For the complete documentation index, see llms.txt. Markdown versions of all docs pages are available by appending .md to any docs URL.

Messages

Send requests through agentgateway using the Anthropic Messages API.

The Anthropic Messages API (/v1/messages) is the native interface for Anthropic Claude models.

About

The Anthropic Messages API is the primary endpoint for Claude models. Agentgateway proxies these requests to your configured providers while providing token usage tracking, observability metrics, and policy enforcement. Agentgateway automatically adds the x-api-key and anthropic-version headers that the Anthropic API requires.

The related /v1/messages/count_tokens endpoint, which estimates token usage before sending a request, is handled by the anthropicTokenCount route type.

Route type configuration

In the simplified llm configuration, agentgateway automatically maps /v1/messages requests to the messages route type, so no explicit route configuration is required.

# yaml-language-server: $schema=https://agentgateway.dev/schema/config
llm:
  models:
  - name: "*"
    provider: anthropic
    params:
      apiKey: "$ANTHROPIC_API_KEY"

To configure the route type explicitly, use the binds/listeners/routes format and set the messages route type in the policies.ai.routes map. To also support token counting, map /v1/messages/count_tokens to the anthropicTokenCount route type.

# yaml-language-server: $schema=https://agentgateway.dev/schema/config
binds:
- port: 4000
  listeners:
  - routes:
    - backends:
      - ai:
          name: anthropic
          provider:
            anthropic: {}
      policies:
        ai:
          routes:
            "/v1/messages": "messages"
            "/v1/messages/count_tokens": "anthropicTokenCount"
        backendAuth:
          key: "$ANTHROPIC_API_KEY"

Using the API

Send a request to the /v1/messages endpoint. The request is forwarded to the Anthropic API and the response is returned to the client.

curl -X POST http://localhost:4000/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-opus-4-6",
    "max_tokens": 100,
    "messages": [{"role": "user", "content": "Hello!"}]
  }'

For Anthropic-specific features such as token counting, extended thinking, and structured outputs, see the Anthropic provider guide.

Token usage tracking

After sending Messages requests, verify that agentgateway recorded token usage metrics.

1. Open the agentgateway metrics endpoint.
2. Look for the agentgateway_gen_ai_client_token_usage metric. The metric includes labels for the token type (input or output) and the model used.

For more information about LLM metrics and observability, see Observe traffic.