Key Points

The post quotes Cursor AI's announcement of the Composer 2 launch, highlighting Kimi-k2.5 as the foundational model behind it.
It notes that Kimi-k2.5 is integrated through Cursor's continued pretraining and high‑compute RL training, illustrating an open model ecosystem.
The integration is delivered via Cursor accessing Kimi-k2.5 through FireworksAI_HQ's hosted RL and inference platform as part of an authorized commercial partnership.
The piece situates the launch within a broader AI industry context, referencing other recent articles like OpenAI's acquisition of Astral and GPT-5.4 updates.

Simon Willison’s Weblog

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20th March 2026

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.

Note: Cursor accesses Kimi-k2.5 via @FireworksAI_HQ hosted RL and inference platform as part of an authorized commercial partnership.

— Kimi.ai @Kimi_Moonshot, responding to reports that Composer 2 was built on top of Kimi K2.5

Posted 20th March 2026 at 8:29 pm