Cursor's new coding model was built on top of Kimi K2.5, a Chinese open-source base

AI coding tool Cursor released its latest model, Composer 2, on March 19, marketing it as delivering "frontier-level coding intelligence".^[1] Within hours, a developer discovered that the model was built on top of Kimi K2.5, an open-source model from Beijing-based Moonshot AI, a detail Cursor had not mentioned anywhere in its announcement.^[2]

The discovery came when X user Fynn (@fynnso) inspected Cursor's API endpoint and found the model identifier accounts/anysphere/models/kimi-k2p5-rl-0317-s515-fast appearing in responses.^[2] Fynn posted a screenshot of the intercepted API call and concluded that Composer 2 appeared to be Kimi K2.5 with additional reinforcement learning applied on top.

Cursor's vice president of developer education, Lee Robinson, confirmed the finding on X, writing that Composer 2 "started from an open-source base" but that only about a quarter of the compute used in the final model came from Kimi K2.5, with the remainder from Cursor's own training.^[3] He added that the company was following the license through its inference partner, Fireworks AI.

In a follow-up post, Robinson named the base model directly: "Since people really want me to say this: 'KIMI K2.5'" [3]. Then, in a separate post, he acknowledged the omission directly, writing that it was "a miss to not mention the Kimi base in our blog from the start" and that the company would fix that for the next model.^[4]

Moonshot AI endorsed the use publicly

The official Kimi account on X congratulated Cursor and confirmed the use was authorized, writing that the integration took place through "an authorized commercial partnership" with Fireworks AI.^[5] The account called the arrangement an example of the open model ecosystem working as intended.

An open licensing question

Kimi K2.5 is released under a modified MIT license that imposes an attribution requirement: any product or service exceeding 100 million monthly active users or $20 million in monthly revenue must prominently display "Kimi K2.5" in user-facing materials.^[6][7] Cursor, which is valued at $29.3 billion following a $2.3 billion funding round last year and reportedly generates over $2 billion in annualized revenue, would appear to exceed that threshold.^[8] No such attribution currently appears in Cursor's interface or documentation. Whether the Fireworks AI partnership satisfies that clause, or whether Cursor's product itself requires the attribution, is not addressed in any of the public statements from either company.

Context: geopolitics and disclosure norms

The omission drew attention partly because of the broader geopolitical context around U.S. companies building on Chinese AI technology. Moonshot AI is a Beijing-based company backed by Alibaba and HongShan, formerly Sequoia China.^[8] The U.S.-China AI competition narrative has intensified since DeepSeek's emergence in early 2025, and disclosing a Chinese model as a foundation layer carries reputational risk that a U.S. or European base would not.

But the incident also raises a more structural question about model provenance in AI products. As open-source models become increasingly competitive at the base layer, the practice of fine-tuning an existing model and shipping it under a product name is likely to become more common. What disclosure norms should apply, and whether current licensing mechanisms are sufficient to enforce them, remains an open question across the industry.

Cursor's Composer 2 is priced at $0.50 per million input tokens and $2.50 per million output tokens and is available within the Cursor coding environment.^[1]