On March 19, Cursor shipped Composer 2 — its most capable proprietary coding model to date. The company marketed it as a breakthrough: faster completions, better multi-file edits, a tenth of the price of Claude Opus 4.6. Elon Musk endorsed it. Developers rushed to try it.
Less than 24 hours later, a developer named Fynn found a model ID hidden in Cursor's API configuration: kimi-k2p5-rl-0317-s515-fast. That string decoded to "Kimi 2.5 plus reinforcement learning" — Kimi being an open-source model from Moonshot AI, a Chinese startup backed by Alibaba and HongShan.
The discovery set off one of the biggest transparency controversies in the AI coding tool space this year. Here's what happened, why it matters, and what it means for every developer who trusts their editor's AI.
The Discovery That Unraveled the Marketing
Cursor's blog post announcing Composer 2 said nothing about Moonshot AI, Kimi, or any open-source foundation. The messaging positioned it as Cursor's own creation — a proprietary model that could outperform Claude Opus 4.6 at a fraction of the cost.
But the model ID in the API told a different story. Within hours of Fynn's discovery, Yulun Du — Moonshot AI's head of pre-training — tested Composer 2's tokenizer and confirmed it matched Kimi K2.5's tokenizer identically. The evidence was undeniable.
Elon Musk weighed in with a characteristically blunt assessment: "Yeah, it's Kimi 2.5."
What Cursor Actually Admitted
Cursor's vice president of developer education, Lee Robinson, acknowledged the base: "Yep, Composer 2 started from an open-source base!" He argued that only about a quarter of the compute spent on the final model came from Kimi K2.5, with the remaining three-quarters from Cursor's own reinforcement learning training.
Co-founder Aman Sanger went further, calling it a "miss" not to mention the Kimi base in their blog post. "We'll fix that for the next model," he said.
The company framed the relationship as an "authorized commercial partnership" through Fireworks AI, which hosted the RL training and inference. Moonshot AI's official Kimi account on X corroborated this, congratulating Cursor on the integration.
The Licensing Problem Nobody Can Ignore
This isn't just about credit. Kimi K2.5's license contains a specific clause: any commercial product with more than 100 million monthly active users or generating more than $20 million in monthly revenue must prominently display "Kimi K2.5" in its user interface.
Cursor's annualized revenue stands at approximately $2 billion — roughly 8x above that $20 million monthly threshold. At no point did Cursor display Kimi K2.5 branding in its interface.
Whether this constitutes a license violation depends on the specifics of Cursor's commercial agreement with Fireworks AI. But the optics are damaging regardless. Developers who chose Cursor partly based on the "proprietary model" narrative now know that narrative was incomplete at best.
Why This Matters for Every Developer
The Cursor-Kimi i
Originally published on Skila AI with full details.
Top comments (0)