Flex tier doesn't work in GPT5 model

What version of Codex is running?

_No response_

Which model were you using?

gpt-5

What platform is your computer?

_No response_

What steps can reproduce the bug?

Hi, regarding the commit:

feat: add flex mode option for cost savings (#372)

I believe that now it has a bug since Flex mode should be available for all the Flex enabled models and now gpt-5 is one of this supported models.

The flex tier was introduced but it seems that currently the gpt-5 model is not supported with this Codex CLI implementation.

Also not sure about if this will work with cohesively with reasoning effort parameter. For example what would happen if we enable flex mode and high reasoning effort, are they compatible?

Thanks.

What is the expected behavior?

_No response_

What do you see instead?

_No response_

Additional information

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