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Cursor AI's Composer 2 Uses Open-Source Base Model with Additional Training

Cursor AI's Composer 2 Uses Open-Source Base Model with Additional Training

@leerob

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 through our inference partner terms.

Mar 20, 2026, 4:46 PM View on X

283 Replies

122 Retweets

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780.1K Views  Lee Robinson @leerob

One Sentence Summary

Lee Robinson reveals that Cursor's Composer 2 started from an open-source base and will undergo full pretraining, with only ~1/4 of compute from the base model.

Summary

This tweet provides rare transparency about AI model training practices. Lee Robinson, a developer relations lead at Cursor AI, discloses that Composer 2 originated from an open-source foundation model. He explains that only about one-quarter of the total compute was spent on the base model, with the majority coming from Cursor's own training efforts. This explains significant differences in evaluation results. He also confirms they are complying with the open-source license through their inference partner terms. This disclosure is notable for shedding light on the often opaque relationship between open-source bases and proprietary fine-tuning in commercial AI products.

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Cursor AI

Composer 2

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Cursor AI's Composer 2 Uses Open-Source Base Model with A...