multi_agent_v1__spawn_agent can deliver work into an existing durable task and change its model

Summary

A multi_agent_v1__spawn_agent call intended to create a temporary subagent instead delivered the assignment into an existing user-owned durable task.

The durable task's model was changed from gpt-6-astra to gpt-5.6-sol as a result.

Environment

• Codex Desktop on macOS
• Local Codex CLI: 0.154.0
• Affected persisted task metadata was created under CLI 0.153.4
• Parent task: gpt-5.6-luna, effective Multi-Agent V1
• Existing durable task: initially gpt-6-astra, effective Multi-Agent V2
• No local multi_agent_v2 configuration change was made

Reproduction

From a user-owned task using Multi-Agent V1, invoke:

{
  "model": "gpt-5.6-sol",
  "reasoning_effort": "medium",
  "fork_context": true,
  "message": "Perform a bounded, read-only critique. Do not edit files."
}

Expected behavior:

• A new subordinate agent is created.
• The child has its own agent/task identity.
• Existing user-owned tasks are unaffected.
• The requested model applies only to the new child.

Actual behavior:

• The assignment appeared inside an existing durable user-owned task.
• The UI displayed “Sent by ChatGPT from another task.”
• The existing task's model changed from gpt-6-astra to gpt-5.6-sol.
• No explicit create_thread or send_message_to_thread call was made by the parent.

Expected safeguards

spawn_agent should never resolve to, reuse, or mutate an existing user-owned durable task.

The system should:

1. Create a new child identity for every spawn.
2. Reject any destination that is not a valid child-agent record owned by the current parent.
3. Keep model overrides scoped to the newly created child.
4. Surface a clear error if child creation cannot be completed.
5. Preserve the existing task's model and lifecycle state.

Evidence

• Parent tool call requested gpt-5.6-sol.
• Existing task metadata shows gpt-6-astra before the delegation and gpt-5.6-sol afterward.
• The target task was an existing durable user-owned task, not a newly created subagent.
• The UI showed the cross-task delegation marker.
• No project files were changed.

Related issues

• #38687 — subagents can access task-management controls and create or message user-owned durable tasks.
• #34821 — V2 subagents can resume with the parent's model.
• #32031 — V2 spawn model overrides and fork behavior are unreliable.

Start by tracing the multi_agent_v1__spawn_agent entry point and how it resolves child identities and destinations. Reproduce the request from the issue, then verify that each spawn creates a child-owned identity, rejects existing user-owned tasks, scopes model overrides to the child, and preserves the existing task.