Cursor ships Composer 2, and the API response strings give away its undisclosed Kimi K2.5 base
On March 20, 2026, developer Fynn was debugging Cursor's OpenAI-compatible endpoint when the returned model ID came back as accounts/anysphere/models/kimi-k2p5-rl-0317-s515-fast — evidence that Composer 2 was post-trained with reinforcement learning on top of Moonshot AI's Kimi K2.5. The tweet hit 44.4 views within a day.
Why it matters: One API debugging session ties together Cursor's undisclosed Kimi base, the licensing attribution dispute, and the cost landscape for Chinese versus U.S. models — a look at how the industry handles disclosure.