Anthropic Structured Outputs

• Profile: anthropic.so.2026-04-30
• API: Native Claude API JSON outputs (output_config.format) and strict tool use (strict: true)
• Model behavior: Strict structured outputs compile supported JSON Schema into a grammar. SDK helpers may remove unsupported constraints before sending schemas to Claude, then validate locally.

Supported Keywords

Anthropic supports standard JSON Schema with documented limitations. Numeric and string constraints such as minimum, maximum, minLength, and maxLength are not sent directly by SDK helpers; they are moved into descriptions and enforced by local validation.

Value Restrictions

• additionalProperties: Must be false
• format: ["date-time", "time", "date", "duration", "email", "hostname", "uri", "ipv4", "ipv6", "uuid"]
• minItems: Only 0 or 1

Structural Limits

Limit	Value
Root schema type	Object not required, but recommended
additionalProperties	Must be false on all objects
Max nesting depth	Unlimited
Max total properties	Unlimited
Max total enum values	Unlimited
Max string length budget	Unlimited
Optional properties	24 across strict schemas
Union parameters (anyOf / type arrays)	16 across strict schemas
External $ref	Allowed
allOf with $ref	Not supported

The native Claude profile is not the same as Anthropic's OpenAI SDK compatibility layer. Anthropic documents that the compatibility layer ignores OpenAI response_format, and ignores strict for function calling; use the native Claude API for guaranteed schema conformance.

Reference

Anthropic Structured Outputs documentation