Hey, thanks for the report, this is a known issue.
What’s happening: when using BYOK with a base URL override, Cursor Agent sends an OpenAI Responses API payload (input, flat tool format, etc.) to the /chat/completions endpoint instead of using the Chat Completions format.
Related threads about the same issue:
- Requests are sent to the wrong endpoint when using a base URL override
- Cursor is sending Responses API requests to the Chat Completions API
The team is aware. There’s no ETA for a fix yet, but your detailed report, especially the breakdown of all the format mismatches, really helps with prioritization.
For now, the main solution is the workaround you already found, intercept and convert the payload on the proxy side. Another approach some users take is to avoid the base URL override and use Cursor’s built-in API routing, if that fits your setup.