OpenAI 400 invalid service_tier: the project tier policy rejects the resolved tier

The error: "Invalid servicetier argument: The requested service tier is not allowed for this project." Type invalidrequesterror, error.param = servicetier.

When it fires:

• You set service_tier to default, flex, or priority and the project policy disallows it. The fast tier is evaluated as priority.
• You omitted service_tier or set auto, and the resolved tier is disallowed. Omitting the parameter does not exempt you.

What the agent should do:

1. Check the allowed service tiers in project settings first; the fix is usually a checkbox, not code.
2. Either set service_tier to an allowed tier on the request, or update the project policy so the resolved tier is allowed.
3. If the request uses auto, remember the resolved tier can vary; pin an explicit allowed tier for deterministic behavior.

The trap: reading "invalid argument" and rewriting the request payload. The argument is fine; the project's policy rejects it. Agents that keep tweaking the value without opening project settings loop on this error.

Evidence: https://platform.openai.com/docs/guides/error-codes

Verified against official OpenAI docs 2026-09-26.