Or, for the model with the same capabilities and no access approval requirement: changedabout-claude/models/migration-guide
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2020
2121## Migrating to Claude Mythos 5 and Claude Fable 5
2222
23[Claude Fable 5](https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5) is Anthropic's most capable widely released model, 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](https://anthropic.com/glasswing) shares the same capabilities and is offered only to approved customers in Project Glasswing.
23[Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) is Anthropic's most capable widely released model, 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](https://anthropic.com/glasswing) shares the same capabilities and is offered only to approved customers in Project Glasswing.
2424
2525The baseline settings shared by `claude-fable-5` and `claude-mythos-5`:
2626
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3838
3939### Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview
4040
41[Claude Mythos 5](https://anthropic.com/glasswing) is the access-gated successor to [Claude Mythos Preview](https://anthropic.com/glasswing), the invitation-only research preview. [Claude Fable 5](https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5) offers the same capabilities and does not require access approval. The changes in this section apply equally to both targets.
41[Claude Mythos 5](https://anthropic.com/glasswing) is the access-gated successor to [Claude Mythos Preview](https://anthropic.com/glasswing), the invitation-only research preview. [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) offers the same capabilities and does not require access approval. The changes in this section apply equally to both targets.
4242
4343Migration is mostly drop-in. Claude Mythos 5 and Claude Fable 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Mythos Preview, and token counts are roughly unchanged because all three models use the same tokenizer. The key changes to check are the features that are no longer available (listed in the next section) and thinking output. If you migrate to Claude Fable 5, also plan for safety classifier refusals, which Claude Mythos Preview and Claude Mythos 5 do not have; see [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
4444
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330330
331331### Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 5
332332
333Claude Fable 5 and Claude Mythos 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Opus 5, with the same [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default and the same [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview). The prefill and sampling-parameter restrictions, and the thinking display behavior, carry over from Claude Opus 5 unchanged. The changes to check are always-on thinking, pricing, Priority Tier, and data retention.
333Claude Fable 5 and Claude Mythos 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Opus 5, with the same [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default and the same [128k max output tokens](https://platform.claude.com/docs/en/models/overview). The prefill and sampling-parameter restrictions, and the thinking display behavior, carry over from Claude Opus 5 unchanged. The changes to check are always-on thinking, pricing, Priority Tier, and data retention.
334334
335335#### Update your model name
336336
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365365 If your code is on Claude Opus 4.7 or earlier, first apply the relevant [Migrating to Claude Opus 5](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-to-claude-opus-5) from-section for the API-level changes from your current model, then the remaining delta in this section.
366366</Note>
367367
368Migration is mostly drop-in. Claude Fable 5 and Claude Mythos 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Opus 4.8, with the same [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default and the same [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview). Token counts are roughly unchanged because the models use the same tokenizer. The key changes to check are always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), thinking output, safety classifier refusals (Claude Fable 5 only), and pricing.
368Migration is mostly drop-in. Claude Fable 5 and Claude Mythos 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Opus 4.8, with the same [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default and the same [128k max output tokens](https://platform.claude.com/docs/en/models/overview). Token counts are roughly unchanged because the models use the same tokenizer. The key changes to check are always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), thinking output, safety classifier refusals (Claude Fable 5 only), and pricing.
369369
370370#### Update your model name
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701701
702702Claude Opus 5 is a step-change improvement over Claude Opus 4.8, strong on deep reasoning, agentic and long-horizon tasks, and test-time compute scaling. For behavioral differences and model-specific prompting patterns, see [Prompting Claude Opus 5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5).
703703
704Claude Opus 5 is a drop-in upgrade for Claude Opus 4.8 at the same pricing of $5 per million input tokens and $25 per million output tokens; see [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing). There are two breaking changes for code already running on Claude Opus 4.8, covered under Breaking changes below. Claude Opus 5 supports the same set of features as Claude Opus 4.8, including the [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) (the default, with no beta header), [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview), [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching), [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side [tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview), with two exceptions: [web fetch](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool) is not available on Claude Opus 5, and [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is not supported on Claude Opus 5. See each tool page for model availability.
