Codex CLI incorrectly omits service_tier="flex" for gpt-5.6 models

What version of Codex CLI is running?

0.147.0

What subscription do you have?

API key

Which model were you using?

gpt-5.6-sol

What platform is your computer?

Darwin 24.5.0 arm64 arm

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

warp

Codex doctor report

What issue are you seeing?

Codex CLI reports that the flex service tier is unsupported for gpt-5.6-sol and omits it from API requests. However, the same model and service tier work correctly when sent directly to the Responses API.

What steps can reproduce the bug?

Run:

 codex exec -m gpt-5.6-sol \
   -c 'service_tier="flex"' \
   -c 'model_reasoning_effort="low"' \
   'Reply with exactly: OK'

Actual behavior

Codex prints:

Configured service tier flex is not advertised as supported
for model gpt-5.6-sol and will be omitted from requests.

The request succeeds, but without Flex processing.

Expected behavior

Codex should pass service_tier: "flex" to the API without displaying a warning.

Direct API verification

The equivalent request sent directly to /v1/responses succeeds:

  {
    "model": "gpt-5.6-sol",
    "input": "Reply with exactly: OK",
    "reasoning": {
      "effort": "low"
    },
    "service_tier": "flex"
  }

The API response confirms that Flex was used:

  {
    "status": "completed",
    "model": "gpt-5.6-sol",
    "service_tier": "flex",
    "output_text": "OK",
    "error": null
  }

This suggests that the Codex CLI model capability metadata or validation logic is outdated or inconsistent with the Responses API.

What is the expected behavior?

No response

Additional information

No response

Reproduce the issue with the provided codex exec command and inspect the model capability metadata or validation logic that decides whether service_tier="flex" is supported. Done means gpt-5.6-sol sends service_tier: "flex" to the Responses API without displaying the unsupported-tier warning.