HomeDocs changes
Documentation changes
What the docs quietly changed.
Anthropic's published documentation, fetched on a schedule and diffed against the last copy. Every edit dated, including the ones that never made a release note.
Last poll read all 4 cleanly: Claude Code CLI, Claude Developer Platform, Claude Documentation, Model Context Protocol. A date here is when this site noticed, not when Anthropic edited.
Claude Developer Platform
821pages tracked
767 changed this week · last read 24 Oct 2026
Claude Code CLI
228pages tracked
146 changed this week · last read 24 Oct 2026
Claude Documentation
280pages tracked
69 changed this week · last read 24 Oct 2026
Model Context Protocol
357pages tracked
171 changed this week · last read 23 Oct 2026
100 on this page
10,173 of the ledger's changes in api. Show every change
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7 Oct 2026
119 pages
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api
api/errors
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api/errors
Claude Haiku 5.5 accepts `thinking: {"type": "disabled"}` and holds it to the same two effort limits that apply to `between_tools`: at `xhigh` or `max` effort, or with a per-messag
On the Claude API and Google Cloud, Claude Opus 5.5 and Claude Sonnet 5.5 support [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) on
On the Claude API and Google Cloud, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 support [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/co
On Claude Fable 5.1, Claude Opus 5.5, and Claude Sonnet 5.5, the API accepts a replayed thinking block only while the `system` prompt, `tools`, and messages that preceded it are un
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agents-and-tools/tool-use/web-search-tool
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agents-and-tools/tool-use/web-search-tool
To check whether a model accepts web search before you send a request, read its `capabilities.server_tools.web_search.supported` value from the Models API. [Using the Models API](h
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agents-and-tools/tool-use/tool-search-tool
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agents-and-tools/tool-use/tool-search-tool
| Claude Haiku 5.5 (claude-haiku-5-5) | `tool_search_tool_regex_20251119`, `tool_sear
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agents-and-tools/tool-use/tool-reference
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agents-and-tools/tool-use/tool-reference
For model compatibility, see each tool's page. Supported models vary by tool and by tool version.
For model compatibility, see each tool's page. Supported models vary by tool and by tool version. To check from your code whether a model accepts web search or code execution, see
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agents-and-tools/tool-use/programmatic-tool-calling
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agents-and-tools/tool-use/programmatic-tool-calling
- claude-haiku-5-5
Programmatic tool calling requires the [code execution tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool) with tool version `code_execution_20
Programmatic tool calling requires the [code execution tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool) with tool version `code_execution_20
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agents-and-tools/tool-use/overview
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agents-and-tools/tool-use/overview
| Claude Haiku 5.5 | 286 tokens
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agents-and-tools/tool-use/define-tools
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agents-and-tools/tool-use/define-tools
| Manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) (`thinking: {type: "enabled"}`) | `any` and `tool` are not supported
| Manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) (`thinking: {type: "enabled"}`) | `any` and `tool` are not supported
 with a `keep` value other than `"all"` shifts the advisor's quoted transcript each turn,
[`clear_thinking`](https://platform.claude.com/docs/en/build-with-claude/context-editing) with a `keep` value other than `"all"` shifts the advisor's quoted transcript each turn,
| Executor models | Advisor models
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agents-and-tools/mcp-connector
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agents-and-tools/mcp-connector
implementation("com.anthropic:anthropic-java:2.68.0")
implementation("com.anthropic:anthropic-java-mcp:2.68.0")
implementation("com.anthropic:anthropic-java:2.69.0")
implementation("com.anthropic:anthropic-java-mcp:2.69.0")
<version>2.68.0</version>
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agents-and-tools/agent-skills/overview
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agents-and-tools/agent-skills/overview
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agents-and-tools/agent-skills/claude-api-skill
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agents-and-tools/agent-skills/claude-api-skill
* **Model migration:** Step-by-step guidance for migrating to newer Claude models (including the breaking changes and behavior shifts on [Claude Opus 5.5](https://platform.claude.c
* **Model migration:** Step-by-step guidance for migrating to newer Claude models (including the breaking changes and behavior shifts on [Claude Opus 5.5](https://platform.claude.c
For the full list of model-specific changes the skill applies, see [Migrating to Claude Opus 5.5 from Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5-5/migration-g
For the full list of model-specific changes the skill applies, see [Migrating to Claude Opus 5.5 from Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5-5/migration-g
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agents-and-tools/agent-skills/best-practices
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agents-and-tools/agent-skills/best-practices
<Frame>
</Frame>
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about-claude/use-case-guides/ticket-routing
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about-claude/use-case-guides/ticket-routing
Many customers have found `claude-haiku-4-5-20251001` an ideal model for ticket routing, as it is the fastest and most cost-effective model in the Claude 4 family while still deliv
Claude Haiku 5.5 (`claude-haiku-5-5`) suits ticket routing: it's the fastest and most cost-effective current model, and at `low` effort it handles simple, high-volume requests such
This prompt is written for Claude Haiku 4.5, which runs here without thinking. On Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, and Claude Sonnet 5.5, ask for
This prompt is written for Claude Haiku 5.5, which runs here at `low` effort. On Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, and Claude Sonnet 5.5, ask for
DEFAULT_MODEL = "claude-haiku-4-5-20251001"
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about-claude/use-case-guides/content-moderation
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about-claude/use-case-guides/content-moderation
When selecting a model, it’s important to consider the size of your data. If costs are a concern, a smaller model such as Claude Haiku 4.5 is an excellent choice because of its cos
