# Claude Mythos 5.1

Claude Mythos 5.1 is an AI model released by Anthropic on Sep 1 2026. It has a 1M context window. At release it scored 60.9% on Terminal-Bench 4.0.

## Facts

| Field | Value |
| --- | --- |
| Model | Claude Mythos 5.1 |
| Developer | Anthropic |
| Release date | Tuesday, Sep 1 2026 |
| Licensing | Proprietary |
| Context window | 1M |

## Benchmark scores published at release

| Benchmark | Score | Source | What it measures |
| --- | --- | --- | --- |
| Terminal-Bench 4.0 | 60.9% | Lab | Can the AI work in a command-line terminal — running commands and finishing technical setup tasks the way a developer would? Version 4.0 recalibrated how much time, CPU and memory each task gets, removed eight tasks and fixed nineteen, so fewer runs fail for reasons that have nothing to do with the model. Scores are not comparable with earlier versions. Higher is better. |

## About Claude Mythos 5.1

Claude Mythos 5.1, released September 1, 2026 alongside Claude Fable 5.1, continued the pairing Anthropic set up with the Claude 5 generation: the same frontier model as that day's Fable release, offered by invitation only to Project Glasswing participants without the dual-use safety measures that let Fable ship to everyone. It succeeded Claude Mythos 5 and shared Fable 5.1's specifications and pricing — a 1M-token context window, 128K output tokens, always-on adaptive thinking, $10 per million input tokens and $50 per million output, with cache reads cut to $0.25 per million.

Access went through Anthropic, AWS or Google Cloud account teams rather than the console, and the model was listed on the Claude API, Amazon Bedrock, Google Cloud and Microsoft Foundry at launch. Like Mythos 5 before it, it required 30-day data retention and was not offered under zero data retention without express authorisation. The one documented behavioural difference from Fable 5.1 was that Mythos 5.1 did not enforce the check that rejects a conversation whose earlier turns were edited after a thinking block was produced. The one figure Anthropic published for it was 60.9% on Terminal-Bench 4.0, five points above Fable 5.1's 55.8% under the same harness.

## Questions and answers

### When was Claude Mythos 5.1 released?

Claude Mythos 5.1 was released by Anthropic on Tuesday, Sep 1 2026.

### Who made Claude Mythos 5.1?

Claude Mythos 5.1 was built by Anthropic. AI safety company building the Claude family of models. Founded in 2021 by former OpenAI researchers.

### What benchmark scores did Claude Mythos 5.1 get?

Claude Mythos 5.1 reports 1 tracked benchmark score — Terminal-Bench 4.0: 60.9%. Scores are the figures published at release by Anthropic. It holds the best score among all models tracked here on Terminal-Bench 4.0.

### What is the context window of Claude Mythos 5.1?

Claude Mythos 5.1 has a context window of 1M. That is the maximum amount of input plus output the model can hold in a single request.

### Is Claude Mythos 5.1 open source?

No. Claude Mythos 5.1 is a proprietary model. The weights are not published — it is available only through the provider's own API, apps, or partner platforms.

### What came before and after Claude Mythos 5.1?

Anthropic's previous tracked release was Claude Fable 5.1 on Sep 1 2026. It is the most recent Anthropic model tracked on AI Release Tracker.


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Canonical page: https://aireleasetracker.com/model/anthropic/claude-mythos-5.1
Full dataset: https://aireleasetracker.com/llms-full.txt · JSON: https://aireleasetracker.com/models.json
Source: AI Release Tracker (https://aireleasetracker.com). Benchmark scores are the figures published by the releasing lab at launch.