In short
How they differ.
Worked out from the figures below. Prices and specs are the makers' own, read Oct 20, 2026.
- No independent board we follow ranks both models yet.
Side by side
The specs.
| Claude Mythos 5.1 | Claude Sonnet 5.5 | |
|---|---|---|
| Made by | Anthropic · US | Anthropic · US |
| Released | Sep 1, 2026 | Sep 28, 2026 |
| Price per 1M tokens | No public price | $2 in / $10 out |
| Context window | Not stated | 1M |
| Takes in | Not stated | text, image |
| Download and run it yourself | No | No |
| Size | Not published | Not published |
| Maker says | “Our most capable model for cybersecurity and biology research.” | “High-performance model for coding and agents” |
| Sources | Prices, Oct 20, 2026 · Description | Prices, Oct 20, 2026 · Description |
Independent tests
Where they place.
Position on each board we follow (1 is best). Epoch AI's and Vectara's scores are shown; the other boards are linked for their own figures.
| Board | Claude Mythos 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Epoch Capabilities IndexEpoch AI, Oct 20, 2026 | Not listed | #6164.45 |
| LMArena WebDevLMArena, Oct 19, 2026 | Not listed | #3 |
| LMArena mathLMArena, Oct 15, 2026 | Not listed | #15 |
| LMArena Agent ArenaLMArena, Oct 19, 2026 | Not listed | #5 |
| Humanity's Last ExamScale AI | Not listed | #6 |
| SWE-Bench ProScale AI, Sep 22, 2026 | Not listed | #8 |
| Vals IndexVals AI, Oct 19, 2026 | Not listed | #1 |
| Terminal-Bench 4.0Vals AI, Oct 19, 2026 | Not listed | #2 |
| Vibe Code BenchVals AI, Oct 18, 2026 | Not listed | #1 |
| ProofBenchVals AI, Oct 12, 2026 | Not listed | #1 |
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