Claude Mythos 5.1 vs Claude Opus 5.5

Pyyan / Compare / Claude Mythos 5.1 vs Claude Opus 5.5

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Language Models · verified 24 Sept 2026

Claude Mythos 5.1Anthropiccurrent

Claude Opus 5.5Anthropiccurrent

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Specification	Claude Mythos 5.1	Claude Opus 5.5
Summary	Fable 5.1 with the safety classifiers removed. You cannot simply buy it.	Fable 5.1's level of work on most tasks, at 40% less to run than Opus 5.
Context	1M	1M
Input ($/Mtok)	$10	$4
Output ($/Mtok)	$50	$20
Licence	Proprietary, gated	Proprietary
Max output	128K	128K
Parameters	Not published	Not published
Released	1 Sep 2026	22 Sep 2026
Model ID	claude-mythos-5-1	claude-opus-5-5
Category	Language Models	Language Models
Official	—	Anthropic ↗

Highlighted rows are where these differ.

The same underlying model as Fable 5.1, at a different safeguard level
US organisations and individuals in Anthropic's trusted access programmes only
Safety classifiers and high-risk fallbacks removed for Project Glasswing participants

Best for vetted frontier research.

66.4% on Terminal-Bench 4.0 against 52.3% for Opus 5, on Anthropic's own harness
Cache reads cut 60% to $0.20, and output more than 30% faster than Opus 5
Attempts to work around its boundaries 85% less often than Opus 5, Anthropic reports

Best for agentic coding at flagship quality.