Claude Mythos 5.1

Claude Mythos 5.1 is Anthropic’s flagship, restricted-access Mythos-class reasoning model built specifically for advanced cybersecurity defense, biodefense research, and complex scientific discovery. Shared on the same underlying model weights as Claude Fable 5.1, Mythos 5.1 operates without standard commercial domain-refusal classifiers, enabling deep vulnerability auditing and life sciences research for vetted partners.

Model Overview

Mythos 5.1 advances the frontier in coding, bio-engineering, and terminal-based scientific reasoning. It delivers substantial performance gains over Mythos 5 and Opus 5 while introducing improved token efficiency and lower per-task execution costs on long-horizon agentic workflows.

• Developer: Anthropic
• Model Family: Mythos / Claude 5.1
• Release Date: September 1, 2026
• Access Program: Restricted Access / Project Glasswing (Vetted Government, Defense, & Life Sciences Organizations)
• Knowledge Cutoff: June 2026
• Primary Target Use Cases: Zero-day software vulnerability detection, automated exploit patching, protein design, bioinformatics, and complex mathematical proof deduction.

Core Capabilities & Architectural Highlights

Cybersecurity & Exploit Research

Mythos 5.1 sets state-of-the-art results across major security frameworks (including ExploitBench, OSS-Fuzz, ExploitGym, and Firefox 147 vulnerability audits). It identifies deep architectural flaws across massive software stacks and generates automated remediation patches natively.

Advanced Life Sciences & Bioinformatics

Delivers lead performance across internal and partner benchmarks in protein folding design, genomic optimization, and organic chemistry synthesis. It performs end-to-end scientific research loops—analyzing raw sequence data, optimizing compute kernels for genome-wide analysis, and verifying molecular binding mechanics.

Always-On Adaptive Thinking & Agentic Execution

• Long-Horizon Persistence: Designed to execute multi-hour, unattended tasks without losing logical direction or hallucinating false completions.
• Dynamic Chain-of-Thought: Internal adaptive reasoning is permanently active, allocating variable thinking depth based on prompt complexity.

Technical Specifications

Mythos 5.1 vs. Fable 5.1 Relationship

Claude Mythos 5.1 and Claude Fable 5.1 share identical base model weights. Performance differences on public benchmarks reflect domain-specific safety interventions: