Cursor's "shell" Kimi controversy takes a turn: from infringement allegations to collaboration.

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

Less than two hours later, developer @fynnso intercepted the actual model ID of Composer 2 while debugging Cursor API requests: `kimi-k2p5-rl-0317-s515-fast`, which literally means "Kimi K2.5 + RL". Du Yulun, head of pre-training at Moon's Dark Side, immediately tweeted that after testing Composer 2's tokenizer, the team found it to be "completely identical to our Kimi tokenizer," and "almost certainly the result of further retraining of our model." He directly questioned Cursor co-founder Michael Truell: "Why don't you respect our license and pay any fees?"

The tweet was subsequently deleted. The controversy quickly escalated on social media, with Elon Musk replying to @fynnso's post, "Yeah, it's Kimi 2.5," further amplifying the topic. Kimi K2.5 uses a modified MIT license, which explicitly states that commercial products with over 100 million monthly active users or over $20 million in monthly revenue must prominently display "Kimi K2.5" on their user interface. Given Cursor's valuation and paid user base, the monthly revenue threshold was almost inevitably triggered. The tide then turned. The official account of The Dark Side of the Moon, @Kimi_Moonshot, posted early this morning, shifting from accusation to congratulations: Congratulations to the Cursor team on the release of Composer 2, "We are proud to see Kimi K2.5 providing the foundation." The statement also clarified that Cursor's access to Kimi K2.5 through the RL and inference platform hosted by Fireworks AI constitutes a licensed commercial collaboration, and the compliance of the license is guaranteed by Fireworks AI's commercial agreement. Following Kimi's official statement, Cursor co-founder Aman Sanger and VP of Developer Education Lee Robinson subsequently followed up. Sanger explained the technology choice: the team conducted perplexity evaluations on multiple pedestals, and Kimi K2.5 "proved to be the strongest." This was followed by further pre-training and high-computational-power reinforcement learning at four times the scale, deployed via Fireworks AI's inference and RL sampler. Robinson added that approximately one-quarter of the computational power in the final model came from the pedestals, with the remaining three-quarters coming from Cursor's own training. Both admitted that not mentioning the Kimi pedestal in the blog post was "a mistake," stating that the next model would be noted immediately. This is the second time Cursor has been found to be using a Chinese open-source model without disclosure. In November 2025, when Composer 1 was released, the community discovered that its tokenizer was identical to DeepSeek's, and the model occasionally output Chinese characters during inference; at that time, Cursor also failed to address this. The discussion sparked by this incident has gone beyond license compliance itself. Clément Delangue, co-founder and CEO of Hugging Face, commented that this is yet another validation of China's open-source efforts, stating that "China's open-source is now the biggest force shaping the global AI technology stack." The competition at the forefront is no longer just about who trains from scratch, but who adapts, fine-tunes, and productizes the fastest. A noteworthy coincidence: On March 15th, Bloomberg reported that Dark Side of the Moon was seeking up to $1 billion in new funding, valuing the company at approximately $18 billion, more than four times its valuation three months prior, with Alibaba and Tencent both participating in the investment. Just five days later, the world's highest-valued AI programming tool was discovered to be based on Kimi K2.5. Anysphere, valued at $29.3 billion, identified Kimi K2.5 as the "strongest base" in its evaluation, building its core products upon it—perhaps the most direct market endorsement of Dark Side of the Moon's technological capabilities. At this juncture, before this round of financing is even complete, the Cursor incident essentially served as a demonstration of Kimi's capabilities to a global developer community. Whether the $18 billion valuation still underestimates the dark side of the moon may need to be re-evaluated.