Cursor acknowledges that the new coding model was built on top of Moonshot AI’s Kim

AI coding company Cursor announced a new model this week called Composer 2, touting it as offering “state-of-the-art coding intelligence.”

However, an X user posting under the name Fynn quickly claimed that Composer 2 was “just Kimi 2.5” with reinforcement learning added. Kimi 2.5 is an open source model recently released by Moonshot AI, a Chinese company backed by Alibaba and HongShan (formerly Sequoia China).

As evidence, Finn points to the code that appears to identify you as a model.

“[A]At least rename the model ID,” they scoffed.

This was a surprising revelation since Cursor is a well-funded US startup that raised $2.3 billion last fall at a valuation of $29.3 billion and reportedly has annual revenue of more than $2 billion. Additionally, the company made no mention of Moonshot AI or Kimi in its announcement.

However, Lee Robinson, Cursor’s vice president of developer education, was quick to admit, “Yes, Composer 2 started with an open source base!” But he said, “Only a quarter of the computations spent on the final model came from the base; the rest came from training.” As a result, Composer 2’s performance on various benchmarks is “very different” from Kimi’s performance, he said.

Robinson also asserted that Cursor’s use of Kimi was consistent with the terms of the license, a point echoed by X’s Kimi account in a subsequent post congratulating Cursor, saying Cursor used Kimi “as part of an authorized commercial partnership” with Fireworks AI.

“We are proud that Kimi-k2.5 provides the foundation,” the Kimi account said. “Seeing our models effectively integrated through Cursor’s continuous pre-training and high-compute RL training is an open model ecosystem we want to support.”

So why not just admit it? Beyond the potential embarrassment of not building a model from scratch, building on China’s model may feel especially difficult at a time when the so-called AI “arms race” is often framed as an existential battle between the United States and China. (See, for example, Silicon Valley’s apparent panic after Chinese company DeepSeek released a competing model early last year.)

Aman Sanger, co-founder of Cursor, admitted, “It was a mistake not to mention Kimibase on the blog from the beginning. We will correct that in the next model.”