service_tier=fast rejected on ChatGPT subscription, API-only parameter confirmed

TrickyPlastic · reddit · 2026-09-21

A user of the Pi coding agent tried enabling fast mode for gpt-5.6-luna via a ChatGPT subscription by injecting servicetier="fast" in a beforeproviderrequest extension hook, but the parameter was rejected as unsupported.

The experiment suggests servicetier is exclusive to API-based billing and cannot be used through the openai-codex subscription path. The post includes the full TypeScript extension code showing how to inject provider parameters by model ID.

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