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Cursor Chapter 3’s Kimi K2.5

by Larry Chiang on March 21, 2026


**Cursor’s “Composer 2” got called out as “fake” (or at least misleadingly presented) mainly because of a quick drama that unfolded right after its launch around March 19-20, 2026.**
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Here’s the breakdown of what happened and why people are saying it’s “fake”:
Cursor launched Composer 2 as their shiny new **proprietary frontier coding model** — hyped as something they built/trained themselves, beating or matching top models like Claude Opus 4.6 on coding benchmarks (e.g. ~61.7% on Terminal-Bench), way cheaper (~1/5–1/10 the cost), faster, better context handling, etc. They marketed it heavily as “their” model, focused on coding data, with no mention of any base model from someone else.
**Within <24 hours**, developers spotted the real model ID leaking through the API: something like **kimi-k2p5-rl-0317-s515-fast** (or very similar variants).  
→ That’s clearly a **fine-tuned / RL-post-trained version of Moonshot AI’s open-weight Kimi K2.5** (a strong Chinese open model).
Moonshot AI (the creators) initially had staff publicly confirm:
– Tokenizer matches exactly
– Behavior / outputs match Kimi K2.5
– Cursor didn’t follow the license’s attribution rule
**The license drama**: Kimi K2.5 has a clause that if your product makes >$20M/month revenue (or huge user numbers), you must **prominently display “Powered by Kimi K2.5″** (or similar) in the UI. Cursor was reportedly doing ~$100M–$167M+ monthly revenue at the time and showed zero attribution → alleged violation.
This blew up fast on X/Twitter/Reddit/HN:
– Accusations of “fake proprietary model”
– “Wrapper + little RL and pretending it’s theirs”
– Hype for $29–50B valuation raise looked deceptive
– Elon Musk and others piled on with quick jabs like “Yeah it’s just Kimi 2.5”
**What happened next (resolution)**:  
Moonshot AI’s official account stepped in and basically said:  
“Congrats Cursor! Kimi K2.5 provides the foundation. We’re proud. Cursor accesses it via Fireworks AI as part of an authorized commercial partnership. All good on our end.”
Cursor (via team members) also clarified they follow the license through their inference partner (Fireworks), and the post-training/RL was real (they claim ~3/4 of the compute was their own).
So the license-violation fire died down quickly once Moonshot gave the green light / partnership confirmation. But the damage to perception was done — a lot of people still call it “fake” or “dishonest marketing” because:
– Cursor never disclosed the base model upfront
– Launched with heavy “our frontier model” framing
– Got caught, only explained after community reverse-engineered it
**Bottom line today (March 21, 2026)**:  
It’s **not fake in the sense of non-functional** — Composer 2 is apparently a legitimately strong coding agent (many users say it performs great).  
But it **is** “fake” in the marketing/PR sense: presented as more original/proprietary than it actually was, until forced to admit the Kimi base via a leak.
Classic 2026 AI startup move — stand on the shoulders of open Chinese frontier models, add targeted RL + product integration, then hype it as “ours”. Works great until someone leaks the model string. 😅

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