Grok 4.1 Fast is xAI's agentic tool-calling model, optimized for real-world use cases such as customer support and deep research. It features a 2M-token context window — the largest among Western frontier models — and focuses on significantly reduced hallucination rates for information-seeking tasks. Reasoning can be enabled or disabled via the API's reasoning parameter, allowing developers to choose between speed-optimized direct answers and deeper analytical responses.