## Create a Message

**post** `/v1/messages`

Send a structured list of input messages with text and/or image content, and the model will generate the next message in the conversation.

The Messages API can be used for either single queries or stateless multi-turn conversations.

Learn more about the Messages API in our [user guide](https://docs.claude.com/en/docs/initial-setup)

### Header Parameters

- `"anthropic-beta": optional array of AnthropicBeta`

  Optional header to specify the beta version(s) you want to use.

  - `string`

  - `"message-batches-2024-09-24" or "prompt-caching-2024-07-31" or "computer-use-2024-10-22" or 25 more`

    - `"message-batches-2024-09-24"`

    - `"prompt-caching-2024-07-31"`

    - `"computer-use-2024-10-22"`

    - `"computer-use-2025-01-24"`

    - `"pdfs-2024-09-25"`

    - `"token-counting-2024-11-01"`

    - `"token-efficient-tools-2025-02-19"`

    - `"output-128k-2025-02-19"`

    - `"files-api-2025-04-14"`

    - `"mcp-client-2025-04-04"`

    - `"mcp-client-2025-11-20"`

    - `"dev-full-thinking-2025-05-14"`

    - `"interleaved-thinking-2025-05-14"`

    - `"code-execution-2025-05-22"`

    - `"extended-cache-ttl-2025-04-11"`

    - `"context-1m-2025-08-07"`

    - `"context-management-2025-06-27"`

    - `"model-context-window-exceeded-2025-08-26"`

    - `"skills-2025-10-02"`

    - `"fast-mode-2026-02-01"`

    - `"output-300k-2026-03-24"`

    - `"user-profiles-2026-03-24"`

    - `"advisor-tool-2026-03-01"`

    - `"managed-agents-2026-04-01"`

    - `"cache-diagnosis-2026-04-07"`

    - `"thinking-token-count-2026-05-13"`

    - `"server-side-fallback-2026-06-01"`

    - `"fallback-credit-2026-06-01"`

### Body Parameters

- `max_tokens: number`

  The maximum number of tokens to generate before stopping.

  Note that our models may stop _before_ reaching this maximum. This parameter only specifies the absolute maximum number of tokens to generate.

  Set to `0` to populate the [prompt cache](https://docs.claude.com/en/docs/build-with-claude/prompt-caching#pre-warming-the-cache) without generating a response.

  Different models have different maximum values for this parameter.  See [models](https://docs.claude.com/en/docs/models-overview) for details.

- `messages: array of BetaMessageParam`

  Input messages.

  Our models are trained to operate on alternating `user` and `assistant` conversational turns. When creating a new `Message`, you specify the prior conversational turns with the `messages` parameter, and the model then generates the next `Message` in the conversation. Consecutive `user` or `assistant` turns in your request will be combined into a single turn.

  Each input message must be an object with a `role` and `content`. You can specify a single `user`-role message, or you can include multiple `user` and `assistant` messages.

  If the final message uses the `assistant` role, the response content will continue immediately from the content in that message. This can be used to constrain part of the model's response.

  Example with a single `user` message:

  ```json
  [{"role": "user", "content": "Hello, Claude"}]
  ```

  Example with multiple conversational turns:

  ```json
  [
    {"role": "user", "content": "Hello there."},
    {"role": "assistant", "content": "Hi, I'm Claude. How can I help you?"},
    {"role": "user", "content": "Can you explain LLMs in plain English?"},
  ]
  ```

  Example with a partially-filled response from Claude:

  ```json
  [
    {"role": "user", "content": "What's the Greek name for Sun? (A) Sol (B) Helios (C) Sun"},
    {"role": "assistant", "content": "The best answer is ("},
  ]
  ```

  Each input message `content` may be either a single `string` or an array of content blocks, where each block has a specific `type`. Using a `string` for `content` is shorthand for an array of one content block of type `"text"`. The following input messages are equivalent:

  ```json
  {"role": "user", "content": "Hello, Claude"}
  ```

  ```json
  {"role": "user", "content": [{"type": "text", "text": "Hello, Claude"}]}
  ```

  See [input examples](https://docs.claude.com/en/api/messages-examples).

  Note that if you want to include a [system prompt](https://docs.claude.com/en/docs/system-prompts), you can use the top-level `system` parameter — there is no `"system"` role for input messages in the Messages API.

  There is a limit of 100,000 messages in a single request.

  - `content: string or array of BetaContentBlockParam`

    - `string`

    - `array of BetaContentBlockParam`

      - `BetaTextBlockParam object { text, type, cache_control, citations }`

        - `text: string`

        - `type: "text"`

          - `"text"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

          - `type: "ephemeral"`

            - `"ephemeral"`

          - `ttl: optional "5m" or "1h"`

            The time-to-live for the cache control breakpoint.

            This may be one the following values:

            - `5m`: 5 minutes
            - `1h`: 1 hour

            Defaults to `5m`.

            - `"5m"`

            - `"1h"`

        - `citations: optional array of BetaTextCitationParam`

          - `BetaCitationCharLocationParam object { cited_text, document_index, document_title, 3 more }`

            - `cited_text: string`

            - `document_index: number`

            - `document_title: string`

            - `end_char_index: number`

            - `start_char_index: number`

            - `type: "char_location"`

              - `"char_location"`

          - `BetaCitationPageLocationParam object { cited_text, document_index, document_title, 3 more }`

            - `cited_text: string`

            - `document_index: number`

            - `document_title: string`

            - `end_page_number: number`

            - `start_page_number: number`

            - `type: "page_location"`

              - `"page_location"`

          - `BetaCitationContentBlockLocationParam object { cited_text, document_index, document_title, 3 more }`

            - `cited_text: string`

              The full text of the cited block range, concatenated.

              Always equals the contents of `content[start_block_index:end_block_index]` joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

            - `document_index: number`

            - `document_title: string`

            - `end_block_index: number`

              Exclusive 0-based end index of the cited block range in the source's `content` array.

              Always greater than `start_block_index`; a single-block citation has `end_block_index = start_block_index + 1`.

            - `start_block_index: number`

              0-based index of the first cited block in the source's `content` array.

            - `type: "content_block_location"`

              - `"content_block_location"`

          - `BetaCitationWebSearchResultLocationParam object { cited_text, encrypted_index, title, 2 more }`

            - `cited_text: string`

            - `encrypted_index: string`

            - `title: string`

            - `type: "web_search_result_location"`

              - `"web_search_result_location"`

            - `url: string`

          - `BetaCitationSearchResultLocationParam object { cited_text, end_block_index, search_result_index, 4 more }`

            - `cited_text: string`

              The full text of the cited block range, concatenated.

              Always equals the contents of `content[start_block_index:end_block_index]` joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

            - `end_block_index: number`

              Exclusive 0-based end index of the cited block range in the source's `content` array.

              Always greater than `start_block_index`; a single-block citation has `end_block_index = start_block_index + 1`.

            - `search_result_index: number`

              0-based index of the cited search result among all `search_result` content blocks in the request, in the order they appear across messages and tool results.

              Counted separately from `document_index`; server-side web search results are not included in this count.

            - `source: string`

            - `start_block_index: number`

              0-based index of the first cited block in the source's `content` array.

            - `title: string`

            - `type: "search_result_location"`

              - `"search_result_location"`

      - `BetaImageBlockParam object { source, type, cache_control }`

        - `source: BetaBase64ImageSource or BetaURLImageSource or BetaFileImageSource`

          - `BetaBase64ImageSource object { data, media_type, type }`

            - `data: string`

            - `media_type: "image/jpeg" or "image/png" or "image/gif" or "image/webp"`

              - `"image/jpeg"`

              - `"image/png"`

              - `"image/gif"`

              - `"image/webp"`

            - `type: "base64"`

              - `"base64"`

          - `BetaURLImageSource object { type, url }`

            - `type: "url"`

              - `"url"`

            - `url: string`

          - `BetaFileImageSource object { file_id, type }`

            - `file_id: string`

            - `type: "file"`

              - `"file"`

        - `type: "image"`

          - `"image"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaRequestDocumentBlock object { source, type, cache_control, 3 more }`

        - `source: BetaBase64PDFSource or BetaPlainTextSource or BetaContentBlockSource or 2 more`

          - `BetaBase64PDFSource object { data, media_type, type }`

            - `data: string`

            - `media_type: "application/pdf"`

              - `"application/pdf"`

            - `type: "base64"`

              - `"base64"`

          - `BetaPlainTextSource object { data, media_type, type }`

            - `data: string`

            - `media_type: "text/plain"`

              - `"text/plain"`

            - `type: "text"`

              - `"text"`

          - `BetaContentBlockSource object { content, type }`

            - `content: string or array of BetaContentBlockSourceContent`

              - `string`

              - `BetaContentBlockSourceContent = array of BetaContentBlockSourceContent`

                - `BetaTextBlockParam object { text, type, cache_control, citations }`

                - `BetaImageBlockParam object { source, type, cache_control }`

            - `type: "content"`

              - `"content"`

          - `BetaURLPDFSource object { type, url }`

            - `type: "url"`

              - `"url"`

            - `url: string`

          - `BetaFileDocumentSource object { file_id, type }`

            - `file_id: string`

            - `type: "file"`

              - `"file"`

        - `type: "document"`

          - `"document"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `citations: optional BetaCitationsConfigParam`

          - `enabled: optional boolean`

        - `context: optional string`

        - `title: optional string`

      - `BetaSearchResultBlockParam object { content, source, title, 3 more }`

        - `content: array of BetaTextBlockParam`

          - `text: string`

          - `type: "text"`

          - `cache_control: optional BetaCacheControlEphemeral`

            Create a cache control breakpoint at this content block.

          - `citations: optional array of BetaTextCitationParam`

        - `source: string`

        - `title: string`

        - `type: "search_result"`

          - `"search_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `citations: optional BetaCitationsConfigParam`

      - `BetaThinkingBlockParam object { signature, thinking, type }`

        - `signature: string`

        - `thinking: string`

        - `type: "thinking"`

          - `"thinking"`

      - `BetaRedactedThinkingBlockParam object { data, type }`

        - `data: string`

        - `type: "redacted_thinking"`

          - `"redacted_thinking"`

      - `BetaToolUseBlockParam object { id, input, name, 3 more }`

        - `id: string`

        - `input: map[unknown]`

        - `name: string`

        - `type: "tool_use"`

          - `"tool_use"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

          Tool invocation directly from the model.

          - `BetaDirectCaller object { type }`

            Tool invocation directly from the model.

            - `type: "direct"`

              - `"direct"`

          - `BetaServerToolCaller object { tool_id, type }`

            Tool invocation generated by a server-side tool.

            - `tool_id: string`

            - `type: "code_execution_20250825"`

              - `"code_execution_20250825"`

          - `BetaServerToolCaller20260120 object { tool_id, type }`

            - `tool_id: string`

            - `type: "code_execution_20260120"`

              - `"code_execution_20260120"`

      - `BetaToolResultBlockParam object { tool_use_id, type, cache_control, 2 more }`

        - `tool_use_id: string`

        - `type: "tool_result"`

          - `"tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `content: optional string or array of BetaTextBlockParam or BetaImageBlockParam or BetaSearchResultBlockParam or 2 more`

          - `string`

          - `array of BetaTextBlockParam or BetaImageBlockParam or BetaSearchResultBlockParam or 2 more`

            - `BetaTextBlockParam object { text, type, cache_control, citations }`

            - `BetaImageBlockParam object { source, type, cache_control }`

            - `BetaSearchResultBlockParam object { content, source, title, 3 more }`

            - `BetaRequestDocumentBlock object { source, type, cache_control, 3 more }`

            - `BetaToolReferenceBlockParam object { tool_name, type, cache_control }`

              Tool reference block that can be included in tool_result content.

