Announcement from Cursor, whose team also trained the model: https://cursor.com/blog/grok-4-5.

Notably:

> Grok 4.5 and Composer 2.5 are two different model weight classes, and we're excited to support both sizes and weights. Composer 2.5 will remain offered, and we will release new models of this size going forward.

Composer 2.5 is 1T total/32B active (based on Kimi 2.5), while Elon publicly said Grok 4.5 is 1.5T parameters total. Hardly a different weight class.

The API cost difference is ~2.5x, probably because xAI has much higher costs to recoup.