.md).
 
diff --git a/packages/subagent/tool-subagent/README.zh.md b/packages/subagent/tool-subagent/README.zh.md
index 1fd88363b3..d64831d7cf 100644
--- a/packages/subagent/tool-subagent/README.zh.md
+++ b/packages/subagent/tool-subagent/README.zh.md
@@ -8,7 +8,7 @@
 
 每个插件实例把一个 `provider` 绑定到一个 `toolName`；模型不会收到提供方选择器。如需公开另一种传输，请加载另一个名称不同的实例。工具只在其提供方存在时注册，从而避免对同级加载顺序和提供方重新加载的依赖。工具描述遵循 `provider.inheritsParentContext`：新建子 agent（智能体）需要独立提示词，而 fork 子 agent 已能看到父级已完成轮次。
 
-前台调用会让执行信号贯穿启动和执行，等待 `run.result`，并且在返回前总会等待 `run.dispose()`。只有 `completed` 会返回规范值 `{ kind: 'foreground', runId, output: JsonValue[] }`，并渲染为相同的最终文本；中止、拒绝、token 上限和其他失败都会变成出错的工具结果，不包含局部输出。如果结果收集与 dispose（资源释放）都 reject，出错的结果会保留两项诊断信息。
+前台调用会让执行信号贯穿启动和执行，等待 `run.result`，并且在返回前总会等待 `run.dispose()`。只有 `completed` 会返回规范值 `{ kind: 'foreground', runId, output: JsonValue[] }`，并渲染为相同的最终文本；中止、拒绝、token 上限和其他失败都会变成出错的工具结果，其消息在终止原因标题之后附带子代理保留下来的部分文本（即 `SubagentResult.output` 的选取结果）——被截断的回答不会被报告为成功，也绝不会被悄悄丢弃。如果结果收集与 dispose（资源释放）都 reject，出错的结果会保留两项诊断信息。
 
 设置 `run_in_background: true` 后，`backgroundMode` 会选择路由。`one-shot` 会注册一个归父级所有的普通 Task，并返回规范值 `{ kind: 'background', taskId }`，渲染为 `started background subagent task <id>`，即使提供方支持可继续子 agent 也不例外；通用 Task 工具负责其后续状态、收集、取消和通知。`continuable` 要求提供方具备 `prepareContinuable` 能力，调用 `ctx.subagents.startContinuable()`，并返回 `{ kind: 'continuable', subagentId }`，渲染为 `started subagent <childId>`。可继续路由在 inbox 接受时结算：子 agent 自此拥有自己的轮次，因此该调用既不等待也不收集结果，而且子 agent 不会回报——通过该 id 查看其 transcript（文本记录）即是其输出来源，可选的全局 `send_message` 工具则向其发送更多工作。启动可继续工作不要求加载 `send_message`。见 [后台 subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md)、[可继续的 subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md)和[服务合并 Agent Note](../../../.agents/notes/implemented/simplification/2026-07-26-merge-subagent-control-service.md)。
 
diff --git a/packages/subagent/tool-subagent/src/index.ts b/packages/subagent/tool-subagent/src/index.ts
index 31df8f9e9c..13781bde91 100644
--- a/packages/subagent/tool-subagent/src/index.ts
+++ b/packages/subagent/tool-subagent/src/index.ts
@@ -134,6 +134,21 @@ function stopReasonError(result: SubagentResult): string | undefined {
   }
 }
 
+/**
+ * Append the child's preserved partial answer to a stop-reason error so a
+ * truncated or cancelled child's real text still reaches the parent model.
+ * @param error - the stop-reason headline.
+ * @param output - the child's selected output (`SubagentResult.output`).
+ * @returns the headline, extended with the partial text when any exists.
+ */
+function withPartialText(error: string, output: ContentBlock[]): string {
+  const text = output
+    .filter((block): block is Extract<ContentBlock, { type: 'text' }> => block.type === 'text')
+    .map(block => block.text)
+    .join('')
+  return text.length === 0 ? error : `${error}\nPartial output before the run ended:\n${text}`
+}
+
 type ForegroundToolResult = {
   readonly kind: 'foreground'
   readonly runId: SubagentRun['id']
@@ -149,8 +164,9 @@ async function settleForegroundRun(run: SubagentRun): Promise<ForegroundToolResu
     run.result.then((result): ForegroundToolResult => {
       const error = stopReasonError(result)
       if (error !== undefined) {
-        // The registry converts this throw to isError; partial output is not success.
-        throw new Error(error)
+        // The registry converts this throw to isError; partial output is not
+        // success, but the preserved partial answer still reaches the parent.
+        throw new Error(withPartialText(error, result.output))
       }
       return {
         kind: 'foreground',
diff --git a/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts b/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
index 8f289048cb..20cb11db2b 100644
--- a/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
+++ b/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
@@ -154,6 +154,9 @@ describe('dsh-tool-subagent', () => {
     const result = await callSubagent(ctx, { description: 'd', prompt: 'p' })
     expect(result.isError).toBe(true)
     expect(text(result)).toContain(fragment)
+    // The failure is not partial success, but the child's preserved partial
+    // answer still reaches the parent model inside the error result.
+    expect(text(result)).toContain('scripted subagent reply')
   })
 
   it('registers under a configurable toolName so multiple providers can coexist', async () => {