introducing-claude-fable-5-and-claude-mythos-5 changedmodels/fable-5/introducing-claude-fable-5-and-claude-mythos-5
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## Prompting ## Working with Claude Fable 5 and Claude Mythos 5 ### Prompting
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44description: Claude Fable 5 and Claude Mythos 5 capabilities, API changes, and availability.
55---
66
7<Note>
8 Claude Fable 5.1 and Claude Mythos 5.1 build on these models. See [What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1).
9</Note>
10
711<Tip>
812 Access to Claude Fable 5 and Claude Mythos 5 has been restored. See [our statement](https://www.anthropic.com/news/redeploying-fable-5) for more information.
913</Tip>
1014
11Claude Fable 5 is Anthropic's most capable widely released model, built for the most demanding reasoning and long-horizon agentic work. Claude Mythos 5 shares the same capabilities and is available only in limited release through [Project Glasswing](https://anthropic.com/glasswing).
15Claude Fable 5 is built for demanding reasoning and long-horizon agentic work. Claude Mythos 5 shares the same capabilities and is available only in limited release through [Project Glasswing](https://anthropic.com/glasswing).
1216
1317The headline change for integrations: Claude Fable 5 includes safety classifiers that can decline requests. Claude Mythos 5 does not include these classifiers. If your integration calls Claude Fable 5, plan for three changes: new response handling for refusals, fallback options for retrying on another Claude model, and new billing rules. [Refusals, fallback, and billing on Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5#refusals-fallback-and-billing-on-claude-fable-5) summarizes all three.
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1620
1721| Model | API model ID | Description |
1822| --------------- | ----------------- | --------------------------------------------------------------------------------------------------------------------------------------------- |
19| Claude Fable 5 | `claude-fable-5` | Anthropic's most capable widely released model, for the most demanding reasoning and long-horizon agentic work |
23| Claude Fable 5 | `claude-fable-5` | Built for demanding reasoning and long-horizon agentic work |
2024| Claude Mythos 5 | `claude-mythos-5` | Shares Claude Fable 5's capabilities without the safety classifiers. Available through Project Glasswing. Successor to Claude Mythos Preview. |
2125
2226Claude Fable 5 and Claude Mythos 5 share the same specs and pricing:
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2832
2933## Refusals, fallback, and billing on Claude Fable 5
3034
31Claude Fable 5 includes safety classifiers that can decline certain requests. Claude Mythos 5 does not include these classifiers, so this section applies to Claude Fable 5 only. The following sections summarize what refusals mean for your integration; each links to the full guide.
35Claude Fable 5 includes safety classifiers that can decline certain requests. Claude Mythos 5 does not include these classifiers, so this section applies to Claude Fable 5 only. The following sections summarize what refusals mean for your integration. Each links to the full guide.
3236
3337### Refusals
3438
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4852
4953## Availability
5054
51Claude Fable 5 and Claude Mythos 5 both become available on June 9, 2026:
52
5355* **Claude Fable 5** is available on the Claude API, [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Google Cloud](https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai), and [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry).
5456* **Claude Mythos 5** is offered only to approved customers in [Project Glasswing](https://anthropic.com/glasswing). For access, contact your Anthropic, AWS, or Google Cloud account team. Customers without access to Claude Mythos 5 can use Claude Fable 5, which does not require access approval and offers the same capabilities.
5557
56Claude Fable 5 and Claude Mythos 5 carry 30-day data retention and are not available under zero data retention: both are designated [Covered Models](https://support.claude.com/en/articles/15425695). See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
58Claude Fable 5 and Claude Mythos 5 carry 30-day data retention and are not available under zero data retention unless expressly authorized by Anthropic. Both are designated [Covered Models](https://support.claude.com/en/articles/15425695). See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
5759
58## Working with Claude Fable 5 and Claude Mythos 5
60## Prompting
5961
60### Prompting
61
6262Claude Fable 5 responds to the same prompting techniques as other Claude models, with a few differences in how to structure long-context prompts and reasoning instructions. See [Prompting Claude Fable 5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5).
6363
6464## Messages API on Claude Fable 5 and Claude Mythos 5
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6565
6666### Adaptive thinking is always on
6767
68Claude Fable 5 and Claude Mythos 5 always have thinking enabled; passing `thinking: {"type": "disabled"}` is not supported. To reduce or otherwise control thinking depth, use the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) parameter.
68Claude Fable 5 and Claude Mythos 5 always have thinking enabled. Passing `thinking: {"type": "disabled"}` is not supported. To reduce or otherwise control thinking depth, use the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) parameter.
6969
7070### Raw thinking content is never returned
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7878
7979## Supported features
8080
81At launch, Claude Fable 5 and Claude Mythos 5 support:
81Claude Fable 5 and Claude Mythos 5 support:
8282
8383* [Effort](https://platform.claude.com/docs/en/build-with-claude/effort)
8484* [Task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets) (beta: set the `task-budgets-2026-03-13` header)
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9999## Next steps
100100
101101<CardGroup>
102 <Card title="Migration guide" icon="arrow-right" href="https://platform.claude.com/docs/en/models/fable-5/migration-guide">
103 Step-by-step upgrade instructions from Claude Opus 4.8 and Claude Mythos Preview.
104 </Card>
105
106102 <Card title="Models overview" icon="settings" href="https://platform.claude.com/docs/en/models/overview">
107103 Specs and comparison for all current Claude models.
108104 </Card>
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