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"