review: one fold implementation, uniform end-edge absence, partial text in tool errors, snapshot scenario

Address ds-review-bot on #2127:

- assistant-output: the rule has ONE implementation, the incremental
  AssistantOutputFold (push/pushText/collect); finalAssistantOutput folds a
  complete suffix, the SDK backend folds notification events, and the ACP
  backend folds raw chunk text into the same streamed fallback.
- subagent/end.lastAssistantMessage: 'no output' is encoded once — absent,
  never [], on both lifecycle shapes (observeRun now omits empty output).
- tool-subagent: a non-completed foreground result stays isError but appends
  the child's preserved partial text after the stop-reason headline.
- Authored keyless snapshot scenario subagent-max-tokens-partial pins the
  assembled transcript: the child's committed log carries the usage-only
  empty message and the parent's tool result carries the partial answer.
- Rule-boundary sentence (message wins over later streamed text) and the
  consumer half recorded in the Agent Note; comments trimmed to pointers.

.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md
-2026-08-10-subagent-empty-terminal-message-output.md: ece2a55930aabfaeaf88ef1757918296b32bfea5
-2026-08-10-subagent-empty-terminal-message-output.zh.md: 00b8f5bd7bd8e71935f68af2c35521f2b2377186
+2026-08-10-subagent-empty-terminal-message-output.md: d90047c07a300a1afbc42c7db1a4fefa25d56764
+2026-08-10-subagent-empty-terminal-message-output.zh.md: 0a5ce02dccef422dc75bc980d104f41f116427f2

.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.md

@@ -10,9 +10,11 @@ The agent loop appends an EMPTY-content `assistant/message` when a `max-tokens`
 
 ## Decision
 
-`dsh-subagent` owns one canonical selection rule in `src/assistant-output.ts`: the last NON-EMPTY assistant message wins; without one, the accumulated `text-delta` stream is the answer; empty-content messages never contribute. `finalAssistantOutput(events)` applies the rule to an event suffix (the in-process `readResult` and the Activation capture), and `assistantMessageOutput(event)` is the same per-event predicate for the SDK backend's incremental fold. The contract is stated once at `SubagentResult.output` and mirrored by the subsystem reference; `subagent/end.lastAssistantMessage` declares it selects by the same rule. A `max-tokens` or `aborted` finish still reports its honest stop reason; only output selection changed.
+`dsh-subagent` owns one canonical selection rule in `src/assistant-output.ts`: the last NON-EMPTY assistant message wins; without one, the accumulated `text-delta` stream is the answer; empty-content messages never contribute. The rule has one implementation, the incremental `AssistantOutputFold` (`push(event)` for session-event transports, `pushText(text)` for chunk-only transports, `collect()` to select), and `finalAssistantOutput(events)` applies it to a complete event suffix (the in-process `readResult` and the Activation capture). The SDK backend folds notification events; the ACP backend, which surfaces no complete assistant messages, folds raw chunk text into the same streamed fallback. The contract is stated once at `SubagentResult.output` and mirrored by the subsystem reference; `subagent/end.lastAssistantMessage` selects by the same rule, and "no output" has one encoding on that edge — the field is absent, never an empty array, on both the one-shot and continuable lifecycle shapes. A `max-tokens` or `aborted` finish still reports its honest stop reason; only output selection changed.
 
-The ACP backend accumulates chunks only and was never affected. The fake SDK runtime gained a `FAKE_EMPTY_MESSAGE` mode so the keyless backend test can script a usage-only terminal message.
+The foreground delegation tool observes the same selection: a non-`completed` result stays an `isError` tool result, but its message appends the child's preserved partial text after the stop-reason headline, so the parent model sees the truncated answer instead of a bare failure.
+
+The fake SDK runtime gained a `FAKE_EMPTY_MESSAGE` mode so the keyless backend test can script a usage-only terminal message, and the authored `subagent-max-tokens-partial` ACP snapshot scenario pins the assembled transcript: a scripted child streams text plus a tool call, is cut off by a tool-only max-tokens step (the empty usage-only message appears in its committed log), and the parent's tool result carries the partial answer.
 
 ## Alternatives considered
 
@@ -24,4 +26,4 @@ The ACP backend accumulates chunks only and was never affected. The fake SDK run
 
 ## Consequences
 
-Multi-step children cut off at max-tokens report their earlier text; cancelled in-process children keep the text streamed before the abort; one-shot and continuable `subagent/end` edges agree with `SubagentResult.output`. A message whose content is non-empty but textless (for example reasoning-only) still wins over streamed text — the rule is about empty content, not text presence. Regression tests in all three packages script the empty-terminal-message and cancel paths and fail under the previous selections.
+Multi-step children cut off at max-tokens report their earlier text; cancelled in-process children keep the text streamed before the abort; one-shot and continuable `subagent/end` edges agree with `SubagentResult.output`. A message whose content is non-empty but textless (for example reasoning-only) still wins over streamed text — the rule is about empty content, not text presence. A non-empty message also wins over text streamed AFTER it: a child cancelled while streaming a later step reports its earlier complete message, matching the SDK backend's documented contract, with the stop reason signalling the truncation. Regression tests in all three packages script the empty-terminal-message and cancel paths and fail under the previous selections.

