diff --git a/fastapi/_compat/shared.py b/fastapi/_compat/shared.py
index 4e1b7bcb6e..c009da8fdc 100644
--- a/fastapi/_compat/shared.py
+++ b/fastapi/_compat/shared.py
@@ -8,6 +8,7 @@ from dataclasses import is_dataclass
 from typing import (
     Annotated,
     Any,
+    TypeVar,
     Union,
 )
 
@@ -15,7 +16,9 @@ from fastapi.types import UnionType
 from pydantic import BaseModel
 from pydantic.version import VERSION as PYDANTIC_VERSION
 from starlette.datastructures import UploadFile
-from typing_extensions import get_args, get_origin
+from typing_extensions import TypeGuard, get_args, get_origin
+
+_T = TypeVar("_T")
 
 # Copy from Pydantic: pydantic/_internal/_typing_extra.py
 if sys.version_info < (3, 10):
@@ -39,13 +42,13 @@ sequence_annotation_to_type = {
     deque: deque,
 }
 
-sequence_types = tuple(sequence_annotation_to_type.keys())
+sequence_types: tuple[type[Any], ...] = tuple(sequence_annotation_to_type.keys())
 
 
-# Copy of Pydantic: pydantic/_internal/_utils.py
+# Copy of Pydantic: pydantic/_internal/_utils.py with added TypeGuard
 def lenient_issubclass(
-    cls: Any, class_or_tuple: Union[type[Any], tuple[type[Any], ...], None]
-) -> bool:
+    cls: Any, class_or_tuple: Union[type[_T], tuple[type[_T], ...], None]
+) -> TypeGuard[type[_T]]:
     try:
         return isinstance(cls, type) and issubclass(cls, class_or_tuple)  # type: ignore[arg-type]
     except TypeError:  # pragma: no cover
diff --git a/fastapi/_compat/v2.py b/fastapi/_compat/v2.py
index 0a1961e707..efc20fa58b 100644
--- a/fastapi/_compat/v2.py
+++ b/fastapi/_compat/v2.py
@@ -12,7 +12,7 @@ from typing import (
     cast,
 )
 
-from fastapi._compat import shared
+from fastapi._compat import lenient_issubclass, shared
 from fastapi.openapi.constants import REF_TEMPLATE
 from fastapi.types import IncEx, ModelNameMap, UnionType
 from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, create_model
@@ -23,7 +23,6 @@ from pydantic._internal._schema_generation_shared import (  # type: ignore[attr-
     GetJsonSchemaHandler as GetJsonSchemaHandler,
 )
 from pydantic._internal._typing_extra import eval_type_lenient
-from pydantic._internal._utils import lenient_issubclass as lenient_issubclass
 from pydantic.fields import FieldInfo as FieldInfo
 from pydantic.json_schema import GenerateJsonSchema as GenerateJsonSchema
 from pydantic.json_schema import JsonSchemaValue as JsonSchemaValue
@@ -416,10 +415,11 @@ def get_flat_models_from_annotation(
     origin = get_origin(annotation)
     if origin is not None:
         for arg in get_args(annotation):
-            if lenient_issubclass(arg, (BaseModel, Enum)) and arg not in known_models:
-                known_models.add(arg)
-                if lenient_issubclass(arg, BaseModel):
-                    get_flat_models_from_model(arg, known_models=known_models)
+            if lenient_issubclass(arg, (BaseModel, Enum)):
+                if arg not in known_models:
+                    known_models.add(arg)  # type: ignore[arg-type]
+                    if lenient_issubclass(arg, BaseModel):
+                        get_flat_models_from_model(arg, known_models=known_models)
             else:
                 get_flat_models_from_annotation(arg, known_models=known_models)
     return known_models