From 1a805a9a425b8d1d958fa9b4270c0fec699276b1 Mon Sep 17 00:00:00 2001
From: Nishchay Mahor <nishchaymahor@gmail.com>
Date: Mon, 27 Jul 2026 14:49:29 -0700
Subject: [PATCH] fix: honor field defaults in nested Pydantic model function
 schemas
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When generating a function-calling schema for a parameter that is a Pydantic
model, each sub-field was built from an inspect.Parameter without a default,
so its schema default stayed unset and _get_required_fields marked every
non-Optional field as required — even fields with a default. This forced the
model to supply values for optional parameters. Forward the field's default
(resolving default_factory) into the sub-field parameter so defaulted fields
are correctly optional and their default is emitted in the schema.
---
 .../genai/_automatic_function_calling_util.py | 18 +++++--
 google/genai/tests/types/test_types.py        | 49 +++++++++++++++++++
 2 files changed, 62 insertions(+), 5 deletions(-)

diff --git a/google/genai/_automatic_function_calling_util.py b/google/genai/_automatic_function_calling_util.py
index ec7f9a702..4c94344a6 100644
--- a/google/genai/_automatic_function_calling_util.py
+++ b/google/genai/_automatic_function_calling_util.py
@@ -301,13 +301,21 @@ def _parse_schema_from_parameter(  # type: ignore[return]
     schema.type = _py_builtin_type_to_schema_type[dict]
     schema.properties = {}
     for field_name, field_info in param.annotation.model_fields.items():
+      field_param = inspect.Parameter(
+          field_name,
+          inspect.Parameter.POSITIONAL_OR_KEYWORD,
+          annotation=field_info.annotation,
+      )
+      # Forward the field's default so fields with a default are not marked
+      # required. Without this the sub-schema default stays unset and every
+      # non-Optional field is treated as required.
+      if not field_info.is_required():
+        field_param = field_param.replace(
+            default=field_info.get_default(call_default_factory=True)
+        )
       schema.properties[field_name] = _parse_schema_from_parameter(
           api_option,
-          inspect.Parameter(
-              field_name,
-              inspect.Parameter.POSITIONAL_OR_KEYWORD,
-              annotation=field_info.annotation,
-          ),
+          field_param,
           func_name,
       )
     schema.required = _get_required_fields(schema)
diff --git a/google/genai/tests/types/test_types.py b/google/genai/tests/types/test_types.py
index 50a3f9058..2ada314d9 100644
--- a/google/genai/tests/types/test_types.py
+++ b/google/genai/tests/types/test_types.py
@@ -1514,6 +1514,55 @@ def func_under_test(
   assert actual_schema_vertex == expected_schema
 
 
+def test_pydantic_model_with_default_fields():
+  class MyPydanticModel(pydantic.BaseModel):
+    a: int
+    b: int = 5
+    c: str = 'hello'
+    d: list[int] = pydantic.Field(default_factory=list)
+
+  def func_under_test(
+      a: MyPydanticModel,
+  ):
+    """test pydantic model with default fields."""
+    pass
+
+  expected_schema = types.FunctionDeclaration(
+      name='func_under_test',
+      parameters=types.Schema(
+          type='OBJECT',
+          properties={
+              'a': types.Schema(
+                  type='OBJECT',
+                  properties={
+                      'a': types.Schema(type='INTEGER'),
+                      'b': types.Schema(type='INTEGER', default=5),
+                      'c': types.Schema(type='STRING', default='hello'),
+                      'd': types.Schema(
+                          type='ARRAY',
+                          default=[],
+                          items=types.Schema(type='INTEGER'),
+                      ),
+                  },
+                  required=['a'],
+              ),
+          },
+          required=['a'],
+      ),
+      description='test pydantic model with default fields.',
+  )
+
+  actual_schema_mldev = types.FunctionDeclaration.from_callable(
+      client=mldev_client, callable=func_under_test
+  )
+  actual_schema_vertex = types.FunctionDeclaration.from_callable(
+      client=vertex_client, callable=func_under_test
+  )
+
+  assert actual_schema_mldev == expected_schema
+  assert actual_schema_vertex == expected_schema
+
+
 @pytest.mark.skip(
     reason=(
         'AFC is in progress of refactoring, this test is failing python 3.14'