AI coding company Cursor launched a new model this week called Composer 2, which it promoted as offering “frontier-level coding intelligence.” 

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

As evidence, Fynn pointed to code that seemed to identify Kimi as the model.

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

It was a surprising revelation, since Cursor is a well-funded U.S. startup that raised a $2.3 billion round last fall at a $29.3 billion valuation, and is reportedly exceeding $2 billion in annualized revenue. Also, the company didn’t mention anything about Moonshot AI or Kimi in its announcement.

However, Cursor’s vice president of developer education Lee Robinson soon acknowledged, “Yep, Composer 2 started from an open-source base!” But he said, “Only ~1/4 of the compute spent on the final model came from the base, the rest is from our training.” As a result, he said Composer 2’s performance on various benchmarks is “very different” from Kimi’s.

Robinson also insisted that Cursor’s use of Kimi was consistent with the terms of its license, a point the Kimi account on X repeated in a subsequent post congratulating Cursor, where it said the Cursor used Kimi “as part of an authorized commercial partnership” with Fireworks AI.

“We are proud to see Kimi-k2.5 provide the foundation,” the Kimi account said. “Seeing our model integrated effectively through Cursor’s continued pretraining & high-compute RL training is the open model ecosystem we love to support.”

So why not acknowledge Kimi upfront? Beyond any potential embarrassment in not creating a model from scratch, building on top of a Chinese model might feel particularly fraught right now, with the so-called AI “arms race” often framed as an existential battle between United States and China. (See, for example, Silicon Valley’s apparent panic after Chinese company DeepSeek released a competitive model early last year.)

Cursor co-founder Aman Sanger acknowledged, “It was a miss to not mention the Kimi base in our blog from the start. We’ll fix that for the next model.”