add anthropic json schema support

[aoki-ryusei]

Summary

Adds native JSON Schema structured output support for the Anthropic provider by mapping the common response_format: :json_schema
API to Anthropic's output_config.format API. This allows agents using Anthropic to get schema-validated JSON responses using the same interface as OpenAI — no provider-specific code required.

The implementation adds normalize_response_format in Transforms, which converts the common format to Anthropic's native structure. json_object emulation (prompt prefill) is unchanged.

How it works

normalize_response_format translates the common response_format hash into Anthropic's output_config.format structure. name and strict from the common format are intentionally dropped — Anthropic's API does not use them. JSON Schema Hashes are passed through as-is and are not converted to Anthropic::BaseModel. No beta header is required; output_config is part of the stable Messages API.

Usage

class ColorsAgent < ApplicationAgent
  generate_with :anthropic, model: "claude-haiku-4-5"

  def primary_colors
    prompt(
      "Return the three primary colors.",
      response_format: :json_schema
    )
  end
end

Place a schema file at app/views/agents/colors_agent/primary_colors.json:

{
  "schema": {
    "type": "object",
    "properties": {
      "colors": { "type": "array", "items": { "type": "string" } }
    },
    "required": ["colors"],
    "additionalProperties": false
  }
}

ActiveAgent serializes the request as:

{
  output_config: {
    format: {
      type: "json_schema",
      schema: { ... }
    }
  }
}

Supported input forms

All four input forms produce the same output_config payload:

Form	Example
Bare symbol	response_format: :json_schema
Type-only hash	response_format: { type: "json_schema" }
Inline schema	response_format: { type: "json_schema", json_schema: { name:, schema:, strict: } }
Named schema	response_format: { type: "json_schema", json_schema: "other" }

Files changed

Production code (2 files):

File	Purpose
anthropic/transforms.rb	Add normalize_response_format to convert json_schema to output_config.format; call it in normalize_params
anthropic/request.rb	Read response_format without deleting so @response_format is preserved for json_object emulation; normalize_params handles deletion and conversion

Test code (1 file, 6 tests):

File	Tests
response_format_test.rb	Enable test_request_builder for all four input forms; enable live VCR test asserting response.format.type == "json_schema" and response.message.parsed_json[:colors] is an Array

Supporting files:

File	Purpose
vcr_cassettes/…/test_agent_response_json_schema_*.yml	VCR cassettes for the four json_schema variants
docs/actions/structured_output.md	Update provider support table (Anthropic json_schema: ❌ → 🟩)
docs/providers/anthropic.md	Add JSON Schema Support section; update Limitations

Design decisions

• Pass-through Hash: JSON Schema Hashes are forwarded as-is to Anthropic. No conversion to Anthropic::BaseModel — keeps the
  implementation provider-agnostic and consistent with how OpenAI structured output works in ActiveAgent.
• name / strict dropped: Anthropic's output_config.format does not support these OpenAI fields. They are accepted in the common API
  but not forwarded.
• json_object unchanged: The existing prompt-prefill emulation for json_object is preserved. output_config.format and message
  prefill are incompatible, so they remain separate code paths.
• No beta header: output_config is part of the stable Messages API. Beta is only triggered by MCP or explicit anthropic_beta.

Test plan

• All four input forms produce the same output_config payload — verified by test_request_builder
• Live VCR test confirms response.format.type == "json_schema" and response.message.parsed_json[:colors] is an Array
• Existing json_object and text tests pass unchanged
• Reviewer: verify CI passes on all matrix combinations

[TonsOfFun]

I'm going to merge this as the only failure is from a deprecation on rails main that is now resolved on activeagent main