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Claude Fable 5 vs. Claude Fable 5.1: What Changed

AgentHiveX Research · 10/22/2026
ReleasedFable 5: Jun 9, 2026 · Fable 5.1: Sep 1, 2026
Price per 1M tokens (input / output)Both: $10 / $50
Cache read per 1M tokensFable 5: $1 · Fable 5.1: $0.25
Reliable knowledge cutoffFable 5: Jan 2026 · Fable 5.1: Jun 2026
StatusFable 5: legacy (until at least Jun 9, 2027) · Fable 5.1: latest

Overview

Claude Fable 5 launched on June 9, 2026 as Anthropic's Mythos-class model for general use. Claude Fable 5.1 followed on September 1, 2026 and is now the current version. If you came here searching for Fable 5, the current model is 5.1.

What stayed the same

  • 1M-token context window and 128K max output.
  • $10 per 1M input tokens and $50 per 1M output tokens.
  • Adaptive thinking that is always on, with default effort high.
  • Text and image input, text output.
  • Availability on the Claude API, Amazon Bedrock, Google Cloud, Microsoft Foundry and Claude Platform on AWS.

    • What changed in 5.1

  • Cache reads cost $0.25 per 1M tokens, down from $1.
  • The reliable knowledge cutoff moved from January 2026 to June 2026.
  • Anthropic says 5.1 is stronger at long-running agentic coding, multistep research, and document, spreadsheet and slide work.
  • There are four breaking changes: forced tool use returns an error, only Fable 5.1 and Mythos 5.1 can read Fable 5.1 thinking blocks, editing earlier turns invalidates thinking blocks, and some advisor tool pairings are not supported.
  • New additions include mid-conversation effort changes (beta), turn-scoped system messages (beta), readable progress updates between tool calls (beta) and content provenance.

    • Safety classifiers

    Fable models include safety classifiers that can decline high-risk requests, for example in cybersecurity or biology. Integrations need refusal handling and a fallback to another Claude model. Claude Mythos 5 and 5.1 share the same capabilities but are offered only to verified organisations.

    Should you migrate?

    Anthropic recommends moving to Fable 5.1, but test the breaking changes first, especially if you force tool use or edit earlier conversation turns.

    No hands-on testing was conducted for this comparison. Every figure above comes from the official pages listed under Research sources, as published on or before October 22, 2026. Where a vendor has not published a number, we say so instead of estimating it.

    Research sources

    Claude Fable 5 model page — Claude docs ↗Introducing Claude Fable 5 and Claude Mythos 5 — Claude docs ↗Claude Fable 5.1 model page — Claude docs ↗Claude models overview — Claude docs ↗