AI coding platform Cursor unveiled a new model this week dubbed Composer 2, which the company promoted as delivering "frontier-level coding intelligence" — a bold claim that quickly drew scrutiny from the developer community.
Shortly after the announcement, an X user posting under the handle Fynn alleged that Composer 2 was "just Kimi 2.5" augmented with additional reinforcement learning. Kimi 2.5 refers to an open-source model recently released by Moonshot AI, a Chinese AI company backed by Alibaba and HongShan, formerly known as Sequoia China.
As supporting evidence, Fynn pointed to embedded code that appeared to surface Kimi as the underlying model identifier.
"[A]t least rename the model ID," they quipped pointedly.
The revelation caught many observers off guard. Cursor is a well-capitalized U.S. startup that closed a $2.3 billion funding round last fall at a $29.3 billion valuation and is reportedly surpassing $2 billion in annualized revenue. Adding to the intrigue, the company's launch announcement made no reference whatsoever to Moonshot AI or its Kimi model.
Cursor's vice president of developer education, Lee Robinson, subsequently confirmed, "Yep, Composer 2 started from an open-source base!" He was quick to add, however, that "only ~1/4 of the compute spent on the final model came from the base, the rest is from our training." He maintained that, as a result of this substantial additional training, Composer 2's performance across various benchmarks is "very different" from the base Kimi model.
Robinson further asserted that Cursor's use of Kimi as a foundation was fully compliant with the terms of Moonshot AI's open-source license — a position echoed by the official Kimi account on X in a subsequent post congratulating Cursor, which clarified that Cursor had used Kimi "as part of an authorized commercial partnership" with inference platform Fireworks AI.
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"We are proud to see Kimi-k2.5 provide the foundation," the Kimi account stated. "Seeing our model integrated effectively through Cursor's continued pretraining & high-compute RL training is the open model ecosystem we love to support."
The question of why Cursor opted against disclosing Kimi's role from the outset is worth examining. Beyond any reputational discomfort associated with not developing a model entirely in-house, building commercial products atop open-source models from Chinese AI labs carries its own set of sensitivities in the current climate — particularly given the ongoing discourse framing AI development as an intensifying competitive dynamic between the United States and China. (The degree to which that narrative shapes industry decisions was illustrated vividly by Silicon Valley's collective reaction when Chinese lab DeepSeek released a highly competitive model early last year.)
Cursor co-founder Aman Sanger ultimately conceded the oversight directly: "It was a miss to not mention the Kimi base in our blog from the start. We'll fix that for the next model."
