fix(subagent): harden continuable persistence

packages/subagent/subagent-control/README.md

@@ -4,7 +4,7 @@ The continuable-subagent control service (`ctx.subagentControl`): the one orches
 
 ## Activation lifecycle
 
-A continuable background subagent is a durable child session with a series of Task-backed activations. `startContinuable()` allocates the stable child session id before Task creation, snapshots the descriptor inputs (a non-JSON input throws with no Task), and registers the initial activation's Task; the provider publishes exactly that child id and appends the versioned `subagent/descriptor` event inside the child's first turn. Every activation — initial or resumed — creates a fresh Task whose settlement awaits the child result, disposes the run, and only then records the `TaskOutcome`: a terminal Task leaves the durable child session but no live child Agent.
+A continuable background subagent is a durable child session with a series of Task-backed activations. `startContinuable()` allocates the stable child session id before Task creation, snapshots the descriptor inputs (a non-JSON input throws with no Task), and registers the initial activation's Task; the provider publishes exactly that child id and appends the versioned `subagent/descriptor` event inside the child's first turn. Every activation — initial or resumed — creates a fresh Task whose settlement awaits the provider's durability-confirmed child result, disposes the run, and only then records the `TaskOutcome`: a terminal Task leaves the durable child session but no live child Agent. A provider rejection with `DURABILITY_FAILED` settles the Task as `failed` and copies the error message into `detail`, so `task_output` reports the failed checkpoint and resumability risk without exposing unconfirmed output.
 
 `sendMessage(parent, childId, message)` owns steer-or-resume routing. A running activation receives live delivery through the run's strict `steer` capability and returns the existing Task id (`steered`); an absent activation starts a fresh Task that loads the persisted child, authorizes the recorded `parentSession` as the direct parent, folds the descriptor, and dispatches `SubagentService.resume()` (`started`). Failure throws and means the message was not delivered: losing a strict-steering race with Task settlement never falls through to cold resume within the same call, and a live registry Agent outside the activation association is an ownership conflict rather than an adoption target.
 

packages/subagent/subagent-control/src/index.ts

@@ -116,6 +116,13 @@ export function runOutcome(result: SubagentResult): TaskOutcome {
   }
 }
 
+/** Render infrastructure failure detail without hiding a durability diagnosis. */
+function runFailureDetail(error: unknown): string {
+  return error instanceof HarnessError && error.code === 'DURABILITY_FAILED'
+    ? error.message
+    : String(error)
+}
+
 /**
  * Await the child result, dispose the run, then return its task outcome. Result
  * and disposal failures become `failed`; when both fail, both details survive.
@@ -127,7 +134,7 @@ export async function settleRun(run: SubagentRun): Promise<TaskOutcome> {
   try {
     outcome = runOutcome(await run.result)
   } catch (error: unknown) {
-    outcome = { status: 'failed', detail: String(error) }
+    outcome = { status: 'failed', detail: runFailureDetail(error) }
   }
   try {
     await run.dispose()

packages/subagent/subagent-control/tests/subagent-control.spec.ts

@@ -16,7 +16,7 @@ import { TaskId } from '@deepseek-ai/dsh-tasks'
 import LocalTaskService from '@deepseek-ai/dsh-tasks-local'
 import * as ToolTasks from '@deepseek-ai/dsh-tool-tasks'
 import type { GenerateOptions, StreamChunk } from '@deepseek-ai/dsh-llm'
-import { createUserMessage, LlmAdapter } from '@deepseek-ai/dsh-llm'
+import { createUserMessage, HarnessError, LlmAdapter } from '@deepseek-ai/dsh-llm'
 import { MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
 import SubagentControlService, { runOutcome, settleRun, SubagentControlError } from '../src/index.ts'
 
@@ -685,16 +685,29 @@ describe('outcome mapping helpers', () => {
     expect(failed).toEqual({ status: 'failed', detail: 'Error: transport gone' })
     expect(disposed).toBe(true)
 
-    const disposeFailed = await settleRun({
+    const durabilityMessage = 'subagent "child-3" durability checkpoint failed; latest state unavailable: disk full'
+    const durabilityFailed = await settleRun({
       id: SessionId('child-3'),
       localAgent: undefined,
+      result: Promise.reject(new HarnessError(
+        durabilityMessage,
+        'DURABILITY_FAILED',
+        { cause: new Error('disk full') },
+      )),
+      dispose: () => Promise.resolve(),
+    })
+    expect(durabilityFailed).toEqual({ status: 'failed', detail: durabilityMessage })
+
+    const disposeFailed = await settleRun({
+      id: SessionId('child-4'),
+      localAgent: undefined,
       result: Promise.resolve({ output: [], stopReason: 'completed' }),
       dispose: () => Promise.reject(new Error('reap failed')),
     })
     expect(disposeFailed).toEqual({ status: 'failed', detail: 'dispose failed: Error: reap failed' })
 
     const bothFailed = await settleRun({
-      id: SessionId('child-4'),
+      id: SessionId('child-5'),
       localAgent: undefined,
       result: Promise.reject(new Error('result failed')),
       dispose: () => Promise.reject(new Error('reap failed')),