From 7845acd5bbbaf36e20cc3687be621d402c06013b Mon Sep 17 00:00:00 2001
From: Amy Wu <wuamy@google.com>
Date: Fri, 25 Sep 2026 16:01:36 -0700
Subject: [PATCH] fix: build a model before reading its field annotations when
 pruning extra fields

`_remove_extra_fields` walks a response against a model's
`model_fields[...].annotation` to find the nested model to recurse into. Models
build on first use, and an unbuilt model's annotations still hold the forward
references they were declared with, so the walk can hand a bare `'Part'` string
to the next level and fail with `AttributeError: 'str' object has no attribute
'model_fields'`. Build the model first; that costs nothing extra, because the
caller validates against the same model immediately afterwards.

PiperOrigin-RevId: 988545915
---
 google/genai/_common.py                  |  8 +++++++
 google/genai/tests/common/test_common.py | 27 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/google/genai/_common.py b/google/genai/_common.py
index 64dd44274..b2a1b4d17 100644
--- a/google/genai/_common.py
+++ b/google/genai/_common.py
@@ -332,6 +332,14 @@ def _remove_extra_fields(model: Any, response: dict[str, object]) -> None:
   Mutates the response in place.
   """
 
+  # Models are built on first use (`defer_build`), and an unbuilt model's field
+  # annotations still hold the forward references they were declared with, e.g.
+  # the string 'Part' rather than the `Part` class. Building the model resolves
+  # them, and costs nothing extra here because the caller validates against
+  # this model immediately afterwards.
+  if isinstance(model, type) and issubclass(model, pydantic.BaseModel):
+    model.model_rebuild(raise_errors=False)
+
   key_values = list(response.items())
 
   for key, value in key_values:
diff --git a/google/genai/tests/common/test_common.py b/google/genai/tests/common/test_common.py
index 1ead6e46d..75ca4b506 100644
--- a/google/genai/tests/common/test_common.py
+++ b/google/genai/tests/common/test_common.py
@@ -952,3 +952,30 @@ class TestModel(_common.BaseModel):
     TestModel.model_validate(data)
 
   assert len(caplog.records) == 0
+
+
+class _ForwardRefOuterModel(_common.BaseModel):
+  # Declared before the model it points at, the way the generated types
+  # sometimes are, so the annotation is a forward reference.
+  items: Optional[List["_ForwardRefInnerModel"]] = None
+
+
+class _ForwardRefInnerModel(_common.BaseModel):
+  value: Optional[str] = None
+
+
+def test_from_response_resolves_forward_references():
+  """Extra fields are pruned from nested models declared as forward references.
+
+  Models are built on first use, so until something builds the outer model its
+  `items` annotation holds an unresolved reference to `_ForwardRefInnerModel`
+  rather than the class, and the pruning walk has nothing to recurse into.
+  """
+  assert not _ForwardRefOuterModel.__pydantic_complete__
+
+  result = _ForwardRefOuterModel._from_response(
+      response={"items": [{"value": "a", "extraField": 1}]}, kwargs={}
+  )
+
+  assert result.items is not None
+  assert result.items[0].value == "a"