When selecting a model, it’s important to consider the size of your data. If costs are a concern, a smaller model such as Claude Haiku 5.5 is an excellent choice because of its cos
* **Claude Haiku 4.5 estimated cost**
* Input token cost: 28,600 MTok \* $1.00/MTok = $28,600 USD
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about-claude/pricing
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about-claude/pricing
| Claude Haiku 5.5 (for prompts up to 100,000 tokens) | $0.10 / MTok | $0.125 / MTok |
| Claude Haiku 5.5 (for prompts over 100,000 tokens) | $0.50 / MTok | $0.625 / MTok |
| Cache operation | Multiplier | Duration |
| -------------------- | -------------------------------------------------------------------------------------------------- | ------------------------------------ |
| 5-minute cache write | 1.25x base input price | Cache valid for 5 minutes |
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about-claude/models/optimizing-for-cost-and-intelligence
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about-claude/models/optimizing-for-cost-and-intelligence
 that trades intelligence for latency and cost within
* **Effort:** Several Claude models support an [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) that trades intelligence for latency and cost within
For many applications, starting with a faster, more cost-effective model like Claude Haiku 4.5 can be the optimal approach:
For many applications, starting with a faster, more cost-effective model like Claude Haiku 5.5 can be the optimal approach:
1. Begin implementation with Claude Haiku 4.5.
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about-claude/model-deprecations
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about-claude/model-deprecations
| claude-haiku-5-5 | Active | N/A | Not sooner than October 7, 2027 |
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models/sonnet-5-5/migration-guide
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models/sonnet-5-5/migration-guide
title: Migrating to Claude Sonnet 5.5
title: Claude Sonnet 5.5 migration guide
description: "Move code to Claude Sonnet 5.5 from Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5, Claude Sonnet 4, Claude 3.7 Sonnet, or Claude Haiku 4.5: settings that retu
description: Switch to Claude Sonnet 5.5 from earlier Sonnet models or Claude Haiku 4.5 with this migration guide. The guidance to enable Claude Sonnet 5.5 includes settings that r
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models/overview
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models/overview
* **Output quality:** When migrating from a previous model generation, you may notice larger improvements in overall performance. If you're on Claude Opus 5 or earlier, see [Migrat
* **Output quality:** When migrating from a previous model generation, you may notice larger improvements in overall performance. If you're on Claude Opus 5 or earlier, see the [Cl
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models/opus-5-5/migration-guide
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models/opus-5-5/migration-guide
title: Migrating to Claude Opus 5.5
title: Claude Opus 5.5 migration guide
description: "Migrate to Claude Opus 5.5 from earlier Opus models or Claude Sonnet 5: request settings that return errors, thinking blocks in every response, and a checklist for ea
description: Switch to Claude Opus 5.5 from earlier Opus models or Claude Sonnet 5 with this migration guide. The guidance to enable Claude Opus 5.5 includes request settings that
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models/haiku-4-5/migration-guide
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models/haiku-4-5/migration-guide
title: Migrating to Claude Haiku 4.5
title: Claude Haiku 4.5 migration guide
description: "Migrate to Claude Haiku 4.5 from earlier Haiku models: model IDs, breaking changes, and a migration checklist."
description: Switch to Claude Haiku 4.5 from earlier Haiku models with this migration guide. The guidance to enable Claude Haiku 4.5 includes model IDs, breaking changes, and a mig
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models/fable-5/migration-guide
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models/fable-5/migration-guide
title: Migrating to Claude Mythos 5 and Claude Fable 5
title: Claude Mythos 5 and Claude Fable 5 migration guide
description: "Migrate to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8: model IDs, API changes, and migration checklists."
description: Switch to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8 with this migration guide. The guidance to enable Claude Myt
If your code is on Claude Opus 4.7 or earlier, first apply the relevant [Migrating to Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) from-s
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models/fable-5-1/whats-new-fable-5-1
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models/fable-5-1/whats-new-fable-5-1
On Claude Fable 5.1 you can change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level mid-conversation without invalidating the prompt cache. Raise it
On Claude Fable 5.1 you can change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level mid-conversation without invalidating the prompt cache. Raise it
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models/fable-5-1/overview
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models/fable-5-1/overview
<Card title="Migrating to Claude Fable 5.1" icon="arrow-right" href="https://platform.claude.com/docs/en/models/fable-5-1/migration-guide">
<Card title="Claude Fable 5.1 migration guide" icon="arrow-right" href="https://platform.claude.com/docs/en/models/fable-5-1/migration-guide">
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models/fable-5-1/migration-guide
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models/fable-5-1/migration-guide
title: Migrating to Claude Fable 5.1 and Claude Mythos 5.1
title: Claude Fable 5.1 and Claude Mythos 5.1 migration guide
description: "Migrate to Claude Fable 5.1 and Claude Mythos 5.1 from Claude Fable 5, Claude Mythos 5, Claude Opus 5, or Claude Opus 4.8: model IDs, breaking changes, and migration
description: Switch to Claude Fable 5.1 and Claude Mythos 5.1 from Claude Fable 5, Claude Mythos 5, Claude Opus 5, or Claude Opus 4.8 with this migration guide. The guidance to ena
* **Thinking:** [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on, unchanged from Claude Fable 5. The model decides when and how much
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managed-agents/tools-web-restrictions
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managed-agents/tools-web-restrictions
These per-tool lists are the way to restrict what the web tools can reach. Two other settings do not affect these tools:
* **Environment networking:** An environment's [`networking`](https://platform.claude.com/docs/en/managed-agents/environments#networking) settings control the sandbox's own outbo
* **Organization settings:** Organization-level web search and web fetch settings in the Claude Console apply to the Messages API. They do not apply to Managed Agents sessions.
