Subagents regression in 0.137.0

What version of Codex CLI is running?

0.137.0

What subscription do you have?

Pro

Which model were you using?

gpt-5.5 medium

What platform is your computer?

Linux 6.17.0-35-generic x86_64

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

VS Code

What issue are you seeing?

This simple subagent launch prompt worked in Codex CLI 0.136.0:
Spawn a 5.4 mini subagent with xhigh reasoning effort. Have it print "Hello world"

<img width="1382" height="655" alt="Image" src="https://github.com/user-attachments/assets/a4e0701f-8321-4373-865f-7705791c89da" />

However the exact same prompt does not work in 0.137.0:

<img width="1361" height="693" alt="Image" src="https://github.com/user-attachments/assets/2a7f7c6b-9bee-4041-a233-26ee65dd5350" />

Version 0.136.0 launched the subagent with the correct model and reasoning effort, but 0.137.0 cannot.

What steps can reproduce the bug?

Uploaded thread: 019e99a6-aa7a-7751-b1ba-ce09d07635fe

What is the expected behavior?

Launch a subagent with the correct model and reasoning effort.

Additional information

_No response_