704Claude Opus 5 is a drop-in upgrade for Claude Opus 4.8 at the same pricing of $5 per million input tokens and $25 per million output tokens; see [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing). There are two breaking changes for code already running on Claude Opus 4.8, covered under Breaking changes below. Claude Opus 5 supports the same set of features as Claude Opus 4.8, including the [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) (the default, with no beta header), [128k max output tokens](https://platform.claude.com/docs/en/models/overview), [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching), [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side [tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview), with two exceptions: [web fetch](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool) is not available on Claude Opus 5, and [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is not supported on Claude Opus 5. See each tool page for model availability.
705705
706706### Migrating to Claude Opus 5 from Claude Opus 4.8
707707
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794794
795795### Migrating to Claude Opus 5 from Claude Opus 4.7
796796
797Claude Opus 5 should have strong out-of-the-box performance on existing Claude Opus 4.7 prompts and evals, at the same pricing of $5 per million input tokens and $25 per million output tokens. It supports the same set of features as Claude Opus 4.7, including the [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows), [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview), [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching), [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side [tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview), with two exceptions: [web fetch](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool) is not available on Claude Opus 5, and [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is not supported on Claude Opus 5. It also adds [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) and publicly documents [refusal stop details](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response). On the Claude API, Claude Opus 5 also supports [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) as the stable `computer_toolset_20260801` toolset and the [browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool) for tasks inside webpages, neither of which Claude Opus 4.7 supports; existing integrations on the earlier `computer_20251124` version continue to work unchanged on both models. To upgrade an existing integration, see [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124).
797Claude Opus 5 should have strong out-of-the-box performance on existing Claude Opus 4.7 prompts and evals, at the same pricing of $5 per million input tokens and $25 per million output tokens. It supports the same set of features as Claude Opus 4.7, including the [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows), [128k max output tokens](https://platform.claude.com/docs/en/models/overview), [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking), [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching), [batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing), the [Files API](https://platform.claude.com/docs/en/build-with-claude/files), [PDF support](https://platform.claude.com/docs/en/build-with-claude/pdf-support), [vision](https://platform.claude.com/docs/en/build-with-claude/vision), and server-side and client-side [tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview), with two exceptions: [web fetch](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool) is not available on Claude Opus 5, and [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is not supported on Claude Opus 5. It also adds [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) and publicly documents [refusal stop details](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response). On the Claude API, Claude Opus 5 also supports [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) as the stable `computer_toolset_20260801` toolset and the [browser use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/browser-use-tool) for tasks inside webpages, neither of which Claude Opus 4.7 supports; existing integrations on the earlier `computer_20251124` version continue to work unchanged on both models. To upgrade an existing integration, see [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124).
798798
799799<Note>
800800 If your code is on Claude Opus 4.6 or earlier, use [Migrating to Claude Opus 5 from Claude Opus 4.6 and earlier Opus models](https://platform.claude.com/docs/en/about-claude/models/migration-guide#migrating-from-claude-opus-46) instead. That section includes breaking changes (sampling parameters rejected, manual extended thinking rejected, new tokenizer) that the upgrade from Claude Opus 4.7 alone does not cover.
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906906Claude Opus 5 should have strong out-of-the-box performance on existing Claude Opus 4.6 prompts and evals at the same pricing, but there are a handful of behavioral and API changes worth knowing about as you migrate. Most of these changes took effect in Claude Opus 4.7; two more, thinking on by default and an effort cap on disabling thinking, take effect on Claude Opus 5. All of them are covered below, so this section is complete for code coming straight from Claude Opus 4.6. Claude Opus 5 supports the same set of features as Claude Opus 4.6, including:
907907
908908* [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) at standard API pricing with no long-context premium
909* [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview)
909* [128k max output tokens](https://platform.claude.com/docs/en/models/overview)
910910* [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking)
911911* [Prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching)
912912* [Batch processing](https://platform.claude.com/docs/en/build-with-claude/batch-processing)
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20122012
20132013### Migrating to Claude Opus 5 from Claude Sonnet 5
20142014
2015Claude Opus 5 and Claude Sonnet 5 share the same API surface: both run with [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) on by default, both default the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) to `high` on the Claude API and Claude Code, both serve a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default with [128k max output tokens](https://platform.claude.com/docs/en/about-claude/models/overview), and neither supports [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Manual extended thinking and non-default sampling parameters return a 400 error on both models, as does assistant prefill.
