Create a Message

POST /anthropic/v1/messages● Supported

Request body

modelstring必填◐ Partially supported
Body param: The model that will complete your prompt. See [models](https://docs.anthropic.com/en/docs/models-overview) for additional details and options.
deepseek-v4-pro, glm-5.3-flash, qwen3.8-flash and claude-opus-5 can be called; deepseek-v4-flash returns 400 on this face.
max_tokensnumber必填
Body param: 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://platform.claude.com/docs/en/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://platform.claude.com/docs/en/about-claude/models/overview) for details.
Limits output tokens, including reasoning.
messagesarray必填◐ Partially supported
Body param: 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://platform.claude.com/docs/en/build-with-claude/working-with-messages). Note that if you want to include a [system prompt](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices#give-claude-a-role), 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.
Text and tool round trips work. Managed document file_id returns 400; qwen3.8-flash can read inline PDFs. Images have model-specific size limits.
▸ 展开子字段（2）
- contentstring | array必填
  Message content: a plain string (shorthand for one text block) or an array of content blocks such as text, image, document, tool_use and tool_result.
- rolestring必填
  Who produced the message. Use user for your turns and assistant to replay earlier model turns; system-role messages are accepted by the shape but system instructions normally go in the top-level system field.
streamboolean可选◐ Partially supported
Body param: Whether to incrementally stream the response using server-sent events. See [streaming](https://platform.claude.com/docs/en/build-with-claude/streaming) for details.
Content and tool streaming work; extra message_delta events may precede content and message_start usage is zero.
systemstring | array可选
Body param: 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://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices#give-claude-a-role).
Verified to take effect.
temperaturenumber可选
top_pnumber可选
top_knumber可选
stop_sequencesarray可选◐ Partially supported
Body param: 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.
Works on deepseek-v4-pro and qwen3.8-flash; glm-5.3-flash truncates text but does not report accurate stop_reason/stop_sequence.
toolsarray可选
Body param: 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://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools), see their individual documentation as each has its own behavior (e.g., the [web search tool](https://platform.claude.com/docs/en/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://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) for more details.
Verified to take effect.
▸ 更多参数（16）
- tool_choiceobject可选◐ Partially supported
  Body param: 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.
  All four modes work on qwen3.8-flash; deepseek-v4-pro ignores any/tool/none and glm-5.3-flash ignores none.
  ▸ 展开子字段（3）
  - typestring必填
    取值 none
  - disable_parallel_tool_useboolean可选○ Currently ignored
    Whether to disable parallel tool use. Defaults to `false`. If set to `true`, the model will output exactly one tool use.
    Accepted but currently has no effect. true may still return multiple tool calls.
  - namestring必填
    The name of the tool to use.
- metadataobject可选◐ Partially supported
  Body param: An object describing metadata about the request.
  Accepts metadata; metadata.user_id is carried on same-protocol lines without promising an upstream echo.
  ▸ 展开子字段（1）
  - user_idstring可选
    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.
- cache_controlobject可选◐ Partially supported
  Body param: Top-level cache control automatically applies a cache_control marker to the last cacheable block in the request.
  Implicit cache hits occur on deepseek-v4-pro, qwen3.8-flash and glm-5.3-flash, but do not establish that the marker controls caching. No hits were observed on claude-opus-5, and ttl=1h had no observed effect.
  ▸ 展开子字段（2）
  - typestring必填
    Always `ephemeral`. Discriminator that marks this object as a CacheControlEphemeral in a `type`-tagged union.
    取值 ephemeral
  - ttlstring可选
    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`. See [prompt caching pricing](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) for details.
- containerstring | object可选○ Currently ignored
  Accepted and ignored; the platform does not implement container.
  ▸ 展开子字段（2）
  - idstring可选
    Container id
  - skillsarray可选
    List of skills to load in the container
- inference_geostring可选○ Currently ignored
  Accepted and ignored; the platform does not implement inference_geo.
- output_configobject可选
  Body param: Configuration options for the model's output, such as the output format.
  ▸ 展开子字段（2）
  - effortstring可选◐ Partially supported
    All possible effort levels.
    Only high/xhigh work on deepseek-v4-pro; other levels return 400. All levels return 400 on qwen3.8-flash. glm-5.3-flash accepts levels but usage is not monotonic.
  - formatobject可选◐ Partially supported
    A schema to specify Claude's output format in responses. See [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs)
    json_schema works on deepseek-v4-pro and qwen3.8-flash; glm-5.3-flash returns 400 when combined with thinking.
- service_tierstring可选○ Currently ignored
  Accepted and ignored; the platform does not implement service_tier.
- thinkingobject可选◐ Partially supported
  Body param: 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://platform.claude.com/docs/en/build-with-claude/extended-thinking) for details.
  disabled works on deepseek-v4-pro and qwen3.8-flash; any thinking parameter returns 400 on glm-5.3-flash. adaptive also returns 400 on tested deepseek-v4-pro and qwen3.8-flash.
  ▸ 展开子字段（3）
  - budget_tokensnumber必填◐ Partially supported
    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://platform.claude.com/docs/en/build-with-claude/extended-thinking) for details.
    Accepted on deepseek-v4-pro and qwen3.8-flash but is not a reliable upper bound; glm-5.3-flash returns 400.
  - typestring必填
    取值 adaptive
  - displaystring可选○ Currently ignored
    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`.
    Accepted but currently has no effect. omitted still returns the full thinking content.
- context_managementobject可选○ Currently ignored
  Accepted and ignored; the platform does not implement context_management.
- diagnosticsobject可选○ Currently ignored
  Accepted and ignored; the platform does not implement diagnostics.
- mcp_serversarray可选○ Currently ignored
  Accepted and ignored; the platform does not implement mcp_servers.
- speedstring可选○ Currently ignored
  Accepted and ignored; the platform does not implement speed.
- output_formatobject可选○ Currently ignored
  Accepted and ignored legacy field; use output_config.format for structured output.
- compactionobject可选○ Unsupported
  Recognized compaction; not implemented, and returns 400 explicitly stating that the parameter is not supported.
- fallback_credit_tokenstring可选○ Unsupported
  Recognized fallback_credit_token; not implemented, and returns 400 explicitly stating that the parameter is not supported.
- fallbacksstring可选○ Unsupported
  Recognized fallbacks; not implemented, and returns 400 explicitly stating that the parameter is not supported.

