introducing-claude-fable-5-and-claude-mythos-5

about-claude/models/introducing-claude-fable-5-and-claude-mythos-5 Page removed · 133 lines, page removed

## Models ## Refusals, fallback, and billing on Claude Fable 5 ### Refusals ### Fallback ### Billing ## Availability ## Working with Claude Fable 5 and Claude Mythos 5 ### Prompting ## Messages API on Claude Fable 5 and Claude Mythos 5 ### Adaptive thinking is always on ### Raw thinking content is never returned ## Supported features ## Migrating from earlier models ## Next steps

The page is gone upstream. What it last said is kept here.

about-claude/models/introducing-claude-fable-5-and-claude-mythos-5 First recorded · 133 lines, first recorded

## Models ## Refusals, fallback, and billing on Claude Fable 5 ### Refusals ### Fallback ### Billing ## Availability ## Working with Claude Fable 5 and Claude Mythos 5 ### Prompting ## Messages API on Claude Fable 5 and Claude Mythos 5 ### Adaptive thinking is always on ### Raw thinking content is never returned ## Supported features ## Migrating from earlier models ## Next steps

The first capture of this source. The page was already there, and this is what it said.

---
title: Introducing Claude Fable 5 and Claude Mythos 5
url: https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5
description: Claude Fable 5 and Claude Mythos 5 capabilities, API changes, and availability.
---

<Tip>
  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.
</Tip>

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](https://anthropic.com/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](https://platform.claude.com/docs/en/about-claude/models/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`  | Anthropic's most capable widely released model, for the most demanding reasoning and long-horizon agentic work                                |
| 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. |

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/about-claude/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](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

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

* **Claude Fable 5** is generally 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 not generally available: it is offered in limited availability 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 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](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).

## 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](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

At launch, 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/about-claude/models/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/about-claude/models/migration-guide#migrating-from-claude-opus-48).

## Next steps

<CardGroup>
  <Card title="Migration guide" icon="arrow-right" href="https://platform.claude.com/docs/en/about-claude/models/migration-guide">
    Step-by-step upgrade instructions from Claude Opus 4.8 and Claude Mythos Preview.
  </Card>

  <Card title="Models overview" icon="settings" href="https://platform.claude.com/docs/en/about-claude/models/overview">
    Specs and comparison for all current Claude models.
  </Card>

  <Card title="Adaptive thinking" icon="brain" href="https://platform.claude.com/docs/en/build-with-claude/thinking">
    The only thinking mode on Claude Fable 5 and Claude Mythos 5.
  </Card>

  <Card title="Refusals and fallback" icon="shield" href="https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback">
    How Claude Fable 5 declines requests, and how to retry on another model.
  </Card>

  <Card title="Fallback credit" icon="coins" href="https://platform.claude.com/docs/en/build-with-claude/fallback-credit">
    Avoid paying the prompt-cache cost twice on a retry.
  </Card>

  <Card title="Fallback and billing cookbook" icon="book-open" href="https://platform.claude.com/cookbook/fable-5-fallback-billing-guide">
    A worked end-to-end example of refusal handling, fallback, and billing.
  </Card>

  <Card title="Effort" icon="sliders" href="https://platform.claude.com/docs/en/build-with-claude/effort">
    Control thinking depth and cost on Claude Fable 5 and Claude Mythos 5.
  </Card>

  <Card title="Prompting Claude Fable 5" icon="terminal" href="https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5">
    Fable-specific prompting techniques.
  </Card>
</CardGroup>