overview changedmodels/fable-5-1/overview
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1616
1717## Overview
1818
19Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still don't meet your requirements. Claude Mythos 5.1 offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
19Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still don’t meet your requirements. Claude Mythos 5.1 offers the same capabilities only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
2020
2121If you already call Claude Fable 5, four changes are breaking:
2222
2323* [Forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported).
24* Only Claude Fable 5.1 and Claude Mythos 5.1 can [read Claude Fable 5.1's thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them).
24* Only Claude Fable 5.1 and Claude Mythos 5.1 can [read Claude Fable 5.1’s thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them).
2525* [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks).
2626* [Some advisor tool pairings are not supported](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#advisor-tool-pairings).
2727
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3333* A [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)
3434* [Content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance)
3535
36[What's new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1)
36[What’s new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1)
3737
3838## Claude Fable 5.1 and Claude Mythos 5.1
3939
40[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). It shares Claude Fable 5.1's specifications and pricing. To request access, apply to the program that covers your use case, or contact your Anthropic, AWS, or Google Cloud account team.
40[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) offers the same capabilities only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). It shares Claude Fable 5.1’s specifications and pricing. To request access, apply to the program that covers your use case, or contact your Anthropic, AWS, or Google Cloud account team.
4141
4242## How it compares
4343
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116116 </Card>
117117
118118 <Card title="Preserved thinking" icon="brain" href="https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-thinking">
119 When this model's thinking blocks stay usable: across model switches and across changes to the conversation.
119 When this model’s thinking blocks stay usable: across model switches and across changes to the conversation.
120120 </Card>
121121
122122 <Card title="Per-message effort" icon="sliders" href="https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta">
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