              - `tool_name: string`

              - `type: "tool_reference"`

                - `"tool_reference"`

              - `cache_control: optional BetaCacheControlEphemeral`

                Create a cache control breakpoint at this content block.

        - `is_error: optional boolean`

      - `BetaServerToolUseBlockParam object { id, input, name, 3 more }`

        - `id: string`

        - `input: map[unknown]`

        - `name: "advisor" or "web_search" or "web_fetch" or 5 more`

          - `"advisor"`

          - `"web_search"`

          - `"web_fetch"`

          - `"code_execution"`

          - `"bash_code_execution"`

          - `"text_editor_code_execution"`

          - `"tool_search_tool_regex"`

          - `"tool_search_tool_bm25"`

        - `type: "server_tool_use"`

          - `"server_tool_use"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

          Tool invocation directly from the model.

          - `BetaDirectCaller object { type }`

            Tool invocation directly from the model.

          - `BetaServerToolCaller object { tool_id, type }`

            Tool invocation generated by a server-side tool.

          - `BetaServerToolCaller20260120 object { tool_id, type }`

      - `BetaWebSearchToolResultBlockParam object { content, tool_use_id, type, 2 more }`

        - `content: BetaWebSearchToolResultBlockParamContent`

          - `ResultBlock = array of BetaWebSearchResultBlockParam`

            - `encrypted_content: string`

            - `title: string`

            - `type: "web_search_result"`

              - `"web_search_result"`

            - `url: string`

            - `page_age: optional string`

          - `BetaWebSearchToolRequestError object { error_code, type }`

            - `error_code: BetaWebSearchToolResultErrorCode`

              - `"invalid_tool_input"`

              - `"unavailable"`

              - `"max_uses_exceeded"`

              - `"too_many_requests"`

              - `"query_too_long"`

              - `"request_too_large"`

            - `type: "web_search_tool_result_error"`

              - `"web_search_tool_result_error"`

        - `tool_use_id: string`

        - `type: "web_search_tool_result"`

          - `"web_search_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

          Tool invocation directly from the model.

          - `BetaDirectCaller object { type }`

            Tool invocation directly from the model.

          - `BetaServerToolCaller object { tool_id, type }`

            Tool invocation generated by a server-side tool.

          - `BetaServerToolCaller20260120 object { tool_id, type }`

      - `BetaWebFetchToolResultBlockParam object { content, tool_use_id, type, 2 more }`

        - `content: BetaWebFetchToolResultErrorBlockParam or BetaWebFetchBlockParam`

          - `BetaWebFetchToolResultErrorBlockParam object { error_code, type }`

            - `error_code: BetaWebFetchToolResultErrorCode`

              - `"invalid_tool_input"`

              - `"url_too_long"`

              - `"url_not_allowed"`

              - `"url_not_in_prior_context"`

              - `"url_not_accessible"`

              - `"unsupported_content_type"`

              - `"too_many_requests"`

              - `"max_uses_exceeded"`

              - `"unavailable"`

            - `type: "web_fetch_tool_result_error"`

              - `"web_fetch_tool_result_error"`

          - `BetaWebFetchBlockParam object { content, type, url, retrieved_at }`

            - `content: BetaRequestDocumentBlock`

            - `type: "web_fetch_result"`

              - `"web_fetch_result"`

            - `url: string`

              Fetched content URL

            - `retrieved_at: optional string`

              ISO 8601 timestamp when the content was retrieved

        - `tool_use_id: string`

        - `type: "web_fetch_tool_result"`

          - `"web_fetch_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

          Tool invocation directly from the model.

          - `BetaDirectCaller object { type }`

            Tool invocation directly from the model.

          - `BetaServerToolCaller object { tool_id, type }`

            Tool invocation generated by a server-side tool.

          - `BetaServerToolCaller20260120 object { tool_id, type }`

      - `BetaAdvisorToolResultBlockParam object { content, tool_use_id, type, cache_control }`

        - `content: BetaAdvisorToolResultErrorParam or BetaAdvisorResultBlockParam or BetaAdvisorRedactedResultBlockParam`

          - `BetaAdvisorToolResultErrorParam object { error_code, type }`

            - `error_code: "max_uses_exceeded" or "prompt_too_long" or "too_many_requests" or 4 more`

              - `"max_uses_exceeded"`

              - `"prompt_too_long"`

              - `"too_many_requests"`

              - `"overloaded"`

              - `"unavailable"`

              - `"execution_time_exceeded"`

              - `"model_not_found"`

            - `type: "advisor_tool_result_error"`

              - `"advisor_tool_result_error"`

          - `BetaAdvisorResultBlockParam object { text, type, stop_reason }`

            - `text: string`

            - `type: "advisor_result"`

              - `"advisor_result"`

            - `stop_reason: optional string`

          - `BetaAdvisorRedactedResultBlockParam object { encrypted_content, type, stop_reason }`

            - `encrypted_content: string`

              Opaque blob produced by a prior response; must be round-tripped verbatim.

            - `type: "advisor_redacted_result"`

              - `"advisor_redacted_result"`

            - `stop_reason: optional string`

        - `tool_use_id: string`

        - `type: "advisor_tool_result"`

          - `"advisor_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaCodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }`

        - `content: BetaCodeExecutionToolResultBlockParamContent`

          Code execution result with encrypted stdout for PFC + web_search results.

          - `BetaCodeExecutionToolResultErrorParam object { error_code, type }`

            - `error_code: BetaCodeExecutionToolResultErrorCode`

              - `"invalid_tool_input"`

              - `"unavailable"`

              - `"too_many_requests"`

              - `"execution_time_exceeded"`

            - `type: "code_execution_tool_result_error"`

              - `"code_execution_tool_result_error"`

          - `BetaCodeExecutionResultBlockParam object { content, return_code, stderr, 2 more }`

            - `content: array of BetaCodeExecutionOutputBlockParam`

              - `file_id: string`

              - `type: "code_execution_output"`

                - `"code_execution_output"`

            - `return_code: number`

            - `stderr: string`

            - `stdout: string`

            - `type: "code_execution_result"`

              - `"code_execution_result"`

          - `BetaEncryptedCodeExecutionResultBlockParam object { content, encrypted_stdout, return_code, 2 more }`

            Code execution result with encrypted stdout for PFC + web_search results.

            - `content: array of BetaCodeExecutionOutputBlockParam`

              - `file_id: string`

              - `type: "code_execution_output"`

            - `encrypted_stdout: string`

            - `return_code: number`

            - `stderr: string`

            - `type: "encrypted_code_execution_result"`

              - `"encrypted_code_execution_result"`

        - `tool_use_id: string`

        - `type: "code_execution_tool_result"`

          - `"code_execution_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaBashCodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }`

        - `content: BetaBashCodeExecutionToolResultErrorParam or BetaBashCodeExecutionResultBlockParam`

          - `BetaBashCodeExecutionToolResultErrorParam object { error_code, type }`

            - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or 2 more`

              - `"invalid_tool_input"`

              - `"unavailable"`

              - `"too_many_requests"`

              - `"execution_time_exceeded"`

              - `"output_file_too_large"`

            - `type: "bash_code_execution_tool_result_error"`

              - `"bash_code_execution_tool_result_error"`

          - `BetaBashCodeExecutionResultBlockParam object { content, return_code, stderr, 2 more }`

            - `content: array of BetaBashCodeExecutionOutputBlockParam`

              - `file_id: string`

              - `type: "bash_code_execution_output"`

                - `"bash_code_execution_output"`

            - `return_code: number`

            - `stderr: string`

            - `stdout: string`

            - `type: "bash_code_execution_result"`

              - `"bash_code_execution_result"`

        - `tool_use_id: string`

        - `type: "bash_code_execution_tool_result"`

          - `"bash_code_execution_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaTextEditorCodeExecutionToolResultBlockParam object { content, tool_use_id, type, cache_control }`

        - `content: BetaTextEditorCodeExecutionToolResultErrorParam or BetaTextEditorCodeExecutionViewResultBlockParam or BetaTextEditorCodeExecutionCreateResultBlockParam or BetaTextEditorCodeExecutionStrReplaceResultBlockParam`

          - `BetaTextEditorCodeExecutionToolResultErrorParam object { error_code, type, error_message }`

            - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or 2 more`

              - `"invalid_tool_input"`

              - `"unavailable"`

              - `"too_many_requests"`

              - `"execution_time_exceeded"`

              - `"file_not_found"`

            - `type: "text_editor_code_execution_tool_result_error"`

              - `"text_editor_code_execution_tool_result_error"`

            - `error_message: optional string`

          - `BetaTextEditorCodeExecutionViewResultBlockParam object { content, file_type, type, 3 more }`

            - `content: string`

            - `file_type: "text" or "image" or "pdf"`

              - `"text"`

              - `"image"`

              - `"pdf"`

            - `type: "text_editor_code_execution_view_result"`

              - `"text_editor_code_execution_view_result"`

            - `num_lines: optional number`

            - `start_line: optional number`

            - `total_lines: optional number`

          - `BetaTextEditorCodeExecutionCreateResultBlockParam object { is_file_update, type }`

            - `is_file_update: boolean`

            - `type: "text_editor_code_execution_create_result"`

              - `"text_editor_code_execution_create_result"`

          - `BetaTextEditorCodeExecutionStrReplaceResultBlockParam object { type, lines, new_lines, 3 more }`

            - `type: "text_editor_code_execution_str_replace_result"`

              - `"text_editor_code_execution_str_replace_result"`

            - `lines: optional array of string`

            - `new_lines: optional number`

            - `new_start: optional number`

            - `old_lines: optional number`

            - `old_start: optional number`

        - `tool_use_id: string`

        - `type: "text_editor_code_execution_tool_result"`

          - `"text_editor_code_execution_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaToolSearchToolResultBlockParam object { content, tool_use_id, type, cache_control }`

        - `content: BetaToolSearchToolResultErrorParam or BetaToolSearchToolSearchResultBlockParam`

          - `BetaToolSearchToolResultErrorParam object { error_code, type, error_message }`

            - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or "execution_time_exceeded"`

              - `"invalid_tool_input"`

              - `"unavailable"`

              - `"too_many_requests"`

              - `"execution_time_exceeded"`

            - `type: "tool_search_tool_result_error"`

              - `"tool_search_tool_result_error"`

            - `error_message: optional string`

          - `BetaToolSearchToolSearchResultBlockParam object { tool_references, type }`

            - `tool_references: array of BetaToolReferenceBlockParam`

              - `tool_name: string`

              - `type: "tool_reference"`

              - `cache_control: optional BetaCacheControlEphemeral`

                Create a cache control breakpoint at this content block.

            - `type: "tool_search_tool_search_result"`

              - `"tool_search_tool_search_result"`

        - `tool_use_id: string`

        - `type: "tool_search_tool_result"`

          - `"tool_search_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaMCPToolUseBlockParam object { id, input, name, 3 more }`

        - `id: string`

        - `input: map[unknown]`

        - `name: string`

        - `server_name: string`

          The name of the MCP server

        - `type: "mcp_tool_use"`

          - `"mcp_tool_use"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaRequestMCPToolResultBlockParam object { tool_use_id, type, cache_control, 2 more }`

        - `tool_use_id: string`

        - `type: "mcp_tool_result"`

          - `"mcp_tool_result"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `content: optional string or array of BetaTextBlockParam`

          - `string`

          - `BetaMCPToolResultBlockParamContent = array of BetaTextBlockParam`

            - `text: string`

            - `type: "text"`

            - `cache_control: optional BetaCacheControlEphemeral`

              Create a cache control breakpoint at this content block.

