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Accessing the API

The API is made available via our web Console. You can use the Workbench to try out the API in the browser and then generate API keys in Account Settings. Use workspaces to segment your API keys and control spend by use case.

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Authentication

All requests to the Anthropic API must include an x-api-key header with your API key. If you are using the Client SDKs, you will set the API when constructing a client, and then the SDK will send the header on your behalf with every request. If integrating directly with the API, you’ll need to send this header yourself.

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Content types

The Anthropic API always accepts JSON in request bodies and returns JSON in response bodies. You will need to send the content-type: application/json header in requests. If you are using the Client SDKs, this will be taken care of automatically.

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Request size limits

The API has a maximum request size of 32 MB for standard endpoints, including the Messages API and Token Counting API. If you exceed this limit, you’ll receive a 413 request_too_large error from Cloudflare. Specific endpoints have different limits:
  • Standard endpoints (Messages, Token Counting): 32 MB
  • Batch API: 256 MB
  • Files API: 500 MB

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Response Headers

The Anthropic API includes the following headers in every response:
  • request-id: A globally unique identifier for the request.
  • anthropic-organization-id: The organization ID associated with the API key used in the request.

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Examples

Shell
curl https://api.anthropic.com/v1/messages \
     --header "x-api-key: $ANTHROPIC_API_KEY" \
     --header "anthropic-version: 2023-06-01" \
     --header "content-type: application/json" \
     --data \
'{
    "model": "claude-opus-4-1-20250805",
    "max_tokens": 1024,
    "messages": [
        {"role": "user", "content": "Hello, world"}
    ]
}'