Hey, thanks for the report. This is a known limitation: BYOK currently only supports /v1/chat/completions, while GPT-5/5.1 models use the /v1/responses format-so the proxy sees an input payload on the chat/completions endpoint and returns the “missing messages” error.
Workarounds:
- Turn off your OpenAI API key and Base URL override and use Cursor’s built-in API for GPT-5/5.1 (Settings > Models > API Keys, or
Cmd+Shift+0). This should unblock you. - Reference: Cursor is sending Response API requests to Chat Completion API and With custom model api key get request error
Could you confirm:
- The model you selected (e.g., GPT-5, GPT-5.1 Codex)
- The exact Base URL you’re proxying
- If switching off the custom key/override fixes it
We’re tracking this limitation-appreciate your patience.