Cursor, with your distinctive features, you're also using a shell of a Chinese model? Even American netizens and Musk can't stand it anymore.

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Cursor was once an essential AI programming tool for every developer. Recently, they released a model called Composer 2.

In their official announcement, they emphasized that this model was improved through “continuous pretraining” and “reinforcement learning.”

However, less than 24 hours later, a developer debugging the API discovered that the model ID was:

kimi-k2p5-rl-0317-s515-fast.

In plain language, the Composer 2 model released by Cursor is essentially Kimi K2.5 with some reinforcement learning added.

Du Yulun, head of pretraining at the Dark Side of the Moon, then posted on social media, stating that tests showed Composer 2’s tokenizer was identical to Kimi’s, and directly questioned why Cursor’s co-founder did not comply with the license or pay fees.

Suddenly, discussions about Cursor “cloning” and violating open-source licenses exploded in the tech community.

But don’t worry, there’s a twist.

As the controversy heated up, Kimi’s official account clarified: Cursor used Kimi K2.5 via the Fireworks AI managed platform under a commercial licensing agreement.

Cursor team member Lee Robinson also admitted that failing to mention that the base model was Kimi at first was a mistake. What seemed like a serious license dispute ultimately turned into a PR issue about “miscommunication.”

01

The discovery process was straightforward.

A developer named Fynn, while debugging Cursor’s OpenAI-compatible interface, saw the API returning a model identifier:

accounts/anysphere/models/kimi-k2p5-rl-0317-s515-fast.

The naming was very direct: k2p5 refers to Kimi K2.5, rl indicates reinforcement learning, 0317 might be the training date, and fast denotes a faster version.

After posting this on social media, it quickly drew attention from the tech community.

Elon Musk himself replied under this tweet, saying “Yes, it’s Kimi K2.5.”

The controversy quickly focused on the license terms. Kimi K2.5 uses a modified MIT license, which includes a clause for commercial use: if a product using the model (including derivatives) exceeds 100 million monthly active users or earns over $20 million monthly, it must prominently display “Kimi K2.5” in the interface.

The issue was that Cursor’s interface only displayed “Composer 2,” with no mention of Kimi. Meanwhile, Cursor was negotiating a new funding round, aiming for a valuation of about $50 billion, up from $29.3 billion last November.

Coincidentally, the Dark Side of the Moon was also raising funds recently!

Media reports said that the Dark Side of the Moon just raised $1 billion in its third round within 90 days, with a valuation of $18 billion. After this news, it was a positive signal for their fundraising, as Cursor’s endorsement of Kimi K2.5 lent credibility: “Kimi K2.5 is indeed effective!”

Around 3 a.m. on March 21, Kimi’s official account posted a statement on X: “Congratulations to the Cursor team on releasing Composer 2. We are proud that Kimi K2.5 provided the foundation. Seeing our model effectively integrated through Cursor’s continuous pretraining and high-performance reinforcement learning is exactly the open-source ecosystem we support.”

The key sentence was at the end: “Cursor accessed Kimi K2.5 via Fireworks AI’s managed reinforcement learning and inference platform, as part of a licensed commercial partnership.”

This clarification changed the entire nature of the incident. What initially appeared to be unauthorized use of open-source models turned into a legitimate business collaboration.

Cursor did not directly use the open-source weights of Kimi K2.5 but obtained authorization through the third-party platform Fireworks AI.

02

This isn’t Cursor’s first controversy.

Back in October 2025, when Cursor released Composer 1, similar issues arose.

Developers from multiple countries found that Composer 1-generated code frequently contained Chinese comments. Some investors posted screenshots, claiming this was strong evidence that Composer 1 was fine-tuned from a Chinese open-source model.

Later, media confirmed that both Cursor and another AI programming tool, Windsurf, used Chinese open-source models. Windsurf admitted to using Zhipu’s GLM.

But Cursor was quite firm, refusing to disclose the base model of Composer 1. They later quietly released Composer 1.5, effectively ending that controversy.

The current controversy over Composer 2 is essentially a repeat of the same issue. Had it not been discovered, it might have continued hidden.

From a technical perspective, Cursor’s approach isn’t inherently wrong.

Using open-source models for secondary development, then improving performance through continuous pretraining and reinforcement learning, is common in the AI industry.

Kimi K2.5 itself is an open-source model, and the Dark Side of the Moon’s goal in releasing it was to encourage more developers and companies to innovate on this foundation.

However, from the open-source community’s perspective, Cursor’s handling was problematic. Their announcement emphasized “initial continuous pretraining” and “large-scale reinforcement learning” but did not mention the origin of the base model.

This narrative could easily lead people to believe that Composer 2 is entirely self-developed, overlooking Kimi K2.5’s contribution.

Many companies promote their models by highlighting their improvements but rarely disclose the underlying open-source models they used. Legally, this may be acceptable, but in the open-source community, it’s seen as disrespectful to contributors.

In fact, copying domestic models has become quite common.

On March 17, Japanese tech giant Rakuten officially announced the release of the large model Rakuten AI 3.0, claiming it to be Japan’s largest and highest-performance AI model.

But upon inspection, it turned out to be DeepSeek-V3.

The story of Cursor, of course, continues. The storm over Composer 2 was also quelled by a post from the Dark Side of the Moon on X.

But market competition has only just begun. Under pressure from Claude Code and Codex, Cursor needs to prove not only its technical strength but also its value as a company valued at $50 billion.

And that might be even more difficult than developing a new model.