Limit custom encoder fallback to custom paths

fastapi/encoders.py

@@ -241,6 +241,7 @@ def jsonable_encoder(
     if exclude is not None and not isinstance(exclude, (set, dict)):
         exclude = set(exclude)  # type: ignore[assignment]  # ty: ignore[invalid-assignment]
     if isinstance(obj, BaseModel):
+        if custom_encoder:
 
             def custom_encoder_fallback(value: Any) -> Any:
                 if type(value) in custom_encoder:
@@ -272,7 +273,7 @@ def jsonable_encoder(
                 exclude_unset=exclude_unset,
                 exclude_none=exclude_none,
                 exclude_defaults=exclude_defaults,
-            fallback=custom_encoder_fallback if custom_encoder else None,
+                fallback=custom_encoder_fallback,
             )
             return jsonable_encoder(
                 obj_dict,
@@ -283,6 +284,21 @@ def jsonable_encoder(
                 custom_encoder=custom_encoder,
                 sqlalchemy_safe=sqlalchemy_safe,
             )
+        obj_dict = obj.model_dump(
+            mode="json",
+            include=include,
+            exclude=exclude,
+            by_alias=by_alias,
+            exclude_unset=exclude_unset,
+            exclude_none=exclude_none,
+            exclude_defaults=exclude_defaults,
+        )
+        return jsonable_encoder(
+            obj_dict,
+            exclude_none=exclude_none,
+            exclude_defaults=exclude_defaults,
+            sqlalchemy_safe=sqlalchemy_safe,
+        )
     if dataclasses.is_dataclass(obj):
         assert not isinstance(obj, type)
         obj_dict = dataclasses.asdict(obj)