Apply custom encoders to model fields

fastapi/encoders.py

@@ -242,7 +242,7 @@ def jsonable_encoder(
         exclude = set(exclude)  # type: ignore[assignment]  # ty: ignore[invalid-assignment]
     if isinstance(obj, BaseModel):
         obj_dict = obj.model_dump(
-            mode="json",
+            mode="python" if custom_encoder else "json",
             include=include,
             exclude=exclude,
             by_alias=by_alias,
@@ -254,6 +254,7 @@ def jsonable_encoder(
             obj_dict,
             exclude_none=exclude_none,
             exclude_defaults=exclude_defaults,
+            custom_encoder=custom_encoder,
             sqlalchemy_safe=sqlalchemy_safe,
         )
     if dataclasses.is_dataclass(obj):

tests/test_jsonable_encoder.py

@@ -12,7 +12,7 @@ import pytest
 from fastapi._compat import Undefined
 from fastapi.encoders import jsonable_encoder
 from fastapi.exceptions import PydanticV1NotSupportedError
-from pydantic import BaseModel, Field, ValidationError
+from pydantic import BaseModel, ConfigDict, Field, ValidationError
 
 
 class Person:
@@ -225,6 +225,21 @@ def test_custom_encoders():
     assert encoded_instance2["dt_field"] == instance["dt_field"].isoformat()
 
 
+def test_custom_encoder_model_field():
+    class CustomValue:
+        pass
+
+    class ModelWithCustomValue(BaseModel):
+        model_config = ConfigDict(arbitrary_types_allowed=True)
+
+        value: CustomValue
+
+    assert jsonable_encoder(
+        ModelWithCustomValue(value=CustomValue()),
+        custom_encoder={CustomValue: lambda _: "encoded"},
+    ) == {"value": "encoded"}
+
+
 def test_custom_enum_encoders():
     def custom_enum_encoder(v: Enum):
         return v.value.lower()