Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations: changedmodels/fable-5-1/migration-guide
Nearest release: v2.1.292, published 11 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
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1818 The skill applies the model ID swap and, as needed, breaking parameter changes, prefill replacement, and effort calibration for your target model across your code base, then produces a checklist of items to verify manually. It asks you to confirm the migration scope (entire working directory, a subdirectory, or a specific file list) before editing any files. The skill also detects Amazon Bedrock and Claude Platform on AWS clients and adjusts model ID formats and feature changes for those platforms.
1919</Tip>
2020
21[Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) succeeds Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost. It's 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 5.1](https://anthropic.com/glasswing) shares the same capabilities and is offered only to approved customers in Project Glasswing. For behavioral differences and prompting patterns, see [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
21[Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1) succeeds Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost. It's 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 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) 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). For behavioral differences and prompting patterns, see [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
2222
2323The baseline settings shared by `claude-fable-5-1` and `claude-mythos-5-1`:
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3333Where the two models diverge:
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35* **Availability:** Claude Fable 5.1 doesn't require access approval. Claude Mythos 5.1 is available only to approved customers in [Project Glasswing](https://anthropic.com/glasswing). Contact your Anthropic account team for access.
35* **Availability:** Claude Fable 5.1 doesn't require access approval. Claude Mythos 5.1 is available only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). To request access, apply to the program that covers your use case, or contact your Anthropic account team.
3636* **Priority Tier:** Neither model is supported on [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Claude Fable 5 is.
3737
3838## Migrating to Claude Fable 5.1 from Claude Fable 5
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4545model = "claude-fable-5" # Before
4646model = "claude-fable-5-1" # After
4747
48# Or, for the Project Glasswing model with the same capabilities:
48# Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations:
4949model = "claude-mythos-5-1" # After
5050```
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15421542model = "claude-opus-5" # Before
15431543model = "claude-fable-5-1" # After
15441544
1545# Or, for the Project Glasswing model with the same capabilities:
1545# Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations:
15461546model = "claude-mythos-5-1" # After
15471547```
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15821582model = "claude-opus-4-8" # Before
15831583model = "claude-fable-5-1" # After
15841584
1585# Or, for the Project Glasswing model with the same capabilities:
1585# Or, for Claude Mythos 5.1, which offers the same capabilities to verified organizations:
15861586model = "claude-mythos-5-1" # After
15871587```
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16001600
16011601## Migrating to Claude Mythos 5.1 from Claude Mythos 5
16021602
1603[Claude Mythos 5.1](https://anthropic.com/glasswing) is the access-gated counterpart to Claude Fable 5.1. Confirm your organization's access with your Anthropic account team before switching model IDs.
1603[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) is the counterpart to Claude Fable 5.1 for organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Confirm that your organization has access to Claude Mythos 5.1 before you switch model IDs.
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16051605The API-level delta matches [Migrating to Claude Fable 5.1 from Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#migrating-from-claude-fable-5-to-claude-fable-5-1): forced tool choice returns a 400 error, and thinking blocks are preserved only for the model that produced them or a newer one (Claude Mythos 5.1 reads Claude Mythos 5's blocks, not the reverse). Unlike Claude Fable 5.1, Claude Mythos 5.1 doesn't run the conversation check, so editing earlier turns doesn't [invalidate thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking), though it still restarts the prompt cache.
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