Claude Mythos 5.1 (Reasoning)

by Anthropic

Claude Mythos 5.1 (Reasoning) is a reasoning model from Anthropic, released on September 1, 2026. It has a 1M-token context window and can generate up to 128K output tokens. API pricing is $10 per 1M input tokens and $50 per 1M output tokens. It ranks #34 of 220 models by Intelligence Score with a score of 43.9.

Claude Mythos 5.1 is Anthropic's trusted-access deployment of the Claude Fable 5.1 weights (cataloged publicly as Fable 5.1). It is targeted for demanding reasoning and long-horizon agentic work with text and image input, tool use, multilingual support, and a 1M-token context window. Benchmarks below are sourced from Anthropic's official Claude Fable/Mythos 5.1 scorecard. It is not generally available via the public Claude API as a cataloged model id.

Key specs

ProviderAnthropic
Release date2026-09-01
Licenseproprietary
Context window1M tokens
Max output128K tokens
Input price$10 / 1M tokens
Output price$50 / 1M tokens
Intelligence Score43.9
Coding index36.1

Quick answers

What is the context window of Claude Mythos 5.1 (Reasoning)?

Claude Mythos 5.1 (Reasoning) has a context window of 1M tokens, with a maximum output of 128K tokens per response.

How much does Claude Mythos 5.1 (Reasoning) cost?

Claude Mythos 5.1 (Reasoning) costs $10 per 1M input tokens and $50 per 1M output tokens through the API.

Is Claude Mythos 5.1 (Reasoning) open source?

No. Claude Mythos 5.1 (Reasoning) is a proprietary model from Anthropic (proprietary license).

When was Claude Mythos 5.1 (Reasoning) released?

Anthropic released Claude Mythos 5.1 (Reasoning) on September 1, 2026.

How does Claude Mythos 5.1 (Reasoning) compare to Claude Opus 5.5 (Reasoning)?

Claude Mythos 5.1 (Reasoning) scores 43.9 on the Intelligence Score versus 60.5 for Claude Opus 5.5 (Reasoning), the current #1. Input pricing is $10 vs $4 per 1M tokens.

Compare on the AI leaderboard · All AI models

More models from Anthropic