`web_search` and `web_fetch` run on Anthropic's servers, not in the sandbox. A cloud environment with `limited` [networking](https://platform.claude.com/docs/en/managed-agents/en
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managed-agents/quickstart
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managed-agents/quickstart
An environment defines the sandbox where your agent runs. This one uses `limited` [networking](https://platform.claude.com/docs/en/managed-agents/environments#networking) with
An environment defines the sandbox where your agent runs. This one uses `limited` [networking](https://platform.claude.com/docs/en/managed-agents/environments#networking) with
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managed-agents/environments
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managed-agents/environments
The `networking` field controls the sandbox's outbound network access. It does not affect the `web_search` or `web_fetch` tools, which run on Anthropic's servers; to restrict the s
The `networking` field controls the sandbox's outbound network access.
With `limited` networking, `allowed_hosts` also applies to the `web_search` and `web_fetch` tools, which run on Anthropic's servers. A `web_fetch` call for a URL on a host that `al
An agent that only uses the `web_search` and `web_fetch` tools does not need `unrestricted` networking if you can list the sites it needs. [Networking](https://platform.claude.com/
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manage-claude/workload-identity-federation
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manage-claude/workload-identity-federation
To manage these resources programmatically, see [Manage WIF with the Admin API](https://platform.claude.com/docs/en/manage-claude/wif-admin-api) for the curl walkthrough, or see th
To manage these resources programmatically, see [Manage WIF with the Admin API](https://platform.claude.com/docs/en/manage-claude/wif-admin-api) for the curl walkthrough, or see th
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manage-claude/wif-reference
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manage-claude/wif-reference
For complete parameter details and response schemas, see the [Service accounts API reference](https://platform.claude.com/docs/en/api/beta/organization/service_accounts), [Federati
For complete parameter details and response schemas, see the [Service accounts API reference](https://platform.claude.com/docs/en/api/organization/service_accounts), [Federation is
| Constraint | Detail
| ----------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------
| Maximum size | The `assertion` JWT must be at most 16 KiB.
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manage-claude/user-management
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manage-claude/user-management
For complete parameter details and response schemas, see [List users](https://platform.claude.com/docs/en/api/beta/organization/users/list) in the API reference.
For complete parameter details and response schemas, see [List users](https://platform.claude.com/docs/en/api/organization/users/list) in the API reference.
For complete parameter details and response schemas, see [Get user](https://platform.claude.com/docs/en/api/beta/organization/users/retrieve) in the API reference.
For complete parameter details and response schemas, see [Get user](https://platform.claude.com/docs/en/api/organization/users/retrieve) in the API reference.
For complete parameter details and response schemas, see [Update user](https://platform.claude.com/docs/en/api/beta/organization/users/update) in the API reference.
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manage-claude/usage-cost-api
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manage-claude/usage-cost-api
To retrieve your organization's API key IDs, use the [List API Keys](https://platform.claude.com/docs/en/api/beta/organization/api_keys/list) endpoint.
To retrieve your organization's workspace IDs, use the [List Workspaces](https://platform.claude.com/docs/en/api/beta/organization/workspaces/list) endpoint, or find your organiz
To retrieve your organization's API key IDs, use the [List API Keys](https://platform.claude.com/docs/en/api/organization/api_keys/list) endpoint.
