Custom .codex/agents no longer selectable in CLI v0.137.0; subagents spawn generic and inherit parent model

What version of Codex CLI is running?

0.137.0

What subscription do you have?

Pro Lite

Which model were you using?

gpt-5.5

What platform is your computer?

Linux (WSL2)

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

VSCode

What issue are you seeing?

Summary

In Codex CLI v0.137.0, project-scoped custom subagents under .codex/agents/*.toml are no longer selectable from the model-visible spawn_agent tool surface. Spawned subagents fall back to generic multi_agent_v1 threads, inherit the parent model/reasoning settings, and do not record agent_role or agent_path in session metadata.

This is a regression from earlier CLI versions where spawn_agent accepted an agent_type argument and correctly spawned the configured custom agent.

## Environment

• Codex CLI version: 0.137.0
• Platform: Linux
• Auth mode: ChatGPT
• Parent model: gpt-5.5
• Custom agents are defined under project scope:
• .codex/agents/researcher.toml

## Custom agent shape

Example custom agent file:

```
name = "researcher"
description = "Read-only source researcher."
model = "gpt-5.4-mini"
model_reasoning_effort = "medium"
sandbox_mode = "read-only"
nickname_candidates = ["Researcher", "Source Researcher"]

developer_instructions = """
You are a read-only researcher. Do not edit files. Return concise findings.
"""

  ## Expected behavior

  When asking Codex to spawn the custom researcher agent, the spawned thread should:

  - Load .codex/agents/researcher.toml
  - Use model = "gpt-5.4-mini"
  - Use model_reasoning_effort = "medium"
  - Record agent_role = "researcher" or equivalent custom-agent identity in session metadata
  - Record an agent_path or otherwise show that the custom agent config was loaded

  This worked in an earlier Codex CLI version. In that version, the model-visible spawn call accepted an argument like:

  {
    "agent_type": "researcher",
    "message": "...",
    "fork_context": false
  }

  ## Actual behavior in v0.137.0

  The exposed tool is multi_agent_v1.spawn_agent, and its schema only accepts:

  fork_context
  items
  message

  There is no agent_type, agent, name, model, or custom-agent selector field.

  When a subagent is spawned with prompt text like “You are acting as researcher,” the spawned session is generic:

  source=subagent:thread_spawn
  agent_role=None
  agent_path=None
  model=gpt-5.5
  effort=high
  multi_agent_version=v1

  So the child thread inherits the parent gpt-5.5/high configuration instead of using the custom agent’s pinned gpt-5.4-mini/medium.

### What steps can reproduce the bug?

## Reproduction Steps

  1. Create a project-scoped custom agent:

  .codex/agents/researcher.toml

  with:

name = "researcher"
description = "Read-only researcher."
model = "gpt-5.4-mini"
model_reasoning_effort = "medium"
sandbox_mode = "read-only"
developer_instructions = "You are a read-only researcher."

  2. Start Codex CLI v0.137.0 in that project.
  3. Ask Codex to spawn the custom researcher agent.
  4. Inspect the available spawn_agent tool schema and the resulting child session metadata.