            - `citations: optional array of BetaTextCitationParam`

        - `is_error: optional boolean`

      - `BetaContainerUploadBlockParam object { file_id, type, cache_control }`

        A content block that represents a file to be uploaded to the container
        Files uploaded via this block will be available in the container's input directory.

        - `file_id: string`

        - `type: "container_upload"`

          - `"container_upload"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaCompactionBlockParam object { type, cache_control, content, encrypted_content }`

        A compaction block containing summary of previous context.

        Users should round-trip these blocks from responses to subsequent requests
        to maintain context across compaction boundaries.

        When content is None, the block represents a failed compaction. The server
        treats these as no-ops. Empty string content is not allowed.

        - `type: "compaction"`

          - `"compaction"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

        - `content: optional string`

          Summary of previously compacted content, or null if compaction failed

        - `encrypted_content: optional string`

          Opaque metadata from prior compaction, to be round-tripped verbatim

      - `BetaMidConversationSystemBlockParam object { content, type, cache_control }`

        System instructions that appear mid-conversation.

        Use this block to provide or update system-level instructions at a specific
        point in the conversation, rather than only via the top-level `system` parameter.

        - `content: array of BetaTextBlockParam`

          System instruction text blocks.

          - `text: string`

          - `type: "text"`

          - `cache_control: optional BetaCacheControlEphemeral`

            Create a cache control breakpoint at this content block.

          - `citations: optional array of BetaTextCitationParam`

        - `type: "mid_conv_system"`

          - `"mid_conv_system"`

        - `cache_control: optional BetaCacheControlEphemeral`

          Create a cache control breakpoint at this content block.

      - `BetaFallbackBlockParam object { from, to, type }`

        A `fallback` block echoed back from a prior response.

        Accepted in `messages[].content` and never rendered into the prompt,
        not validated against the request's `fallbacks` chain or top-level
        `model`, and stripped before the sticky-routing cache key is computed.

        Callers should echo the assistant turn verbatim — block included. The
        block's position is load-bearing for thinking verification: the thinking
        runs on either side of a fallback hop carry independently-rooted
        verification hash chains, and this block is the only record of where one
        chain ends and the next begins. When thinking runs flank the boundary,
        omitting the block merges the runs into one contiguous span whose hashes
        cannot verify (the request is rejected), and moving it into the middle of
        a single run splits that run's chain and is likewise rejected; between
        non-thinking blocks the block's placement has no verification effect.

        - `from: BetaFallbackInfoParam`

          Identifies one hop of a fallback transition.

          - `model: Model`

            The model that will complete your prompt.

            See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

            - `"claude-fable-5" or "claude-mythos-5" or "claude-opus-4-8" or 17 more`

              The model that will complete your prompt.

              See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

              - `"claude-fable-5"`

                Next generation of intelligence for the hardest knowledge work and coding problems

              - `"claude-mythos-5"`

                Most capable model for cybersecurity and biology research

              - `"claude-opus-4-8"`

                Frontier intelligence for long-running agents and coding

              - `"claude-opus-4-7"`

                Frontier intelligence for long-running agents and coding

              - `"claude-mythos-preview"`

                New class of intelligence, strongest in coding and cybersecurity

              - `"claude-opus-4-6"`

                Frontier intelligence for long-running agents and coding

              - `"claude-sonnet-4-6"`

                Best combination of speed and intelligence

              - `"claude-haiku-4-5"`

                Fastest model with near-frontier intelligence

              - `"claude-haiku-4-5-20251001"`

                Fastest model with near-frontier intelligence

              - `"claude-opus-4-5"`

                Premium model combining maximum intelligence with practical performance

              - `"claude-opus-4-5-20251101"`

                Premium model combining maximum intelligence with practical performance

              - `"claude-sonnet-4-5"`

                High-performance model for agents and coding

              - `"claude-sonnet-4-5-20250929"`

                High-performance model for agents and coding

              - `"claude-opus-4-1"`

                Exceptional model for specialized complex tasks

              - `"claude-opus-4-1-20250805"`

                Exceptional model for specialized complex tasks

              - `"claude-opus-4-0"`

                Powerful model for complex tasks

              - `"claude-opus-4-20250514"`

                Powerful model for complex tasks

              - `"claude-sonnet-4-0"`

                High-performance model with extended thinking

              - `"claude-sonnet-4-20250514"`

                High-performance model with extended thinking

              - `"claude-3-haiku-20240307"`

                Fast and cost-effective model

            - `string`

        - `to: BetaFallbackInfoParam`

          Identifies one hop of a fallback transition.

        - `type: "fallback"`

          - `"fallback"`

  - `role: "user" or "assistant" or "system"`

    - `"user"`

    - `"assistant"`

    - `"system"`

- `model: Model`

  The model that will complete your prompt.

  See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

- `cache_control: optional BetaCacheControlEphemeral`

  Top-level cache control automatically applies a cache_control marker to the last cacheable block in the request.

- `container: optional BetaContainerParams or string`

  Container identifier for reuse across requests.

  - `BetaContainerParams object { id, skills }`

    Container parameters with skills to be loaded.

    - `id: optional string`

      Container id

    - `skills: optional array of BetaSkillParams`

      List of skills to load in the container

      - `skill_id: string`

        Skill ID

      - `type: "anthropic" or "custom"`

        Type of skill - either 'anthropic' (built-in) or 'custom' (user-defined)

        - `"anthropic"`

        - `"custom"`

      - `version: optional string`

        Skill version or 'latest' for most recent version

  - `string`

- `context_management: optional BetaContextManagementConfig`

  Context management configuration.

  This allows you to control how Claude manages context across multiple requests, such as whether to clear function results or not.

  - `edits: optional array of BetaClearToolUses20250919Edit or BetaClearThinking20251015Edit or BetaCompact20260112Edit`

    List of context management edits to apply

    - `BetaClearToolUses20250919Edit object { type, clear_at_least, clear_tool_inputs, 3 more }`

      - `type: "clear_tool_uses_20250919"`

        - `"clear_tool_uses_20250919"`

      - `clear_at_least: optional BetaInputTokensClearAtLeast`

        Minimum number of tokens that must be cleared when triggered. Context will only be modified if at least this many tokens can be removed.

        - `type: "input_tokens"`

          - `"input_tokens"`

        - `value: number`

      - `clear_tool_inputs: optional boolean or array of string`

        Whether to clear all tool inputs (bool) or specific tool inputs to clear (list)

        - `boolean`

        - `array of string`

      - `exclude_tools: optional array of string`

        Tool names whose uses are preserved from clearing

      - `keep: optional BetaToolUsesKeep`

        Number of tool uses to retain in the conversation

        - `type: "tool_uses"`

          - `"tool_uses"`

        - `value: number`

      - `trigger: optional BetaInputTokensTrigger or BetaToolUsesTrigger`

        Condition that triggers the context management strategy

        - `BetaInputTokensTrigger object { type, value }`

          - `type: "input_tokens"`

            - `"input_tokens"`

          - `value: number`

        - `BetaToolUsesTrigger object { type, value }`

          - `type: "tool_uses"`

            - `"tool_uses"`

          - `value: number`

    - `BetaClearThinking20251015Edit object { type, keep }`

      - `type: "clear_thinking_20251015"`

        - `"clear_thinking_20251015"`

      - `keep: optional BetaThinkingTurns or BetaAllThinkingTurns or "all"`

        Number of most recent assistant turns to keep thinking blocks for. Older turns will have their thinking blocks removed.

        - `BetaThinkingTurns object { type, value }`

          - `type: "thinking_turns"`

            - `"thinking_turns"`

          - `value: number`

        - `BetaAllThinkingTurns object { type }`

          - `type: "all"`

            - `"all"`

        - `"all"`

          - `"all"`

    - `BetaCompact20260112Edit object { type, instructions, pause_after_compaction, trigger }`

      Automatically compact older context when reaching the configured trigger threshold.

      - `type: "compact_20260112"`

        - `"compact_20260112"`

      - `instructions: optional string`

        Additional instructions for summarization.

      - `pause_after_compaction: optional boolean`

        Whether to pause after compaction and return the compaction block to the user.

      - `trigger: optional BetaInputTokensTrigger`

        When to trigger compaction. Defaults to 150000 input tokens.

- `diagnostics: optional BetaDiagnosticsParam`

  Request-level diagnostics. Currently carries the previous response
  id for prompt-cache divergence reporting.

  - `previous_message_id: optional string`

    The `id` (`msg_...`) from this client's previous /v1/messages response. The server compares that request's prompt fingerprint against this one and returns `diagnostics.cache_miss_reason` when the prompt-cache prefix could not be reused. Pass `null` on the first turn to opt in without a prior message to compare.

- `fallback_credit_token: optional string`

  The `fallback_credit_token` from a prior refusal's `stop_details`.

  When a preceding request was refused and returned a `fallback_credit_token`,
  pass that code here on the retry to have the retry's cache-creation tokens
  for the prefix that was warm on the refused model billed at the cache-read
  rate. Must be redeemed by the same organization and workspace, with the same
  request body (optionally extended by one appended `assistant` message whose
  content is the partial text — with any trailing whitespace stripped from
  the final text block — and paired server-tool blocks streamed before the
  refusal; the appended-assistant form is not available for requests with
  `output_format` set or forced `tool_choice`), on an eligible fallback
  model, on the same platform,
  and within 5 minutes of the refusal; a mismatch is a 400. A token minted
  mid-server-tool-loop whose partial content was continuable may only be
  redeemed with the appended-assistant form — if an exact-body retry is
  rejected with a 400 saying the token must be redeemed by continuing the
  partial response, retry with the appended-assistant form instead.

  When the appended-assistant form is used on a model that otherwise disallows
  assistant-turn prefill, this token also authorizes that one prefill.

- `fallbacks: optional array of BetaFallbackParam`

  Opt-in server-side retry on one or more substitute models when the requested model declines for policy reasons. Tried in order: if the first entry also declines, the second is tried, and so on.

  - `model: Model`

    The model that will complete your prompt.

    See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

  - `max_tokens: optional number`

  - `output_config: optional BetaOutputConfig`

    - `effort: optional "low" or "medium" or "high" or 2 more`

      All possible effort levels.

      - `"low"`

      - `"medium"`

      - `"high"`

      - `"xhigh"`

      - `"max"`

    - `format: optional BetaJSONOutputFormat`

      A schema to specify Claude's output format in responses. See [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs)

      - `schema: map[unknown]`

        The JSON schema of the format

      - `type: "json_schema"`

        - `"json_schema"`

    - `task_budget: optional BetaTokenTaskBudget`

      User-configurable total token budget across contexts.

      - `total: number`

        Total token budget across all contexts in the session.

      - `type: "tokens"`

        The budget type. Currently only 'tokens' is supported.

        - `"tokens"`

      - `remaining: optional number`

        Remaining tokens in the budget. Use this to track usage across contexts when implementing compaction client-side. Defaults to total if not provided.

