fix(subagent): keep output past an empty terminal message with one selection rule

A max-tokens step that assembled only tool-call blocks appends an
EMPTY-content assistant/message (the usage host). Three consumers each
hand-rolled output selection and all let it erase the child's real
answer: the in-process readResult and the Activation subagent/end
capture took the last message unfiltered, and the SDK backend let any
message beat its streamed-text fallback; the in-process driver also had
no streamed-text fallback for cancelled turns.

dsh-subagent now owns the canonical rule in src/assistant-output.ts
(last non-empty assistant message, else the accumulated text-delta
stream) and all three consumers apply it. Regression tests in all three
packages fail under the previous selections.

Closes #1514

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: 80f5c40a2c949b7c2a638ec19c02950e8cd69f0b
+README.zh.md: b34421dbbf2d06b7c9236776aabf19ba8005204e

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 hosts only usage and is skipped), or the `text-delta` stream accumulated before the activity was cut short — a partial answer survives cancel and error paths.
 
 `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.
 

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`（空内容消息仅承载 usage，会被跳过），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。
 
 `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。
 

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 { assistantMessageOutput, 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). */
@@ -163,17 +163,21 @@ 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).
+  // The child's final answer, folded incrementally under the seam's canonical
+  // rule (`finalAssistantOutput`): the last NON-EMPTY complete assistant
+  // message when one exists, else the text streamed so far (a partial answer
+  // surviving cancel). An empty-content message hosts only usage (a max-tokens
+  // step that assembled no text blocks), so it never erases streamed text.
   let lastMessage: ContentBlock[] | undefined
   const partial: string[] = []
   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') {
+    const content = assistantMessageOutput(event)
+    if (content !== undefined) {
+      lastMessage = content
+    } else 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[] => {

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())