In the early morning of March 20, AI programming tool Cursor (parent company Anysphere, latest valuation $29.3 billion) released its self-developed model Composer 2. The blog stated that the performance improvement came from "the first continuation pre-training of the base model, combined with reinforcement learning," without mentioning the source of the base model throughout the entire process.

Less than two hours later, developer @fynnso intercepted the actual model ID of Composer 2 while debugging the Cursor API request: `kimi-k2p5-rl-0317-s515-fast`, which literally means "Kimi K2.5 + RL". Du Yulun, the person in charge of pre-training at Moon’s Dark Side, immediately tweeted that the team found after testing Composer 2's tokenizer that "it is completely consistent with our Kimi tokenizer," and "it can almost be confirmed that this is the result of our model being further fine-tuned," directly questioning Cursor co-founder Michael Truell: "Why not respect our license and pay any fees?"

The tweet was subsequently deleted. The controversy quickly fermented on social media, with Elon Musk replying to @fynnso's post "Yeah, it's Kimi 2.5," further amplifying the topic's heat.

Kimi K2.5 employs a modified MIT license, explicitly stating: commercial products with over 100 million monthly active users or more than $20 million in monthly revenue must prominently label "Kimi K2.5" in the user interface. Given Cursor's valuation and paid user scale, the monthly revenue threshold is almost certainly triggered.

Then the tide turned. The official account of Moon’s Dark Side @Kimi_Moonshot posted this morning, shifting the tone from accusation to congratulations: congratulating the Cursor team on releasing Composer 2, "we are proud to see Kimi K2.5 providing the foundation." The statement also clarified that Cursor accesses Kimi K2.5 through Fireworks AI's hosted RL and inference platform, which belongs to an authorized commercial partnership, and the license compliance is ensured by Fireworks AI's commercial agreement.

After Kimi's official statement, Cursor co-founder Aman Sanger and Vice President of Developer Education Lee Robinson followed up. Sanger explained the technical choice: the team conducted perplexity evaluations on multiple bases, and Kimi K2.5 "proved to be the strongest," then overlaying continuation pre-training and high-performance reinforcement learning at a scale four times larger, deployed through Fireworks AI's inference and RL sampler.

Robinson added that about 1/4 of the computational power in the final model came from the base, while the remaining 3/4 came from training by Cursor itself. Both admitted that not mentioning the Kimi base when the blog was released "was a mistake" and stated that the next model would indicate this at the earliest opportunity.

This is the second time Cursor has been found using a Chinese open-source model without disclosure. In November 2025, when Composer 1 was released, the community found its tokenizer consistent with DeepSeek, and the model occasionally output Chinese during the inference process; at that time, Cursor also did not provide an explanation.

The discussions sparked by this incident have gone beyond license compliance itself. Clément Delangue, co-founder and CEO of Hugging Face, commented that this is another validation of Chinese open-source, "Today, Chinese open-source is the greatest force shaping the global AI technology stack," stating that frontier competition is no longer just about who trains from scratch, but who adapts, fine-tunes, and productizes the quickest.

A noteworthy timing coincidence: on March 15, Bloomberg reported that Moon’s Dark Side was seeking up to $1 billion in a new funding round, with a valuation of approximately $18 billion, more than quadrupling from three months prior, with Alibaba and Tencent both participating in the betting. Just five days later, the world's highest-valued AI programming tool was found to be based on Kimi K2.5. Anysphere, valued at $29.3 billion, identified Kimi K2.5 as the "strongest base" in its evaluations, upon which it built its most core product; this may represent the most direct market endorsement of Moon’s Dark Side's technological capacity.

At this point, before the completion of this funding round, the Cursor incident effectively served as a capability demonstration for Kimi aimed at global developers; whether the $18 billion valuation still underestimates Moon’s Dark Side may need to be re-evaluated.

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