  - `speed: optional "standard" or "fast"`

    - `"standard"`

    - `"fast"`

  - `thinking: optional BetaThinkingConfigEnabled or BetaThinkingConfigDisabled or BetaThinkingConfigAdaptive`

    - `BetaThinkingConfigEnabled object { budget_tokens, type, display }`

      - `budget_tokens: number`

        Determines how many tokens Claude can use for its internal reasoning process. Larger budgets can enable more thorough analysis for complex problems, improving response quality.

        Must be ≥1024 and less than `max_tokens`.

        See [extended thinking](https://docs.claude.com/en/docs/build-with-claude/extended-thinking) for details.

      - `type: "enabled"`

        - `"enabled"`

      - `display: optional "summarized" or "omitted"`

        Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

        - `"summarized"`

        - `"omitted"`

    - `BetaThinkingConfigDisabled object { type }`

      - `type: "disabled"`

        - `"disabled"`

    - `BetaThinkingConfigAdaptive object { type, display }`

      - `type: "adaptive"`

        - `"adaptive"`

      - `display: optional "summarized" or "omitted"`

        Controls how thinking content appears in the response. When set to `summarized`, thinking is returned normally. When set to `omitted`, thinking content is redacted but a signature is returned for multi-turn continuity. Defaults to `summarized`.

        - `"summarized"`

        - `"omitted"`

- `inference_geo: optional string`

  Specifies the geographic region for inference processing. If not specified, the workspace's `default_inference_geo` is used.

- `mcp_servers: optional array of BetaRequestMCPServerURLDefinition`

  MCP servers to be utilized in this request

  - `name: string`

  - `type: "url"`

    - `"url"`

  - `url: string`

  - `authorization_token: optional string`

  - `tool_configuration: optional BetaRequestMCPServerToolConfiguration`

    - `allowed_tools: optional array of string`

    - `enabled: optional boolean`

- `metadata: optional BetaMetadata`

  An object describing metadata about the request.

  - `user_id: optional string`

    An external identifier for the user who is associated with the request.

    This should be a uuid, hash value, or other opaque identifier. Anthropic may use this id to help detect abuse. Do not include any identifying information such as name, email address, or phone number.

- `output_config: optional BetaOutputConfig`

  Configuration options for the model's output, such as the output format.

- `output_format: optional BetaJSONOutputFormat`

  Deprecated: Use `output_config.format` instead. See [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs)

  A schema to specify Claude's output format in responses. This parameter will be removed in a future release.

- `service_tier: optional "auto" or "standard_only"`

  Determines whether to use priority capacity (if available) or standard capacity for this request.

  Anthropic offers different levels of service for your API requests. See [service-tiers](https://docs.claude.com/en/api/service-tiers) for details.

  - `"auto"`

  - `"standard_only"`

- `speed: optional "standard" or "fast"`

  The inference speed mode for this request. `"fast"` enables high output-tokens-per-second inference.

  - `"standard"`

  - `"fast"`

- `stop_sequences: optional array of string`

  Custom text sequences that will cause the model to stop generating.

  Our models will normally stop when they have naturally completed their turn, which will result in a response `stop_reason` of `"end_turn"`.

  If you want the model to stop generating when it encounters custom strings of text, you can use the `stop_sequences` parameter. If the model encounters one of the custom sequences, the response `stop_reason` value will be `"stop_sequence"` and the response `stop_sequence` value will contain the matched stop sequence.

- `stream: optional boolean`

  Whether to incrementally stream the response using server-sent events.

  See [streaming](https://docs.claude.com/en/api/messages-streaming) for details.

- `system: optional string or array of BetaTextBlockParam`

  System prompt.

  A system prompt is a way of providing context and instructions to Claude, such as specifying a particular goal or role. See our [guide to system prompts](https://docs.claude.com/en/docs/system-prompts).

  - `string`

  - `array of BetaTextBlockParam`

    - `text: string`

    - `type: "text"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `citations: optional array of BetaTextCitationParam`

- `temperature: optional number`

  Amount of randomness injected into the response.

  Defaults to `1.0`. Ranges from `0.0` to `1.0`. Use `temperature` closer to `0.0` for analytical / multiple choice, and closer to `1.0` for creative and generative tasks.

  Note that even with `temperature` of `0.0`, the results will not be fully deterministic.

- `thinking: optional BetaThinkingConfigParam`

  Configuration for enabling Claude's extended thinking.

  When enabled, responses include `thinking` content blocks showing Claude's thinking process before the final answer. Requires a minimum budget of 1,024 tokens and counts towards your `max_tokens` limit.

  See [extended thinking](https://docs.claude.com/en/docs/build-with-claude/extended-thinking) for details.

  - `BetaThinkingConfigEnabled object { budget_tokens, type, display }`

  - `BetaThinkingConfigDisabled object { type }`

  - `BetaThinkingConfigAdaptive object { type, display }`

- `tool_choice: optional BetaToolChoice`

  How the model should use the provided tools. The model can use a specific tool, any available tool, decide by itself, or not use tools at all.

  - `BetaToolChoiceAuto object { type, disable_parallel_tool_use }`

    The model will automatically decide whether to use tools.

    - `type: "auto"`

      - `"auto"`

    - `disable_parallel_tool_use: optional boolean`

      Whether to disable parallel tool use.

      Defaults to `false`. If set to `true`, the model will output at most one tool use.

  - `BetaToolChoiceAny object { type, disable_parallel_tool_use }`

    The model will use any available tools.

    - `type: "any"`

      - `"any"`

    - `disable_parallel_tool_use: optional boolean`

      Whether to disable parallel tool use.

      Defaults to `false`. If set to `true`, the model will output exactly one tool use.

  - `BetaToolChoiceTool object { name, type, disable_parallel_tool_use }`

    The model will use the specified tool with `tool_choice.name`.

    - `name: string`

      The name of the tool to use.

    - `type: "tool"`

      - `"tool"`

    - `disable_parallel_tool_use: optional boolean`

      Whether to disable parallel tool use.

      Defaults to `false`. If set to `true`, the model will output exactly one tool use.

  - `BetaToolChoiceNone object { type }`

    The model will not be allowed to use tools.

    - `type: "none"`

      - `"none"`

- `tools: optional array of BetaToolUnion`

  Definitions of tools that the model may use.

  If you include `tools` in your API request, the model may return `tool_use` content blocks that represent the model's use of those tools. You can then run those tools using the tool input generated by the model and then optionally return results back to the model using `tool_result` content blocks.

  There are two types of tools: **client tools** and **server tools**. The behavior described below applies to client tools. For [server tools](https://docs.claude.com/en/docs/agents-and-tools/tool-use/overview#server-tools), see their individual documentation as each has its own behavior (e.g., the [web search tool](https://docs.claude.com/en/docs/agents-and-tools/tool-use/web-search-tool)).

  Each tool definition includes:

  * `name`: Name of the tool.
  * `description`: Optional, but strongly-recommended description of the tool.
  * `input_schema`: [JSON schema](https://json-schema.org/draft/2020-12) for the tool `input` shape that the model will produce in `tool_use` output content blocks.

  For example, if you defined `tools` as:

  ```json
  [
    {
      "name": "get_stock_price",
      "description": "Get the current stock price for a given ticker symbol.",
      "input_schema": {
        "type": "object",
        "properties": {
          "ticker": {
            "type": "string",
            "description": "The stock ticker symbol, e.g. AAPL for Apple Inc."
          }
        },
        "required": ["ticker"]
      }
    }
  ]
  ```

  And then asked the model "What's the S&P 500 at today?", the model might produce `tool_use` content blocks in the response like this:

  ```json
  [
    {
      "type": "tool_use",
      "id": "toolu_01D7FLrfh4GYq7yT1ULFeyMV",
      "name": "get_stock_price",
      "input": { "ticker": "^GSPC" }
    }
  ]
  ```

  You might then run your `get_stock_price` tool with `{"ticker": "^GSPC"}` as an input, and return the following back to the model in a subsequent `user` message:

  ```json
  [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01D7FLrfh4GYq7yT1ULFeyMV",
      "content": "259.75 USD"
    }
  ]
  ```

  Tools can be used for workflows that include running client-side tools and functions, or more generally whenever you want the model to produce a particular JSON structure of output.

  See our [guide](https://docs.claude.com/en/docs/tool-use) for more details.

  - `BetaTool object { input_schema, name, allowed_callers, 7 more }`

    - `input_schema: object { type, properties, required }`

      [JSON schema](https://json-schema.org/draft/2020-12) for this tool's input.

      This defines the shape of the `input` that your tool accepts and that the model will produce.

      - `type: "object"`

        - `"object"`

      - `properties: optional map[unknown]`

      - `required: optional array of string`

    - `name: string`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `description: optional string`

      Description of what this tool does.

      Tool descriptions should be as detailed as possible. The more information that the model has about what the tool is and how to use it, the better it will perform. You can use natural language descriptions to reinforce important aspects of the tool input JSON schema.

    - `eager_input_streaming: optional boolean`

      Enable eager input streaming for this tool. When true, tool input parameters will be streamed incrementally as they are generated, and types will be inferred on-the-fly rather than buffering the full JSON output. When false, streaming is disabled for this tool even if the fine-grained-tool-streaming beta is active. When null (default), uses the default behavior based on beta headers.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

    - `type: optional "custom"`

      - `"custom"`

  - `BetaToolBash20241022 object { name, type, allowed_callers, 4 more }`

    - `name: "bash"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"bash"`

    - `type: "bash_20241022"`

      - `"bash_20241022"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolBash20250124 object { name, type, allowed_callers, 4 more }`

    - `name: "bash"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"bash"`

    - `type: "bash_20250124"`

      - `"bash_20250124"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaCodeExecutionTool20250522 object { name, type, allowed_callers, 3 more }`

    - `name: "code_execution"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"code_execution"`

    - `type: "code_execution_20250522"`

      - `"code_execution_20250522"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaCodeExecutionTool20250825 object { name, type, allowed_callers, 3 more }`

    - `name: "code_execution"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"code_execution"`

    - `type: "code_execution_20250825"`

      - `"code_execution_20250825"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaCodeExecutionTool20260120 object { name, type, allowed_callers, 3 more }`

    Code execution tool with REPL state persistence (daemon mode + gVisor checkpoint).

    - `name: "code_execution"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"code_execution"`

    - `type: "code_execution_20260120"`

      - `"code_execution_20260120"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolComputerUse20241022 object { display_height_px, display_width_px, name, 7 more }`

    - `display_height_px: number`

      The height of the display in pixels.

    - `display_width_px: number`

      The width of the display in pixels.

    - `name: "computer"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"computer"`

    - `type: "computer_20241022"`

      - `"computer_20241022"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `display_number: optional number`

      The X11 display number (e.g. 0, 1) for the display.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaMemoryTool20250818 object { name, type, allowed_callers, 4 more }`

    - `name: "memory"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"memory"`

    - `type: "memory_20250818"`

      - `"memory_20250818"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolComputerUse20250124 object { display_height_px, display_width_px, name, 7 more }`

    - `display_height_px: number`

      The height of the display in pixels.

    - `display_width_px: number`

      The width of the display in pixels.

    - `name: "computer"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"computer"`

    - `type: "computer_20250124"`

      - `"computer_20250124"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `display_number: optional number`

      The X11 display number (e.g. 0, 1) for the display.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolTextEditor20241022 object { name, type, allowed_callers, 4 more }`

    - `name: "str_replace_editor"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"str_replace_editor"`

    - `type: "text_editor_20241022"`

      - `"text_editor_20241022"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolComputerUse20251124 object { display_height_px, display_width_px, name, 8 more }`

    - `display_height_px: number`

      The height of the display in pixels.