Returns

200

With stream=true the reply streams as server-sent events (text/event-stream).

Preserves top-level cache_control, top_k, metadata.user_id, output_config.format, thinking.display and anthropic-user-profile-id. Native-only options restrict routing; requests fail when no provider can preserve them. Nonempty anthropic-workspace-id returns 400. container, inference_geo and service_tier are accepted and ignored; see field support for other extensions. Unknown top-level fields are forwarded unchanged to same-protocol upstream providers, which decide whether to accept them. Declared fields still undergo type and range validation; invalid values return 400 naming the field.

Errors follow the Anthropic format — see error codes

Response

HTTP 200

Message created

application/json

Name	Type	Description
id	string	Unique object identifier. The format and length of IDs may change over time.
container	null \| object	Information about the container used in the request (for the code execution tool)
container.id	string	Identifier for the container used in this request
container.expires_at	string	The time at which the container will expire.
container.skills	null \| array<object>	Skills loaded in the container
content	array<object>	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)" }] ```
content[].citations	null \| array<object>	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`.
content[].text	string	The text generated by the model.
content[].type	string	Always `text`. Discriminator that marks this object as a TextBlock in a `type`-tagged union. / Always `thinking`. Discriminator that marks this object as a ThinkingBlock in a `type`-tagged union. / Always `redacted_thinking`. Discriminator that marks this object as a RedactedThinkingBlock in a `type`-tagged union. / Always `tool_use`. Discriminator that marks this object as a ToolUseBlock in a `type`-tagged union. / Always `server_tool_use`. Discriminator that marks this object as a ServerToolUseBlock in a `type`-tagged union. / Always `web_search_tool_result`. Discriminator that marks this object as a WebSearchToolResultBlock in a `type`-tagged union. / Always `web_fetch_tool_result`. Discriminator that marks this object as a WebFetchToolResultBlock in a `type`-tagged union. / Always `code_execution_tool_result`. Discriminator that marks this object as a CodeExecutionToolResultBlock in a `type`-tagged union. / Always `bash_code_execution_tool_result`. Discriminator that marks this object as a BashCodeExecutionToolResultBlock in a `type`-tagged union. / Always `text_editor_code_execution_tool_result`. Discriminator that marks this object as a TextEditorCodeExecutionToolResultBlock in a `type`-tagged union. / Always `tool_search_tool_result`. Discriminator that marks this object as a ToolSearchToolResultBlock in a `type`-tagged union. / Always `container_upload`. Discriminator that marks this object as a ContainerUploadBlock in a `type`-tagged union.
content[].signature	string	A value used to verify that this thinking block was generated by Claude when it is passed back to the API. This is an opaque field and should not be interpreted or parsed. When passing thinking blocks back to the API (required when using tools with extended thinking), pass them back exactly as received, with this field intact. See extended thinking for details.
content[].thinking	string	The text of Claude's thinking process for this block.
content[].data	string	The contents of this redacted thinking block, returned when portions of the model's thinking were safety-redacted. This field is opaque and encrypted, with no readable content. Pass `redacted_thinking` blocks back to the API unchanged when continuing a multi-turn conversation. See extended thinking for details.
content[].id	string	Id of the tool call; send it back as tool_use_id in the tool_result block that answers it. / Id of the server tool call; the matching result block repeats it as tool_use_id.
content[].caller	object	Which party invoked the tool: the model directly or a code execution tool. / Which party invoked the server tool: the model directly or a code execution tool. / Which party invoked the search: the model directly or a code execution tool. / Which party invoked the fetch: the model directly or a code execution tool.