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manage-claude/data-residency
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manage-claude/data-residency
The `inference_geo` parameter is supported on Claude 4.6 and later models. Requests with `inference_geo` on Claude Opus 4.5, Claude Sonnet 4.5, Claude Haiku 4.5, or earlier models
The `inference_geo` parameter is supported on Claude 4.6 and later models. Requests with `inference_geo` on Claude Opus 4.5, Claude Sonnet 4.5 (deprecated), Claude Haiku 4.5, or ea
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manage-claude/authentication
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manage-claude/authentication
Store API keys in a secrets manager, rotate them periodically, and disable or delete any key you suspect has leaked. On the [API keys page](https://platform.claude.com/settings/key
Store API keys in a secrets manager, rotate them periodically, and disable or delete any key you suspect has leaked. On the [API keys page](https://platform.claude.com/settings/key
You can find a workspace's ID in the **ID** column of [Settings → Workspaces](https://platform.claude.com/settings/workspaces) in the Claude Console, or by calling the [List Worksp
You can find a workspace's ID in the **ID** column of [Settings → Workspaces](https://platform.claude.com/settings/workspaces) in the Claude Console, or by calling the [List Worksp
The Console API keys table shows each key's expiration, and the Admin API reports each key's `expires_at` timestamp on the [List API Keys](https://platform.claude.com/docs/en/api/b
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get-api-key
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get-api-key
The [Admin API](https://platform.claude.com/docs/en/api/beta/organization) includes endpoints for managing your organization's API keys programmatically, such as [Retrieve API Key]
The [Admin API](https://platform.claude.com/docs/en/api/organization) includes endpoints for managing your organization's API keys programmatically, such as [Retrieve API Key](http
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cli-sdks-libraries/libraries/openai-sdk
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cli-sdks-libraries/libraries/openai-sdk
You can enable [thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) by adding the `thinking` parameter. On current models thinking is adaptive, with Claude de
You can enable [thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) by adding the `thinking` parameter. On current models thinking is adaptive, with Claude de
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claude_api_primer
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claude_api_primer
Thinking can sometimes help Claude with very hard tasks. The current mechanism is [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {"
Thinking can sometimes help Claude with very hard tasks. The current mechanism is [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {"
* Claude Sonnet 4.5 (`claude-sonnet-4-5-20250929`, legacy manual thinking only)
* Claude Sonnet 4.5 (`claude-sonnet-4-5-20250929`, [deprecated](https://platform.claude.com/docs/en/about-claude/model-deprecations), legacy manual thinking only)
Interleaved thinking enables Claude to think between tool calls, reasoning about tool results before deciding the next step.
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build-with-claude/thinking-steering-and-cost
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build-with-claude/thinking-steering-and-cost
Claude's thinking is adaptive: the model evaluates each request and decides for itself whether to think and how much. You set an intent, optionally specify the effort, and the mode
Claude's thinking is adaptive: the model evaluates each request and determines whether to think and how much. You set an intent, optionally specify the effort, and the model alloca
To learn how to turn thinking on, how to read thinking output, and about [thinking output on Claude Fable 5 and Claude Mythos 5](https://platform.claude.com/docs/en/build-with-clau
## How Claude decides when to think
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build-with-claude/thinking
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build-with-claude/thinking
![Diagram of how thinking works: Claude evaluates the request and decides whether to think up front; with tool use, thinking can recur between tool calls; one response returns th
![Diagram: Claude determines whether to think up front, may think between tool calls, and returns thinking blocks before text](https://platform.claude.com/docs/images/how-thinkin
On Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6, thinking is off until you set `thinking: {type: "adaptive"}`, which lets Claude decide when and how dee
On Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6, thinking is off until you set `thinking: {type: "adaptive"}`, which lets Claude determine when and how
* Reason about the results of a tool call before deciding what to do next
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build-with-claude/task-budgets
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build-with-claude/task-budgets
Let Claude decide when and how much to use extended thinking.
Let Claude determine when and how much to use extended thinking.
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build-with-claude/prompt-engineering/prompting-claude-opus-5-5
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build-with-claude/prompt-engineering/prompting-claude-opus-5-5
In chat applications, if your system prompt contains instructions that tell Claude to think carefully before answering, consider removing them for Claude Opus 5.5. The model decide
In chat applications, if your system prompt contains instructions that tell Claude to think carefully before answering, consider removing them for Claude Opus 5.5. The model determ
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build-with-claude/prompt-engineering/claude-prompting-best-practices
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build-with-claude/prompt-engineering/claude-prompting-best-practices
Claude 4.6 and later models and Claude Mythos Preview use [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {type: "adaptive"}`), wher
Claude 4.6 and later models and Claude Mythos Preview use [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {type: "adaptive"}`), wher
Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5, and Claude Haiku 4.5 feature [context awareness](https://platform.claude.com/docs/en/build-with-claude/context-windows#contex
Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5 (deprecated), and Claude Haiku 4.5 feature [context awareness](https://platform.claude.com/docs/en/build-with-claude/context-w
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build-with-claude/prompt-caching
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* 1,024 tokens for Claude Opus 4.8, Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5, Claude Opus 4.1 ([retired, except on Bedrock and Google Cloud](https://platform.claude.co
* 1,024 tokens for Claude Opus 4.8, Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5 ([deprecated](https://platform.claude.com/docs/en/about-claude/model-deprecations)), Claud
# On earlier Opus/Sonnet and all Haiku models, non-tool-result user block causes prior thinking blocks to be stripped; on Opus 4.5+/Sonnet 4.6+ they are kept
# Depending on the model, this non-tool-result user block either keeps prior thinking blocks or strips them (see the next paragraph)
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Preserved thinking is a property of newer Claude models that guards against distillation. It decides whether the model can use a thinking block that you send back from an earlier t
Preserved thinking is a property of newer Claude models that guards against distillation. It determines whether the model can use a thinking block that you send back from an earlie
To use several betas in one request, combine the values in one `anthropic-beta` header. Beta names are the same on Amazon Bedrock and Google Cloud wherever the beta is available th
To use several betas in one request on the Claude API, combine their names in one `anthropic-beta` header:
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| [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking)
| [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking)
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| Language | Claude Sonnet 4.51 | Claude Haiku 4.51 |
| --------------------------------- | ------------------ | ----------------- |
