The AI coding firm Cursor introduced a new model this week named Composer 2, which it touted as delivering “frontier-level coding intelligence.” 

Nevertheless, an X user known as Fynn quickly contended that Composer 2 was merely “Kimi 2.5” enhanced with extra reinforcement learning — Kimi 2.5 is an open-source model that was recently unveiled by Moonshot AI, a Chinese enterprise supported by Alibaba and HongShan (previously Sequoia China). 

To support this claim, Fynn referenced code that appeared to reveal Kimi as the underlying model.

“[A]t least change the model ID,” they derided.

This was an unexpected turn, considering Cursor is a well-capitalized U.S. startup that secured a $2.3 billion funding round last autumn at a $29.3 billion valuation, and is said to be generating over $2 billion in annualized revenue. Moreover, the firm did not mention Moonshot AI or Kimi in its announcement.

However, Lee Robinson, Cursor’s vice president of developer education, soon admitted, “Yep, Composer 2 began from an open-source foundation!” But he clarified, “Only about 1/4 of the compute allocated to the final model came from the base, with the remainder sourced from our training.” Consequently, he noted that Composer 2’s performance across various benchmarks is “very different” from that of Kimi.

Robinson also asserted that Cursor’s application of Kimi aligned with the terms of its license, a notion reiterated by the Kimi account on X in a subsequent message congratulating Cursor, stating that Cursor utilized Kimi “as part of an authorized commercial partnership” with Fireworks AI.

“We are delighted to see Kimi-k2.5 serving as the foundation,” the Kimi account stated. “Witnessing our model seamlessly integrated through Cursor’s ongoing pretraining & intensive RL training is the open model ecosystem we cherish supporting.”

So why not recognize Kimi from the beginning? Beyond any potential embarrassment of not developing a model independently, leveraging a Chinese model might feel particularly sensitive at this moment, especially with the so-called AI “arms race” often depicted as a critical struggle between the United States and China. (For instance, observe Silicon Valley’s noticeable anxiety after the Chinese company DeepSeek launched a competing model early last year.)

Cursor co-founder Aman Sanger acknowledged, “Not mentioning the Kimi base in our blog initially was a mistake. We’ll rectify that for the next model.”