content[].input	any	Arguments the model chose, as a JSON object that follows the tool's input_schema. / Arguments the model passed to the server tool, as a JSON object.
content[].name	string	Name of the tool the model wants to call. / Name of the server tool the model called, such as web_search, web_fetch or code_execution.
content[].toolset_name	null \| string	For a toolset member tool_use, the toolset family.
content[].content	object \| array<object> \| object \| object \| object \| object \| object	Search results (array of web_search_result blocks) or an error object. / The fetched document or an error object. / Result of the code execution: a result block, an encrypted result block, or an error object. / Result of the bash command: a result block or an error object. / Result of the text editor command: a view, create or str_replace result block, or an error object. / Result of the tool search: a search result block that lists tool references, or an error object.
content[].tool_use_id	string	Id of the web_search server tool call this result belongs to. / Id of the web_fetch server tool call this result belongs to. / Id of the code_execution server tool call this result belongs to. / Id of the bash_code_execution server tool call this result belongs to. / Id of the text_editor_code_execution server tool call this result belongs to. / Id of the tool search server tool call this result belongs to.
content[].file_id	string	Id of the file that was uploaded into the code execution container.
model	string	The model that will complete your prompt. See models for additional details and options.
role	string	Conversational role of the generated message. This will always be `"assistant"`.
stop_details	null \| object	Structured information about a refusal.
stop_details.category	null \| string	The policy category that triggered a refusal. - `cyber` - The request could enable cyber harm, such as malware or exploit development. Benign cybersecurity work can also trigger this category. - `bio` - The request could enable biological harm, such as dangerous lab methods. Beneficial life sciences work can also trigger this category. - `frontier_llm` - The request could assist the development of competing AI models, which is restricted under Anthropic's commercial terms. Benign machine learning work can also trigger this category. - `reasoning_extraction` - The request asks the model to reproduce its internal reasoning in the response text. To get reasoning in a structured form instead, use adaptive thinking. - `general_harms` - The request could be related to an area that was determined as harmful. Benign work might sometimes trigger this category.
stop_details.explanation	null \| string	Human-readable explanation of the refusal. This text is not guaranteed to be stable. `null` when no explanation is available for the category.
stop_details.type	string	Always `refusal`. Discriminator that marks this object as a RefusalStopDetails in a `type`-tagged union.
stop_reason	null \| string	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 - `"model_context_window_exceeded"`: we exceeded the model's context window 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.
stop_sequence	null \| 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	string	Object type. For Messages, this is always `"message"`.
usage	object	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`.
usage.cache_creation	null \| object	Breakdown of cached tokens by TTL
usage.cache_creation_input_tokens	null \| number	The number of input tokens used to create the cache entry.
usage.cache_read_input_tokens	null \| number	The number of input tokens read from the cache.
usage.inference_geo	null \| string	The geographic region where inference was performed for this request.
usage.input_tokens	number	The number of input tokens which were used.
usage.output_tokens	number	The number of output tokens which were used.
usage.output_tokens_details	null \| object	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.
usage.server_tool_use	null \| object	The number of server tool requests.
usage.service_tier	null \| string	If the request used the priority, standard, or batch tier.

