fix(server): steering no longer ends running subagents

[shivamhwp]

Problem

Sending a steer to a thread with running subagents sometimes ended those subagents. It looked random because it depends on how the subagent was launched and on the steer's model settings.

• Claude: T3 sends steers with priority "now", which cancels the parent's running tool calls. A subagent launched in the foreground (Agent tool without run_in_background) is one of those calls, so it died with Request interrupted by user for tool use. Background subagents were fine.
• Codex: if a direct steer's model selection differed from the running turn's in any way (even just the reasoning effort, or one extra default option from the composer), T3 stopped and restarted the turn instead of steering. Stopping a Codex turn interrupts every subagent under it.

Change

• Claude: right before a steer, the adapter calls the SDK's backgroundTasks(toolUseId) for each running subagent the parent launched. It targets only those subagents. With no argument, backgroundTasks() also backgrounds the subagent's own shell commands, and the subagent quits early. The steer is answered right away and the subagent keeps running. A subagent moved this way is recorded like one Claude started in the background, so its result wakes the parent when it finishes.

  Edge cases, each with a test that fails without its fix:

  • No answer: the steer waits at most 5 seconds for the moves, then goes out anyway. A later steer reuses a move that's still pending, so unanswered requests don't pile up.
  • Late answer: a move answered after the steer went out still marks the subagent, so its end still wakes the parent.
  • Turn already over: if the turn ends while the moves are being answered, the steer fails and the orchestrator sends the message as a new turn, as it does for any steer that misses its turn.
  • Nested subagents: subagents started by another subagent aren't moved; they follow their owner. This includes ones whose launch the adapter never saw, which are identified by spawn depth.
  • Finished or re-run in between: a subagent that ended, or ended and ran again under a new tool call, before its move was answered is not marked.
  • Backgrounding refused: if background tasks are disabled, the steer goes through as it did before.

• Codex: a different model selection no longer turns a steer into a restart. The steer joins the running turn and the new selection is saved for the next turn. Claude already worked this way. OpenCode gets the same behavior, since it also steers live and applies selections on the next turn.

This is a deliberate trade. Applying a new model or effort instantly meant restarting the turn, and that kills every subagent the turn is running. If a maintainer wants instant apply back, the safer version is to restart only when the turn has no live subagents.

Restarts still happen when you switch provider instance, when the adapter says a selection needs a new session, or when a client explicitly asks for a restart. Stop still ends everything.

Scope and approval

No tracking issue. This is a focused fix for a reproducible bug: steering is meant to keep the turn running, not end its subagents. Server-only. No contract, client, or docs changes.

Verification

Recordings (real T3 Code web app, real Claude Code and Codex CLIs)

Re-recorded on 2026-10-15 after merging the latest main (Claude Code 2.1.300, Codex 0.162.1). Each clip is the same flow on main and on this PR. A subagent runs a 45–60 second job, and a steer ("what is 2+2?") is sent while it runs. GIFs play at 4x; the MP4 links play at 2x.

Claude, foreground subagent, then steer

Before (main)	After (this PR)
The subagent dies. Claude says "The agent isn't running, though. The call was interrupted", and the panel shows 1 failed. mp4	Claude answers the steer while "Slow job" keeps Running. It finishes with FINISHED, then the parent replies DONE. mp4

Codex (GPT-6.1-Sol), effort changed Low → Medium mid-run, then direct steer (Ctrl-click send)

Before (main)	After (this PR)
The subagent goes Stopped 10 seconds in, at the steer. The parent is still "Working" more than 3 minutes later, waiting on a dead subagent. mp4	The parent answers the steer ("2 + 2 = 4") while the subagent keeps Running. The subagent finishes, and the parent replies DONE on the new Medium effort. mp4

The web composer's default "queue, then press Steer" path sends the thread's saved selection, so it didn't trigger the Codex restart on main. The restart happens on direct steers that carry the composer's selection: Ctrl-click send, the "Steer" follow-up setting, and steers from agents via t3_thread_send.

Real CLIs through T3's orchestrator (temporary live test, not committed)

Re-run on 2026-10-15 on the branch merged with the latest main, with Claude Code 2.1.300 and Codex 0.162.1 (claude-sonnet-5-5, gpt-6.1-sol). Each subagent ran a 35–70 second command. All 10 scenarios ran at the same time, and all 10 passed. The "Before" column is from the same scenarios run on main; the recordings above re-confirm it on today's main.

