From 80d69ae0bb393c728c61c41086b770237b7b676c Mon Sep 17 00:00:00 2001
From: Victorien <65306057+Viicos@users.noreply.github.com>
Date: Tue, 2 Dec 2025 17:59:38 +0100
Subject: [PATCH 1/2] =?UTF-8?q?=F0=9F=90=9B=20Fix=20optional=20sequence=20?=
 =?UTF-8?q?handling=20with=20new=20union=20syntax=20from=20Python=203.10?=
 =?UTF-8?q?=20(#14430)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
---
 fastapi/_compat/v2.py |  4 ++--
 tests/test_compat.py  | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/fastapi/_compat/v2.py b/fastapi/_compat/v2.py
index 543a42dda..0faa7d5a8 100644
--- a/fastapi/_compat/v2.py
+++ b/fastapi/_compat/v2.py
@@ -17,7 +17,7 @@ from typing import (
 
 from fastapi._compat import may_v1, shared
 from fastapi.openapi.constants import REF_TEMPLATE
-from fastapi.types import IncEx, ModelNameMap
+from fastapi.types import IncEx, ModelNameMap, UnionType
 from pydantic import BaseModel, TypeAdapter, create_model
 from pydantic import PydanticSchemaGenerationError as PydanticSchemaGenerationError
 from pydantic import PydanticUndefinedAnnotation as PydanticUndefinedAnnotation
@@ -381,7 +381,7 @@ 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):
diff --git a/tests/test_compat.py b/tests/test_compat.py
index c3a97209a..26537c5ab 100644
--- a/tests/test_compat.py
+++ b/tests/test_compat.py
@@ -14,7 +14,7 @@ from fastapi.testclient import TestClient
 from pydantic import BaseModel, ConfigDict
 from pydantic.fields import FieldInfo
 
-from .utils import needs_py_lt_314, needs_pydanticv2
+from .utils import needs_py310, needs_py_lt_314, needs_pydanticv2
 
 
 @needs_pydanticv2
@@ -148,6 +148,19 @@ def test_serialize_sequence_value_with_optional_list():
     assert isinstance(result, list)
 
 
+@needs_pydanticv2
+@needs_py310
+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."""

From 930b27e5fa32ce150ddcea61f0c913435ae71c83 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]" <github-actions[bot]@users.noreply.github.com>
Date: Tue, 2 Dec 2025 17:00:02 +0000
Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=93=9D=20Update=20release=20notes?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

[skip ci]
---
 docs/en/docs/release-notes.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/docs/en/docs/release-notes.md b/docs/en/docs/release-notes.md
index e80df1709..6fffdc58c 100644
--- a/docs/en/docs/release-notes.md
+++ b/docs/en/docs/release-notes.md
@@ -11,6 +11,10 @@ hide:
 
 * ✨ Handle wrapped dependencies. PR [#9555](https://github.com/fastapi/fastapi/pull/9555) by [@phy1729](https://github.com/phy1729).
 
+### Fixes
+
+* 🐛 Fix optional sequence handling with new union syntax from Python 3.10. PR [#14430](https://github.com/fastapi/fastapi/pull/14430) by [@Viicos](https://github.com/Viicos).
+
 ### Refactors
 
 * 🔥 Remove dangling extra condiitonal no longer needed. PR [#14435](https://github.com/fastapi/fastapi/pull/14435) by [@tiangolo](https://github.com/tiangolo).