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This guide covers migrating Messages API code. If you use Claude Managed Agents, no changes beyond updating the model name are required.

Automate your migration with the Claude API skill. In Claude Code, run /claude-api migrate to invoke the bundled Claude API skill. It works for any current Claude model as the target:

text
/claude-api migrate this project to claude-fable-5-1

The skill applies the model ID swap and, as needed, breaking parameter changes, prefill replacement, and effort calibration for your target model across your code base, then produces a checklist of items to verify manually. It asks you to confirm the migration scope (entire working directory, a subdirectory, or a specific file list) before editing any files. The skill also detects Amazon Bedrock and Claude Platform on AWS clients and adjusts model ID formats and feature changes for those platforms.

Claude Fable 5.1 succeeds Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost. It's available on the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry. Claude Mythos 5.1 shares the same capabilities and is offered only to approved customers in Project Glasswing. For behavioral differences and prompting patterns, see Prompting Claude Fable 5.1.

The baseline settings shared by claude-fable-5-1 and claude-mythos-5-1:

  • Thinking: Adaptive thinking is always on, unchanged from Claude Fable 5. The model decides when and how much to think. No thinking configuration is required. Both thinking: \{type: "disabled"\} and manual extended thinking (thinking: \{type: "enabled", budget_tokens: N\}) return a 400 error.
  • Prefill: Prefilling the assistant message returns a 400 error, unchanged from Claude Fable 5. Use system prompt instructions instead.
  • Tool choice: \{type: "auto"\} (the default) and \{type: "none"\} are supported. Forcing a tool call with \{type: "any"\} or \{type: "tool", name: "..."\} returns a 400 error. See Breaking changes.
  • Preserved thinking across models: Claude Fable 5.1 reads thinking blocks from Claude Opus 5, Claude Fable 5, Claude Mythos 5, and earlier Claude models. None of those models can read Claude Fable 5.1's blocks. See Breaking changes.
  • Context window and output: A 1M token context window by default, and up to 128k output tokens per request.
  • Pricing: $10 USD per million input tokens and $50 USD per million output tokens, the same as Claude Fable 5. Prompt cache reads are $0.25 USD per million tokens, a quarter of the Claude Fable 5 rate. See Claude pricing.
  • Data retention: Both models require 30-day data retention, aren't available under zero data retention (ZDR) arrangements unless expressly authorized by Anthropic, and are designated Covered Models, the same as Claude Fable 5 and Claude Mythos 5. On the Claude API, a request from an organization or workspace without 30-day retention returns a 400 invalid_request_error. Organizations with a ZDR arrangement should contact their Anthropic account team, or configure retention per workspace. See Model-specific data retention requirements for per-platform details.

Where the two models diverge:

  • Availability: Claude Fable 5.1 doesn't require access approval. Claude Mythos 5.1 is available only to approved customers in Project Glasswing. Contact your Anthropic account team for access.
  • Safety classifiers: Claude Fable 5.1 runs safety classifiers covering the same stop_details categories as Claude Fable 5. A declined request returns stop_reason: "refusal" with a stop_details.category, and can fall back to another model with the fallbacks parameter or a client-side retry. See Refusals and fallback.
  • Priority Tier: Neither model is supported on Priority Tier. Claude Fable 5 is.

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