    - `display_width_px: number`

      The width of the display in pixels.

    - `name: "computer"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"computer"`

    - `type: "computer_20251124"`

      - `"computer_20251124"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `display_number: optional number`

      The X11 display number (e.g. 0, 1) for the display.

    - `enable_zoom: optional boolean`

      Whether to enable an action to take a zoomed-in screenshot of the screen.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolTextEditor20250124 object { name, type, allowed_callers, 4 more }`

    - `name: "str_replace_editor"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"str_replace_editor"`

    - `type: "text_editor_20250124"`

      - `"text_editor_20250124"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolTextEditor20250429 object { name, type, allowed_callers, 4 more }`

    - `name: "str_replace_based_edit_tool"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"str_replace_based_edit_tool"`

    - `type: "text_editor_20250429"`

      - `"text_editor_20250429"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolTextEditor20250728 object { name, type, allowed_callers, 5 more }`

    - `name: "str_replace_based_edit_tool"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"str_replace_based_edit_tool"`

    - `type: "text_editor_20250728"`

      - `"text_editor_20250728"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `input_examples: optional array of map[unknown]`

    - `max_characters: optional number`

      Maximum number of characters to display when viewing a file. If not specified, defaults to displaying the full file.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaWebSearchTool20250305 object { name, type, allowed_callers, 7 more }`

    - `name: "web_search"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"web_search"`

    - `type: "web_search_20250305"`

      - `"web_search_20250305"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `allowed_domains: optional array of string`

      If provided, only these domains will be included in results. Cannot be used alongside `blocked_domains`.

    - `blocked_domains: optional array of string`

      If provided, these domains will never appear in results. Cannot be used alongside `allowed_domains`.

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

    - `user_location: optional BetaUserLocation`

      Parameters for the user's location. Used to provide more relevant search results.

      - `type: "approximate"`

        - `"approximate"`

      - `city: optional string`

        The city of the user.

      - `country: optional string`

        The two letter [ISO country code](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2) of the user.

      - `region: optional string`

        The region of the user.

      - `timezone: optional string`

        The [IANA timezone](https://nodatime.org/TimeZones) of the user.

  - `BetaWebFetchTool20250910 object { name, type, allowed_callers, 8 more }`

    - `name: "web_fetch"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"web_fetch"`

    - `type: "web_fetch_20250910"`

      - `"web_fetch_20250910"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `allowed_domains: optional array of string`

      List of domains to allow fetching from

    - `blocked_domains: optional array of string`

      List of domains to block fetching from

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `citations: optional BetaCitationsConfigParam`

      Citations configuration for fetched documents. Citations are disabled by default.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_content_tokens: optional number`

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaWebSearchTool20260209 object { name, type, allowed_callers, 7 more }`

    - `name: "web_search"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"web_search"`

    - `type: "web_search_20260209"`

      - `"web_search_20260209"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `allowed_domains: optional array of string`

      If provided, only these domains will be included in results. Cannot be used alongside `blocked_domains`.

    - `blocked_domains: optional array of string`

      If provided, these domains will never appear in results. Cannot be used alongside `allowed_domains`.

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

    - `user_location: optional BetaUserLocation`

      Parameters for the user's location. Used to provide more relevant search results.

  - `BetaWebFetchTool20260209 object { name, type, allowed_callers, 8 more }`

    - `name: "web_fetch"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"web_fetch"`

    - `type: "web_fetch_20260209"`

      - `"web_fetch_20260209"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `allowed_domains: optional array of string`

      List of domains to allow fetching from

    - `blocked_domains: optional array of string`

      List of domains to block fetching from

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `citations: optional BetaCitationsConfigParam`

      Citations configuration for fetched documents. Citations are disabled by default.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_content_tokens: optional number`

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaWebFetchTool20260309 object { name, type, allowed_callers, 9 more }`

    Web fetch tool with use_cache parameter for bypassing cached content.

    - `name: "web_fetch"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"web_fetch"`

    - `type: "web_fetch_20260309"`

      - `"web_fetch_20260309"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `allowed_domains: optional array of string`

      List of domains to allow fetching from

    - `blocked_domains: optional array of string`

      List of domains to block fetching from

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `citations: optional BetaCitationsConfigParam`

      Citations configuration for fetched documents. Citations are disabled by default.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_content_tokens: optional number`

      Maximum number of tokens used by including web page text content in the context. The limit is approximate and does not apply to binary content such as PDFs.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

    - `use_cache: optional boolean`

      Whether to use cached content. Set to false to bypass the cache and fetch fresh content. Only set to false when the user explicitly requests fresh content or when fetching rapidly-changing sources.

  - `BetaAdvisorTool20260301 object { model, name, type, 7 more }`

    - `model: Model`

      The model that will complete your prompt.

      See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

    - `name: "advisor"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"advisor"`

    - `type: "advisor_20260301"`

      - `"advisor_20260301"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `caching: optional BetaCacheControlEphemeral`

      Caching for the advisor's own prompt. When set, each advisor call writes a cache entry at the given TTL so subsequent calls in the same conversation read the stable prefix. When omitted, the advisor prompt is not cached.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `max_tokens: optional number`

      Bounds the advisor's total output (thinking + text) per call. When the advisor hits this cap, the returned advisor_result or advisor_redacted_result block carries stop_reason='max_tokens', and a truncation note is appended to the advice text the worker model sees (inside the encrypted blob in redacted mode). When set, the server also emits a remaining-tokens budget block in the advisor's prompt so the advisor self-shapes toward the cap. When omitted, the advisor model's default output cap applies and no budget block is emitted.

    - `max_uses: optional number`

      Maximum number of times the tool can be used in the API request.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolSearchToolBm25_20251119 object { name, type, allowed_callers, 3 more }`

    - `name: "tool_search_tool_bm25"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"tool_search_tool_bm25"`

    - `type: "tool_search_tool_bm25_20251119" or "tool_search_tool_bm25"`

      - `"tool_search_tool_bm25_20251119"`

      - `"tool_search_tool_bm25"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaToolSearchToolRegex20251119 object { name, type, allowed_callers, 3 more }`

    - `name: "tool_search_tool_regex"`

      Name of the tool.

      This is how the tool will be called by the model and in `tool_use` blocks.

      - `"tool_search_tool_regex"`

    - `type: "tool_search_tool_regex_20251119" or "tool_search_tool_regex"`

      - `"tool_search_tool_regex_20251119"`

      - `"tool_search_tool_regex"`

    - `allowed_callers: optional array of "direct" or "code_execution_20250825" or "code_execution_20260120"`

      - `"direct"`

      - `"code_execution_20250825"`

      - `"code_execution_20260120"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `defer_loading: optional boolean`

      If true, tool will not be included in initial system prompt. Only loaded when returned via tool_reference from tool search.

    - `strict: optional boolean`

      When true, guarantees schema validation on tool names and inputs

  - `BetaMCPToolset object { mcp_server_name, type, cache_control, 2 more }`

    Configuration for a group of tools from an MCP server.

    Allows configuring enabled status and defer_loading for all tools
    from an MCP server, with optional per-tool overrides.

    - `mcp_server_name: string`

      Name of the MCP server to configure tools for

    - `type: "mcp_toolset"`

      - `"mcp_toolset"`

    - `cache_control: optional BetaCacheControlEphemeral`

      Create a cache control breakpoint at this content block.

    - `configs: optional map[BetaMCPToolConfig]`

      Configuration overrides for specific tools, keyed by tool name

      - `defer_loading: optional boolean`

      - `enabled: optional boolean`

    - `default_config: optional BetaMCPToolDefaultConfig`

      Default configuration applied to all tools from this server

      - `defer_loading: optional boolean`

      - `enabled: optional boolean`

- `top_k: optional number`

  Only sample from the top K options for each subsequent token.

  Used to remove "long tail" low probability responses. [Learn more technical details here](https://towardsdatascience.com/how-to-sample-from-language-models-682bceb97277).

  Recommended for advanced use cases only.

- `top_p: optional number`

  Use nucleus sampling.

  In nucleus sampling, we compute the cumulative distribution over all the options for each subsequent token in decreasing probability order and cut it off once it reaches a particular probability specified by `top_p`.

  Recommended for advanced use cases only.

- `user_profile_id: optional string`

  The user profile ID to attribute this request to. Use when acting on behalf of a party other than your organization.

### Returns

- `BetaMessage object { id, container, content, 9 more }`

  - `id: string`

    Unique object identifier.

    The format and length of IDs may change over time.

  - `container: BetaContainer`

    Information about the container used in the request (for the code execution tool)

    - `id: string`

      Identifier for the container used in this request

    - `expires_at: string`

      The time at which the container will expire.

    - `skills: array of BetaSkill`

      Skills loaded in the container

      - `skill_id: string`

        Skill ID

      - `type: "anthropic" or "custom"`

        Type of skill - either 'anthropic' (built-in) or 'custom' (user-defined)

        - `"anthropic"`

        - `"custom"`

      - `version: string`

        Skill version or 'latest' for most recent version

  - `content: array of BetaContentBlock`

    Content generated by the model.

    This is an array of content blocks, each of which has a `type` that determines its shape.

    Example:

    ```json
    [{"type": "text", "text": "Hi, I'm Claude."}]
    ```

    If the request input `messages` ended with an `assistant` turn, then the response `content` will continue directly from that last turn. You can use this to constrain the model's output.

    For example, if the input `messages` were:

    ```json
    [
      {"role": "user", "content": "What's the Greek name for Sun? (A) Sol (B) Helios (C) Sun"},
      {"role": "assistant", "content": "The best answer is ("}
    ]
    ```

    Then the response `content` might be:

    ```json
    [{"type": "text", "text": "B)"}]
    ```

    - `BetaTextBlock object { citations, text, type }`

      - `citations: array of BetaTextCitation`

        Citations supporting the text block.

        The type of citation returned will depend on the type of document being cited. Citing a PDF results in `page_location`, plain text results in `char_location`, and content document results in `content_block_location`.

        - `BetaCitationCharLocation object { cited_text, document_index, document_title, 4 more }`

          - `cited_text: string`

          - `document_index: number`

          - `document_title: string`

          - `end_char_index: number`

          - `file_id: string`

          - `start_char_index: number`

          - `type: "char_location"`

            - `"char_location"`

        - `BetaCitationPageLocation object { cited_text, document_index, document_title, 4 more }`

          - `cited_text: string`

          - `document_index: number`

          - `document_title: string`

          - `end_page_number: number`

          - `file_id: string`

          - `start_page_number: number`

          - `type: "page_location"`

            - `"page_location"`

        - `BetaCitationContentBlockLocation object { cited_text, document_index, document_title, 4 more }`

          - `cited_text: string`

            The full text of the cited block range, concatenated.

            Always equals the contents of `content[start_block_index:end_block_index]` joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

          - `document_index: number`

          - `document_title: string`

          - `end_block_index: number`

            Exclusive 0-based end index of the cited block range in the source's `content` array.

            Always greater than `start_block_index`; a single-block citation has `end_block_index = start_block_index + 1`.

          - `file_id: string`

          - `start_block_index: number`

            0-based index of the first cited block in the source's `content` array.

          - `type: "content_block_location"`

            - `"content_block_location"`

        - `BetaCitationsWebSearchResultLocation object { cited_text, encrypted_index, title, 2 more }`

          - `cited_text: string`

          - `encrypted_index: string`

          - `title: string`

          - `type: "web_search_result_location"`

            - `"web_search_result_location"`

          - `url: string`

        - `BetaCitationSearchResultLocation object { cited_text, end_block_index, search_result_index, 4 more }`

          - `cited_text: string`

            The full text of the cited block range, concatenated.

