Kimi k1.5
By Moonshot AI
5 is a reasoning-focused large language model from Moonshot AI, trained with reinforcement learning to produce extended chains of reasoning before answering, targeting improved performance on math, coding, and multi-step logical problems…
Definition
Kimi k1.5 is a reasoning-focused large language model from Moonshot AI, trained with reinforcement learning to produce extended chains of reasoning before answering, targeting improved performance on math, coding, and multi-step logical problems compared to standard chat-tuned models. Moonshot AI has also positioned it as multimodal, capable of reasoning over both text and visual inputs, and it is available through Moonshot AI's Kimi consumer platform and developer API.
Overview
Kimi k1.5 addresses the gap between fluent conversational responses and reliable multi-step problem solving: standard chat-tuned models often produce plausible-sounding answers to math or coding questions without necessarily reasoning through them correctly from first principles, so Moonshot AI trained k1.5 specifically to generate extended reasoning traces before committing to a final answer for a given problem submitted by a user rather than answering immediately. Moonshot AI has also emphasized k1.5's ability to handle long-context reasoning tasks, extending the benefits of extended reasoning to problems involving lengthy input documents. Mechanically, k1.5 is trained with reinforcement learning that rewards correct multi-step reasoning, particularly on domains with checkable answers such as mathematics and programming, where a verifier can confirm whether the model's final result is actually correct rather than relying only on a human preference judgment between two candidate responses. This lets the model be rewarded directly for reaching a correct outcome through its reasoning process, following an industry pattern of allocating more computation to reasoning at inference time rather than only during the original training run, a technique also seen in other reasoning-focused releases from competing labs. The reinforcement learning approach used for k1.5 draws on techniques for reward shaping that have since become common across other reasoning-focused model releases. Kimi k1.5 is part of the same reasoning-focused wave as other extended-inference-time models released around the same period by competing labs, and Moonshot AI has additionally positioned it as multimodal, capable of reasoning over visual as well as text inputs, distinguishing it from reasoning models limited strictly to text-only input and output formats and broadening the range of problems it can be applied to. Kimi's consumer platform has used k1.5's reasoning strength as a headline feature to differentiate it from earlier, non-reasoning chat products in the same market. In practice, k1.5 is used for mathematical problem solving requiring multi-step reasoning, code generation and verification tasks where correctness can be checked programmatically, tutoring and step-by-step explanation for learners working through difficult material, and scientific or logical reasoning tasks, made available through Moonshot AI's Kimi consumer platform and a separate developer API for integration into other products and services. Developers integrating k1.5 via API typically need to account for its variable response time, since harder problems trigger longer reasoning traces than simple ones. As with other reasoning-focused models, k1.5's extended reasoning traces typically increase inference latency and cost compared to standard chat models, so it is best suited to tasks where accuracy on complex problems outweighs the need for fast, low-cost responses, and less suited to simple queries where a standard chat model would answer just as reliably for considerably less compute and lower cost per request to the API. As with other reasoning models, cost estimates should be based on typical reasoning-trace length for a given task rather than on simple per-token pricing alone.
Key Features
- Reasoning-focused model trained with reinforcement learning for multi-step problem solving
- Optimized for math, coding, and logical reasoning with checkable rewards
- Multimodal capability reasoning over both text and visual inputs
- Developed by Moonshot AI, maker of the Kimi chat assistant
- Available through Moonshot AI's Kimi platform and developer API
- Higher inference latency and cost typical of extended-reasoning models