Anthropic Messages API
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 supports text, images, tool use, extended thinking, streaming, structured outputs, prompt caching, compaction, and the new advisor tool for pairing executor and higher-intelligence advisor models.
Anthropic Messages API is one of 24 APIs that Anthropic publishes on the APIs.io network, described by a machine-readable OpenAPI specification and an AsyncAPI event-driven specification.
This API exposes 2 JSON Schema definitions.
Tagged areas include AI, Artificial Intelligence, and Messages. The published artifact set on APIs.io includes API documentation, an OpenAPI specification, an AsyncAPI specification, a JSON-LD context, and 2 JSON Schemas.
This API exposes 1 operation across 1 path, and defines 26 schemas. It is described by OpenAPI 3.1.0, at version 1.0.0.
Requests are made against a single base URL, https://api.anthropic.com/v1.
Metadata
The identity and technical contract details declared by the specification.
Authentication & Security 1
Anthropic Messages API declares
1 security scheme
for authenticating requests.
An API key is passed in the header as x-api-key (AdminApiKeyAuth).
By default, every request must be authenticated.
AdminApiKeyAuth— Your Admin API key for authentication (starts with sk-ant-admin...).
Paths & Operations 1
Across 1 path, the API surfaces 1 operation — 1 POST. Each is listed below with its method, path, parameters, and response codes.
Create messages using Claude models
Schemas 26
The contract defines 26 schemas that model the data the API accepts and returns. The most detailed are CreateMessageRequest (15 properties), MessageResponse (8 properties), Citation (7 properties), DocumentSource (5 properties). Each schema is shown below with its type and property counts.
Specification
The full machine-readable OpenAPI contract behind this narrative.
Source
More from Anthropic 12
Other APIs Anthropic publishes across the network.
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