            Always equals the contents of `content[start_block_index:end_block_index]` joined together. The text block is the minimal citable unit; this field is never a substring of a single block. Not counted toward output tokens, and not counted toward input tokens when sent back in subsequent turns.

          - `end_block_index: number`

            Exclusive 0-based end index of the cited block range in the source's `content` array.

            Always greater than `start_block_index`; a single-block citation has `end_block_index = start_block_index + 1`.

          - `search_result_index: number`

            0-based index of the cited search result among all `search_result` content blocks in the request, in the order they appear across messages and tool results.

            Counted separately from `document_index`; server-side web search results are not included in this count.

          - `source: string`

          - `start_block_index: number`

            0-based index of the first cited block in the source's `content` array.

          - `title: string`

          - `type: "search_result_location"`

            - `"search_result_location"`

      - `text: string`

      - `type: "text"`

        - `"text"`

    - `BetaThinkingBlock object { signature, thinking, type }`

      - `signature: string`

      - `thinking: string`

      - `type: "thinking"`

        - `"thinking"`

    - `BetaRedactedThinkingBlock object { data, type }`

      - `data: string`

      - `type: "redacted_thinking"`

        - `"redacted_thinking"`

    - `BetaToolUseBlock object { id, input, name, 2 more }`

      - `id: string`

      - `input: map[unknown]`

      - `name: string`

      - `type: "tool_use"`

        - `"tool_use"`

      - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

        Tool invocation directly from the model.

        - `BetaDirectCaller object { type }`

          Tool invocation directly from the model.

          - `type: "direct"`

            - `"direct"`

        - `BetaServerToolCaller object { tool_id, type }`

          Tool invocation generated by a server-side tool.

          - `tool_id: string`

          - `type: "code_execution_20250825"`

            - `"code_execution_20250825"`

        - `BetaServerToolCaller20260120 object { tool_id, type }`

          - `tool_id: string`

          - `type: "code_execution_20260120"`

            - `"code_execution_20260120"`

    - `BetaServerToolUseBlock object { id, input, name, 2 more }`

      - `id: string`

      - `input: map[unknown]`

      - `name: "advisor" or "web_search" or "web_fetch" or 5 more`

        - `"advisor"`

        - `"web_search"`

        - `"web_fetch"`

        - `"code_execution"`

        - `"bash_code_execution"`

        - `"text_editor_code_execution"`

        - `"tool_search_tool_regex"`

        - `"tool_search_tool_bm25"`

      - `type: "server_tool_use"`

        - `"server_tool_use"`

      - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

        Tool invocation directly from the model.

        - `BetaDirectCaller object { type }`

          Tool invocation directly from the model.

        - `BetaServerToolCaller object { tool_id, type }`

          Tool invocation generated by a server-side tool.

        - `BetaServerToolCaller20260120 object { tool_id, type }`

    - `BetaWebSearchToolResultBlock object { content, tool_use_id, type, caller }`

      - `content: BetaWebSearchToolResultBlockContent`

        - `BetaWebSearchToolResultError object { error_code, type }`

          - `error_code: BetaWebSearchToolResultErrorCode`

            - `"invalid_tool_input"`

            - `"unavailable"`

            - `"max_uses_exceeded"`

            - `"too_many_requests"`

            - `"query_too_long"`

            - `"request_too_large"`

          - `type: "web_search_tool_result_error"`

            - `"web_search_tool_result_error"`

        - `array of BetaWebSearchResultBlock`

          - `encrypted_content: string`

          - `page_age: string`

          - `title: string`

          - `type: "web_search_result"`

            - `"web_search_result"`

          - `url: string`

      - `tool_use_id: string`

      - `type: "web_search_tool_result"`

        - `"web_search_tool_result"`

      - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

        Tool invocation directly from the model.

        - `BetaDirectCaller object { type }`

          Tool invocation directly from the model.

        - `BetaServerToolCaller object { tool_id, type }`

          Tool invocation generated by a server-side tool.

        - `BetaServerToolCaller20260120 object { tool_id, type }`

    - `BetaWebFetchToolResultBlock object { content, tool_use_id, type, caller }`

      - `content: BetaWebFetchToolResultErrorBlock or BetaWebFetchBlock`

        - `BetaWebFetchToolResultErrorBlock object { error_code, type }`

          - `error_code: BetaWebFetchToolResultErrorCode`

            - `"invalid_tool_input"`

            - `"url_too_long"`

            - `"url_not_allowed"`

            - `"url_not_in_prior_context"`

            - `"url_not_accessible"`

            - `"unsupported_content_type"`

            - `"too_many_requests"`

            - `"max_uses_exceeded"`

            - `"unavailable"`

          - `type: "web_fetch_tool_result_error"`

            - `"web_fetch_tool_result_error"`

        - `BetaWebFetchBlock object { content, retrieved_at, type, url }`

          - `content: BetaDocumentBlock`

            - `citations: BetaCitationConfig`

              Citation configuration for the document

              - `enabled: boolean`

            - `source: BetaBase64PDFSource or BetaPlainTextSource`

              - `BetaBase64PDFSource object { data, media_type, type }`

                - `data: string`

                - `media_type: "application/pdf"`

                  - `"application/pdf"`

                - `type: "base64"`

                  - `"base64"`

              - `BetaPlainTextSource object { data, media_type, type }`

                - `data: string`

                - `media_type: "text/plain"`

                  - `"text/plain"`

                - `type: "text"`

                  - `"text"`

            - `title: string`

              The title of the document

            - `type: "document"`

              - `"document"`

          - `retrieved_at: string`

            ISO 8601 timestamp when the content was retrieved

          - `type: "web_fetch_result"`

            - `"web_fetch_result"`

          - `url: string`

            Fetched content URL

      - `tool_use_id: string`

      - `type: "web_fetch_tool_result"`

        - `"web_fetch_tool_result"`

      - `caller: optional BetaDirectCaller or BetaServerToolCaller or BetaServerToolCaller20260120`

        Tool invocation directly from the model.

        - `BetaDirectCaller object { type }`

          Tool invocation directly from the model.

        - `BetaServerToolCaller object { tool_id, type }`

          Tool invocation generated by a server-side tool.

        - `BetaServerToolCaller20260120 object { tool_id, type }`

    - `BetaAdvisorToolResultBlock object { content, tool_use_id, type }`

      - `content: BetaAdvisorToolResultError or BetaAdvisorResultBlock or BetaAdvisorRedactedResultBlock`

        - `BetaAdvisorToolResultError object { error_code, type }`

          - `error_code: "max_uses_exceeded" or "prompt_too_long" or "too_many_requests" or 4 more`

            - `"max_uses_exceeded"`

            - `"prompt_too_long"`

            - `"too_many_requests"`

            - `"overloaded"`

            - `"unavailable"`

            - `"execution_time_exceeded"`

            - `"model_not_found"`

          - `type: "advisor_tool_result_error"`

            - `"advisor_tool_result_error"`

        - `BetaAdvisorResultBlock object { stop_reason, text, type }`

          - `stop_reason: string`

            The advisor sub-inference's stop reason (same values as the top-level message `stop_reason`). `max_tokens` indicates the advisor's output was truncated at the tool's `max_tokens` value or the advisor model's policy cap.

          - `text: string`

          - `type: "advisor_result"`

            - `"advisor_result"`

        - `BetaAdvisorRedactedResultBlock object { encrypted_content, stop_reason, type }`

          - `encrypted_content: string`

            Opaque blob containing the advisor's output. Round-trip verbatim; do not inspect or modify.

          - `stop_reason: string`

            The advisor sub-inference's stop reason (same values as the top-level message `stop_reason`).

          - `type: "advisor_redacted_result"`

            - `"advisor_redacted_result"`

      - `tool_use_id: string`

      - `type: "advisor_tool_result"`

        - `"advisor_tool_result"`

    - `BetaCodeExecutionToolResultBlock object { content, tool_use_id, type }`

      - `content: BetaCodeExecutionToolResultBlockContent`

        Code execution result with encrypted stdout for PFC + web_search results.

        - `BetaCodeExecutionToolResultError object { error_code, type }`

          - `error_code: BetaCodeExecutionToolResultErrorCode`

            - `"invalid_tool_input"`

            - `"unavailable"`

            - `"too_many_requests"`

            - `"execution_time_exceeded"`

          - `type: "code_execution_tool_result_error"`

            - `"code_execution_tool_result_error"`

        - `BetaCodeExecutionResultBlock object { content, return_code, stderr, 2 more }`

          - `content: array of BetaCodeExecutionOutputBlock`

            - `file_id: string`

            - `type: "code_execution_output"`

              - `"code_execution_output"`

          - `return_code: number`

          - `stderr: string`

          - `stdout: string`

          - `type: "code_execution_result"`

            - `"code_execution_result"`

        - `BetaEncryptedCodeExecutionResultBlock object { content, encrypted_stdout, return_code, 2 more }`

          Code execution result with encrypted stdout for PFC + web_search results.

          - `content: array of BetaCodeExecutionOutputBlock`

            - `file_id: string`

            - `type: "code_execution_output"`

          - `encrypted_stdout: string`

          - `return_code: number`

          - `stderr: string`

          - `type: "encrypted_code_execution_result"`

            - `"encrypted_code_execution_result"`

      - `tool_use_id: string`

      - `type: "code_execution_tool_result"`

        - `"code_execution_tool_result"`

    - `BetaBashCodeExecutionToolResultBlock object { content, tool_use_id, type }`

      - `content: BetaBashCodeExecutionToolResultError or BetaBashCodeExecutionResultBlock`

        - `BetaBashCodeExecutionToolResultError object { error_code, type }`

          - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or 2 more`

            - `"invalid_tool_input"`

            - `"unavailable"`

            - `"too_many_requests"`

            - `"execution_time_exceeded"`

            - `"output_file_too_large"`

          - `type: "bash_code_execution_tool_result_error"`

            - `"bash_code_execution_tool_result_error"`

        - `BetaBashCodeExecutionResultBlock object { content, return_code, stderr, 2 more }`

          - `content: array of BetaBashCodeExecutionOutputBlock`

            - `file_id: string`

            - `type: "bash_code_execution_output"`

              - `"bash_code_execution_output"`

          - `return_code: number`

          - `stderr: string`

          - `stdout: string`

          - `type: "bash_code_execution_result"`

            - `"bash_code_execution_result"`

      - `tool_use_id: string`

      - `type: "bash_code_execution_tool_result"`

        - `"bash_code_execution_tool_result"`

    - `BetaTextEditorCodeExecutionToolResultBlock object { content, tool_use_id, type }`

      - `content: BetaTextEditorCodeExecutionToolResultError or BetaTextEditorCodeExecutionViewResultBlock or BetaTextEditorCodeExecutionCreateResultBlock or BetaTextEditorCodeExecutionStrReplaceResultBlock`

        - `BetaTextEditorCodeExecutionToolResultError object { error_code, error_message, type }`

          - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or 2 more`

            - `"invalid_tool_input"`

            - `"unavailable"`

            - `"too_many_requests"`

            - `"execution_time_exceeded"`

            - `"file_not_found"`

          - `error_message: string`

          - `type: "text_editor_code_execution_tool_result_error"`

            - `"text_editor_code_execution_tool_result_error"`

        - `BetaTextEditorCodeExecutionViewResultBlock object { content, file_type, num_lines, 3 more }`

