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Introducing Claude Fable 5 and Claude Mythos 5

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Claude Fable 5 and Claude Mythos 5 capabilities, API changes, and availability.

Claude Fable 5 is built for demanding reasoning and long-horizon agentic work. Claude Mythos 5 shares the same capabilities and is available only to organizations verified through Anthropic's verification programs, such as the Cyber Verification Program.

The headline change for integrations: Claude Fable 5 includes safety classifiers that can decline requests. 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 summarizes all three.

Models

Claude Fable 5 and Claude Mythos 5 share the same specs and pricing:

• 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.

For specs across all current models, see the models overview.

Refusals, fallback, and billing on Claude Fable 5

Claude Fable 5 includes safety classifiers that can decline certain requests. The following sections summarize what refusals mean for your integration. Each links to the full guide.

Refusals

When Claude Fable 5 declines a request, the Messages API returns stop_reason: "refusal" as a successful HTTP 200 response, not an error. The response also reports which classifier declined the request. See Refusals and fallback for response shapes and handling guidance.

Fallback

A request that Claude Fable 5 refuses can usually be served by another Claude model. There are three ways to retry:

• Server-side: Pass the fallbacks parameter to have the API retry for you, using its "default" mode for Anthropic's recommended models or naming your own (in beta on the Claude API). See Server-side fallback.
• Client-side: Use the SDK middleware to retry from the client on any platform. See Client-side fallback.
• Manual: Build the retry yourself, on any platform and in any language. See Fallback credit.

Billing

A refusal that arrives before any output is billed when it is in a category with low volumes of false positives, to disrupt attempts to circumvent Anthropic's safeguards at scale. Before September 24, 2026, these refusals were not billed. A mid-stream refusal bills the input tokens and the output already streamed at normal rates. For the billed categories, see How refusals are billed. When you retry on another model, fallback credit refunds the prompt-cache cost of switching, so you avoid paying that cost twice.

Availability

• Claude Fable 5 is available on the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry.
• Claude Mythos 5 is available only to organizations verified through Anthropic's verification programs, such as the Cyber Verification Program. Customers without access to Claude Mythos 5 can use Claude Fable 5, which does not require access approval and offers the same capabilities.

Claude 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. See Model-specific data retention requirements.

Prompting

Claude 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.

Messages API on Claude Fable 5 and Claude Mythos 5

Adaptive thinking is always on

Claude 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 parameter.

Raw thinking content is never returned

The raw chain of thought is never returned on Claude Fable 5 and Claude Mythos 5. The thinking.display setting controls what thinking blocks contain instead:

• "summarized" returns thinking blocks with a readable summary of the reasoning.
• "omitted" (the default) returns thinking blocks with an empty thinking field.

Pass thinking blocks back unchanged in multi-turn conversations on the same model. See thinking output on Claude Fable 5 and Claude Mythos 5 for cross-model handling.

Supported features

Claude Fable 5 and Claude Mythos 5 support:

• Effort
• Task budgets (beta: set the task-budgets-2026-03-13 header)
• The memory tool
• Code execution
• Programmatic tool calling
• Tool result clearing through context editing (beta: set the context-management-2025-06-27 header)
• Compaction
• Vision

Migrating from earlier models

Step-by-step instructions live in the migration guide:

• From Claude Mythos Preview: see Migrating from Claude Mythos Preview to Claude Mythos 5.
• From Claude Opus 4.8: see Migrating from Claude Opus 4.8 to Claude Fable 5.

Next steps