Native resume_agent silently changes a subagent’s model

Start by locating the native resume_agent handling and trace how a spawned child’s model is carried into resume. Reproduce the case with gpt-5.6-luna under a gpt-5.6-sol parent, then verify that the resumed child retains its original model rather than silently switching.

What version of Codex CLI is running?

codex-cli 0.146.0

What subscription do you have?

pro

Which model were you using?

Child spawned as gpt-5.6-luna; parent/default was gpt-5.6-sol. On native resume, the child silently resumed as gpt-5.6-sol.

What platform is your computer?

Linux x86_64

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

VScode over ssh

Codex doctor report

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}

What issue are you seeing?

A child explicitly spawned with gpt-5.6-luna was completed, closed, and
later resumed using the native resume_agent multi-agent tool. It
retained the same child identity but silently resumed as gpt-5.6-sol,
the parent/config default.

The native resume tool accepts only id. It provides no model or
reasoning-effort override and does not report that the effective model
changed.

This affects review provenance and model/cost-routing policy because one
child session identity can silently represent work performed by different
model classes.

The relevant implementation is in:

codex-rs/core/src/tools/handlers/multi_agents/resume_agent.rs

What steps can reproduce the bug?

1. Set the parent/default model to gpt-5.6-sol.
2. Enable native multi-agent support.
3. Spawn a child explicitly pinned to gpt-5.6-luna.
4. Let the child complete and close.
5. Resume it using native resume_agent({"id":"<child-thread-id>"}).
6. Observe that the same child identity resumes as gpt-5.6-sol, rather
   than its persisted gpt-5.6-luna model.

The native resume_agent schema accepts only id, so there is no way to
preserve or explicitly restore the intended model through this tool.

What is the expected behavior?

Native subagent resume should preserve the child thread's persisted model
and reasoning effort by default.

If explicit overrides are supported, resume_agent should accept model
and reasoning-effort fields. Any unavoidable substitution should report
the persisted, requested, and effective model before running the resumed
turn.

Additional information

Ordinary CLI resume exposes explicit model selection:

• codex resume --model <MODEL>
• codex exec resume --model <MODEL>

Related issues:

• #32061: ordinary session resume should restore persisted model and
  effort.
• #31814: subagent model fields hidden under some multi-agent
  configurations.

This report is specifically about native multi-agent resume_agent
silently changing the child model while retaining the child identity.