Add information about class in error message of schema generation

pydantic/_internal/_generate_schema.py

@@ -2160,7 +2160,7 @@ def _extract_get_pydantic_json_schema(tp: Any, schema: CoreSchema) -> GetJsonSch
         if not has_custom_v2_modify_js_func:
             raise PydanticUserError(
                 'The `__modify_schema__` method is not supported in Pydantic v2. '
-                'Use `__get_pydantic_json_schema__` instead.',
+                'Use `__get_pydantic_json_schema__` instead for class' + str(tp),
                 code='custom-json-schema',
             )
 

tests/test_internal.py

@@ -206,3 +206,19 @@ def test_schema_is_valid():
         collect_invalid_schemas(cs.nullable_schema(cs.int_schema(metadata={HAS_INVALID_SCHEMAS_METADATA_KEY: True})))
         is True
     )
+
+
+def test_model_name_in_schema_error_message():
+    from pydantic import BaseModel, ConfigDict, PydanticUserError
+
+    class SomeLongName:
+        @classmethod
+        def __modify_schema__(cls, field_schema):
+            field_schema['title'] = 'SomeLongName'
+
+    with pytest.raises(PydanticUserError, match='The `__modify_schema__`.*SomeLongName.*'):
+
+        class B(BaseModel):
+            model_config = ConfigDict(arbitrary_types_allowed=True)
+
+            a: SomeLongName