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

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.

When using the Anthropic provider, Agentgateway automatically handles additional requirements, such as the x-api-key and anthropic-version headers that the Anthropic API requires.

The related /v1/messages/count_tokens endpoint estimates token usage before sending a request and 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 gateways and 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: $schema=https://agentgateway.dev/schema/config
gateways:
  default:
    port: 4000
routes:
- backends:
  - ai:
      name: anthropic
      provider:
        anthropic: {}
  policies:
    ai:
      routes:
        "/v1/messages": "messages"
        "/v1/messages/count_tokens": "anthropicTokenCount"
    backendAuth:
      key: "$ANTHROPIC_API_KEY"

Provider format conversion

A Messages request does not need a provider that speaks the Anthropic Messages format. When the selected provider advertises a different format, agentgateway converts the request on the way out and converts the reply back into the Messages shape, so the client receives Anthropic responses either way.

Agentgateway uses the first of these formats that the provider supports.

The first three rows are values that a custom provider declares in its formats list. Bedrock Converse is not one of those values. A bedrock provider supports the Converse format and nothing else, so a Messages request that is routed to a Bedrock provider always takes the Converse conversion.

Because completions comes before responses, a provider that advertises both is unaffected by the Responses conversion. That conversion applies to a provider that advertises responses and not completions.

Converting to the Responses format

The Responses conversion covers a common agent subset:

• Text and system instructions
• Image inputs, supplied by URL, base64 data, or file ID
• Function tools, tool choice, and the parallel tool-call preference
• Assistant tool-use history, and tool results that are text
• Structured output JSON schemas
• Prompt cache breakpoints
• Streaming and usage reporting

A Messages feature that the Responses format cannot represent at all fails before the upstream request is sent, and returns a 400 with an unsupported conversion message. These features fail this way:

• Thinking and redacted-thinking history
• Document, search-result, and server-tool content blocks
• Tool results that are not text

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.