App-server broker retains thread subscriptions after task clients disconnect

Summary

The app-server broker shares one upstream connection across plugin tasks.

Each task starts or resumes a thread, which subscribes that upstream connection.

The downstream task socket closes after completion.

The broker clears routing state but never sends thread/unsubscribe.

Completed threads and their MCP runtimes remain loaded until unsubscribe or app-server shutdown.

Observed impact

• Fresh tasks accumulate MCP runtimes inside one broker session.
• One app-server retained 11 Basic Memory runtimes.
• Other app-servers retained seven, five, and five runtimes.
• Basic Memory only made the generic broker defect visible.
• The proposed fix contains no Basic Memory dependency.

Reproduction

1. Configure any stdio MCP server in Codex.
2. Start Claude Code with the Codex plugin.
3. Run several fresh Codex tasks in one workspace.
4. Let every task finish normally.
5. Inspect the app-server process tree.

Observed result

• Each fresh task creates another thread and MCP runtime.
• The downstream task client disconnects.
• The upstream thread subscription remains active.
• Completed tasks therefore accumulate MCP processes.

Expected behavior

The broker should track thread ownership for each downstream socket.

It should send thread/unsubscribe after the final owner disconnects.

Cause

thread/start subscribes the shared upstream connection to thread events.

The broker does not associate that subscription with its downstream owner.

Socket closure therefore removes routing state without removing the subscription.

The final unsubscribe starts Codex's documented 30-minute inactivity period.

Codex unloads the thread and MCP runtime after that period.

App-server documentation

Proposed fix

• Track subscribed threads for each downstream socket.
• Preserve subscriptions while another downstream owner remains.
• Cover started, resumed, forked, review, and automatically created child threads.
• Unsubscribe after normal or abrupt final-owner disconnection.
• Log upstream unsubscribe failures.
• Add broker ownership regression tests.

Real-process verification

Tested with Codex CLI 0.150.1 and Basic Memory.

Case	Probe after client closure	Meaning	MCP process result
Control broker	unsubscribed	The broker retained the subscription	Exited approximately 30 minutes after the probe unsubscribed
Patched broker	notSubscribed	The broker had already unsubscribed	Exited approximately 30 minutes after client closure

Both cases started a real Basic Memory wrapper and Python server.

Every isolated test descendant exited during cleanup.

Related work

• #543 tracks retained broker and app-server process trees.
• #660 proposes broader broker supervision and cleanup.
• #680 proposes broker idle shutdown.

Those changes bound session lifetime.

This issue addresses per-task thread retention inside a running broker.

Start by tracing the app-server broker's handling of thread/start, resumed, forked, review, and automatically created child threads, then follow downstream socket closure through routing cleanup. Add broker ownership regression tests covering shared owners and normal or abrupt final-owner disconnection. Done means final owners trigger thread/unsubscribe, shared subscriptions remain active, failures are logged, and all listed thread paths are covered.