diff --git a/setup.cfg b/setup.cfg
index fb809bca..6c32d48f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -51,7 +51,8 @@ install_requires =
     importlib-metadata; python_version<"3.8"
     aiohttp
     pandas
-    pydantic<=2.1.1
+    pydantic>=1.10.8,<2.0
+
 
 [options.packages.find]
 where = src
diff --git a/src/flexmeasures_client/s2/__init__.py b/src/flexmeasures_client/s2/__init__.py
index acbe2bf5..723bc3d0 100644
--- a/src/flexmeasures_client/s2/__init__.py
+++ b/src/flexmeasures_client/s2/__init__.py
@@ -42,7 +42,7 @@ def wrap(*args, **kwargs):
                 self = args[0]
 
                 # TODO: implement function __hash__ in ID that returns
-                # the value of root, this way we would be able to use
+                # the value of __root__, this way we would be able to use
                 # the ID as key directly
                 self.incoming_messages[
                     get_message_id(incoming_message)
@@ -63,7 +63,7 @@ def wrap(*args, **kwargs):
                     status=ReceptionStatusValues.INVALID_DATA,
                 )  # TODO: Discuss status
 
-        message_type = schema.model_fields.get("message_type").default
+        message_type = schema.__fields__.get("message_type").default
         setattr(wrap, "_tag", Tag(message_type))
 
         return wrap
@@ -178,7 +178,7 @@ def handle_message(self, message: pydantic.BaseModel | str | Dict) -> dict:
         """
 
         if isinstance(message, pydantic.BaseModel):
-            message = json.loads(message.model_dump_json())
+            message = json.loads(message.json())
 
         if isinstance(message, str):
             message = json.loads(message)
@@ -200,8 +200,10 @@ def handle_response_status(self, message: ReceptionStatus):
         callback_store = None
 
         # save acknowledgement status code
-        # TODO: implement function __hash__ in ID that returns the value of root
-        self.outgoing_messages_status[message.subject_message_id.root] = message.status
+        # TODO: implement function __hash__ in ID that returns the value of __root__
+        self.outgoing_messages_status[
+            message.subject_message_id.__root__
+        ] = message.status
 
         # choose which callback to call, depending on the ReceptionStatus value
         if message.status == ReceptionStatusValues.OK:
@@ -210,12 +212,12 @@ def handle_response_status(self, message: ReceptionStatus):
             callback_store = self.failure_callbacks
 
         # pop callback from callback_store and run it, if there exists one
-        if callback := callback_store.pop(message.subject_message_id.root, None):
+        if callback := callback_store.pop(message.subject_message_id.__root__, None):
             callback()
 
         # delete success callback related to this message
         if callback is None and (message.status != ReceptionStatusValues.OK):
-            self.success_callbacks.pop(message.subject_message_id.root, None)
+            self.success_callbacks.pop(message.subject_message_id.__root__, None)
 
     @register(RevokeObject)
     def handle_revoke_object(self, message: RevokeObject):
@@ -223,7 +225,7 @@ def handle_revoke_object(self, message: RevokeObject):
         Stores the revoked object ID into the objects_revoked list
         """
 
-        self.objects_revoked.append(message.object_id.root)
+        self.objects_revoked.append(message.object_id.__root__)
 
         return ReceptionStatus(
             subject_message_id=message.message_id, status=ReceptionStatusValues.OK
diff --git a/src/flexmeasures_client/s2/cem.py b/src/flexmeasures_client/s2/cem.py
index 056540b3..dd6645d7 100644
--- a/src/flexmeasures_client/s2/cem.py
+++ b/src/flexmeasures_client/s2/cem.py
@@ -105,7 +105,7 @@ async def handle_message(self, message: Dict | pydantic.BaseModel | str):
         response = None
 
         if isinstance(message, pydantic.BaseModel):
-            message = json.loads(message.model_dump_json())
+            message = json.loads(message.json())
 
         if isinstance(message, str):
             message = json.loads(message)
@@ -158,7 +158,7 @@ async def get_message(self) -> str:
 
         # Pending for pydantic V2 to implement model.model_dump(mode="json") in
         # PR #1409 (https://github.com/pydantic/pydantic/issues/1409)
-        message = json.loads(message.model_dump_json())
+        message = json.loads(message.json())
 
         return message
 
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/__init__.py b/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
index 0b1fca8f..efce7c29 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
@@ -59,7 +59,7 @@ def __init__(self, max_size: int = 100) -> None:
     def handle_system_description(
         self, message: FRBCSystemDescription
     ) -> pydantic.BaseModel:
-        system_description_id = message.message_id.root
+        system_description_id = message.message_id.__root__
 
