Cursor 「In-house」 Composer 2 Embeds Kimi K2.5, Far Side of the Moon Alleges: Non-compliance with License, Unpaid Fees

According to 1M AI News monitoring, developer @fynnso discovered during Cursor API request debugging that the actual model ID of Composer 2 is `kimi-k2p5-rl-0317-s515-fast`, literally meaning "Kimi K2.5 + RL." Moonshot AI Pre-training Lead Du Yulun (@Yulun_Du) immediately tweeted, stating that after testing Composer 2's tokenizer, they found it to be "completely identical to our Kimi tokenizer" and that they could "almost confirm that this is the result of our model being further trained," and directly mentioned Cursor Co-founder Michael Truell, questioning, "Why didn't you respect our license and did not pay any fees."

When Cursor released Composer 2, they claimed that the performance improvement came from "continuing pre-training on the base model for the first time, combined with reinforcement learning," but did not mention Kimi K2.5 throughout. These two points are not contradictory: continued pre-training and RL are both applied on top of a certain base, and Cursor simply did not disclose the base's origin.

Kimi K2.5 adopts a modified MIT license, which explicitly states: for commercial products with over 100 million MAU or monthly revenue exceeding 20 million USD, "Kimi K2.5" must be prominently displayed in the user interface. Given Cursor's $29.3 billion valuation and the scale of paid users, the monthly revenue threshold is almost certainly triggered. As of the time of writing, Cursor has not publicly responded.

This is the second time Cursor's self-developed model has been suspected of being a shell of a Chinese open-source model. In November 2025, when Composer 1 was released, the community discovered that its tokenizer was identical to DeepSeek, and the model would output Chinese during the inference process, at which time Cursor similarly did not disclose the base's origin.