fix(event_handler): normalize Union and RootModel sequences in body validation

aws_lambda_powertools/event_handler/middlewares/openapi_validation.py

@@ -3,10 +3,11 @@
 import dataclasses
 import json
 import logging
-from typing import TYPE_CHECKING, Any, Callable, Mapping, MutableMapping, Sequence, cast
+from typing import TYPE_CHECKING, Any, Callable, Mapping, MutableMapping, Sequence, Union, cast
 from urllib.parse import parse_qs
 
 from pydantic import BaseModel
+from typing_extensions import get_args, get_origin
 
 from aws_lambda_powertools.event_handler.middlewares import BaseMiddlewareHandler
 from aws_lambda_powertools.event_handler.openapi.compat import (
@@ -25,6 +26,7 @@
     ResponseValidationError,
 )
 from aws_lambda_powertools.event_handler.openapi.params import Param
+from aws_lambda_powertools.event_handler.openapi.types import UnionType
 
 if TYPE_CHECKING:
     from pydantic.fields import FieldInfo
@@ -431,9 +433,41 @@ def _handle_missing_field_value(
         values[field.name] = field.get_default()
 
 
+def _is_or_contains_sequence(annotation: Any) -> bool:
+    """
+    Check if annotation is a sequence or Union/RootModel containing a sequence.
+
+    This function handles complex type annotations like:
+    - List[Model] - direct sequence
+    - Union[Model, List[Model]] - checks if any Union member is a sequence
+    - Optional[List[Model]] - Union[List[Model], None]
+    - RootModel[List[Model]] - checks if the RootModel wraps a sequence
+    - Optional[RootModel[List[Model]]] - Union member that is a RootModel
+    - RootModel[Union[Model, List[Model]]] - RootModel wrapping a Union with a sequence
+    """
+    # Direct sequence check
+    if field_annotation_is_sequence(annotation):
+        return True
+
+    # Check Union members — recurse so we catch RootModel inside Union
+    origin = get_origin(annotation)
+    if origin is Union or origin is UnionType:
+        for arg in get_args(annotation):
+            if _is_or_contains_sequence(arg):
+                return True
+
+    # Check if it's a RootModel wrapping a sequence (or Union containing a sequence)
+    if lenient_issubclass(annotation, BaseModel) and getattr(annotation, "__pydantic_root_model__", False):
+        if hasattr(annotation, "model_fields") and "root" in annotation.model_fields:
+            root_annotation = annotation.model_fields["root"].annotation
+            return _is_or_contains_sequence(root_annotation)
+
+    return False
+
+
 def _normalize_field_value(value: Any, field_info: FieldInfo) -> Any:
     """Normalize field value, converting lists to single values for non-sequence fields."""
-    if field_annotation_is_sequence(field_info.annotation):
+    if _is_or_contains_sequence(field_info.annotation):
         return value
     elif isinstance(value, list) and value:
         return value[0]