Claude Mythos 5.1 vs GPT-5.5 Pro
Claude Mythos 5.1 and GPT-5.5 Pro are closely matched at 45.1 and 44.9 on the LLM Stats Score. Claude Mythos 5.1 is 3.4x cheaper per token.
Anthropic · OpenAI · Updated for 2026
Which is better?
Claude Mythos 5.1 and GPT-5.5 Pro are closely matched on the overall LLM Stats Score at 45.1 and 44.9.
On price, Claude Mythos 5.1 is roughly 3.4x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
GPT-5.5 Pro also accepts a larger context window (1,050,000 input tokens), making it the stronger choice for long documents and large codebases.
Based on current LLM Stats indexes, shared benchmarks, pricing, and model metadata for 2026.
Choose Claude Mythos 5.1
- cost matters — it's about 3.4x cheaper per token
- you want the most recent training data — it shipped Sep 2026
Choose GPT-5.5 Pro
- you process long inputs — it offers a 1,050,000 token context window
At a glance
The differences that matter most.
Individual benchmarks
1 reported for Claude Mythos 5.1 · 5 for GPT-5.5 Pro
Claude Mythos 5.1 and GPT-5.5 Prodon't have any common benchmark datasets to compare. They may have been evaluated on different testing suites.
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 3.0x cheaper than GPT-5.5 Pro ($30.00/1M tokens).
For output processing, Claude Mythos 5.1 ($50.00/1M tokens) is 3.6x cheaper than GPT-5.5 Pro ($180.00/1M tokens).
In conclusion, GPT-5.5 Pro is more expensive than Claude Mythos 5.1.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
GPT-5.5 Pro accepts 1,050,000 input tokens compared to Claude Mythos 5.1's 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 GPT-5.5 Pro support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
Claude Mythos 5.1
GPT-5.5 Pro
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 GPT-5.5 Pro was released on 2026-04-23.
Claude Mythos 5.1 is 4 months newer than GPT-5.5 Pro.
Sep 1, 2026
1 weeks ago
4mo newerApr 23, 2026
4 months ago
Knowledge Cutoff
When training data ends
GPT-5.5 Pro has a documented knowledge cutoff of 2025-12-01, while Claude Mythos 5.1's cutoff date is not specified.
We can confirm GPT-5.5 Pro's training data extends to 2025-12-01, but cannot make a direct comparison without Claude Mythos 5.1's cutoff date.
—
Dec 2025
Provider Availability
Claude Mythos 5.1 is available from Anthropic. GPT-5.5 Pro is available from OpenAI.
Claude Mythos 5.1
GPT-5.5 Pro
Outputs Comparison
Judge for yourself.
Run your own prompts against Claude Mythos 5.1 and GPT-5.5 Pro side-by-side, then vote on the output you prefer.
FAQ
Common questions about Claude Mythos 5.1 vs GPT-5.5 Pro.