I Found a Hidden Model ID in Cursors API. It Wasnt What They Said It Was.

On March 19, Cursor shipped Composer 2 — its most capable proprietary coding model to date. The company marketed it as a breakthrough: faster completions, better multi-file edits, a tenth of the price of Claude Opus 4.6. Elon Musk endorsed it. Developers rushed to try it. Less than 24 hours later, a developer named Fynn found a model ID hidden in Cursor's API configuration: kimi-k2p5-rl-0317-s515-fast. That string decoded to "Kimi 2.5 plus reinforcement learning" — Kimi being an open-source model from Moonshot AI, a Chinese startup backed by Alibaba and HongShan. The discovery set off one of the biggest transparency controversies in the AI coding tool space this year. Here's what happened, why it matters, and what it means for every developer who trusts their editor's AI. The Discovery That Unraveled the Marketing Cursor's blog post announcing Composer 2 said nothing about Moonshot AI, Kimi, or any open-source foundation. The messaging positioned it as Cursor's own creation — a proprie