anthropic/structured-outputs (Anthropic Structured Outputs)

This example demonstrates Anthropic's structured outputs feature, which ensures Claude's responses follow a specific schema. It shows both JSON outputs for structured data extraction and strict tool use for guaranteed schema validation on tool calls.

What You'll Learn

• How to use output_format to constrain Claude's responses to a JSON schema
• How to use strict: true on tools to guarantee type-safe function parameters
• The difference between JSON outputs and strict tool use
• When to use each approach

Setup

export ANTHROPIC_API_KEY=your_api_key_here
npx promptfoo@latest init --example anthropic/structured-outputs
npx promptfoo@latest eval

Features Demonstrated

JSON Outputs

The first provider configuration shows how to extract structured data from unstructured text using a JSON schema:

providers:
  - id: anthropic:messages:claude-sonnet-4-6
    config:
      output_format:
        type: json_schema
        schema:
          type: object
          properties:
            customer_name:
              type: string
            customer_email:
              type: string
            # ... more fields
          required:
            - customer_name
            - customer_email
          additionalProperties: false

Use JSON outputs when:

• Extracting data from images or text
• Generating structured reports
• Formatting API responses
• You need Claude's response in a specific format

Strict Tool Use

The second provider configuration demonstrates how to ensure tool parameters exactly match your schema:

providers:
  - id: anthropic:messages:claude-sonnet-4-6
    config:
      tools:
        - name: book_demo
          strict: true # Enable strict mode
          input_schema:
            type: object
            properties:
              customer_email:
                type: string
              # ... more properties
            required:
              - customer_email
              - customer_name
            additionalProperties: false

Use strict tool use when:

• Building agentic workflows
• Ensuring type-safe function calls
• Complex tools with many/nested properties
• You need validated parameters and tool names

Key Benefits

• Always valid: No more JSON.parse() errors
• Type safe: Guaranteed field types and required fields
• Reliable: No retries needed for schema violations
• Production-ready: Build agents that work consistently at scale

Schema Requirements

Both modes share these JSON Schema limitations:

✅ Supported:

• Basic types: object, array, string, integer, number, boolean, null
• enum (primitives only)
• required and additionalProperties: false
• Array minItems (only 0 and 1)

❌ Not supported:

• Recursive schemas
• Numerical constraints (minimum, maximum)
• String constraints (minLength, maxLength)
• Complex {n,m} quantifiers in regex patterns

Test Cases

The example includes comprehensive tests:

1. Schema validation: Ensures all required fields are present
2. Data accuracy: Verifies extracted values match the input
3. No extra fields: Confirms additionalProperties: false is enforced
4. Tool calling: Validates tool names and parameters are correct
5. Type safety: Checks that types match schema definitions

Supported Models

Structured outputs are available for:

• Claude Sonnet 4.6 (claude-sonnet-4-6)
• Claude Opus 4.1 (claude-opus-4-1-20250805)

Learn More

• Anthropic Structured Outputs Documentation
• promptfoo Anthropic Provider Documentation