Fix structured output with nested definitions with Gemini via OpenRouter

pydantic_ai_slim/pydantic_ai/models/openrouter.py

@@ -399,7 +399,10 @@ class _OpenRouterChoice(chat_completion.Choice):
 class _OpenRouterCostDetails:
     """OpenRouter specific cost details."""
 
-    upstream_inference_cost: int | None = None
+    upstream_inference_cost: float | None = None
+
+    # TODO rework fields, tests/models/cassettes/test_openrouter/test_openrouter_google_nested_schema.yaml
+    # shows an `upstream_inference_completions_cost` field as well
 
 
 class _OpenRouterPromptTokenDetails(completion_usage.PromptTokensDetails):

pydantic_ai_slim/pydantic_ai/providers/openrouter.py

@@ -1,12 +1,14 @@
 from __future__ import annotations as _annotations
 
 import os
+from dataclasses import replace
 from typing import overload
 
 import httpx
 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
@@ -31,6 +33,64 @@
     ) 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.
+    """
+    profile = google_model_profile(model_name)
+    if profile is None:  # pragma: no cover
+        return None
+    return replace(profile, json_schema_transformer=_OpenRouterGoogleJsonSchemaTransformer)
+
+
 class OpenRouterProvider(Provider[AsyncOpenAI]):
     """Provider for OpenRouter API."""
 
@@ -48,7 +108,7 @@ 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,