text/event-stream — chunk / event object

Name	Type	Description
delta	object	Final fields of the message sent with message_delta: why generation stopped and the container. / The incremental change to the content block at index.
delta.container	null \| object	Information about the container used in the request (for the code execution tool)
delta.stop_details	null \| object	Structured information about a refusal.
delta.stop_reason	null \| string	Why the model stopped; null while the message is still generating.
delta.stop_sequence	null \| string	The custom stop sequence that was matched; null unless stop_reason is stop_sequence.
delta.text	string	Text fragment to append to the text block.
delta.type	string	Always `text_delta`. Discriminator that marks this object as a TextDelta in a `type`-tagged union. / Always `input_json_delta`. Discriminator that marks this object as a InputJSONDelta in a `type`-tagged union. / Always `citations_delta`. Discriminator that marks this object as a CitationsDelta in a `type`-tagged union. / Always `thinking_delta`. Discriminator that marks this object as a ThinkingDelta in a `type`-tagged union. / Always `signature_delta`. Discriminator that marks this object as a SignatureDelta in a `type`-tagged union.
delta.partial_json	string	Fragment of the tool input JSON string; concatenate all fragments of a block and parse the result when the block stops.
delta.citation	object	The citation to attach to the text block being streamed.
delta.thinking	string	The incremental `thinking` text for this content block. Concatenate the `thinking` values of successive `thinking_delta` events to assemble the block's full `thinking` value.
delta.signature	string	The `signature` for this thinking block: an opaque value used to verify that the block was generated by Claude when it is passed back to the API. Delivered in a `signature_delta` event just before the block's `content_block_stop` event.
type	string	Always `message_delta`. Discriminator that marks this object as a RawMessageDeltaEvent in a `type`-tagged union. / Always `content_block_delta`. Discriminator that marks this object as a RawContentBlockDeltaEvent in a `type`-tagged union. / Always `message_start`. Discriminator that marks this object as a RawMessageStartEvent in a `type`-tagged union. / Always `message_stop`. Discriminator that marks this object as a RawMessageStopEvent in a `type`-tagged union. / Always `content_block_start`. Discriminator that marks this object as a RawContentBlockStartEvent in a `type`-tagged union. / Always `content_block_stop`. Discriminator that marks this object as a RawContentBlockStopEvent in a `type`-tagged union.
usage	object	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`.
usage.cache_creation_input_tokens	null \| number	The cumulative number of input tokens used to create the cache entry.
usage.cache_read_input_tokens	null \| number	The cumulative number of input tokens read from the cache.
usage.input_tokens	null \| number	The cumulative number of input tokens which were used.
usage.output_tokens	number	The cumulative number of output tokens which were used.
usage.output_tokens_details	null \| object	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.
usage.server_tool_use	null \| object	The number of server tool requests.
index	number	Zero-based index of the content block this delta applies to. / Zero-based index of the content block that starts. / Zero-based index of the content block that ends.
message	object	The complete Message object being started; its content is empty at message_start and filled by the following content_block events.
message.id	string	Unique object identifier. The format and length of IDs may change over time.
message.container	null \| object	Information about the container used in the request (for the code execution tool)
message.content	array<object>	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)" }] ```
message.model	string	The model that will complete your prompt. See models for additional details and options.
message.role	string	Conversational role of the generated message. This will always be `"assistant"`.
message.stop_details	null \| object	Structured information about a refusal.
message.stop_reason	null \| string	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 - `"model_context_window_exceeded"`: we exceeded the model's context window 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.
message.stop_sequence	null \| 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.
message.type	string	Object type. For Messages, this is always `"message"`.
message.usage	object	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`.
