Grok 4 Fast
xAI · 2025-09-19
Grok 4 Fast is xAI's speed-optimized tier of the Grok 4 generation, released in September 2025, designed to deliver frontier-level reasoning and tool use at lower latency and cost than the full Grok 4 flagship. It shares the Grok 4 family's reinforcement-learning-based training and native function-calling support while trading some performance headroom for faster inference. It targets developers and production use cases requiring cost-efficient access to the Grok 4 capability tier. The xAI API serves the same weights as two model IDs, `grok-4-fast-reasoning` and `grok-4-fast-non-reasoning`.