Fix jsonable_encoder custom encoders for Pydantic models

fastapi/encoders.py

@@ -224,8 +224,11 @@ def jsonable_encoder(
     if exclude is not None and not isinstance(exclude, (set, dict)):
         exclude = set(exclude)  # type: ignore[assignment]  # ty: ignore[unused-ignore-comment]
     if isinstance(obj, BaseModel):
+        model_dump_mode: str = "json"
+        if custom_encoder:
+            model_dump_mode = "python"
         obj_dict = obj.model_dump(
-            mode="json",
+            mode=model_dump_mode,
             include=include,
             exclude=exclude,
             by_alias=by_alias,
@@ -237,6 +240,7 @@ 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

@@ -225,6 +225,25 @@ def test_custom_encoders():
     assert encoded_instance2["dt_field"] == instance["dt_field"].isoformat()
 
 
+def test_custom_encoders_for_pydantic_model_arbitrary_type():
+    class CustomType:
+        def __init__(self, value: str):
+            self.value = value
+
+    class ModelWithCustomType(BaseModel):
+        custom: CustomType
+
+        model_config = {"arbitrary_types_allowed": True}
+
+    instance = ModelWithCustomType(custom=CustomType("encoded"))
+
+    encoded_instance = jsonable_encoder(
+        instance,
+        custom_encoder={CustomType: lambda o: {"value": o.value}},
+    )
+    assert encoded_instance == {"custom": {"value": "encoded"}}
+
+
 def test_custom_enum_encoders():
     def custom_enum_encoder(v: Enum):
         return v.value.lower()