---
title: "Claude Mythos 5.1"
shortTitle: "Mythos 5.1"
modelId: "claude-mythos-5-1"
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releaseDate: "2026-09-01"
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trainingDataCutoff: "2026-06-30"
reliableKnowledgeCutoff: "2026-06-30"
contextWindow: 1000000
maxOutput: 128000
inputModalities:
  - "Text"
  - "Image"
  - "PDF"
outputModalities:
  - "Text"
pricing:
  input: 10
  output: 50
toolUse: true
capabilities:
  batch: {"supported":true}
  citations: {"supported":true}
  code_execution: {"supported":true}
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  pdf_input: {"supported":true}
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  sampling_params: {"supported":false}
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---

# Claude Mythos 5.1

Canonical URL: https://ghost.fail/models/claude-mythos-5-1

## Overview

**Claude Mythos 5.1** is a large language model by [Anthropic](/anthropic), released September 1, 2026. Like [Mythos 5](/models/claude-mythos-5) before it, it ships in two forms: Mythos 5.1, available only through Anthropic's trusted access programs — including [Project Glasswing](https://anthropic.com/glasswing) — and **Claude Fable 5.1**, which is generally available. Anthropic calls them "the same model, but with different levels of safeguards," with Mythos 5.1's "specifically designed to support work in cybersecurity and the life sciences."[^announcement]

The announcement pitches it less as a capability jump than as an answer to customer complaints, naming "price, data retention, and safeguards" as the feedback it addresses. Input and output prices are unchanged; the cost reduction comes entirely from cache reads.[^announcement]

## Technical details

- **Pricing:** $10/mtok input, $50/mtok output, identical to [Fable 5](/models/claude-mythos-5). Cache reads drop 75% to $0.25/mtok — around 25% cheaper for typical workloads and up to ~45% for context- and tool-heavy agentic ones, by Anthropic's own indexed measurements over four weeks of August 2026 usage.[^announcement]
- **Context:** 1M tokens in, 128K out. Both the reliable knowledge cutoff and the training data cutoff are June 2026.[^docs]
- **Reasoning:** Adaptive thinking, always on. Effort defaults to `high` on the API and in Claude Code, and to `medium` in Claude Cowork and on Claude.ai.[^docs][^announcement]
- **Data retention:** Enterprise Frontier Safeguards (EFS) stores customer data on the customer's own cloud infrastructure rather than Anthropic's, which Anthropic describes as "complete privacy (the same as a zero data retention policy)" while still permitting misuse detection. It rolls out in phases from fall 2026; until then, eligible customers get zero data retention.[^announcement]
- **Safeguards:** Retuned for false positives. Anthropic reports Claude Code users can expect around 60% fewer interventions per session from its cyber safeguards than under Fable 5.[^announcement]
- **API changes:** Three breaking — forced tool use now returns an error, thinking blocks are tied to the model that produced them, and editing earlier turns invalidates them. Five additive — per-message effort, turn-scoped system messages, readable progress updates between tool calls, the lower cache-read price, and content provenance.[^whatsnew]

[^announcement]: [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) (Anthropic, Sep 1, 2026)
[^docs]: [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview) (Claude Platform Docs, retrieved Sep 1, 2026)
[^whatsnew]: [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) (Claude Platform Docs, retrieved Sep 1, 2026)