Kimi K3.1 Pro: Benchmarks, API Changes, and Open Weights

Kimi K3.1 Pro is Moonshot AI’s post-trained, open-weight 3T-class Kimi K3 model, featuring native vision, a one-million-token context window, always-on reasoning, and availability through Kimi’s applications, API, and an accelerated highspeed variant, with full weights planned for release by November 4, 2026. Moonshot reports that it approaches leading proprietary models in coding and agentic tasks, scoring 65.7% on Terminal-Bench 4.0 versus 66.4% for Claude Opus 5.5 and 58.2% for GPT-6 Astra, while outperforming some competitors on benchmarks for web research, office-document analysis, automation, and selected long-horizon tasks. However, its own benchmark table shows notable weaknesses on ProgramBench, Terminal-Bench-Science, some knowledge evaluations, and long-horizon coding relative to proprietary alternatives, making workload-specific retesting important. Moonshot attributes improvements in coding, 3D creation, finance, factual accuracy, and business simulation to a larger reinforcement-learning post-training process, reporting a reduced adversarial hallucination rate from 23.4% for Kimi K3 to 8.3% for K3.1 Pro. The company also removed cyber-related training material from the release pipeline to limit offensive security capability while retaining defensive code-auditing utility. Users migrating from K2 models should account for K3.1 Pro’s larger context window, mandatory reasoning, fixed sampling settings, configurable reasoning effort, and K2’s planned retirement by the end of December 2026.

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