Moonshot says Kimi K2.5 is “the foundation” of Cursor's Composer 2 model; Cursor says it “should have mentioned” Composer 2 started from the base of Kimi K2.5
Congrats to the @cursor_ai team on the launch of Composer 2! We are proud to see Kimi-k2.5 provide the foundation. Seeing our model integrated effectively through Cursor's continued pretraining & high-compute RL training is the open model ecosystem we love to support.
@kimi_moonshot
Context & Ripple Effects
Moonshot had already publicly described Kimi K2.5 as Composer 2's foundation and said Cursor accessed it through Fireworks AI in an earlier disclosure about the model’s role. Cursor’s acknowledgment turns that claim into a shared account of Composer 2’s starting point.
The attribution also frames Cursor’s added pretraining and high-compute reinforcement learning as the layer built atop Kimi K2.5, rather than presenting Composer 2 as originating solely within Cursor. Later coverage keeps that lineage visible: Composer 2.5 was also described as built on Kimi K2.5.
First-order effects
Cursor must clarify Composer 2’s provenance, while Moonshot gains explicit credit for supplying its base model.
The disclosure separates the base-model contribution from Cursor’s continued pretraining and reinforcement-learning work, giving users and buyers a clearer account of the model stack.
Second-order effects
Model developers and AI-product vendors face stronger pressure to disclose when a branded model begins from an external open-weight base, particularly when subsequent training materially changes the result.
Moonshot can point to Composer as evidence that Kimi K2.5 is usable as a foundation for downstream products; competitors may emphasize provenance and post-training differentiation more explicitly.
Third-order effects
If such disclosures become routine, frontier-product competition may increasingly split between base-model suppliers and application vendors that differentiate through post-training, compute, and product integration.
That split would make model provenance a more important procurement and trust signal, even where the shipped model has been substantially modified from its starting base.
The trend: This is one data point in the growing unbundling of AI models into reusable base weights and differentiated downstream training and product layers.
was messing with the OpenAI base URL in Cursor and caught this accounts/anysphere/models/kimi-k2p5- rl-0317-s515-fast so composer 2 is just Kimi K2.5 with RL at least rename the model ID [image]
Since people really want me to say this: “KIMI K2.5” ‼️ Yes, that is the base we started from. And we are following the license through inference partner terms (e.g. Fireworks) I'm thankful for OSS models personally, good for the ecosystem.
🚨NEWS: Cursor's $50B “in-house model” is literally Kimi K2.5 with RL on top. Got caught in 24 hours >be Moonshot AI >spend hundreds of millions training Kimi K2.5 >1 trillion parameters, 15 trillion tokens, agent swarm architecture >beat GPT-5.2 and Opus 4.5 on real benchmarks [i…
We've evaluated a lot of base models on perplexity-based evals and Kimi k2.5 proved to be the strongest! After that, we do continued pre-training and high-compute RL (a 4x scale-up). The combination of the strong base, CPT and RL, and Fireworks' inference and RL samplers make
Knowing Cursor is a hack job and built on Kimi K2.5, pretty much cancelling my subscription right now. Zero innovation from them the past few months, it went from a pretty useful tool (nice having IDE), to really providing zero value.
Looks like it's confirmed Cursor's new model is based on Kimi! It reinforces a couple of things: - open-source keeps being the greatest competition enabler - another validation for chinese open-source that is now the biggest force shaping the global AI stack - the frontier is no…
People dunk on Cursor like: “it's just Kimi K2.5,” “look inside, it's a Chinese model.” There's no shame in building on top of strong base models and doing your own post-training or RL (as long as you respect the license). In most cases you don't need to pretrain from scratch.
“1/4 of the compute spent on the final model came from the base, the rest is from our training” k2 and k2.5 (a continued pretrain of k2) each used 15T tokens k2 is 32B active; by 6np, kimi did 5.76e+24 flops if that's 1/4, cursor did 1.7e25 flops i.e. same as gpt-4 flops
I'm a big believer in open source, especially as AI improves. It was a miss to not mention the Kimi base in our blog from the start. We'll fix that for the next model 🙏 Their team clarified our usage was licensed in the tweet below. https://x.com/...
The responses from @Kimi_Moonshot & @cursor_ai both are gracious. So wonderful to see. 1. There is absolutely no shame in building on top of open source model. Way better than repeating the same stacks in walled garden. Open-source > closed 2. Chinese open source models are [imag…
Here's confirmation the license is correct from the Kimi team. Agree with the feedback we should have mentioned the base up front, we will do that for the next model! https://x.com/...
Class Notice how well they've played this. Engineers reacted, deleted tweets, the community did everything else, mounting pressure until Cursor attributed the model. No need for scandal. [image]
Cursor keeps showing poor judgment with comms - behaving not like a $10B+ company, but like an early-stage startup Hikes prices for many enterprise customers without notice, or comms or transparency Big bang Composer 2.0 release w/o sharing that it's based on Kimi 2.5
Cursor's Composer 2 is likely built on Kimi K2.5. The model URL + tokenizer are strong signals. I love this direction: companies mid-train and post-train on top of OSS LLMs. Prediction: open-source model labs will monetize by taking a cut when others build on top of their
Yep, Composer 2 started from an open-source base! We will do full pretraining in the future. Only ~1/4 of the compute spent on the final model came from the base, the rest is from our training. This is why evals are very different. And yes, we are following the license throug…
Moonshot is being incredibly diplomatic here considering Cursor spelled “Kimi K2.5” as “continued pre-training and reinforcement learning.” The model ID was literally kimi-k2p5-rl-0317-s515-fast. They forgot to file the serial numbers off. Nailed it.
whether this is true or not it's going to cause every company producing open source models to re-evaluate if they should continue to do so that is incredibly frustrating
So let me get this right: 1. MoonshotAI distills Claude from Anthropic with 3.4 million exchanges on their API. 2. Releases Kimi K2.5 as OSS. 3. Cursor RL fine-tunes Kimi K2.5 into Composer 2 Cursor now delivers Opus 4.6 performance for a fraction of the price. [image]