Claude Mythos 5.1
Anthropic · September 1, 2026
● activeCloseddecoder onlymultimodal
Context Window1M tokens
Description
Anthropic's massive agentic reasoning system released with a 75% reduction in prompt caching costs, designed for persistent deep-research and multi-day planning.
Benchmark Scores
MMLUMassive Multitask Language Understanding — 57 subjects
93.4%HumanEvalCode generation pass@1 — Python problems
95.8%MATHMATH benchmark — competition-level problems
97.9%GPQAGraduate-level science QA
81.9%SWE-benchReal-world software engineering
73.6%Key Innovations
Agentic
AgenticModels that can autonomously plan, execute multi-step tasks, use tools, and self-correct without human intervention.
Reasoning
ReasoningStructured step-by-step problem solving, often using chain-of-thought or tree-of-thought approaches.
Constitutional AI
Constitutional AIAnthropic's approach to AI safety where the model critiques and revises its own outputs against a set of principles.
Tool Use
Tool UseAbility to call external tools, APIs, and functions — enabling web browsing, code execution, and real-world actions.
Long Context
Long ContextAbility to process very long inputs (100K+ tokens), enabling analysis of entire codebases or books.