400 'Instructions are not valid

What version of Codex is running?

https://github.com/openai/codex/commit/3f40fbc0a886ef54f494a1ff5963971bc42c036a

Which model were you using?

gpt-5

What platform is your computer?

Darwin 24.6.0 arm64 arm

What steps can reproduce the bug?

When I enter “hello,” the following error appears.
400 'Instructions are not valid'

What is the expected behavior?

Hello, and a response comes from ChatGPT.

What do you see instead?

ChatGPT exhibits behavior where it only accepts “authorized system prompts (instructions)”, and was being blocked by one of the following:
It was sending “instructions differing from the default” via config overrides (experimental_instructions_file/base_instructions).
During retries, even if the authentication mode changed, it did not regenerate the instructions and instead resubmitted “the previous ones (content flagged as invalid)”.

Additional information

This occurs when using ChatGPT without an API key.