Claude Mythos 5.1 vs Claude Opus 5.5
Claude Mythos 5.1 and Claude Opus 5.5 are closely matched at 44.5 and 59.7 on the LLM Stats Score. Claude Opus 5.5 is 2.5x cheaper per token.
Anthropic · Anthropic · Updated for 2026
Which is better?
Claude Mythos 5.1 and Claude Opus 5.5 are closely matched on the overall LLM Stats Score at 44.5 and 59.7.
In the 1 individual benchmarks reported for both models, Claude Opus 5.5 wins 1; this is a narrower head-to-head signal than the composite indexes.
On price, Claude Opus 5.5 is roughly 2.5x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
Based on current LLM Stats indexes, shared benchmarks, pricing, and model metadata for 2026.
Choose Claude Mythos 5.1
- you want predictable pricing at $10.00/M input and $50.00/M output
Choose Claude Opus 5.5
- you value its reported benchmark strengths — it wins 1 of 1 exact shared results
- cost matters — it's about 2.5x cheaper per token
- you want the most recent training data — it shipped Sep 2026
At a glance
The differences that matter most.
Individual benchmarks
1 reported for Claude Mythos 5.1 · 31 for Claude Opus 5.5
Claude Mythos 5.1 outperforms in 0 benchmarks, while Claude Opus 5.5 is better at 1 benchmark (Terminal-Bench 4.0).
Claude Opus 5.5 significantly outperforms across most benchmarks.
Human preference
Blind head-to-head votes and playground preference scores
Pricing Analysis
Price comparison per million tokens
For input processing, Claude Mythos 5.1 ($10.00/1M tokens) is 2.5x more expensive than Claude Opus 5.5 ($4.00/1M tokens).
For output processing, Claude Mythos 5.1 ($50.00/1M tokens) is 2.5x more expensive than Claude Opus 5.5 ($20.00/1M tokens).
In conclusion, Claude Mythos 5.1 is more expensive than Claude Opus 5.5.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
Both models have the same input context window of 1,000,000 tokens. Both models can generate responses up to 128,000 tokens.
Input capabilities
Documented input modalities across available providers
Both Claude Mythos 5.1 and Claude Opus 5.5 support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
Claude Mythos 5.1
Claude Opus 5.5
License
Usage and distribution terms
Both models are licensed under proprietary licenses.
Both models have usage restrictions defined by their respective organizations.
Proprietary
Closed source
Proprietary
Closed source
Release Timeline
When each model was launched
Claude Mythos 5.1 was released on 2026-09-01, while Claude Opus 5.5 was released on 2026-09-22.
Claude Opus 5.5 is 1 month newer than Claude Mythos 5.1.
Sep 1, 2026
3 weeks ago
Sep 22, 2026
0 days ago
3w newerKnowledge Cutoff
When training data ends
Claude Opus 5.5 has a documented knowledge cutoff of 2026-06-01, while Claude Mythos 5.1's cutoff date is not specified.
We can confirm Claude Opus 5.5's training data extends to 2026-06-01, but cannot make a direct comparison without Claude Mythos 5.1's cutoff date.
—
Jun 2026
Provider Availability
Claude Mythos 5.1 is available from Anthropic. Claude Opus 5.5 is available from Anthropic.
Claude Mythos 5.1
Claude Opus 5.5
Outputs Comparison
Judge for yourself.
Run your own prompts against Claude Mythos 5.1 and Claude Opus 5.5 side-by-side, then vote on the output you prefer.
FAQ
Common questions about Claude Mythos 5.1 vs Claude Opus 5.5.