| English (baseline, fixed to 100%) | 100% | 100% |
| Spanish | 98.2% | 96.4% |
| Portuguese (Brazil) | 97.8% | 96.1% |
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A `role: "system"` message can also carry `output_config.effort` to change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level from the next `user` tur
A `role: "system"` message can also carry `output_config.effort` to change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level partway through a conver
The first is the error returned without the beta header. On Amazon Bedrock and Google Cloud, pass the beta value as described in [Beta headers](https://platform.claude.com/docs/en/
The first is the error returned without the beta header. On Google Cloud and [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedro
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If you encounter `refusal` stop reasons frequently while using Claude Sonnet 4.5 or Claude Opus 4.1 (the latter [retired, except on Bedrock and Google Cloud](https://platform.cla
If you encounter `refusal` stop reasons frequently while using Claude Sonnet 4.5 ([deprecated](https://platform.claude.com/docs/en/about-claude/model-deprecations)) or Claude Opu
This stop reason is currently typed only in the SDK's `beta` (csharp, go: `Beta`; java: `beta()`) namespace, so the following examples call `client.beta.messages` (csharp, go: `c
This stop reason is currently typed only in the SDK's `beta` (csharp, go: `Beta`; java: `beta()`) namespace, so the following examples call `client.beta.messages` (csharp, go: `c
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Interleaved thinking lets Claude think between tool calls within a single assistant turn, reasoning about each tool result before deciding what to do next. For the concept, the tur
On Claude Opus 4.5, Claude Sonnet 4.5, and earlier Claude 4 models, add the `interleaved-thinking-2025-05-14` [beta header](https://platform.claude.com/docs/en/api/beta-headers) to
Interleaved thinking lets Claude think between tool calls within a single assistant turn, reasoning about each tool result before determining what to do next. For the concept, the
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Claude Opus 5 also supports [changing effort mid-conversation](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) with a per-message
Claude Opus 5 also supports [changing effort mid-conversation](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) with a per-message
Per-message effort is in beta and requires the [beta header](https://platform.claude.com/docs/en/api/beta-headers) `mid-conversation-output-config-2026-07-01`. Models without per-m
Per-message effort is in beta. On the Claude API and [Google Cloud](https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai), it's available on Claude Fable 5.1,
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Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude S
Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude S
Most current Claude models support [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking), which lets Claude think between to
Most current Claude models support [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking), which lets Claude think between to
Claude Sonnet 5, Claude Sonnet 4.6, Claude Sonnet 4.5, and Claude Haiku 4.5 have **context awareness:** these models track their remaining context window (their "token budget") thr
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| Model class | Keep all prior thinking | Keep only the last turn's thinking |
| ---------------- | --------------------------- | ----------------------------------- |
| Opus | Claude Opus 4.5 and later | Claude Opus 4.1 and earlier |
| Sonnet | Claude Sonnet 4.6 and later | Claude Sonnet 4.5 and earlier |
| Haiku | (none) | All models through Claude Haiku 4.5 |
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| **Beta features** | Pass through with `anthropic-beta` headers (see [feature limitations](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws#
| **Beta features** | Pass through with `anthropic-beta` headers (see [feature limitations](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws#
The `inference_geo` parameter is supported on Claude 4.6 and later models. Requests with `inference_geo` on Claude Opus 4.5, Claude Sonnet 4.5, or Claude Haiku 4.5 return a 400 e
The `inference_geo` parameter is supported on Claude 4.6 and later models. Requests with `inference_geo` on Claude Opus 4.5, Claude Sonnet 4.5 (deprecated), or Claude Haiku 4.5 r
* Beta features through the `anthropic-beta` header (see [feature limitations](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws#features-not-supported))
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build-with-claude/claude-on-vertex-ai
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Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Sonnet 4.6 have
Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Sonnet 4.6 have
This applies to Claude Sonnet 4.5 and future models only. Older models (Claude Sonnet 4 (deprecated), Opus 4 (deprecated), and earlier) maintain their existing pricing structures
This applies to Claude Sonnet 4.5 (deprecated) and future models only. Older models (Claude Sonnet 4 (deprecated), Opus 4 (deprecated), and earlier) maintain their existing prici
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[Mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) are available through the InvokeModel API for Claude Fabl
[Mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) are available through the InvokeModel API for Claude Fabl
A `role: "system"` message can also set `output_config.effort` to [change effort mid-conversation](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-co
Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Sonnet 4.6 have
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build-with-claude/claude-in-microsoft-foundry
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Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Sonnet 4.6 have
Claude Fable 5.1, Claude Fable 5, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Sonnet 4.6 have
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*3 Sonnet 4.x rate limit is a total limit that applies to combined traffic across Sonnet 4.6 and Sonnet 4.5. Claude Sonnet 5.5 and Claude Sonnet 5 each have a separate rate limit a
*3 Sonnet 4.x rate limit is a total limit that applies to combined traffic across Sonnet 4.6 and Sonnet 4.5 ([deprecated](https://platform.claude.com/docs/en/about-claude/model-dep
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### Beta features on other platforms
Beta names are the same on every platform that accepts them, but not every platform accepts every beta. [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-clau
On Amazon Bedrock, where beta names go depends on the API you call:
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These organization-level settings in the Claude Console apply to Messages API requests only. [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview) se
These organization-level settings in the Claude Console apply to Messages API requests only. [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview) se
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| Model | Tool versions |
| ---------------------------------------------- | ------------------------------------------------------------------- |
| Claude Fable 5.1 (claude-fable-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
| Claude Mythos 5.1 (claude-mythos-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
| Claude Fable 5 (claude-fable-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
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Organization-level web search and web fetch settings in the Claude Console apply to Messages API requests only; they do not apply to Managed Agents sessions, which use only the per
Organization-level web search and web fetch settings in the Claude Console apply to Messages API requests only. They do not apply to Managed Agents sessions, which use the per-tool
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* Any important caveats or limitations, such as what information the tool does not return if the tool name is unclear. The more context you can give Claude about your tools, the
* Any important caveats or limitations, such as what information the tool does not return if the tool name is unclear. The more context you can give Claude about your tools, the
* `auto` allows Claude to decide whether to call any provided tools or not. This is the default value when `tools` are provided.
