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Or, for the model with the same capabilities and no access approval requirement: changedmodels/fable-5/migration-guide

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11---
2title: Migrating to Claude Mythos 5 and Claude Fable 5
2title: Claude Mythos 5 and Claude Fable 5 migration guide
33url: https://platform.claude.com/docs/en/models/fable-5/migration-guide
4description: "Migrate to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8: model IDs, API changes, and migration checklists."
4description: Switch to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8 with this migration guide. The guidance to enable Claude Mythos 5 and Claude Fable 5 includes model IDs, API changes, and migration checklists.
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77<Note>
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362362## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 4.8
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364364<Note>
365 If your code is on Claude Opus 4.7 or earlier, first apply the relevant [Migrating to Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) from-section for the API-level changes from your current model, then the remaining delta in this section.
365 If your code is on Claude Opus 4.7 or earlier, first apply the relevant [Claude Opus 5.5 migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) from-section for the API-level changes from your current model, then the remaining delta in this section.
366366</Note>
367367 
368368Migration is mostly drop-in. Claude Fable 5 and Claude Mythos 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Opus 4.8, with the same [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default and the same [128k max output tokens](https://platform.claude.com/docs/en/models/overview). Token counts are roughly unchanged because the models use the same tokenizer. The key changes to check are always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), thinking output, safety classifier refusals, and pricing.
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