---
title: Introducing Claude Fable 5 and Claude Mythos 5
url: https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5
description: Claude Fable 5 and Claude Mythos 5 capabilities, API changes, and availability.
---
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).
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.
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](https://support.claude.com/en/articles/14604842).
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](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.
## Models
| Model | API model ID | Description |
| --------------- | ----------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Claude Fable 5 | `claude-fable-5` | Built for demanding reasoning and long-horizon agentic work |
| Claude Mythos 5 | `claude-mythos-5` | Shares Claude Fable 5's capabilities. Available only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Successor to Claude Mythos Preview. |
Claude Fable 5 and Claude Mythos 5 share the same specs and pricing:
* **Context window and output:** a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) 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](https://platform.claude.com/docs/en/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](https://platform.claude.com/docs/en/build-with-claude/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](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback).
* **Client-side:** Use the [SDK middleware](https://platform.claude.com/docs/en/cli-sdks-libraries/middleware) to retry from the client on any platform. See [Client-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback).
* **Manual:** Build the retry yourself, on any platform and in any language. See [Fallback credit](https://platform.claude.com/docs/en/build-with-claude/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](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed). When you retry on another model, [fallback credit](https://platform.claude.com/docs/en/build-with-claude/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](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).
* **Claude Mythos 5** is available only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). 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](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).
## 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](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/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](https://platform.claude.com/docs/en/build-with-claude/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](https://platform.claude.com/docs/en/build-with-claude/thinking#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](https://platform.claude.com/docs/en/build-with-claude/effort)
* [Task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets) (beta: set the `task-budgets-2026-03-13` header)
* The [memory tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/memory-tool)
* [Code execution](https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool)
* [Programmatic tool calling](https://platform.claude.com/docs/en/agents-and-tools/tool-use/programmatic-tool-calling)
* Tool result clearing through [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) (beta: set the `context-management-2025-06-27` header)
* [Compaction](https://platform.claude.com/docs/en/build-with-claude/compaction)
* [Vision](https://platform.claude.com/docs/en/build-with-claude/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](https://platform.claude.com/docs/en/models/fable-5/migration-guide#migrating-from-claude-mythos-preview).
* From Claude Opus 4.8: see [Migrating from Claude Opus 4.8 to Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/migration-guide#migrating-from-claude-opus-48).
## Next steps
Specs and comparison for all current Claude models.
The only thinking mode on Claude Fable 5 and Claude Mythos 5.
How Claude Fable 5 declines requests, and how to retry on another model.
Avoid paying the prompt-cache cost twice on a retry.
A worked end-to-end example of refusal handling, fallback, and billing.
Control thinking depth and cost on Claude Fable 5 and Claude Mythos 5.
Fable-specific prompting techniques.