Hey, yeah, I’ve seen this issue. DeepSeek V4, same as R1 and Kimi K2.5, requires reasoning_content from previous assistant responses to be sent back to the API in multi-turn requests that include tool calls. On the BYOK path, Cursor currently doesn’t store this field for the DeepSeek API directly, so DeepSeek returns 400.
Until we have a fix on our side, you can use a community proxy that sits between Cursor and DeepSeek and fills in the missing field: GitHub - yxlao/deepseek-cursor-proxy: Fixes reasoning_content issue of DeepSeek V4, supprting deepseek-v4-pro and deepseek-v4-flash · GitHub
Related thread with the same root cause and a workaround discussion: Compatibility with DeepSeek model's design to return reasoning content after tool calls?