PyyanThe AI Index

Pyyan / Compare / Claude Mythos 5 vs Claude Opus 5.5

Claude Mythos 5 vs Claude Opus 5.5

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Language Models · verified 24 Sept 2026

Claude Mythos 5Anthropicpreview
Claude Opus 5.5Anthropiccurrent
3 slots left
SpecificationClaude Mythos 5Claude Opus 5.5
SummaryFable 5's capabilities via Project Glasswing.Fable 5.1's level of work on most tasks, at 40% less to run than Opus 5.
Context1M1M
Input ($/Mtok)$10$4
Output ($/Mtok)$50$20
LicenceProprietaryProprietary
Max output128K128K
Parameters—Not published
Released—22 Sep 2026
Model IDclaude-mythos-5claude-opus-5-5
CategoryLanguage ModelsLanguage Models
OfficialAnthropic ↗Anthropic ↗

Highlighted rows are where these differ.

Claude Mythos 5

  • Same capabilities, pricing and API surface as Fable 5
  • Access only through Project Glasswing

Best for Glasswing participants.

Full spec sheet →

Claude Opus 5.5

  • 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.

Full spec sheet →