# Custom api provider url/payload format error

**URL:** <https://forum.cursor.com/t/custom-api-provider-url-payload-format-error/156215>\
**Category:** Bug Reports\
**Tags:** byok, openai\
**Created:** [March 30, 2026, 8:50am UTC](https://forum.cursor.com/t/custom-api-provider-url-payload-format-error/156215 "2026-03-30T08:50:59Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![deanrie](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/deanrie/32/1263_2.png) [@deanrie](https://forum.cursor.com/u/deanrie)\
**Post date:** [March 30, 2026, 9:08am UTC](https://forum.cursor.com/t/custom-api-provider-url-payload-format-error/156215/4 "2026-03-30T09:08:55Z")

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Hey, this is a known issue. When you use a custom API provider, Cursor sends a payload in the Responses API format (`input` instead of `messages`, flat tools format, etc.) to the `/chat/completions` endpoint. That’s why you get the error “field messages is required”.

More details here: [Cursor Agent sends Responses API format to /chat/completions endpoint](https://forum.cursor.com/t/cursor-agent-sends-responses-api-format-to-chat-completions-endpoint/153019)

The team is aware. There’s no timeline for a fix yet, but your report helps with prioritization.

For now, the main workaround is to intercept and convert the payload on the proxy side (Responses API to Chat Completions format). If your provider can’t support that, you can try using Cursor’s built-in routing instead of a base URL override.

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_[View the full topic](https://forum.cursor.com/t/custom-api-provider-url-payload-format-error/156215)._