Structured outputs

Without structured outputs, Claude can generate malformed JSON responses or invalid tool inputs that break your applications. Even with careful prompting, you may encounter:

• Parsing errors from invalid JSON syntax
• Missing required fields
• Inconsistent data types
• Schema violations requiring error handling and retries

Structured outputs guarantee schema-compliant responses through constrained decoding:

• Always valid: No more JSON.parse() errors
• Type safe: Guaranteed field types and required fields
• Reliable: No retries needed for schema violations

JSON outputs

JSON outputs control Claude's response format, ensuring Claude returns valid JSON matching your schema. Use JSON outputs when you need to:

• Control Claude's response format
• Extract data from images or text
• Generate structured reports
• Format API responses

Quick start

Response format: Valid JSON matching your schema in response.content[0].text

How it works

Working with JSON outputs in SDKs

The SDKs provide helpers that make it easier to work with JSON outputs, including schema transformation, automatic validation, and integration with popular schema libraries.

Using native schema definitions

Instead of writing raw JSON schemas, you can use familiar schema definition tools in your language:

• Python: Pydantic models with client.messages.parse()
• TypeScript: Zod schemas with zodOutputFormat()
• Java: Plain Java classes with automatic schema derivation via outputConfig(Class<T>)
• Ruby: Anthropic::BaseModel classes with output_config: {format: Model}
• C#, Go, PHP: Raw JSON schemas passed via output_config

SDK-specific methods

Each SDK provides helpers that make working with structured outputs easier. See individual SDK pages for full details.

How SDK transformation works

The Python and TypeScript SDKs automatically transform schemas with unsupported features:

1. Remove unsupported constraints (e.g., minimum, maximum, minLength, maxLength)
2. Update descriptions with constraint info (e.g., "Must be at least 100"), when the constraint is not directly supported with structured outputs
3. Add additionalProperties: false to all objects
4. Filter string formats to supported list only
5. Validate responses against your original schema (with all constraints)

This means Claude receives a simplified schema, but your code still enforces all constraints through validation.

Example: A Pydantic field with minimum: 100 becomes a plain integer in the sent schema, but the description is updated to "Must be at least 100", and the SDK validates the response against the original constraint.

Common use cases

Strict tool use

For enforcing JSON Schema compliance on tool inputs with grammar-constrained sampling, see Strict tool use.

Using both features together

JSON outputs and strict tool use solve different problems and can be used together:

• JSON outputs control Claude's response format (what Claude says)
• Strict tool use validates tool parameters (how Claude calls your functions)

When combined, Claude can call tools with guaranteed-valid parameters AND return structured JSON responses. This is useful for agentic workflows where you need both reliable tool calls and structured final outputs.

Important considerations

Grammar compilation and caching

Structured outputs use constrained sampling with compiled grammar artifacts. This introduces some performance characteristics to be aware of:

• First request latency: The first time you use a specific schema, there is additional latency while the grammar compiles
• Automatic caching: Compiled grammars are cached for 24 hours from last use, making subsequent requests much faster
• Cache invalidation: The cache is invalidated if you change:
  • The JSON schema structure
  • The set of tools in your request (when using both structured outputs and tool use)
  • Changing only name or description fields does not invalidate the cache

Prompt modification and token costs

When using structured outputs, Claude automatically receives an additional system prompt explaining the expected output format. This means:

• Your input token count is slightly higher
• The injected prompt costs you tokens like any other system prompt
• Changing the output_config.format parameter will invalidate any prompt cache for that conversation thread

JSON Schema limitations

Structured outputs support standard JSON Schema with some limitations. Both JSON outputs and strict tool use share these limitations.

Property ordering

When using structured outputs, properties in objects maintain their defined ordering from your schema, with one important caveat: required properties appear first, followed by optional properties.

For example, given this schema:

The output will order properties as:

1. name (required, in schema order)
2. email (required, in schema order)
3. notes (optional, in schema order)
4. age (optional, in schema order)

This means the output might look like:

If property order in the output is important to your application, ensure all properties are marked as required, or account for this reordering in your parsing logic.

Invalid outputs

While structured outputs guarantee schema compliance in most cases, there are scenarios where the output may not match your schema:

Refusals (stop_reason: "refusal")

Claude maintains its safety and helpfulness properties even when using structured outputs. If Claude refuses a request for safety reasons:

• The response has stop_reason: "refusal"
• You'll receive a 200 status code
• You'll be billed for the tokens generated
• The output may not match your schema because the refusal message takes precedence over schema constraints

Token limit reached (stop_reason: "max_tokens")

If the response is cut off due to reaching the max_tokens limit:

• The response has stop_reason: "max_tokens"
• The output may be incomplete and not match your schema
• Retry with a higher max_tokens value to get the complete structured output

Schema complexity limits

Structured outputs work by compiling your JSON schemas into a grammar that constrains Claude's output. More complex schemas produce larger grammars that take longer to compile. To protect against excessive compilation times, the API enforces several complexity limits.

Explicit limits

The following limits apply to all requests with output_config.format or strict: true:

Additional internal limits

Beyond the explicit limits above, there are additional internal limits on the compiled grammar size. These limits exist because schema complexity doesn't reduce to a single dimension: features like optional parameters, union types, nested objects, and number of tools interact with each other in ways that can make the compiled grammar disproportionately large.

When these limits are exceeded, you'll receive a 400 error with the message "Schema is too complex for compilation." These errors mean the combined complexity of your schemas exceeds what can be efficiently compiled, even if each individual limit above is satisfied. As a final stop-gap, the API also enforces a compilation timeout of 180 seconds. Schemas that pass all explicit checks but produce very large compiled grammars may hit this timeout.

Tips for reducing schema complexity

If you're hitting complexity limits, try these strategies in order:

1. Mark only critical tools as strict. If you have many tools, reserve it for tools where schema violations cause real problems, and rely on Claude's natural adherence for simpler tools.

2. Reduce optional parameters. Make parameters required where possible. Each optional parameter roughly doubles a portion of the grammar's state space. If a parameter always has a reasonable default, consider making it required and having Claude provide that default explicitly.

3. Simplify nested structures. Deeply nested objects with optional fields compound the complexity. Flatten structures where possible.

4. Split into multiple requests. If you have many strict tools, consider splitting them across separate requests or sub-agents.

For persistent issues with valid schemas, contact support with your schema definition.

Data retention

Prompts and responses are processed with ZDR when using structured outputs. However, the JSON schema itself is temporarily cached for up to 24 hours since last use for optimization purposes. No prompt or response data is retained beyond the API response.

For ZDR eligibility across all features, see API and data retention.

Feature compatibility

Works with:

• Batch processing: Process structured outputs at scale with 50% discount
• Token counting: Count tokens without compilation
• Streaming: Stream structured outputs like normal responses
• Combined usage: Use JSON outputs (output_config.format) and strict tool use (strict: true) together in the same request

Incompatible with:

• Citations: Citations require interleaving citation blocks with text, which conflicts with strict JSON schema constraints. Returns 400 error if citations enabled with output_config.format.
• Message Prefilling: Incompatible with JSON outputs