diff --git a/fastapi/_compat/__init__.py b/fastapi/_compat/__init__.py
index 5d772e583..b2ae5adc7 100644
--- a/fastapi/_compat/__init__.py
+++ b/fastapi/_compat/__init__.py
@@ -33,6 +33,7 @@ from .main import (
 from .model_field import ModelField as ModelField
 from .shared import PYDANTIC_V2 as PYDANTIC_V2
 from .shared import PYDANTIC_VERSION_MINOR_TUPLE as PYDANTIC_VERSION_MINOR_TUPLE
+from .shared import annotation_is_pydantic_v1 as annotation_is_pydantic_v1
 from .shared import field_annotation_is_scalar as field_annotation_is_scalar
 from .shared import (
     is_uploadfile_or_nonable_uploadfile_annotation as is_uploadfile_or_nonable_uploadfile_annotation,
diff --git a/fastapi/_compat/main.py b/fastapi/_compat/main.py
index 6cc10dd38..f45f8d33d 100644
--- a/fastapi/_compat/main.py
+++ b/fastapi/_compat/main.py
@@ -205,8 +205,9 @@ def _model_rebuild(model: Type[BaseModel]) -> None:
 
 
 def get_compat_model_name_map(fields: List[ModelField]) -> ModelNameMap:
-    if fields and isinstance(fields[0], v1.ModelField):
-        models = v1.get_flat_models_from_fields(fields, known_models=set())
+    v1_models = [field for field in fields if isinstance(field, v1.ModelField)]
+    if v1_models:
+        models = v1.get_flat_models_from_fields(v1_models, known_models=set())
         return v1.get_model_name_map(models)  # type: ignore[no-any-return]
     return {}
 
diff --git a/fastapi/utils.py b/fastapi/utils.py
index 18c2a4b97..4c74d8d9d 100644
--- a/fastapi/utils.py
+++ b/fastapi/utils.py
@@ -23,11 +23,12 @@ from fastapi._compat import (
     Undefined,
     UndefinedType,
     Validator,
+    annotation_is_pydantic_v1,
     lenient_issubclass,
     v1,
 )
 from fastapi.datastructures import DefaultPlaceholder, DefaultType
-from pydantic import BaseModel, create_model
+from pydantic import BaseModel
 from pydantic.fields import FieldInfo
 from typing_extensions import Literal
 
@@ -86,7 +87,7 @@ def create_model_field(
 ) -> ModelField:
     class_validators = class_validators or {}
 
-    if lenient_issubclass(type_, v1.BaseModel) or version == "1":
+    if annotation_is_pydantic_v1(type_) or version == "1":
         model_config = v1.BaseConfig
         field_info = field_info or v1.FieldInfo()
         kwargs = {
@@ -124,6 +125,7 @@ def create_cloned_field(
 ) -> ModelField:
     if PYDANTIC_V2:
         from ._compat import v2
+
         if isinstance(field, v2.ModelField):  # type: ignore[name-defined]
             return field
     # cloned_types caches already cloned types to support recursive models and improve
@@ -143,7 +145,8 @@ def create_cloned_field(
             cloned_types[original_type] = use_type
             for f in original_type.__fields__.values():
                 use_type.__fields__[f.name] = create_cloned_field(
-                    f, cloned_types=cloned_types,
+                    f,
+                    cloned_types=cloned_types,
                 )
     new_field = create_model_field(name=field.name, type_=use_type, version="1")
     new_field.has_alias = field.has_alias  # type: ignore[attr-defined]