content_block	object	The content block as it starts, with empty text or input to be filled by deltas.
content_block.citations	null \| array<object>	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`.
content_block.text	string	The text generated by the model.
content_block.type	string	Always `text`. Discriminator that marks this object as a TextBlock in a `type`-tagged union. / Always `thinking`. Discriminator that marks this object as a ThinkingBlock in a `type`-tagged union. / Always `redacted_thinking`. Discriminator that marks this object as a RedactedThinkingBlock in a `type`-tagged union. / Always `tool_use`. Discriminator that marks this object as a ToolUseBlock in a `type`-tagged union. / Always `server_tool_use`. Discriminator that marks this object as a ServerToolUseBlock in a `type`-tagged union. / Always `web_search_tool_result`. Discriminator that marks this object as a WebSearchToolResultBlock in a `type`-tagged union. / Always `web_fetch_tool_result`. Discriminator that marks this object as a WebFetchToolResultBlock in a `type`-tagged union. / Always `code_execution_tool_result`. Discriminator that marks this object as a CodeExecutionToolResultBlock in a `type`-tagged union. / Always `bash_code_execution_tool_result`. Discriminator that marks this object as a BashCodeExecutionToolResultBlock in a `type`-tagged union. / Always `text_editor_code_execution_tool_result`. Discriminator that marks this object as a TextEditorCodeExecutionToolResultBlock in a `type`-tagged union. / Always `tool_search_tool_result`. Discriminator that marks this object as a ToolSearchToolResultBlock in a `type`-tagged union. / Always `container_upload`. Discriminator that marks this object as a ContainerUploadBlock in a `type`-tagged union.
content_block.signature	string	A value used to verify that this thinking block was generated by Claude when it is passed back to the API. This is an opaque field and should not be interpreted or parsed. When passing thinking blocks back to the API (required when using tools with extended thinking), pass them back exactly as received, with this field intact. See extended thinking for details.
content_block.thinking	string	The text of Claude's thinking process for this block.
content_block.data	string	The contents of this redacted thinking block, returned when portions of the model's thinking were safety-redacted. This field is opaque and encrypted, with no readable content. Pass `redacted_thinking` blocks back to the API unchanged when continuing a multi-turn conversation. See extended thinking for details.
content_block.id	string	Id of the tool call; send it back as tool_use_id in the tool_result block that answers it. / Id of the server tool call; the matching result block repeats it as tool_use_id.
content_block.caller	object	Which party invoked the tool: the model directly or a code execution tool. / Which party invoked the server tool: the model directly or a code execution tool. / Which party invoked the search: the model directly or a code execution tool. / Which party invoked the fetch: the model directly or a code execution tool.
content_block.input	any	Arguments the model chose, as a JSON object that follows the tool's input_schema. / Arguments the model passed to the server tool, as a JSON object.
content_block.name	string	Name of the tool the model wants to call. / Name of the server tool the model called, such as web_search, web_fetch or code_execution.
content_block.toolset_name	null \| string	For a toolset member tool_use, the toolset family.
content_block.content	object \| array<object> \| object \| object \| object \| object \| object	Search results (array of web_search_result blocks) or an error object. / The fetched document or an error object. / Result of the code execution: a result block, an encrypted result block, or an error object. / Result of the bash command: a result block or an error object. / Result of the text editor command: a view, create or str_replace result block, or an error object. / Result of the tool search: a search result block that lists tool references, or an error object.
content_block.tool_use_id	string	Id of the web_search server tool call this result belongs to. / Id of the web_fetch server tool call this result belongs to. / Id of the code_execution server tool call this result belongs to. / Id of the bash_code_execution server tool call this result belongs to. / Id of the text_editor_code_execution server tool call this result belongs to. / Id of the tool search server tool call this result belongs to.
content_block.file_id	string	Id of the file that was uploaded into the code execution container.