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Claude Mythos 5.1 vs Claude Sonnet 5.5

Claude Mythos 5.1 and Claude Sonnet 5.5 are closely matched at 43.5 and 51.5 on the LLM Stats Score. Claude Sonnet 5.5 is 5.0x cheaper per token.

Anthropic · Anthropic · Updated for 2026

Which is better?

Claude Mythos 5.1 and Claude Sonnet 5.5 are closely matched on the overall LLM Stats Score at 43.5 and 51.5.

In the 1 individual benchmarks reported for both models, Claude Sonnet 5.5 wins 1; this is a narrower head-to-head signal than the composite indexes.

On price, Claude Sonnet 5.5 is roughly 5.0x 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 Sonnet 5.5

  • you value its reported benchmark strengths — it wins 1 of 1 exact shared results
  • cost matters — it's about 5.0x cheaper per token
  • you want the most recent training data — it shipped Sep 2026

At a glance

The differences that matter most.

Claude Mythos 5.1
Anthropic
Claude Sonnet 5.5
Anthropic
Core performance indexes
LLM Stats Score
43.5
#45
51.5
#12
Reasoning
42.4
#48
49.7
#16
Coding
36.6
#21
42.6
#7
Agents
33.2
#28
39.3
#9
Cost, coverage & limits
Benchmark wins
0 of 1
1 of 1
Input price
$10.00 / M
$2.00 / M
Output price
$50.00 / M
$10.00 / M
Context window
1,000,000
1,000,000

Individual benchmarks

1 reported for Claude Mythos 5.1 · 8 for Claude Sonnet 5.5

1 shared

Claude Mythos 5.1 outperforms in 0 benchmarks, while Claude Sonnet 5.5 is better at 1 benchmark (Terminal-Bench 4.0).

Claude Sonnet 5.5 significantly outperforms across most benchmarks.

Mon Sep 28 2026 • llm-stats.com

Human preference

Blind head-to-head votes and playground preference scores

Pricing Analysis

Price comparison per million tokens

Claude Sonnet 5.5 costs less

For input processing, Claude Mythos 5.1 ($10.00/1M tokens) is 5.0x more expensive than Claude Sonnet 5.5 ($2.00/1M tokens).

For output processing, Claude Mythos 5.1 ($50.00/1M tokens) is 5.0x more expensive than Claude Sonnet 5.5 ($10.00/1M tokens).

In conclusion, Claude Mythos 5.1 is more expensive than Claude Sonnet 5.5.*

* Using a 3:1 ratio of input to output tokens

Lowest available price from all providers
Mon Sep 28 2026 • llm-stats.com
Anthropic
Claude Mythos 5.1
Input tokens$10.00
Output tokens$50.00
Best providerAnthropic
Anthropic
Claude Sonnet 5.5
Input tokens$2.00
Output tokens$10.00
Best providerAnthropic
Notice missing or incorrect data?Start an Issue→

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.

Anthropic
Claude Mythos 5.1
Input1,000,000 tokens
Output128,000 tokens
Anthropic
Claude Sonnet 5.5
Input1,000,000 tokens
Output128,000 tokens
Mon Sep 28 2026 • llm-stats.com

Input capabilities

Documented input modalities across available providers

Both Claude Mythos 5.1 and Claude Sonnet 5.5 support multimodal inputs.

They are both capable of processing various types of data, offering versatility in application.

Claude Mythos 5.1

Text
Images
Audio
Video

Claude Sonnet 5.5

Text
Images
Audio
Video

License

Usage and distribution terms

Both models are licensed under proprietary licenses.

Both models have usage restrictions defined by their respective organizations.

Claude Mythos 5.1

Proprietary

Closed source

Claude Sonnet 5.5

Proprietary

Closed source

Release Timeline

When each model was launched

Claude Mythos 5.1 was released on 2026-09-01, while Claude Sonnet 5.5 was released on 2026-09-28.

Claude Sonnet 5.5 is 1 month newer than Claude Mythos 5.1.

Claude Mythos 5.1

Sep 1, 2026

3 weeks ago

Claude Sonnet 5.5

Sep 28, 2026

0 days ago

3w newer

Knowledge Cutoff

When training data ends

Claude Sonnet 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 Sonnet 5.5's training data extends to 2026-06-01, but cannot make a direct comparison without Claude Mythos 5.1's cutoff date.

Claude Mythos 5.1

—

Claude Sonnet 5.5

Jun 2026

Provider Availability

Claude Mythos 5.1 is available from Anthropic. Claude Sonnet 5.5 is available from Anthropic.

Claude Mythos 5.1

anthropic logo
Anthropic
Input Price:Input: $10.00/1MOutput Price:Output: $50.00/1M

Claude Sonnet 5.5

anthropic logo
Anthropic
Input Price:Input: $2.00/1MOutput Price:Output: $10.00/1M
* Prices shown are per million tokens

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Judge for yourself.

Run your own prompts against Claude Mythos 5.1 and Claude Sonnet 5.5 side-by-side, then vote on the output you prefer.

Claude Mythos 5.1
✓ Preferred
Claude Sonnet 5.5
Open in Playground

FAQ

Common questions about Claude Mythos 5.1 vs Claude Sonnet 5.5.

Which is better, Claude Mythos 5.1 or Claude Sonnet 5.5?

Claude Mythos 5.1 and Claude Sonnet 5.5 are closely matched on the LLM Stats Score at 43.5 and 51.5. Claude Mythos 5.1 is made by Anthropic and Claude Sonnet 5.5 is made by Anthropic. The best choice depends on your use case — compare their capability indexes, individual benchmarks, pricing, and limits above.

How does Claude Mythos 5.1 compare to Claude Sonnet 5.5 in benchmarks?

Claude Mythos 5.1 scores Terminal-Bench 4.0: 60.9%. Claude Sonnet 5.5 scores OSWorld 2.1 (partial): 80.1%, Terminal-Bench 4.0: 70.6%, Humanity's Last Exam (with tools): 64.5%, Chartography (no tools): 61.6%, GDPval-AA 2.1: 61.5%.

Is Claude Mythos 5.1 cheaper than Claude Sonnet 5.5?

Claude Sonnet 5.5 is 5.0x cheaper for input tokens. Claude Mythos 5.1 costs $10.00/M input and $50.00/M output via anthropic. Claude Sonnet 5.5 costs $2.00/M input and $10.00/M output via anthropic.

What are the context window sizes for Claude Mythos 5.1 and Claude Sonnet 5.5?

Claude Mythos 5.1 supports 1.0M tokens and Claude Sonnet 5.5 supports 1.0M tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

What are the main differences between Claude Mythos 5.1 and Claude Sonnet 5.5?

Key differences include LLM Stats Score (43.5 vs 51.5), input pricing ($10.00 vs $2.00/M). See the full comparison above for benchmark-by-benchmark results.

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