2015Claude Opus 5 and Claude Sonnet 5 share the same API surface: both run with [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) on by default, both default the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) to `high` on the Claude API and Claude Code, both serve a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default with [128k max output tokens](https://platform.claude.com/docs/en/models/overview), and neither supports [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Manual extended thinking and non-default sampling parameters return a 400 error on both models, as does assistant prefill.
20162016
20172017#### Update your model name
20182018
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20272027
202820282. **Disabling thinking is capped at `high` effort:** On Claude Sonnet 5, `thinking: {type: "disabled"}` is accepted at any effort level. On Claude Opus 5, it is accepted only at an [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level of `high` or below; a request that combines `thinking: {type: "disabled"}` with effort `xhigh` or `max` returns a 400 error, enforced on each request. Audit requests that disable thinking before you migrate.
20292029
20303. **Mid-conversation system messages:** Claude Opus 5 accepts `role: "system"` messages immediately after a user turn in the `messages` array (subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)). This feature is not available on Claude Sonnet 5; use the top-level `system` field instead. If you maintain code paths that rebuild the full message history to update instructions, you can simplify them and preserve [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) hits on earlier turns.
20303. **Mid-conversation system messages:** Claude Opus 5 accepts `role: "system"` messages immediately after a user turn in the `messages` array (subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)). This feature is not available on Claude Sonnet 5. If you maintain code paths that rebuild the full message history to update instructions, you can simplify them and preserve [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) hits on earlier turns.
20312031
203220324. **Web fetch is not available:** The [web fetch](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool) tool is available on Claude Sonnet 5 but not on Claude Opus 5.
20332033
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26512651
26522652### Migrating to Claude Sonnet 5 from Claude Haiku 4.5
26532653
2654Claude Haiku 4.5 and Claude Sonnet 5 differ more at the API level than adjacent models within one class: Claude Haiku 4.5 uses manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) (off by default), a 200k token context window, and up to 64k output tokens, while Claude Sonnet 5 runs with [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) on by default, serves a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default, and supports up to [128k output tokens](https://platform.claude.com/docs/en/about-claude/models/overview).
2654Claude Haiku 4.5 and Claude Sonnet 5 differ more at the API level than adjacent models within one class: Claude Haiku 4.5 uses manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) (off by default), a 200k token context window, and up to 64k output tokens, while Claude Sonnet 5 runs with [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) on by default, serves a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default, and supports up to [128k output tokens](https://platform.claude.com/docs/en/models/overview).
26552655
26562656#### Update your model name
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26912691
26922692Claude Haiku 4.5 is the fastest and most intelligent Haiku model with near-frontier performance, delivering premium model quality for interactive applications and high-volume processing.
26932693
2694For a complete overview of capabilities, see the [models overview](https://platform.claude.com/docs/en/about-claude/models/overview).
2694For a complete overview of capabilities, see the [models overview](https://platform.claude.com/docs/en/models/overview).
26952695
26962696<Note>
26972697 For Claude Haiku 4.5 pricing, see [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing).
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27192719
27202720**Review new rate limits:** Haiku 4.5 has separate rate limits from Haiku 3.5. See [Rate limits](https://platform.claude.com/docs/en/api/rate-limits) documentation for details.
27212721
2722**Explore new capabilities:** See the [models overview](https://platform.claude.com/docs/en/about-claude/models/overview) for details on context awareness, increased output capacity (64k tokens), higher intelligence, and improved speed.
2722**Explore new capabilities:** See the [models overview](https://platform.claude.com/docs/en/models/overview) for details on context awareness, increased output capacity (64k tokens), higher intelligence, and improved speed.
27232723
27242724#### Breaking changes
27252725
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27662766## Get help
27672767
27682768* Check the [API documentation](https://platform.claude.com/docs/en/api/overview) for detailed specifications
2769* Review [model capabilities](https://platform.claude.com/docs/en/about-claude/models/overview) for performance comparisons
2769* Review [model capabilities](https://platform.claude.com/docs/en/models/overview) for performance comparisons
27702770* Review [API release notes](https://platform.claude.com/docs/en/release-notes/api) for API updates
27712771* Contact support if you encounter any issues during migration
27722772
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