          - `content: string`

          - `file_type: "text" or "image" or "pdf"`

            - `"text"`

            - `"image"`

            - `"pdf"`

          - `num_lines: number`

          - `start_line: number`

          - `total_lines: number`

          - `type: "text_editor_code_execution_view_result"`

            - `"text_editor_code_execution_view_result"`

        - `BetaTextEditorCodeExecutionCreateResultBlock object { is_file_update, type }`

          - `is_file_update: boolean`

          - `type: "text_editor_code_execution_create_result"`

            - `"text_editor_code_execution_create_result"`

        - `BetaTextEditorCodeExecutionStrReplaceResultBlock object { lines, new_lines, new_start, 3 more }`

          - `lines: array of string`

          - `new_lines: number`

          - `new_start: number`

          - `old_lines: number`

          - `old_start: number`

          - `type: "text_editor_code_execution_str_replace_result"`

            - `"text_editor_code_execution_str_replace_result"`

      - `tool_use_id: string`

      - `type: "text_editor_code_execution_tool_result"`

        - `"text_editor_code_execution_tool_result"`

    - `BetaToolSearchToolResultBlock object { content, tool_use_id, type }`

      - `content: BetaToolSearchToolResultError or BetaToolSearchToolSearchResultBlock`

        - `BetaToolSearchToolResultError object { error_code, error_message, type }`

          - `error_code: "invalid_tool_input" or "unavailable" or "too_many_requests" or "execution_time_exceeded"`

            - `"invalid_tool_input"`

            - `"unavailable"`

            - `"too_many_requests"`

            - `"execution_time_exceeded"`

          - `error_message: string`

          - `type: "tool_search_tool_result_error"`

            - `"tool_search_tool_result_error"`

        - `BetaToolSearchToolSearchResultBlock object { tool_references, type }`

          - `tool_references: array of BetaToolReferenceBlock`

            - `tool_name: string`

            - `type: "tool_reference"`

              - `"tool_reference"`

          - `type: "tool_search_tool_search_result"`

            - `"tool_search_tool_search_result"`

      - `tool_use_id: string`

      - `type: "tool_search_tool_result"`

        - `"tool_search_tool_result"`

    - `BetaMCPToolUseBlock object { id, input, name, 2 more }`

      - `id: string`

      - `input: map[unknown]`

      - `name: string`

        The name of the MCP tool

      - `server_name: string`

        The name of the MCP server

      - `type: "mcp_tool_use"`

        - `"mcp_tool_use"`

    - `BetaMCPToolResultBlock object { content, is_error, tool_use_id, type }`

      - `content: string or array of BetaTextBlock`

        - `string`

        - `BetaMCPToolResultBlockContent = array of BetaTextBlock`

          - `citations: array of BetaTextCitation`

            Citations supporting the text block.

            The type of citation returned will depend on the type of document being cited. Citing a PDF results in `page_location`, plain text results in `char_location`, and content document results in `content_block_location`.

          - `text: string`

          - `type: "text"`

      - `is_error: boolean`

      - `tool_use_id: string`

      - `type: "mcp_tool_result"`

        - `"mcp_tool_result"`

    - `BetaContainerUploadBlock object { file_id, type }`

      Response model for a file uploaded to the container.

      - `file_id: string`

      - `type: "container_upload"`

        - `"container_upload"`

    - `BetaCompactionBlock object { content, encrypted_content, type }`

      A compaction block returned when autocompact is triggered.

      When content is None, it indicates the compaction failed to produce a valid
      summary (e.g., malformed output from the model). Clients may round-trip
      compaction blocks with null content; the server treats them as no-ops.

      - `content: string`

        Summary of compacted content, or null if compaction failed

      - `encrypted_content: string`

        Opaque metadata from prior compaction, to be round-tripped verbatim

      - `type: "compaction"`

        - `"compaction"`

    - `BetaFallbackBlock object { from, to, type }`

      Marks the point in `content` where one model's output gives way to the next.

      One block appears per hop where a preceding model actually ran this turn and
      declined. A turn routed directly by the sticky decision has no such boundary
      and carries no block — the signal for whether a fallback model served the
      response is the presence of a `fallback_message` entry in
      `usage.iterations`, not this block.

      The block is treated like a server-tool content block for streaming: it
      arrives via the standard `content_block_start` / `content_block_stop`
      pair and carries no deltas.

      - `from: BetaFallbackInfo`

        The model whose output ends at this point — the model that declined at this hop. When the declining hop is the requested model, its `model` echoes the top-level `model` string the caller sent (alias or canonical); when the declining hop is a fallback model, its `model` is that model's canonical id.

        - `model: Model`

          The model that will complete your prompt.

          See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

          - `"claude-fable-5" or "claude-mythos-5" or "claude-opus-4-8" or 17 more`

            The model that will complete your prompt.

            See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

            - `"claude-fable-5"`

              Next generation of intelligence for the hardest knowledge work and coding problems

            - `"claude-mythos-5"`

              Most capable model for cybersecurity and biology research

            - `"claude-opus-4-8"`

              Frontier intelligence for long-running agents and coding

            - `"claude-opus-4-7"`

              Frontier intelligence for long-running agents and coding

            - `"claude-mythos-preview"`

              New class of intelligence, strongest in coding and cybersecurity

            - `"claude-opus-4-6"`

              Frontier intelligence for long-running agents and coding

            - `"claude-sonnet-4-6"`

              Best combination of speed and intelligence

            - `"claude-haiku-4-5"`

              Fastest model with near-frontier intelligence

            - `"claude-haiku-4-5-20251001"`

              Fastest model with near-frontier intelligence

            - `"claude-opus-4-5"`

              Premium model combining maximum intelligence with practical performance

            - `"claude-opus-4-5-20251101"`

              Premium model combining maximum intelligence with practical performance

            - `"claude-sonnet-4-5"`

              High-performance model for agents and coding

            - `"claude-sonnet-4-5-20250929"`

              High-performance model for agents and coding

            - `"claude-opus-4-1"`

              Exceptional model for specialized complex tasks

            - `"claude-opus-4-1-20250805"`

              Exceptional model for specialized complex tasks

            - `"claude-opus-4-0"`

              Powerful model for complex tasks

            - `"claude-opus-4-20250514"`

              Powerful model for complex tasks

            - `"claude-sonnet-4-0"`

              High-performance model with extended thinking

            - `"claude-sonnet-4-20250514"`

              High-performance model with extended thinking

            - `"claude-3-haiku-20240307"`

              Fast and cost-effective model

          - `string`

      - `to: BetaFallbackInfo`

        The fallback model producing the content that follows this block. Its `model` is always the canonical id.

      - `type: "fallback"`

        - `"fallback"`

  - `context_management: BetaContextManagementResponse`

    Context management response.

    Information about context management strategies applied during the request.

    - `applied_edits: array of BetaClearToolUses20250919EditResponse or BetaClearThinking20251015EditResponse`

      List of context management edits that were applied.

      - `BetaClearToolUses20250919EditResponse object { cleared_input_tokens, cleared_tool_uses, type }`

        - `cleared_input_tokens: number`

          Number of input tokens cleared by this edit.

        - `cleared_tool_uses: number`

          Number of tool uses that were cleared.

        - `type: "clear_tool_uses_20250919"`

          The type of context management edit applied.

          - `"clear_tool_uses_20250919"`

      - `BetaClearThinking20251015EditResponse object { cleared_input_tokens, cleared_thinking_turns, type }`

        - `cleared_input_tokens: number`

          Number of input tokens cleared by this edit.

        - `cleared_thinking_turns: number`

          Number of thinking turns that were cleared.

        - `type: "clear_thinking_20251015"`

          The type of context management edit applied.

          - `"clear_thinking_20251015"`

  - `diagnostics: BetaDiagnostics`

    Response envelope for request-level diagnostics. Present (possibly
    null) whenever the caller supplied `diagnostics` on the request.

    - `cache_miss_reason: BetaCacheMissModelChanged or BetaCacheMissSystemChanged or BetaCacheMissToolsChanged or 3 more`

      Explains why the prompt cache could not fully reuse the prefix from the request identified by `diagnostics.previous_message_id`. `null` means diagnosis is still pending — the response was serialized before the background comparison completed.

      - `BetaCacheMissModelChanged object { cache_missed_input_tokens, type }`

        - `cache_missed_input_tokens: number`

          Approximate number of input tokens that would have been read from cache had the prefix matched the previous request.

        - `type: "model_changed"`

          - `"model_changed"`

      - `BetaCacheMissSystemChanged object { cache_missed_input_tokens, type }`

        - `cache_missed_input_tokens: number`

          Approximate number of input tokens that would have been read from cache had the prefix matched the previous request.

        - `type: "system_changed"`

          - `"system_changed"`

      - `BetaCacheMissToolsChanged object { cache_missed_input_tokens, type }`

        - `cache_missed_input_tokens: number`

          Approximate number of input tokens that would have been read from cache had the prefix matched the previous request.

        - `type: "tools_changed"`

          - `"tools_changed"`

      - `BetaCacheMissMessagesChanged object { cache_missed_input_tokens, type }`

        - `cache_missed_input_tokens: number`

          Approximate number of input tokens that would have been read from cache had the prefix matched the previous request.

        - `type: "messages_changed"`

          - `"messages_changed"`

      - `BetaCacheMissPreviousMessageNotFound object { type }`

        - `type: "previous_message_not_found"`

          - `"previous_message_not_found"`

      - `BetaCacheMissUnavailable object { type }`

        - `type: "unavailable"`

          - `"unavailable"`

  - `model: Model`

    The model that will complete your prompt.

    See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

  - `role: "assistant"`

    Conversational role of the generated message.

    This will always be `"assistant"`.

    - `"assistant"`

  - `stop_details: BetaRefusalStopDetails`

    Structured information about a refusal.

    - `category: "cyber" or "bio" or "reasoning_extraction"`

      The policy category that triggered the refusal.

      `null` when the refusal doesn't map to a named category.

      - `"cyber"`

      - `"bio"`

      - `"reasoning_extraction"`

    - `explanation: string`

      Human-readable explanation of the refusal.

      This text is not guaranteed to be stable. `null` when no explanation is available for the category.

    - `fallback_credit_token: string`

      Opaque code that refunds the cache-miss cost when retrying this refused
      request on the fallback model. Pass it as `fallback_credit_token` on the
      retry request. Expires 5 minutes after the refusal.

      The retry is sent either with the same request body (`system`, `messages`,
      `tools`, and other render-shaping fields), or with the same body plus one
      appended `assistant` message whose content is the partial text (with any
      trailing whitespace stripped from the final text block) and paired
      server-tool blocks from this refusal — which also authorizes that
      appended turn as an assistant-prefill continuation on models that otherwise
      disallow prefill. A token minted mid-server-tool-loop whose partial content
      was continuable may only be redeemed the second way — if a same-body retry
      is rejected with a 400 saying the token must be redeemed by continuing the
      partial response, retry the second way instead. Either way: same workspace,
      same platform; a mismatch is a 400. Resending a token for an already-warm
      prefix is permitted but yields no additional credit.

      `null` when the refused model isn't eligible for a fallback credit.

    - `fallback_has_prefill_claim: boolean`

      Whether the accompanying `fallback_credit_token` may be redeemed with the
      appended-assistant retry form. Only set when `fallback_credit_token` is
      present.

