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Claude Fable

Anthropic’s peak widely released Claude (Fable 5.1) for long-horizon agents and deep reasoning.

Anthropic’s Claude Fable 5.1 — the most capable widely released Claude for long-horizon agents, deep reasoning, and demanding coding workflows. Mythos 5.1 is the same underlying model with relaxed cyber/life-sciences safeguards for trusted-access programs.

Why Claude Fable matters

Fable 5.1 is what teams reach for when Sonnet and Opus are not enough—long-running agents, hard research, and frontier coding. It is the public capability ceiling of the Claude family (Mythos 5.1 is trusted-access). Cache-read pricing is lower than Fable 5; list input/output rates are unchanged.

Vision · Tool calling · Thinking · MCP · Coding

Last reviewed: 2 September 2026

Official pricing →

When to choose Claude Fable

Decision guidance for architects—not a feature list.

Best for

  • Long-horizon agentic workflows
  • Deep multi-step reasoning
  • Hardest agentic coding tasks
  • Peak-capability Claude deployments

Avoid if

  • Sonnet or Opus already meets quality targets
  • Cost and latency dominate over peak capability

Strengths

Qualitative snapshot for architects—not a public ranking.

  • Peak Claude capability★★★★★
  • Long-horizon agents★★★★★
  • Deep reasoning★★★★★
  • Cost efficiency★★☆☆☆
  • Open weights★☆☆☆☆

Ecosystem

Built by

  • Anthropic

    Fable is Anthropic’s peak widely released Claude capability tier.

Competes with

  • GPT-5

    Primary rival for peak closed-model agent and coding capability.

Works with

  • LangGraph

    Long-horizon LangGraph agents benefit from Fable’s deep reasoning.

  • Cursor

    Chosen for the hardest repo-scale coding agent tasks.

Recommended for

  • AI Agents

    Built for demanding long-horizon agentic workflows.

Often paired with

  • Claude Opus

    Teams often keep Opus for high-stakes work and escalate to Fable only when needed.

  • Claude Sonnet

    Sonnet handles everyday production traffic; Fable handles the hardest agent threads.

How Claude Fable evolved

Key moments in chronological order.

  1. Model

    Claude Fable 5.1

    Successor to Fable 5 for long-running agents, coding, and research. API id claude-fable-5-1; 1M context, 128k max output, always-on adaptive thinking. High effort default in Claude Code; Medium in Cowork and Claude.ai.

  2. Product

    Mythos 5.1 trusted access

    Same underlying model as Fable 5.1 with more permissive cyber and life-sciences safeguards for Cyber Verification and Life Sciences Verification programs.

  3. Platform

    Fable 5.1 cache reads $0.25/MTok

    Cache reads cut 75% vs Fable 5. List price remains $10 input / $50 output per 1M tokens. Anthropic estimates ~25% lower typical-workload cost vs Fable 5, up to ~45% on cache-heavy agentic work.

  4. API

    Computer use GA + browser use on Fable 5

    Fable 5 supports GA computer_toolset_20260801 and browser_toolset_20260801 on the Claude API.

  5. Model

    Claude Fable 5 launch

    Most capable widely released Claude for long-horizon agents and deep reasoning.

  6. Product

    Mythos 5 (Glasswing)

    Limited-access peer of Fable for approved Project Glasswing customers.

  7. API

    claude-fable-5 API

    Generally available on Claude API and major cloud partners at premium rates.

  8. Product

    Adaptive thinking + effort

    Effort parameter replaces manual thinking budgets for depth control.

  9. Product

    1M context default

    Million-token context and large max outputs for long agent threads.

Overview

Anthropic’s Claude Fable 5.1 — the most capable widely released Claude for long-horizon agents, deep reasoning, and demanding coding workflows. Mythos 5.1 is the same underlying model with relaxed cyber/life-sciences safeguards for trusted-access programs.

Capabilities

  • Vision: Yes
  • Audio: No
  • Tool calling: Yes
  • Thinking: Yes
  • MCP: Yes
  • Coding: Yes
  • Structured output: Yes

Technical specifications

Provider
Anthropic
License
Proprietary
Context window
1M
Parameters
Undisclosed
Architecture
Transformer (proprietary)
Release
2026-09
Modalities
Text, Image
Vision
Yes
Audio
No
Tool calling
Yes
Thinking
Yes
MCP
Yes
Open weights
No
API
Yes
Pricing (input)
~$10 per 1M tokens (claude-fable-5-1)
Pricing (output)
~$50 per 1M tokens

Supported modalities

Text · Image

Context window

1M (1,000,000 tokens)

Pricing

Input: ~$10 per 1M tokens (claude-fable-5-1)
Output: ~$50 per 1M tokens

List price matches Fable 5. Cache reads $0.25 / 1M tokens (75% below Fable 5). Anthropic estimates ~25% lower typical-workload cost vs Fable 5, up to ~45% on cache-heavy agentic work. Verify live pricing.

Availability

API: Yes
Chat UI: Yes
Open weights: No

Claude API model ID: claude-fable-5-1 (1M context, 128k max output, always-on adaptive thinking). Also on AWS Bedrock, Google Cloud, and Microsoft Azure/Foundry. Defaults: High effort in Claude Code; Medium in Cowork and Claude.ai. Mythos 5.1 is trusted-access only (Cyber and Life Sciences Verification Programs). Covered Model with extra retention/safety policy. Enterprise Frontier Safeguards (customer-controlled review) announced for a phased fall rollout; eligible customers may use ZDR until EFS is available. SynthID-Text watermarking on models launched on/after 2026-08-02; detection API in private preview.

Use cases

  • Long-horizon agentic workflows
  • Deep multi-step reasoning and research
  • Hard agentic coding and systems work
  • Peak-capability Claude deployments
  • Computer/browser automation at frontier capability

Strengths

  • Peak widely released Claude capability
  • Strong long-horizon agent performance
  • GA computer use + browser toolsets
  • 1M-token context with adaptive thinking

Limitations

  • Highest Claude cost among widely released tiers
  • Closed weights
  • Overkill when Sonnet or Opus is enough

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