* `auto` allows Claude to determine whether to call any provided tools or not. This is the default value when `tools` are provided.
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Claude typically finishes a batch with `screenshot` so it can observe the outcome before deciding what to do next. When a batch doesn't end with one, your application can attach a
Claude typically finishes a batch with `screenshot` so it can observe the outcome before determining what to do next. When a batch doesn't end with one, your application can attach
| Tool version | Beta header | Use with
| ------------------- | ------------------------- | --------------------------------------------------------------------------------------------------------------------------------
| `computer_20251124` | `computer-use-2025-11-24` | Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5, Claude Sonnet 5, Claude Opus 4.8, Claude Op
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For the full list of model-specific changes the skill applies, see [Migrating to Claude Opus 5.5 from Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5-5/migration-g
For the full list of model-specific changes the skill applies, see [Migrating to Claude Opus 5.5 from Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5-5/migration-g
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**Scope:** This pricing structure applies to Claude Sonnet 4.5, Haiku 4.5, Opus 4.5, and all future models. Earlier models (Claude Opus 4.1 and prior releases) retain their exist
**Scope:** This pricing structure applies to Claude Sonnet 4.5 (deprecated), Haiku 4.5, Opus 4.5, and all future models. Earlier models (Claude Opus 4.1 and prior releases) retai
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These levers set where a single model sits between cost and intelligence: model choice, effort, re-running failures at a higher setting, the budgets and caps it works within, and w
These levers set where a single model sits between cost and intelligence: model choice, effort, re-running failures at a higher setting, the budgets and caps it works within, and w
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* In Python SDK 1.10.0, TypeScript SDK 0.130.0, C# SDK 12.52.0, Go SDK 1.77.0, Java SDK 2.67.0, PHP SDK 0.53.0, Ruby SDK 1.75.0, and version 1.37.0 of the `ant` CLI, the [Admin API
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description: "Claude Mythos 5 reference: the same model as Claude Fable 5, offered by invitation only through Project Glasswing for defensive cybersecurity work. Model IDs, specifi
description: "Claude Mythos 5 reference: the same model as Claude Fable 5, offered for defensive cybersecurity work to organizations verified through Anthropic's verification progr
**Invite only.** Released June 9, 2026.
**Verification required.** Released June 9, 2026.
Claude Mythos 5 is offered separately, by invitation only, as part of Project Glasswing. It shares Claude Fable 5’s specifications and pricing. For access, contact your Anthropic,
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description: "Claude Mythos 5.1 at a glance: the same model as Claude Fable 5.1, offered by invitation only through Project Glasswing. Model IDs, specifications, pricing, and how t
description: "Claude Mythos 5.1 at a glance: the same model as Claude Fable 5.1, available only to organizations verified through Anthropic's verification programs. Model IDs, spec
**Invite only.** Released September 1, 2026.
**Verification required.** Released September 1, 2026.
Claude Fable 5.1 for Project Glasswing participants
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[Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview) is offered separately, by invitation only, for defensive cybersecurity workflows as part of [Project
[Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview) is offered separately for defensive cybersecurity workflows and is available only to organizations v
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[Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) is built for demanding reasoning and long-horizon agentic work.
[Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) is built for demanding reasoning and long-horizon agentic work.
* **Availability:** Claude Fable 5 does not require access approval. Claude Mythos 5 is available only to approved customers in [Project Glasswing](https://anthropic.com/glasswing)
* **Availability:** Claude Fable 5 does not require access approval. Claude Mythos 5 is available only to organizations verified through Anthropic's verification programs, such as
[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.
