Multi-agent v2: completed agent remains resident and prevents replacement spawn after follow-up turns

What version of Codex CLI is running?

0.153.4

What subscription do you have?

Pro 5x

Which model were you using?

gpt-5.6-sol

What platform is your computer?

Linux 6.18.49-3-lts x86_64 unknown

What terminal emulator and version are you using (if applicable)?

Kitty

Codex doctor report

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}

What issue are you seeing?

• multi_agent_version: v2
• Three-worker capacity was full.
• One worker completed after receiving repeated followup_task calls.
• list_agents reported it completed.
• After roughly 80 seconds, spawn_agent still failed with:
  collab spawn failed: agent thread limit reached

What steps can reproduce the bug?

Candidate Reproducer

Configuration

Use multi-agent v2 with four total resident threads: root plus three workers.

[features.multi_agent_v2]
enabled = true

[agents]
enabled = true
max_concurrent_threads_per_session = 4

Start a new root session so the configuration is definitely applied.

Reproduction sequence

1. Spawn two long-running workers, B and C, so they continue occupying residency slots.

   Example task:

   Run sleep 180, then reply DONE. Do not spawn agents.

2. Spawn a short worker A:

   Reply with A0.

   Residency is now full: root + A + B + C.

3. Wait until A reports completed.

4. Reuse A with followup_task:

   Reply with A1.

5. Wait until A reports completed again.

6. Reuse A a second time:

   Reply with A2.

7. Wait until A reports completed.

8. Call list_agents and confirm:

   • A is completed.
   • B and C are running.

9. Wait another 30–90 seconds to rule out an ordinary completion-cleanup race.

10. Spawn a new agent D using a unique task name:

 Reply with D0.

Expected result

A is terminal and idle, so the v2 residency manager evicts A and successfully creates D.

Buggy result

The spawn fails:

collab spawn failed: agent thread limit reached

Despite list_agents reporting A as completed.

This mirrors the observed session almost exactly:

• A completed its initial task.
• A completed two subsequent followup_task turns.
• list_agents showed A completed.
• A replacement spawn roughly 80 seconds later failed.
• B and C were still running.

Control experiment

Repeat the sequence without reusing A:

1. Spawn A, B, and C.
2. Let A complete once.
3. Spawn D.

What is the expected behavior?

If list_agents reports a worker as completed and no new work was sent to it, spawn_agent should be able to reclaim its slot.

If internal cleanup is still underway, completed may remain technically accurate—but returning only agent thread limit reached is inadequate. It should say that completed residents are temporarily non- evictable, preferably with the reason.

Additional information

No response

Start by reproducing the multi-agent flow described in the title: complete an agent, send follow-up turns, and attempt to spawn a replacement. Trace the multi_agent feature and subagent:thread_spawn activity shown in the report; done means completed agents no longer remain resident and replacement agents can spawn successfully.