Scenario	Before (main)	After (this PR)
Claude, foreground subagent, steer	subagent failed (Request interrupted by user for tool use)	completed; steer answered at once; parent woken with "Slow job finished", replied DONE
Claude, two foreground subagents in parallel, steer	n/a	both completed; parent woken for each
Claude, foreground subagent, two steers (effort changed, then back)	n/a	both steers answered; subagent completed; parent woken
Claude, nested subagents, steer while both run	n/a	both kept running; outer waited for the inner one and returned OUTER_FINISHED; parent woken
Claude, subagent finishes during the user's next message	n/a	Claude picked up its result in that message
Claude, background subagent	n/a	completed (unchanged)
Claude, steer with no subagent	n/a	answered (no subagent path involved)
Codex, steer with same selection	n/a	completed (unchanged)
Codex, steer with effort low → medium	subagent interrupted; parent relaunched it from scratch (~3.5 min lost)	completed; one attempt, no restart; thread selection now medium
Codex, two steers (medium, then high)	n/a	completed; one attempt; thread selection now high

Tests

• New: ClaudeAdapterV2.test.ts, "moves the root's foreground subagents to the background before a steer". Six cases: the move succeeds, fails, never answers (including two more steers that reuse the pending move), answers after the steer went out, answers after the turn ended, and answers after the subagent ran again. Each case checks that only the parent's running subagent is moved (not a nested one, including one with no seen launch, and not a finished one). It also checks whether the steer goes through, and whether the subagent's later end wakes the parent with its name. Each case fails without its fix.
• Updated: SteeringCompletion.integration.test.ts, "steers a changed turn-scoped selection into the running turn" now also runs for a provider that can restart. That case fails without the fix.
• Passing after the main merge (325 tests): ClaudeAdapterV2.test.ts, ClaudeAgentSdkQueryRunner.test.ts, ClaudeAutomaticDelivery.integration.test.ts, SelectionRestart.integration.test.ts, SteeringCompletion.integration.test.ts, CommandPolicy.test.ts, EffectWorker.test.ts, ProviderTurnControlService.test.ts, Orchestrator.test.ts. Server typecheck is clean.

Known, unchanged queue behavior: if a message is queued with effort A and a later steer picks effort B, the queued run still starts on A and the thread keeps A afterwards. That is how queued runs already work on every provider (a queued message keeps the selection it was sent with). It isn't changed here.

Reviews: CodeRabbit's three findings and gpt-6.1-sol's findings across five review rounds are all fixed. gpt-6.1-sol's final review of the whole diff at 52d25f8: nothing blocks merging. After merging the latest main (09e7997), it re-checked the main changes that touch the same code (Claude effort changes while background agents run, steered-turn token usage, restart recovery, update-all background work): nothing blocks merging, and 143 more session-release and restart-recovery tests pass.

Not changed here: a steer still waits for the parent's own running shell command (not a subagent) to finish before Claude reads it, as on main. That's #15720.

Remaining gap: on Claude, if a steer lands after Claude has decided to launch a subagent but before that subagent has started, there's nothing to background yet, so that launch gets cancelled. T3 can't see a task before Claude reports it.

Fixed by Claude Opus 5.5 in Claude Code (via T3 Code).

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This server-side fix changes live steering and subagent lifecycle behavior for Claude, Codex, and OpenCode, including asynchronous backgrounding and restart-policy changes. The production impact and concurrency interactions extend beyond a small self-contained bug fix and merit human review.

No code changes detected at e3eeced. Prior analysis still applies.

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Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	4.9 KiB	−23 B (−0.5%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	−23 B (−1.9%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.8 KiB	−41 B (−0.2%)	29.3 KiB	✅
Codex	Live turn messages	2	1	−1 (−50.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	+24 B (+0.5%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	+24 B (+2.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	+41 B (+0.2%)	29.3 KiB	✅
Claude	Live turn messages	1	2	+1 (+100.0%)	8	✅

Baseline: 0e1431f · PR result: e3eeced · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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• apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts
• apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts

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📝 Walkthrough

Claude steering now backgrounds eligible foreground subagents before offering a steer. Deferred model-selection changes remain on the active turn, including for providers that support interrupt-and-restart steering.

Changes: Steering behavior

Layer / File(s)	Summary
Background-task protocol and test support apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts, apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.testkit.ts, apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts, apps/server/src/orchestration-v2/ClaudeAutomaticDelivery.integration.test.ts	The Claude query session and protocol log support background-task requests. Replay validation checks the outbound frame, and query-runner test doubles implement the new method.
Background foreground subagents before steering apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts, apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts	Before offering a steer, the adapter requests backgrounding for eligible running root subagents from the current process. It skips nested and completed subagents, bounds the wait, and logs failures. Tests cover backgrounding outcomes, repeated steers during pending requests, and responses after turn or task changes.
Keep deferred selection changes on the active turn apps/server/src/orchestration-v2/Orchestrator.ts, apps/server/src/orchestration-v2/SteeringCompletion.integration.test.ts	A changed selection that applies on the provider’s next turn no longer forces a restart based on restart capability. Integration tests cover saved-selection behavior with both capability settings.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant ClaudeAdapterV2
  participant ClaudeAgentSdkQuerySession
  participant ClaudeSDK
  ClaudeAdapterV2->>ClaudeAgentSdkQuerySession: Request backgroundTasks for eligible subagent
  ClaudeAgentSdkQuerySession->>ClaudeSDK: Send query.background_tasks
  ClaudeSDK-->>ClaudeAgentSdkQuerySession: Return background-task result
  ClaudeAdapterV2->>ClaudeAdapterV2: Offer steer after background requests

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Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to 52d25

No actionable merge-blocking issue is established; the change is mergeable after normal checks.

