Fix optional sequence handling with new union syntax

fastapi/_compat/v2.py

@@ -3,6 +3,7 @@ import warnings
 from copy import copy, deepcopy
 from dataclasses import dataclass
 from enum import Enum
+from types import UnionType
 from typing import (
     Any,
     Dict,
@@ -371,7 +372,7 @@ def copy_field_info(*, field_info: FieldInfo, annotation: Any) -> FieldInfo:
 
 def serialize_sequence_value(*, field: ModelField, value: Any) -> Sequence[Any]:
     origin_type = get_origin(field.field_info.annotation) or field.field_info.annotation
-    if origin_type is Union:  # Handle optional sequences
+    if origin_type is Union or origin_type is UnionType:  # Handle optional sequences
         union_args = get_args(field.field_info.annotation)
         for union_arg in union_args:
             if union_arg is type(None):

tests/test_compat.py

@@ -1,3 +1,6 @@
+import sys
+import pytest
+
 from typing import Any, Dict, List, Union
 
 from fastapi import FastAPI, UploadFile
@@ -148,6 +151,22 @@ def test_serialize_sequence_value_with_optional_list():
     assert isinstance(result, list)
 
 
+@needs_pydanticv2
+@pytest.mark.skipif(
+    sys.version_info < (3, 10),
+    reason='Requires new union syntax',
+)
+def test_serialize_sequence_value_with_optional_list_pipe_union():
+    """Test that serialize_sequence_value handles optional lists correctly (with new syntax)."""
+    from fastapi._compat import v2
+
+    field_info = FieldInfo(annotation=list[str] | None)
+    field = v2.ModelField(name="items", field_info=field_info)
+    result = v2.serialize_sequence_value(field=field, value=["a", "b", "c"])
+    assert result == ["a", "b", "c"]
+    assert isinstance(result, list)
+
+
 @needs_pydanticv2
 def test_serialize_sequence_value_with_none_first_in_union():
     """Test that serialize_sequence_value handles Union[None, List[...]] correctly."""