Maybe better?

src/ert/config/ensemble_config.py

@@ -58,7 +58,8 @@ class Refcase:
     values: List[Any]
 
     def __post_init__(self):
-        self.values = self.values.tolist()
+        if hasattr(self.values, "tolist"):
+            self.values = self.values.tolist()
 
     def __eq__(self, other: object) -> bool:
         if not isinstance(other, Refcase):
@@ -78,7 +79,9 @@ def all_dates(self) -> List[datetime]:
 @dataclass
 class EnsembleConfig:
     grid_file: Optional[str] = None
-    response_configs: Dict[str, ResponseConfig] = field(default_factory=dict)
+    response_configs: Dict[str, Union[SummaryConfig, GenDataConfig]] = field(
+        default_factory=dict
+    )
     parameter_configs: Dict[str, ParameterConfig] = field(default_factory=dict)
     refcase: Optional[Refcase] = None
     eclbase: Optional[str] = None

src/ert/config/ert_config.py

@@ -87,9 +87,7 @@ class ErtConfig:
     DEFAULT_RUNPATH_FILE: ClassVar[str] = ".ert_runpath_list"
     PREINSTALLED_FORWARD_MODEL_STEPS: ClassVar[Dict[str, ForwardModelStep]] = {}
 
-    substitution_list: Union[SubstitutionList, Dict[str, str]] = field(
-        default_factory=dict
-    )
+    substitution_list: SubstitutionList = field(default_factory=SubstitutionList)
     ensemble_config: EnsembleConfig = field(default_factory=EnsembleConfig)
     ens_path: str = DEFAULT_ENSPATH
     env_vars: Dict[str, str] = field(default_factory=dict)
@@ -112,7 +110,7 @@ class ErtConfig:
         Tuple[str, Union[HistoryValues, SummaryValues, GenObsValues]]
     ] = field(default_factory=list)
 
-    @field_validator("substitution_list")
+    @field_validator("substitution_list", mode="before")
     @classmethod
     def convert_to_substitution_list(cls, v: Dict[str, str]) -> SubstitutionList:
         if isinstance(v, SubstitutionList):

src/ert/substitution_list.py

@@ -4,9 +4,9 @@
 import re
 from typing import TYPE_CHECKING, Any, Optional
 
-from pydantic import GetCoreSchemaHandler
+from pydantic import GetCoreSchemaHandler, GetJsonSchemaHandler
+from pydantic.json_schema import JsonSchemaValue
 from pydantic_core import core_schema
-from pydantic_core.core_schema import CoreSchema
 
 logger = logging.getLogger(__name__)
 _PATTERN = re.compile("<[^<>]+>")
@@ -82,9 +82,41 @@ def __str__(self) -> str:
 
     @classmethod
     def __get_pydantic_core_schema__(
-        cls, source_type: Any, handler: GetCoreSchemaHandler
-    ) -> CoreSchema:
-        return core_schema.no_info_after_validator_function(cls, handler(str))
+        cls,
+        _source_type: Any,
+        _handler: GetCoreSchemaHandler,
+    ) -> core_schema.CoreSchema:
+        def _serialize(instance: Any, info: Any) -> Any:
+            # if info.mode == 'json':
+            #     # return json.dumps(dict(instance))
+            #     return dict(instance)
+            return dict(instance)
+
+        from_str_schema = core_schema.chain_schema(
+            [
+                core_schema.str_schema(),
+                core_schema.no_info_plain_validator_function(cls),
+            ]
+        )
+
+        return core_schema.json_or_python_schema(
+            json_schema=from_str_schema,
+            python_schema=core_schema.union_schema(
+                [
+                    from_str_schema,
+                    core_schema.is_instance_schema(cls),
+                ]
+            ),
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                _serialize, info_arg=True
+            ),
+        )
+
+    @classmethod
+    def __get_pydantic_json_schema__(
+        cls, _core_schema: core_schema.CoreSchema, handler: GetJsonSchemaHandler
+    ) -> JsonSchemaValue:
+        return handler(core_schema.str_schema())
 
 
 def _replace_strings(subst_list: SubstitutionList, string: str) -> Optional[str]:

tests/unit_tests/config/test_ert_config.py

@@ -530,7 +530,7 @@ def test_that_ert_config_is_serializable(tmp_path_factory, config_generator):
             config_values.to_config_dict("config.ert", os.getcwd())
         )
         config_json = json.loads(RootModel[ErtConfig](ert_config).model_dump_json())
-        from_json = ErtConfig(config_json)
+        from_json = ErtConfig(**config_json)
         assert from_json == ert_config