### Architecture Summary

Architecture risk: 🔵 Low · up to 52d25

The change affects 1 system.

Changed systems: apps/server

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — apps/server (service) was modified; 6 changed files map to changed impact.

Before / after behavior

• observed — Modified behavior in apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.testkit.ts: ClaudeOutboundFrame now includes query.background_tasks with a toolUseId field.
• observed — Modified behavior in apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.testkit.ts: The replay query session adds backgroundTasks(toolUseId), which asserts the matching outbound frame and maps successful validation to true.
• observed — Modified behavior in apps/server/src/orchestration-v2/ClaudeAutomaticDelivery.integration.test.ts: The query runner mock adds backgroundTasks, which resolves to false.
• observed — Modified behavior in apps/server/src/orchestration-v2/Orchestrator.ts: Updated the comment to state that selections applied on the provider’s next turn remain a steer and wait for the next turn, rather than restarting and ending native subagents.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely identifies the primary fix: steering no longer ends running subagents.
Description check		The description follows the required sections and provides clear problem context, implementation details, scope justification, focused automated tests, manual verification, recordings, known limitatio…

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Review comments at
@apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts:
- Around line 8847-8872: In backgroundForegroundSubagents, add a bounded timeout
to each live.query.backgroundTasks(toolUseId) request before its error handler
so a hung request cannot block the steer; ensure timeout failures are handled by
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@apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts:
- Line 8866: Update the `live.query.backgroundTasks(toolUseId)` flow to add the
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⚠️ Outside diff range comments (1)

🟡 Minor · Preserve successful backgroundTasks responses that arrive after… · ClaudeAdapterV2.ts:8860-8896

apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts:8860-8896
🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win

Preserve successful backgroundTasks responses that arrive after the timeout.

Effect.timeout("5 seconds") interrupts the wrapper effect, but queryRuntime.backgroundTasks(toolUseId) is passed to Effect.tryPromise without an abort signal. The SDK request can therefore resolve true later, after the timeout has discarded the flatMap that adds taskId to backgroundedSubagentTaskIds. The later task_notification then has no subagent wake entry and the parent does not wake. Keep a late-result observer alive independently of the five-second steer deadline, while retaining the existing task/run identity guard before recording the ID.

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Review comment at @apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts
around lines 8860 - 8896:
Update the `backgroundTasks` handling in the `Effect.forEach` flow so a
successful SDK response is observed even if it arrives after the five-second
timeout; keep the timeout behavior for the steer and preserve the existing task
status and `runToolUseId` guard before adding `taskId` to
`backgroundedSubagentTaskIds`.

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Outside diff comments:
Review comments at
@apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts:
- Around line 8860-8896: Update the `backgroundTasks` handling in the
`Effect.forEach` flow so a successful SDK response is observed even if it
arrives after the five-second timeout; keep the timeout behavior for the steer
and preserve the existing task status and `runToolUseId` guard before adding
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[shivamhwp]

@coderabbitai Fixed the outside-diff finding (late backgroundTasks answers) in e8ab9f1. Each move now runs forked in the session scope. Only the steer's wait is bounded, with Fiber.joinAll(...).pipe(Effect.timeoutOption("5 seconds")), so a move answered after the steer went out still marks the subagent. The task-status and runToolUseId guard is unchanged. New test case "answers after the steer went out": the answer lands at 6 s, the steer goes out at 5 s, and the subagent's later end still wakes the root with its name. That case fails on the previous commit.

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🏁 Script executed:

#!/bin/bash
set -euo pipefail
fd '^ClaudeAdapterV2(\.test)?\.ts$'
ast-grep outline apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts --match 'steer|background' --view expanded
ast-grep outline apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts --match 'background|steer' --view expanded
rg -n -C 35 'Fiber\.joinAll|forkIn|runToolUseId ===|backgroundTasks\(' apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts
rg -n -C 65 'answers after the steer went out' apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts

Length of output: 37099

---

@shivamhwp The inspected code addresses the late-answer finding. Each move runs in sessionScope, while the five-second timeout applies only to Fiber.joinAll(moves). A late successful answer can still mark the running subagent, and the runToolUseId guard excludes a later run.

The new regression case checks that the steer proceeds at five seconds and that completion after the six-second answer produces a wake with the subagent’s name. I verified the code and assertions but did not run the tests.

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