Cursor Built Composer 2 on a Chinese Open-Source Model — and Didn't Say So
Cursor launched Composer 2 as a proprietary frontier coding model. Within hours, a developer found it was built on Moonshot AI's Kimi K2.5. Cursor admitted the omission — but the licensing implications are still unresolved.
Cursor launched Composer 2 on March 19 calling it “frontier-level coding intelligence” — positioning the company as a serious AI research lab, not just a well-designed IDE. The launch blog omitted one detail: the model is built on top of Kimi K2.5, an open-source model from Moonshot AI, a Chinese startup backed by Alibaba, Tencent, and HongShan.
A developer named Fynn uncovered this within hours. By routing Cursor’s API traffic through a local debug proxy, Fynn found the model ID in plain text: accounts/anysphere/models/kimi-k2p5-rl-0317-s515-fast. The post got 2.6 million views on X before most people had heard Composer 2 existed.
Moonshot AI’s head of pretraining confirmed the tokenizer match before deleting the post. A second Moonshot employee also confirmed, then deleted. The pattern of confirm-and-delete suggested internal pressure to stay quiet about a high-profile customer relationship.
Cursor co-founder Aman Sanger acknowledged the omission directly: “It was a miss to not mention the Kimi base in our blog from the start.” VP of developer education Lee Robinson provided the technical breakdown: roughly 25% of Composer 2’s compute came from the Kimi K2.5 base, 75% from Cursor’s own reinforcement learning. Cursor accesses the model through Fireworks AI under a commercial partnership with Moonshot AI.
The technical work Cursor did is real. RL-finetuning a base model for a specific domain — in this case, coding tasks within an IDE context — requires significant compute and engineering. Composer 2 is not simply a Kimi K2.5 wrapper. But the framing at launch implied substantially more independence from the underlying model than actually exists.
The licensing issue is harder to dismiss. Kimi K2.5 ships under a Modified MIT License with a conspicuous branding requirement: any product with more than 100 million monthly active users or more than $20 million in monthly revenue must “prominently display ‘Kimi K2.5’” in its UI. Cursor’s annual recurring revenue has been reported above $2 billion — approximately $167 million per month, more than eight times the licensing trigger. Cursor’s interface displayed only “Composer 2.”
Moonshot AI said publicly that Cursor’s use is authorized under a commercial agreement and congratulated Anysphere on the release. What the commercial agreement says about branding obligations isn’t known.
The broader fallout is less about this specific incident and more about what it reveals. Every AI product has a foundation model somewhere. Most don’t disclose it. Cursor got caught because the API call was visible, and a developer had the curiosity to look. The community’s reaction — a week of Hacker News and X threads — signals that developers increasingly care about model lineage and aren’t satisfied with marketing language that implies ownership of underlying research that was done elsewhere.
For Cursor specifically, the reputational hit comes at a sensitive moment. The company is reportedly valued near $50 billion and has positioned itself as the AI-native IDE for serious engineers. The credibility of that story depends partly on developers believing the company is building meaningful AI, not reselling it with a better UX.
Whether or not this changes how Cursor handles future launches, it has already changed how developers will read the next coding tool announcement.