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Migrating to Claude Mythos 5 and Claude Fable 5

Anthropic has released a guide for migrating to Claude Mythos 5 and Claude Fable 5 from previous models, detailing API changes and providing migration checklists.

By Illumora Editorial

Source · Sep 22, 2026, 1:00 AM · On Illumora · Sep 22, 2026, 1:03 AM

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Read the source →Anthropic Docs (prompting) — Migrating to Claude Mythos 5 and Claude Fable 5
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Anthropic has published a migration guide for developers transitioning to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8. The guide covers model IDs, API changes, and provides checklists to assist with the migration process.

Key Points

  • Claude Fable 5 is designed for demanding reasoning and long-horizon agentic work.
  • Claude Fable 5 is available on the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry.
  • Claude Mythos 5 shares the same capabilities as Claude Fable 5 but is offered only to approved customers in Project Glasswing.
  • Migration is largely drop-in, as Claude Mythos 5 and Claude Fable 5 use the same Messages API and tool use patterns as Claude Mythos Preview.
  • Token counts are roughly unchanged when migrating from Claude Mythos Preview because all three models use the same tokenizer.
  • Manual extended thinking (thinking: {type: "enabled", budget_tokens: N}) is not supported on Claude Mythos 5 or Claude Fable 5 and will return a 400 error.
  • Adaptive thinking is always on for Claude Mythos 5 and Claude Fable 5; the model determines when and how much to think on each request.
  • Assistant prefill is not supported on Claude Mythos 5 or Claude Fable 5; system prompt instructions should be used instead.

Context

According to Anthropic, the Claude API skill can automate migration tasks. In Claude Code, running /claude-api migrate invokes this bundled skill. It applies model ID swaps, parameter changes, prefill replacements, and effort calibration across a codebase, then generates a checklist for manual verification. The skill also detects Amazon Bedrock and Claude Platform on AWS clients, adjusting model ID formats and feature changes for those platforms.

Why It Matters

Developers need to understand the specific changes in API behavior and feature availability when upgrading to Claude Mythos 5 or Claude Fable 5 to ensure their applications continue to function correctly and leverage the new models' capabilities.

What To Do

  • Review the migration guide for specific model ID changes and API adjustments.
  • Utilize the Claude API skill in Claude Code by running /claude-api migrate to automate parts of the migration process.
  • Note that manual extended thinking and assistant prefill are no longer supported; adjust code to remove these configurations.
  • Plan for potential safety classifier refusals if migrating to Claude Fable 5, as Claude Mythos Preview and Claude Mythos 5 do not have them.

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