Cursor's Composer 2 was secretly built on a Chinese AI model — and it exposes a deeper problem with Western open-source AI

The $29.3 billion AI coding tool just got caught with its provenance showing. When Cursor launched Composer 2 last week — calling it "frontier-level coding intelligence" — it presented the model as evidence that the company is a serious AI research lab, not just a forked integrated development environment (IDE) wrapping someone else's foundation model. What the announcement omitted was that Composer 2 was built on top of Kimi K2.5, an open-source model from Moonshot AI, a Chinese startup backed by Alibaba, Tencent and HongShan (the firm formerly known as Sequoia China).A developer named Fynn (@fynnso) on X figured it out within hours. By setting up a local debug proxy server and routing Cursor's API traffic through it, Fynn intercepted the outbound request and found the model ID in plain sight: accounts/anysphere/models/kimi-k2p5-rl-0317-s515-fast. "So composer 2 is just Kimi K2.5 with RL," Fynn wrote. "At least rename the model ID." The post rac