The ChatGPT client is incompatible with Microsoft's OpenAI API key GPT-5.6

What version of the Codex App are you using (From “About Codex” dialog)?

Codex Desktop/0.144.2 (Codex Desktop; 26.707.62119)

What subscription do you have?

Microsoft openai apikey

What platform is your computer?

Windows 10.0.26200; x86_64

What issue are you seeing?

The newer version of Codex Desktop, renamed ChatGPT, defaults to sending stream_options.reasoning_summary_delivery, but Microsoft does not support this keyword, causing communication failure. GPT-5.6 also does not allow forcing Chat Completions mode, making it impossible for the ChatGPT client to communicate with Microsoft's large models.

Modifying the configuration file:

[features]
concurrent_reasoning_summaries = false

Then restarting ChatGPT and creating a new session still fails to achieve communication.

Using other clients, even the Codex CLI, works without issue because they all default to concurrent_reasoning_summaries = false.

What steps can reproduce the bug?

You can reproduce the issue immediately by configuring the Microsoft key directly in ChatGPT.

What is the expected behavior?

default to concurrent_reasoning_summaries = false.

Additional information

_No response_