Kimi K3 Fast

Kimi K3 Fast is a serving-tier variant of Moonshot AI's flagship K3 model, designed by the same team behind the K-series line and routed through external inference partners. The underlying K3 base is described as Kimi's most capable flagship to date, built on a Kimi Delta Attention (KDA) architecture with 2.8 trillion parameters and positioned as the world's first open-source model in the 3-trillion-parameter class, with vendor-stated performance comparable to leading closed-source systems. That scale, combined with an unusually long context window, signals a model aimed at workloads that need both breadth of knowledge and long-horizon reasoning rather than narrow, low-latency chat tasks.

For practical deployment, Kimi K3 Fast is engineered for latency-sensitive use, sitting on a Fast serverless tier that prioritizes speed over the higher-throughput Priority tier native to the same family. It supports text-and-image input with text output, giving it built-in visual understanding alongside the cataloged API limit context, which makes it well suited to long document analysis, multi-image reasoning, and coding or research sessions that span large codebases or transcripts. Its open-weights status, reasoning capability, tool calling, and structured outputs further broaden its fit for teams that want to self-host, fine-tune, or wire the model into agent pipelines rather than rely on a black-box API.