Complete a background subagent that settles after its turn

[Ryan Ewen (RyanEwen)]

Problem

A subagent spawned in the background keeps its row spinning for the rest of the session even though it finished, and its entry is never evicted from the subagent registry.

Root cause

ClaudeSdkPipeline derives the turn id for every SDK message from the prompt queue head:

const parent = this._queue.peekParent();
const turnId = parent?.turnId;

and ClaudeSdkMessageRouter.handle returns early when there is none, so once the queue drains every message is dropped before reaching the mapper. A background subagent settles after the turn that spawned it has ended, which is precisely when the queue is empty, so its system.task_notification never becomes a subagent_completed signal.

This contradicts the contract SubagentRegistry documents for itself, that background spawns survive across turns by design because their completion arrives later via system.task_notification. The foreground path is unaffected, since its tool_result arrives inside the turn.

Fix

Route the subagent system messages when there is no turn. They carry their own chat and tool call id, so they never needed one, and completeSubagentSession resolves the turn on the subagent's own chat rather than the parent's, so it works while the parent is idle. Turn-scoped messages are still dropped exactly as before. The mapping is pulled into a small helper so both paths share the existing swallow-and-log behaviour.

Testing

Added a router test that records a background spawn, delivers a terminal task_notification with no turn id, and asserts a subagent_completed signal is produced and the spawn is evicted. The existing turn-less assertion is kept, narrowed to the turn-scoped message it actually covers. I could not find existing coverage for a background subagent completing while the parent is idle, and the e2e subagent suite has no background case at all.

AI disclosure: this comment and the related code were written with the assistance of AI.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

TylerLeonhardt

Matched files:

• src/vs/platform/agentHost/node/claude/claudeSdkMessageRouter.ts

[Copilot]

Pull request overview

Routes turn-less Claude SDK subagent lifecycle messages so background subagents complete and registry entries are evicted.

Changes:

• Handles subagent system messages after the parent turn ends.
• Centralizes signal mapping and error handling.
• Adds regression coverage for background completion without a turn ID.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
claudeSdkMessageRouter.ts	Routes turn-less subagent messages and extracts signal production.
claudeSdkMessageRouter.test.ts	Tests deferred background-subagent completion and eviction.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Mohammad javad Dianat (dianatofficial)]

Well done. The optimization reduces unnecessary allocations.