.agents/notes/implemented/bug-fix/2026-08-10-subagent-empty-terminal-message-output.zh.md

@@ -10,9 +10,11 @@ Status: implemented
 
 ## 决策
 
-`dsh-subagent` 在 `src/assistant-output.ts` 中拥有唯一的规范选取规则：最后一条**非空** assistant 消息优先；没有时，累积的 `text-delta` 流就是回答；空内容消息从不参与。`finalAssistantOutput(events)` 把该规则应用于事件后缀（进程内 `readResult` 与 Activation capture），`assistantMessageOutput(event)` 是同一规则的逐事件谓词，供 SDK 后端的增量折叠使用。契约在 `SubagentResult.output` 处声明一次，并由子系统参考文档镜像；`subagent/end.lastAssistantMessage` 声明按同一规则选取。`max-tokens` 或 `aborted` 终止仍然如实上报其终止原因；只有输出选取发生了变化。
+`dsh-subagent` 在 `src/assistant-output.ts` 中拥有唯一的规范选取规则：最后一条**非空** assistant 消息优先；没有时，累积的 `text-delta` 流就是回答；空内容消息从不参与。规则只有一个实现，即增量的 `AssistantOutputFold`（会话事件传输用 `push(event)`，仅分块传输用 `pushText(text)`，`collect()` 完成选取）；`finalAssistantOutput(events)` 把它应用于完整的事件后缀（进程内 `readResult` 与 Activation capture）。SDK 后端折叠通知事件；ACP 后端不产生完整 assistant 消息，因此把原始分块文本折叠进同一个流式兜底。契约在 `SubagentResult.output` 处声明一次，并由子系统参考文档镜像；`subagent/end.lastAssistantMessage` 按同一规则选取，且"无输出"在该边沿只有一种编码——字段缺省，绝不是空数组，一次性与 continuable 两种生命周期形态一致。`max-tokens` 或 `aborted` 终止仍然如实上报其终止原因；只有输出选取发生了变化。
 
-ACP 后端只累积分块，从未受影响。fake SDK runtime 新增 `FAKE_EMPTY_MESSAGE` 模式，使无密钥后端测试能够脚本化一条仅承载 usage 的终止消息。
+前台委派工具观察同一选取结果：非 `completed` 的结果仍是 `isError` 工具结果，但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，父模型看到的是被截断的回答而不是一句干巴巴的失败。
+
+fake SDK runtime 新增 `FAKE_EMPTY_MESSAGE` 模式，使无密钥后端测试能够脚本化一条仅承载 usage 的终止消息；authored 的 `subagent-max-tokens-partial` ACP snapshot 场景钉住了组装后的 transcript：脚本化的子代理先流式输出文本和一次工具调用，再被仅含工具调用的 max-tokens 步骤截断（空的 usage-only 消息出现在其提交的日志中），父侧工具结果携带部分回答。
 
 ## 考虑过的替代方案
 
@@ -24,4 +26,4 @@ ACP 后端只累积分块，从未受影响。fake SDK runtime 新增 `FAKE_EMPT
 
 ## 后果
 
-被 max-tokens 截断的多步子代理会报告其更早的文本；被取消的进程内子代理保留中止前已流式的文本；一次性与 continuable 的 `subagent/end` 边沿与 `SubagentResult.output` 一致。内容非空但不含文本的消息（例如仅含 reasoning）仍然优先于流式文本——规则针对的是内容为空，而非文本缺失。三个包中的回归测试脚本化了空终止消息与取消路径，并在先前的选取实现下失败。
+被 max-tokens 截断的多步子代理会报告其更早的文本；被取消的进程内子代理保留中止前已流式的文本；一次性与 continuable 的 `subagent/end` 边沿与 `SubagentResult.output` 一致。内容非空但不含文本的消息（例如仅含 reasoning）仍然优先于流式文本——规则针对的是内容为空，而非文本缺失。非空消息同样优先于**其后**才流式出的文本：子代理在流式后续步骤时被取消，报告的是更早那条完整消息，与 SDK 后端文档化的契约一致，截断由终止原因示意。三个包中的回归测试脚本化了空终止消息与取消路径，并在先前的选取实现下失败。