Cursor's self-developed new model surpasses Opus 4.6, mainly promoting "one-tenth the price," netizens mock it as a "Kimi 2.5 rebrand," endorsed by Musk.
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AI programming tool Cursor made a high-profile release of its self-developed model Composer 2, claiming performance surpassing Claude Opus 4.6 with much lower pricing, but was exposed by developers in less than three hours—the foundation underneath is actually China’s open-source model Kimi K2.5 from Moonshot AI.
This “self-developed” storm quickly swept through the AI community. Elon Musk personally weighed in to verify, ending with Cursor’s co-founder publicly apologizing and Kimi’s official account posting congratulations.
On March 21st, according to Hard AI news, Cursor co-founder Aman Sanger admitted after the incident escalated, “It was our omission not to mention Kimi as the base model from the start in the blog. We will correct this in the next model.”
Moonshot AI’s official account promptly replied: "Congratulations to Cursor on launching Composer 2. Very proud to see Kimi K2.5 become the foundational model. This is the open-source ecosystem we love." Moonshot AI also clarified that Cursor accesses Kimi K2.5 via Fireworks AI’s hosted RL and inference platform, as an authorized commercial partnership.
Performance surpasses Opus 4.6, pricing at “ankle-cut” levels
Cursor officially launched Composer 2 this Friday, claiming in its release blog that the model achieved significant improvements across all measured benchmarks, including Terminal-Bench 2.0 and SWE-bench Multilingual.
On Terminal-Bench 2.0, which measures agent terminal operation ability, Composer 2 performed between GPT-5.4 and Claude Opus 4.6, while its price-performance ratio on CursorBench was clearly better than both models.
Pricing is the central highlight of this Cursor release. Standard version Composer 2 is priced at $0.5 per million input tokens, and $2.5 per million output tokens—almost at “ankle-cut” levels compared to Claude Opus 4.6.
Cursor also launched the faster variant Composer 2 Fast, priced at $1.5 per million input tokens and $7.5 per million output tokens, maintaining a price advantage while targeting response speed.
Cursor attributed this breakthrough in price-performance to a new reinforcement learning method, emphasizing it was “genuinely trained ability, not just inference tricks.”
Foundation exposed less than 3 hours after launch
However, Composer 2’s highlight was extremely brief. Less than 3 hours after launch, X user @fynnso discovered the model ID was kimi-k2p5-rl-0317-s515-fast, concluding: “Composer 2 is really just a reinforcement-trained Kimi K2.5.”
This finding quickly spread across X and Hacker News, with memes and discussions flying. Elon Musk also directly replied “Yeah, it’s Kimi 2.5” under @fynnso’s post, further heating up the topic.
Reddit’s r/singularity community was equally lively. One user commented:
“Funniest thing is, everyone praised Composer 2 as a huge leap, but it was using someone else’s model the whole time. Makes you wonder, how many so-called ‘proprietary models’ are actually just open-source fine-tuned versions with a new logo?”
Some opinions argue that Cursor’s real moat lies in solving tasks with data accumulated from numerous developer users, not in pre-training itself. “Every investor knows they aren’t building their own foundational model; they should have been transparent from the beginning.”
Cursor apologizes; Kimi confirms authorized partnership
Facing public pressure, the Cursor team responded directly.
Aman Sanger publicly confirmed that the team assessed several base models for perplexity, with Kimi K2.5 "proving to be the strongest." Continuous pre-training and 4x scale RL with high compute were then applied, deployed via Fireworks AI’s inference and RL sampler.
Cursor VP of Developer Education Lee Robinson disclosed further details: About 1/4 of the computation in the final model comes from the foundational base, the remaining 3/4 is from Cursor’s own training.
Robinson also stated that although Composer 2 is based on an open-source model, the team will undertake full pre-training in the future.
Moonshot AI’s official account then made it clear: this partnership complies with licensing and is authorized commercial cooperation, congratulating Cursor on releasing Composer 2.
So, the legal and authorization aspects of the controversy are mostly clarified, but Cursor’s deliberate avoidance of mentioning the foundation in its release still causes ripples in the developer community.
“Taking notes” reinforcement learning: Cursor’s technical account
Despite controversy over the foundation, Cursor’s technical work retains independent value.
Cursor’s blog details its core method—a reinforcement learning mechanism called "self-summary," designed to address the pain point of AI coding assistants losing context in ultra-long, complex tasks due to limited context windows.
Specifically, during task execution, the model actively pauses at fixed token-length triggers, generates a stage summary, and then continues the task based on compressed context. This summarization ability is implemented as part of the RL reward: the higher the summary quality and subsequent task success rate, the higher the reward; otherwise, punishment is applied.
Internal tests published by Cursor show that token usage is just 1/5 that of traditional summarization, and errors from compression are reduced by about 50%.
Using the difficult task of “running Doom on MIPS architecture” as an example, Composer found an exact solution after 170 rounds of interaction, compressing more than 100,000 tokens of context to about 1,000.
Open source ecosystem and the transparency debate
The deeper discussion points to mutual trust issues between AI application layers and the open-source ecosystem.
Hugging Face co-founder and CEO Clement Delangue saw the value of open source in this event, noting that Chinese open-source models are now the largest force shaping the global AI tech stack.
Competitor Windsurf quickly seized the opportunity, announcing free access to Kimi K2.5 for all users within the next week, leveraging the momentum to attract Cursor users.
Analysis noted that for Cursor, this storm brought extra public pressure at a key fundraising moment. Reportedly, Cursor is conducting a new round of funding at a $50 billion valuation.
Cursor CEO Aman Sanger previously stated that Cursor is “a new type of company, neither purely an application developer nor a model provider.”
This event shows that as open-source foundations increasingly approach top closed models in performance, how downstream application vendors balance commercial packaging and technical transparency will become an unavoidable industry issue.
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