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Structured Output /v1/messages

Use LiteLLM to call Anthropic's structured output feature via the /v1/messages endpoint.

Supported Providers​

ProviderSupportedNotes
Anthropic✅Native support
Azure AI (Anthropic models)✅Claude models on Azure AI
Bedrock (Converse Anthropic models)✅Claude models via Bedrock Converse API
Bedrock (Invoke Anthropic models)✅Claude models via Bedrock Invoke API

Usage​

LiteLLM Proxy Server​

  1. Setup config.yaml
model_list:
  - model_name: claude-sonnet
    litellm_params:
      model: anthropic/claude-sonnet-4-5-20250514
      api_key: os.environ/ANTHROPIC_API_KEY
  1. Start proxy
litellm --config /path/to/config.yaml
  1. Test it!
curl http://localhost:4000/v1/messages \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $LITELLM_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet",
    "max_tokens": 1024,
    "messages": [
      {
        "role": "user",
        "content": "Extract the key information from this email: John Smith (john@example.com) is interested in our Enterprise plan and wants to schedule a demo for next Tuesday at 2pm."
      }
    ],
    "output_format": {
      "type": "json_schema",
      "schema": {
        "type": "object",
        "properties": {
          "name": {"type": "string"},
          "email": {"type": "string"},
          "plan_interest": {"type": "string"},
          "demo_requested": {"type": "boolean"}
        },
        "required": ["name", "email", "plan_interest", "demo_requested"],
        "additionalProperties": false
      }
    }
  }'

Example Response​

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "{\"name\":\"John Smith\",\"email\":\"john@example.com\",\"plan_interest\":\"Enterprise\",\"demo_requested\":true}"
    }
  ],
  "model": "claude-sonnet-4-5-20250514",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 75,
    "output_tokens": 28
  }
}

Request Format​

output_format​

The output_format parameter specifies the structured output format.

{
  "output_format": {
    "type": "json_schema",
    "schema": {
      "type": "object",
      "properties": {
        "field_name": {"type": "string"},
        "another_field": {"type": "integer"}
      },
      "required": ["field_name", "another_field"],
      "additionalProperties": false
    }
  }
}

Fields​

  • type (string): Must be "json_schema"
  • schema (object): A JSON Schema object defining the expected output structure
    • type (string): The root type, typically "object"
    • properties (object): Defines the fields and their types
    • required (array): List of required field names
    • additionalProperties (boolean): Set to false to enforce strict schema adherence