Introducing Claude Fable 5 and Claude Mythos 5

Claude 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.

The 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 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 per million input tokens and $50 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. 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.

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 (in beta on the Claude API and Claude Platform on AWS). See Server-side fallback.
• Client-side: Use the SDK middleware (TypeScript, Python, Go, Java, and C#) 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

You are not billed for a request that is refused before any output is generated. 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 and Claude Mythos 5 both become available on June 9, 2026:

• Claude Fable 5 is generally available on the Claude API, Claude Platform on AWS, Amazon Bedrock, Vertex AI, and Microsoft Foundry.
• Claude Mythos 5 is not generally available: it is offered in limited availability to approved customers in Project 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 is generally available 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: both are designated Covered Models. See Model-specific data retention requirements.

Working with Claude Fable 5 and Claude Mythos 5

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

Adaptive thinking is the only thinking mode on Claude Fable 5 and Claude Mythos 5. It applies whenever the thinking parameter is unset. thinking: {"type": "disabled"} is not supported. Use the effort parameter to control thinking depth.

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

At launch, 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