Claude Mythos 5.1 vs GPT OSS 20B
Claude Mythos 5.1 leads the LLM Stats Score 45.1 to 18.1. GPT OSS 20B is 347.8x cheaper per token.
Anthropic · OpenAI · Updated for 2026
Which is better?
Claude Mythos 5.1 leads the overall LLM Stats Score 45.1 to 18.1, ranking #31 overall.
On price, GPT OSS 20B is roughly 347.8x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
Claude Mythos 5.1 also accepts a larger context window (1,000,000 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
- overall performance matters — it scores 45.1 and ranks #31 on LLM Stats
- your work emphasizes reasoning — it leads those capability indexes
- you process long inputs — it offers a 1,000,000 token context window
- you want the most recent training data — it shipped Sep 2026
Choose GPT OSS 20B
- cost matters — it's about 347.8x cheaper per token
- you need open weights you can self-host or fine-tune
At a glance
The differences that matter most.
Individual benchmarks
1 reported for Claude Mythos 5.1 · 7 for GPT OSS 20B
Claude Mythos 5.1 and GPT OSS 20Bdon'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 333.3x more expensive than GPT OSS 20B ($0.03/1M tokens).
For output processing, Claude Mythos 5.1 ($50.00/1M tokens) is 357.1x more expensive than GPT OSS 20B ($0.14/1M tokens).
In conclusion, Claude Mythos 5.1 is more expensive than GPT OSS 20B.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
Claude Mythos 5.1 accepts 1,000,000 input tokens compared to GPT OSS 20B's 131,072 tokens. GPT OSS 20B can generate longer responses up to 131,072 tokens, while Claude Mythos 5.1 is limited to 128,000 tokens.
Input capabilities
Documented input modalities across available providers
Claude Mythos 5.1 supports multimodal inputs, whereas GPT OSS 20B does not.
Claude Mythos 5.1 can handle both text and other forms of data like images, making it suitable for multimodal applications.
Claude Mythos 5.1
GPT OSS 20B
License
Usage and distribution terms
Claude Mythos 5.1 is licensed under a proprietary license, while GPT OSS 20B uses Apache 2.0.
License differences may affect how you can use these models in commercial or open-source projects.
Proprietary
Closed source
Apache 2.0
Open weights
Release Timeline
When each model was launched
Claude Mythos 5.1 was released on 2026-09-01, while GPT OSS 20B was released on 2025-08-05.
Claude Mythos 5.1 is 13 months newer than GPT OSS 20B.
Sep 1, 2026
1 weeks ago
1.1yr newerAug 5, 2025
1.1 years ago
Knowledge Cutoff
When training data ends
Neither model specifies a knowledge cutoff date.
Unable to compare the recency of their training data.
Provider Availability
Claude Mythos 5.1 is available from Anthropic. GPT OSS 20B is available from DeepInfra, Novita, Fireworks, Groq, OpenAI.
Claude Mythos 5.1
GPT OSS 20B
Outputs Comparison
Judge for yourself.
Run your own prompts against Claude Mythos 5.1 and GPT OSS 20B side-by-side, then vote on the output you prefer.
FAQ
Common questions about Claude Mythos 5.1 vs GPT OSS 20B.