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models/fable-5/introducing-claude-fable-5-and-claude-mythos-5
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Claude Fable 5 is built for demanding reasoning and long-horizon agentic work. Claude Mythos 5 shares the same capabilities and is available only in limited release through [Projec
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
| Model | API model ID | Description |
| --------------- | ----------------- | -------------------------------------------------------------------------------------------------------------- |
| Claude Fable 5 | `claude-fable-5` | Built for demanding reasoning and long-horizon agentic work |
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Claude 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 r
Claude 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 r
| Model | Claude API ID | Description | Availability
| ----------------- | ----------------- | ------------------------------------------------------------------------------------------ | ---------------------------------------------
| Claude Fable 5.1 | claude-fable-5-1 | Successor to Claude Fable 5, for long-running agentic coding, knowledge work, and research | All customers, on the Claude API and partner
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models/fable-5-1/overview
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models/fable-5-1/overview
Claude 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 r
Claude 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 r
[Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) offers the same capabilities by invitation only, as part of [Project Glasswing](https://anthropi
[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 pro
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models/fable-5-1/migration-guide
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[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 quar
[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 quar
* **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/glasswi
* **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
# Or, for the Project Glasswing model with the same capabilities:
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* **Default Workspace** has a `wrkspc_` ID like any other workspace (returned in the [`anthropic-workspace-id` response header](https://platform.claude.com/docs/en/manage-claude/wo
* **Default Workspace** has a `wrkspc_` ID like any other workspace (returned in the [`anthropic-workspace-id` response header](https://platform.claude.com/docs/en/manage-claude/wo
The following SDK and CLI examples construct the default client, which reads the Admin API key from the `ANTHROPIC_API_KEY` environment variable; the SDKs expose these endpoints un
The following SDK and CLI examples construct the default client, which reads the Admin API key from the `ANTHROPIC_API_KEY` environment variable; the SDKs expose these endpoints un
ant beta:organization:workspaces create --name Production
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In the SDKs these endpoints are `client.beta.organization.service_accounts`, `client.beta.organization.federation.issuers`, and `client.beta.organization.federation.rules` (`ant be
In the SDKs these endpoints are `client.organization.service_accounts` (typescript: `client.organization.serviceAccounts`; csharp, go: `client.Organization.ServiceAccounts`; java:
ant beta:organization:service-accounts list
ant organization:service-accounts list
service_accounts = client.beta.organization.service_accounts.list()
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The SDK and CLI examples on this page construct the default client, which reads the Admin API key from the `ANTHROPIC_API_KEY` environment variable. The SDKs expose these endpoints
The SDK and CLI examples on this page construct the default client, which reads the Admin API key from the `ANTHROPIC_API_KEY` environment variable. The SDKs expose these endpoints
ant beta:organization:rate-limits list
ant organization:rate-limits list
rate_limits = client.beta.organization.rate_limits.list()
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| Field | Present | Description |
| ----------------- | -------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
| `type` | Always | `"third_party"` |
| `toolset_name` | Optional | The name the request gives the server. |
| `origin` | Optional | The scheme, host, and non-default port of the server's URL. An absent `origin` means unknown, not safe. |
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api
manage-claude/inference-hooks-configuration
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manage-claude/inference-hooks-configuration
| Private or internal IP | The host resolves to a private or internal address. Use a publicly routable host.
| Private or internal IP | The URL's host is a private or internal IP address, an IPv6 address, or `localhost`. Use a publicly routable host.
| Transport error | DNS resolution, the TLS handshake, or the connection failed.
| Transport error | DNS resolution, the TLS handshake, or the connection failed, or the hostname resolves to a private address.
* **Recent errors:** each entry is reduced to a timestamp, an error type, and a one-line reason. Entries never include request content or your endpoint URL.
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api
manage-claude/inference-hooks
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manage-claude/inference-hooks
Governed requests are the inference requests behind the user's conversation. Ancillary requests aren't sent to your endpoint. These include conversation title generation and a meas
Governed requests are the inference requests behind the user's conversation. Ancillary requests aren't sent to your endpoint. These include conversation title generation and a meas
With **Validate tool calls** on, `tool_call` events may leave out calls to some of claude.ai's own tools, such as tools that suggest connectors or skills, give Claude the user's lo
With **Validate tool calls** on, `tool_call` events may leave out calls to some of claude.ai's own tools, such as tools that list, search for, or suggest connectors, plugins, or sk
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api
manage-claude/cmek-google-cloud-kms
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manage-claude/cmek-google-cloud-kms
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
ant beta:organization:external-keys create <<'YAML'
ant organization:external-keys create <<'YAML'
external_key = client.beta.organization.external_keys.create(
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api
manage-claude/cmek-azure-key-vault
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manage-claude/cmek-azure-key-vault
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
ant beta:organization:external-keys create <<'YAML'
ant organization:external-keys create <<'YAML'
external_key = client.beta.organization.external_keys.create(
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api
manage-claude/cmek-aws-kms
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manage-claude/cmek-aws-kms
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
**Finding your organization ID:** Copy the **Organization ID** field under **Settings > Organization** in the Claude Console, or under **Organization settings > Organization*
**Finding your compartment ID:** Each workspace has a compartment ID that scopes its CMEK data. To find it in the Claude Console, go to [Manage > Security](https://platform.c
**Finding your compartment ID:** Each workspace has a compartment ID that scopes its CMEK data. To find it in the Claude Console, go to [Manage > Security](https://platform.c
ant beta:organization:external-keys create <<'YAML'
