Fix structured output with nested definitions with Gemini via OpenRouter

pydantic_ai_slim/pydantic_ai/providers/openrouter.py

@@ -7,13 +7,14 @@
 from openai import AsyncOpenAI
 
 from pydantic_ai import ModelProfile
+from pydantic_ai._json_schema import JsonSchema, JsonSchemaTransformer
 from pydantic_ai.exceptions import UserError
 from pydantic_ai.models import cached_async_http_client
 from pydantic_ai.profiles.amazon import amazon_model_profile
 from pydantic_ai.profiles.anthropic import anthropic_model_profile
 from pydantic_ai.profiles.cohere import cohere_model_profile
 from pydantic_ai.profiles.deepseek import deepseek_model_profile
-from pydantic_ai.profiles.google import google_model_profile
+from pydantic_ai.profiles.google import GoogleModelProfile
 from pydantic_ai.profiles.grok import grok_model_profile
 from pydantic_ai.profiles.meta import meta_model_profile
 from pydantic_ai.profiles.mistral import mistral_model_profile
@@ -31,6 +32,70 @@
     ) from _import_error
 
 
+class OpenRouterGoogleJsonSchemaTransformer(JsonSchemaTransformer):
+    """Legacy Google JSON schema transformer for OpenRouter compatibility.
+
+    OpenRouter's compatibility layer doesn't fully support modern JSON Schema features
+    like $defs/$ref and anyOf for nullable types. This transformer restores v1.19.0
+    behavior by inlining definitions and simplifying nullable unions.
+
+    See: https://github.com/pydantic/pydantic-ai/issues/3617
+    """
+
+    def __init__(self, schema: JsonSchema, *, strict: bool | None = None):
+        super().__init__(schema, strict=strict, prefer_inlined_defs=True, simplify_nullable_unions=True)
+
+    def transform(self, schema: JsonSchema) -> JsonSchema:
+        # Remove properties not supported by Gemini
+        schema.pop('$schema', None)
+        schema.pop('title', None)
+        schema.pop('discriminator', None)
+        schema.pop('examples', None)
+        schema.pop('exclusiveMaximum', None)
+        schema.pop('exclusiveMinimum', None)
+
+        if (const := schema.pop('const', None)) is not None:
+            schema['enum'] = [const]
+
+        # Convert enums to string type (legacy Gemini requirement)
+        if enum := schema.get('enum'):
+            schema['type'] = 'string'
+            schema['enum'] = [str(val) for val in enum]
+
+        # Convert oneOf to anyOf for discriminated unions
+        if 'oneOf' in schema and 'type' not in schema:
+            schema['anyOf'] = schema.pop('oneOf')
+
+        # Handle string format -> description
+        type_ = schema.get('type')
+        if type_ == 'string' and (fmt := schema.pop('format', None)):
+            description = schema.get('description')
+            if description:
+                schema['description'] = f'{description} (format: {fmt})'
+            else:
+                schema['description'] = f'Format: {fmt}'
+
+        return schema
+
+
+def openrouter_google_model_profile(model_name: str) -> ModelProfile | None:
+    """Get the model profile for a Google model accessed via OpenRouter.
+
+    Uses the legacy transformer to maintain compatibility with OpenRouter's
+    translation layer, which doesn't fully support modern JSON Schema features.
+    """
+    is_image_model = 'image' in model_name
+    is_3_or_newer = 'gemini-3' in model_name
+    return GoogleModelProfile(
+        json_schema_transformer=OpenRouterGoogleJsonSchemaTransformer,
+        supports_image_output=is_image_model,
+        supports_json_schema_output=is_3_or_newer or not is_image_model,
+        supports_json_object_output=is_3_or_newer or not is_image_model,
+        supports_tools=not is_image_model,
+        google_supports_native_output_with_builtin_tools=is_3_or_newer,
+    )
+
+
 class OpenRouterProvider(Provider[AsyncOpenAI]):
     """Provider for OpenRouter API."""
 
@@ -48,7 +113,7 @@ def client(self) -> AsyncOpenAI:
 
     def model_profile(self, model_name: str) -> ModelProfile | None:
         provider_to_profile = {
-            'google': google_model_profile,
+            'google': openrouter_google_model_profile,
             'openai': openai_model_profile,
             'anthropic': anthropic_model_profile,
             'mistralai': mistral_model_profile,