Grok 4, released July 9, 2025, was xAI's reasoning-first flagship, trained with large-scale reinforcement learning on the Colossus cluster. It launched with claims of leading scores on Humanity's Last Exam and ARC-AGI-2, and shipped alongside Grok 4 Heavy, a multi-agent variant that runs several instances in parallel and compares their answers.

The launch cemented xAI's pattern of aggressive benchmark competition and rapid iteration: Grok 4 Fast followed in September 2025, the coding-focused Grok Code Fast 1 in August, and Grok 4.1 in November 2025. The line culminated in Grok 4.5 in July 2026.