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api
manage-claude/admin-api
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manage-claude/admin-api
The [Admin API](https://platform.claude.com/docs/en/api/beta/organization) lets you manage your organization's members, workspaces, invites, and API keys programmatically instead o
The [Admin API](https://platform.claude.com/docs/en/api/organization) lets you manage your organization's members, workspaces, invites, and API keys programmatically instead of by
The Python, TypeScript, C#, Go, Java, PHP, and Ruby SDKs expose the Admin API under `client.beta.organization`, and the `ant` CLI under `ant beta:organization`. The examples on thi
The Python, TypeScript, C#, Go, Java, PHP, and Ruby SDKs expose the endpoints on this page under `client.organization` (csharp, go: `client.Organization`; java: `client.organizatio
ant beta:organization retrieve
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api
build-with-claude/working-with-messages
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build-with-claude/working-with-messages
On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Opus 5.5, Claude Opus 4.8, Cla
On Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/mode
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build-with-claude/prompt-caching
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build-with-claude/prompt-caching
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | $10 / MTok | $12.50 / MTok | $20
| Claude Mythos 5.1 ([limited availability](https://support.claude.com/en/articles/14604842)) | $10 / MTok | $12.50 / MTok | $20
| Claude Mythos 5 ([limited availability](https://anthropic.com/glasswing)) | $10 / MTok | $12.50 / MTok | $20
| Claude Mythos 5 ([limited availability](https://support.claude.com/en/articles/14604842)) | $10 / MTok | $12.50 / MTok | $20
* 512 tokens for Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, Claude Fable 5, and [Claude Mythos 5](https://anthropic.com/glasswing)
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api
build-with-claude/mid-conversation-system-messages
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build-with-claude/mid-conversation-system-messages
This feature is available on Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Op
This feature is available on Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://plat
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api
build-with-claude/claude-on-vertex-ai
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build-with-claude/claude-on-vertex-ai
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | `claude-mythos-5-1` |
| Claude Mythos 5.1 ([limited availability](https://support.claude.com/en/articles/14604842)) | `claude-mythos-5-1` |
| Claude Mythos 5 ([limited availability](https://anthropic.com/glasswing)) | `claude-mythos-5` |
| Claude Mythos 5 ([limited availability](https://support.claude.com/en/articles/14604842)) | `claude-mythos-5` |
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build-with-claude/claude-in-microsoft-foundry
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build-with-claude/claude-in-microsoft-foundry
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | `claude-mythos-5-1` | | ✓ |
| Claude Mythos 5.1 ([limited availability](https://support.claude.com/en/articles/14604842)) | `claude-mythos-5-1` | | ✓ |
| Claude Mythos 5 ([limited availability](https://anthropic.com/glasswing)) | `claude-mythos-5` | | ✓ |
| Claude Mythos 5 ([limited availability](https://support.claude.com/en/articles/14604842)) | `claude-mythos-5` | | ✓ |
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api
build-with-claude/claude-in-amazon-bedrock
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build-with-claude/claude-in-amazon-bedrock
| Model | Model ID | Access
| :------------------------------------------------------------------------------ | :-------------------------------- | :-----------------------------------------------------------
| Claude Fable 5.1 | `anthropic.claude-fable-5-1` | Open
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | `anthropic.claude-mythos-5-1` | Invitation only
| Claude Fable 5 | `anthropic.claude-fable-5` | Open
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build-with-claude/batch-processing
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build-with-claude/batch-processing
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | $5 / MTok | $25 / MTok |
| Claude Mythos 5.1 ([limited availability](https://support.claude.com/en/articles/14604842)) | $5 / MTok | $25 / MTok |
| Claude Mythos 5 ([limited availability](https://anthropic.com/glasswing)) | $5 / MTok | $25 / MTok |
| Claude Mythos 5 ([limited availability](https://support.claude.com/en/articles/14604842)) | $5 / MTok | $25 / MTok |
* **Limited access:** Offered separately, by invitation only, as part of [Project Glasswing](https://anthropic.com/glasswing). For access, contact your Anthropic, AWS, or Google Cl
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api
api/errors
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api/errors
On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Opus 5.5, and [Claude Mythos P
On Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/mode
Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://anthropic.com/glasswing) don't support forced tool use. Sending `tool_choice: {"type": "any"}`
Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) don't support forced tool use. Sending
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agents-and-tools/tool-use/define-tools
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agents-and-tools/tool-use/define-tools
| Model or setting | Restriction
| ----------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------
| Manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) (`thinking: {type: "enabled"}`) | `any` and `tool` are not supported and resul
| Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://anthropic.com/glasswing) | `any` and `tool` return a [400 error](https:
| Model or setting | Restriction
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about-claude/pricing
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about-claude/pricing
| Claude Mythos 5.1 ([limited availability](https://anthropic.com/glasswing)) | $10 / MTok | $12.50 / MTok |
| Claude Mythos 5.1 ([limited availability](https://support.claude.com/en/articles/14604842)) | $10 / MTok | $12.50 / MTok |
| Claude Mythos 5 ([limited availability](https://anthropic.com/glasswing)) | $10 / MTok | $12.50 / MTok |
| Claude Mythos 5 ([limited availability](https://support.claude.com/en/articles/14604842)) | $10 / MTok | $12.50 / MTok |
* **Limited access:** Offered separately, by invitation only, as part of [Project Glasswing](https://anthropic.com/glasswing). For access, contact your Anthropic, AWS, or Google Cl
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about-claude/models/choosing-a-model
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about-claude/models/choosing-a-model
**Claude Fable 5.1** (`claude-fable-5-1`) is Anthropic's most capable model open to all customers. It extends Claude Fable 5 with stronger long-running agentic coding, knowledge wo
**Claude Fable 5.1** (`claude-fable-5-1`) is Anthropic's most capable model open to all customers. It extends Claude Fable 5 with stronger long-running agentic coding, knowledge wo
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