Using service_tier = "flex" in CLI produces error 400 on message

What version of Codex CLI is running?

0.115.0

What subscription do you have?

Pro

Which model were you using?

gpt-5.4

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

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

Windows Terminal PS7

What issue are you seeing?

Error {"type":"error","status":400,"error":{"type":"invalid_request_error","message":"Unsupported service_tier: flex"}} when sending messages or establishing websocket with service_tier = "flex"

What steps can reproduce the bug?

1. set service_tier = "flex" in config.toml as per official OpenAI Codex documention
2. start codex-cli
3. send agent message

What is the expected behavior?

No Error 400 and agent response

Additional information

Same behaviour in Codex App, though it falls back to "normal" so this is masked to the user.