      `true`: retry by resending the same request body plus one appended
      `assistant` message whose content is this response's `content` with any
      trailing whitespace stripped from the final text block and unpaired
      `tool_use` blocks omitted (the same appended-turn shape described on
      `fallback_credit_token`), with the token attached. `false`: retry by
      resending the original request body unchanged, with the token attached —
      the appended-assistant form is not available for this refusal (no
      continuable partial content, or the request uses `output_format` or a
      `tool_choice` that forces tool use). One exception: when the request used
      `output_format` or a forced `tool_choice` and the refusal arrived after
      server tools (including MCP connector tools) had already executed, the
      token may not be redeemable by either retry form; if the exact-body retry
      is then rejected with a 400 saying the token must be redeemed by
      continuing the partial response, discard the token and retry without it.

      Advisory: if an appended-assistant retry is rejected with a 400 despite
      `true`, fall back to resending the original request body with the token.

    - `recommended_model: string`

      The server's suggested retry target for this refusal. Populated when a fallback attempt could not be made (the fallback model's rate limit was exhausted, or it was overloaded); names the fallback model the caller can retry directly. Null otherwise.

    - `type: "refusal"`

      - `"refusal"`

  - `stop_reason: BetaStopReason`

    The reason that we stopped.

    This may be one the following values:

    * `"end_turn"`: the model reached a natural stopping point
    * `"max_tokens"`: we exceeded the requested `max_tokens` or the model's maximum
    * `"stop_sequence"`: one of your provided custom `stop_sequences` was generated
    * `"tool_use"`: the model invoked one or more tools
    * `"pause_turn"`: we paused a long-running turn. You may provide the response back as-is in a subsequent request to let the model continue.
    * `"refusal"`: when streaming classifiers intervene to handle potential policy violations

    In non-streaming mode this value is always non-null. In streaming mode, it is null in the `message_start` event and non-null otherwise.

    - `"end_turn"`

    - `"max_tokens"`

    - `"stop_sequence"`

    - `"tool_use"`

    - `"pause_turn"`

    - `"compaction"`

    - `"refusal"`

    - `"model_context_window_exceeded"`

  - `stop_sequence: string`

    Which custom stop sequence was generated, if any.

    This value will be a non-null string if one of your custom stop sequences was generated.

  - `type: "message"`

    Object type.

    For Messages, this is always `"message"`.

    - `"message"`

  - `usage: BetaUsage`

    Billing and rate-limit usage.

    Anthropic's API bills and rate-limits by token counts, as tokens represent the underlying cost to our systems.

    Under the hood, the API transforms requests into a format suitable for the model. The model's output then goes through a parsing stage before becoming an API response. As a result, the token counts in `usage` will not match one-to-one with the exact visible content of an API request or response.

    For example, `output_tokens` will be non-zero, even for an empty string response from Claude.

    Total input tokens in a request is the summation of `input_tokens`, `cache_creation_input_tokens`, and `cache_read_input_tokens`.

    - `cache_creation: BetaCacheCreation`

      Breakdown of cached tokens by TTL

      - `ephemeral_1h_input_tokens: number`

        The number of input tokens used to create the 1 hour cache entry.

      - `ephemeral_5m_input_tokens: number`

        The number of input tokens used to create the 5 minute cache entry.

    - `cache_creation_input_tokens: number`

      The number of input tokens used to create the cache entry.

    - `cache_read_input_tokens: number`

      The number of input tokens read from the cache.

    - `inference_geo: string`

      The geographic region where inference was performed for this request.

    - `input_tokens: number`

      The number of input tokens which were used.

    - `iterations: BetaIterationsUsage`

      Per-iteration token usage breakdown.

      Each entry represents one sampling iteration, with its own input/output token counts and cache statistics. This allows you to:

      - Determine which iterations exceeded long context thresholds (>=200k tokens)
      - Calculate the true context window size from the last iteration
      - Understand token accumulation across server-side tool use loops

      - `BetaMessageIterationUsage object { cache_creation, cache_creation_input_tokens, cache_read_input_tokens, 4 more }`

        Token usage for a sampling iteration.

        - `cache_creation: BetaCacheCreation`

          Breakdown of cached tokens by TTL

        - `cache_creation_input_tokens: number`

          The number of input tokens used to create the cache entry.

        - `cache_read_input_tokens: number`

          The number of input tokens read from the cache.

        - `input_tokens: number`

          The number of input tokens which were used.

        - `model: Model`

          The model that will complete your prompt.

          See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

        - `output_tokens: number`

          The number of output tokens which were used.

        - `type: "message"`

          Usage for a sampling iteration

          - `"message"`

      - `BetaCompactionIterationUsage object { cache_creation, cache_creation_input_tokens, cache_read_input_tokens, 3 more }`

        Token usage for a compaction iteration.

        - `cache_creation: BetaCacheCreation`

          Breakdown of cached tokens by TTL

        - `cache_creation_input_tokens: number`

          The number of input tokens used to create the cache entry.

        - `cache_read_input_tokens: number`

          The number of input tokens read from the cache.

        - `input_tokens: number`

          The number of input tokens which were used.

        - `output_tokens: number`

          The number of output tokens which were used.

        - `type: "compaction"`

          Usage for a compaction iteration

          - `"compaction"`

      - `BetaAdvisorMessageIterationUsage object { cache_creation, cache_creation_input_tokens, cache_read_input_tokens, 4 more }`

        Token usage for an advisor sub-inference iteration.

        - `cache_creation: BetaCacheCreation`

          Breakdown of cached tokens by TTL

        - `cache_creation_input_tokens: number`

          The number of input tokens used to create the cache entry.

        - `cache_read_input_tokens: number`

          The number of input tokens read from the cache.

        - `input_tokens: number`

          The number of input tokens which were used.

        - `model: Model`

          The model that will complete your prompt.

          See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

        - `output_tokens: number`

          The number of output tokens which were used.

        - `type: "advisor_message"`

          Usage for an advisor sub-inference iteration

          - `"advisor_message"`

      - `BetaFallbackMessageIterationUsage object { cache_creation, cache_creation_input_tokens, cache_read_input_tokens, 4 more }`

        Token usage for the fallback-model attempt of a server-side fallback request.

        Produced in place of a `message` entry for whichever hop served the
        response. A declined hop produces the existing `message` entry. Whether
        a fallback model served the response is signalled by the presence of this
        entry in `usage.iterations`.

        - `cache_creation: BetaCacheCreation`

          Breakdown of cached tokens by TTL

        - `cache_creation_input_tokens: number`

          The number of input tokens used to create the cache entry.

        - `cache_read_input_tokens: number`

          The number of input tokens read from the cache.

        - `input_tokens: number`

          The number of input tokens which were used.

        - `model: Model`

          The model that will complete your prompt.

          See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.

        - `output_tokens: number`

          The number of output tokens which were used.

        - `type: "fallback_message"`

          Usage for the fallback-model attempt that served the response

          - `"fallback_message"`

    - `output_tokens: number`

      The number of output tokens which were used.

    - `output_tokens_details: BetaOutputTokensDetails`

      Breakdown of output tokens by category.

      `output_tokens` remains the inclusive, authoritative total used for billing.
      This object provides a read-only decomposition for observability — for example,
      how many of the billed output tokens were spent on internal reasoning that may
      have been summarized before being returned to you.

      - `thinking_tokens: number`

        Number of output tokens the model generated as internal reasoning, including
        the thinking-block delimiter tokens.

        Reflects the raw reasoning the model produced, not the (possibly shorter)
        summarized thinking text returned in the response body. Computed by
        re-tokenizing the raw reasoning text, so it may differ from the model's exact
        generation count by a small number of tokens. Always ≤ `output_tokens`;
        `output_tokens - thinking_tokens` approximates the non-reasoning output.

    - `server_tool_use: BetaServerToolUsage`

      The number of server tool requests.

      - `web_fetch_requests: number`

        The number of web fetch tool requests.

      - `web_search_requests: number`

        The number of web search tool requests.

    - `service_tier: "standard" or "priority" or "batch"`

      If the request used the priority, standard, or batch tier.

      - `"standard"`

      - `"priority"`

      - `"batch"`

    - `speed: "standard" or "fast"`

      The inference speed mode used for this request.

      - `"standard"`

      - `"fast"`

### Example

```http
curl https://api.anthropic.com/v1/messages \
    -H 'Content-Type: application/json' \
    -H 'anthropic-version: 2023-06-01' \
    -H "X-Api-Key: $ANTHROPIC_API_KEY" \
    --max-time 600 \
    -d "{
          \"max_tokens\": 1024,
          \"messages\": [
            {
              \"content\": \"Hello, world\",
              \"role\": \"user\"
            }
          ],
          \"model\": \"claude-opus-4-6\",
          \"system\": [
            {
              \"text\": \"Today's date is 2024-06-01.\",
              \"type\": \"text\"
            }
          ],
          \"temperature\": 1,
          \"thinking\": {
            \"type\": \"adaptive\"
          },
          \"tools\": [
            {
              \"input_schema\": {
                \"type\": \"object\",
                \"properties\": {
                  \"location\": \"bar\",
                  \"unit\": \"bar\"
                },
                \"required\": [
                  \"location\"
                ]
              },
              \"name\": \"name\"
            }
          ],
          \"top_k\": 5,
          \"top_p\": 0.7
        }"
```

#### Response

```json
{
  "id": "msg_013Zva2CMHLNnXjNJJKqJ2EF",
  "container": {
    "id": "id",
    "expires_at": "2019-12-27T18:11:19.117Z",
    "skills": [
      {
        "skill_id": "pdf",
        "type": "anthropic",
        "version": "latest"
      }
    ]
  },
  "content": [
    {
      "citations": [
        {
          "cited_text": "cited_text",
          "document_index": 0,
          "document_title": "document_title",
          "end_char_index": 0,
          "file_id": "file_id",
          "start_char_index": 0,
          "type": "char_location"
        }
      ],
      "text": "Hi! My name is Claude.",
      "type": "text"
    }
  ],
  "context_management": {
    "applied_edits": [
      {
        "cleared_input_tokens": 0,
        "cleared_tool_uses": 0,
        "type": "clear_tool_uses_20250919"
      }
    ]
  },
  "diagnostics": {
    "cache_miss_reason": {
      "cache_missed_input_tokens": 0,
      "type": "model_changed"
    }
  },
  "model": "claude-opus-4-6",
  "role": "assistant",
  "stop_details": {
    "category": "cyber",
    "explanation": "explanation",
    "fallback_credit_token": "fallback_credit_token",
    "fallback_has_prefill_claim": true,
    "recommended_model": "recommended_model",
    "type": "refusal"
  },
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "type": "message",
  "usage": {
    "cache_creation": {
      "ephemeral_1h_input_tokens": 0,
      "ephemeral_5m_input_tokens": 0
    },
    "cache_creation_input_tokens": 2051,
    "cache_read_input_tokens": 2051,
    "inference_geo": "inference_geo",
    "input_tokens": 2095,
    "iterations": [
      {
        "cache_creation": {
          "ephemeral_1h_input_tokens": 0,
          "ephemeral_5m_input_tokens": 0
        },
        "cache_creation_input_tokens": 0,
        "cache_read_input_tokens": 0,
        "input_tokens": 0,
        "model": "claude-fable-5",
        "output_tokens": 0,
        "type": "message"
      }
    ],
    "output_tokens": 503,
    "output_tokens_details": {
      "thinking_tokens": 0
    },
    "server_tool_use": {
      "web_fetch_requests": 2,
      "web_search_requests": 0
    },
    "service_tier": "standard",
    "speed": "standard"
  }
}
```