         # store system_description message for later
         self._system_description_history[system_description_id] = message
@@ -77,7 +77,7 @@ async def send_actuator_status(self, status: FRBCActuatorStatus):
 
     @register(FRBCStorageStatus)
     def handle_storage_status(self, message: FRBCStorageStatus) -> pydantic.BaseModel:
-        message_id = message.message_id.root
+        message_id = message.message_id.__root__
 
         self._storage_status_history[message_id] = message
 
@@ -87,7 +87,7 @@ def handle_storage_status(self, message: FRBCStorageStatus) -> pydantic.BaseMode
 
     @register(FRBCActuatorStatus)
     def handle_actuator_status(self, message: FRBCActuatorStatus) -> pydantic.BaseModel:
-        message_id = message.message_id.root
+        message_id = message.message_id.__root__
 
         self._actuator_status_history[message_id] = message
 
@@ -129,8 +129,8 @@ async def trigger_schedule(self, system_description_id: str):
         instruction = FRBCInstruction(
             message_id=get_unique_id(),
             id=get_unique_id(),
-            actuator_id=actuator.id.root,
-            operation_mode=actuator.operation_modes[0].id.root,
+            actuator_id=actuator.id.__root__,
+            operation_mode=actuator.operation_modes[0].id.__root__,
             operation_mode_factor=0.5,
             execution_time=system_description.valid_from,
             abnormal_condition=False,
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/utils.py b/src/flexmeasures_client/s2/control_types/FRBC/utils.py
index f6878862..e3857e78 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/utils.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/utils.py
@@ -94,8 +94,8 @@ def fm_schedule_to_instructions(
             instruction = FRBCInstruction(
                 message_id=get_unique_id(),
                 id=get_unique_id(),
-                actuator_id=actuator.id.root,
-                operation_mode=operation_mode.id.root,
+                actuator_id=actuator.id.__root__,
+                operation_mode=operation_mode.id.__root__,
                 operation_mode_factor=operation_mode_factor,
                 execution_time=start,
                 abnormal_condition=False,
diff --git a/src/flexmeasures_client/s2/control_types/__init__.py b/src/flexmeasures_client/s2/control_types/__init__.py
index 2ce1d3b0..1e0aaa7f 100644
--- a/src/flexmeasures_client/s2/control_types/__init__.py
+++ b/src/flexmeasures_client/s2/control_types/__init__.py
@@ -31,7 +31,7 @@ def __init__(self, max_size: int = 100) -> None:
 
     @register(InstructionStatusUpdate)
     def handle_instruction_status_update(self, message: InstructionStatusUpdate):
-        instruction_id: str = cast(str, message.instruction_id.root)
+        instruction_id: str = cast(str, message.instruction_id.__root__)
 
         self._instruction_status_history[instruction_id] = message.status_type
 
diff --git a/src/flexmeasures_client/s2/python_s2_protocol/FRBC/messages.py b/src/flexmeasures_client/s2/python_s2_protocol/FRBC/messages.py
index 6c0c8d3a..d955128b 100644
--- a/src/flexmeasures_client/s2/python_s2_protocol/FRBC/messages.py
+++ b/src/flexmeasures_client/s2/python_s2_protocol/FRBC/messages.py
@@ -3,9 +3,9 @@
 from __future__ import annotations
 
 from datetime import datetime
-from typing import List, Literal, Optional
+from typing import List, Optional
 
-from pydantic import BaseModel, ConfigDict, Extra, Field, constr
+from pydantic import BaseModel, Extra, Field, constr
 
 from flexmeasures_client.s2.python_s2_protocol.common.schemas import (
     ID,
@@ -22,8 +22,11 @@
 
 
 class FRBCActuatorStatus(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.ActuatorStatus"] = Field(default="FRBC.ActuatorStatus")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.ActuatorStatus", const=True)
     message_id: ID = Field(..., description="ID of this message")
     actuator_id: ID = Field(
         ..., description="ID of the actuator this messages refers to"
@@ -46,10 +49,11 @@ class FRBCActuatorStatus(BaseModel):
 
 
 class FRBCFillLevelTargetProfile(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.FillLevelTargetProfile"] = Field(
-        default="FRBC.FillLevelTargetProfile"
-    )
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.FillLevelTargetProfile", const=True)
     message_id: ID = Field(..., description="ID of this message")
     start_time: datetime = Field(
         ..., description="Time at which the FRBC.FillLevelTargetProfile starts."
@@ -57,14 +61,17 @@ class FRBCFillLevelTargetProfile(BaseModel):
     elements: List[FRBCFillLevelTargetProfileElement] = Field(
         ...,
         description="List of different fill levels that have to be targeted within a given duration. There shall be at least one element. Elements must be placed in chronological order.",
-        max_length=288,
-        min_length=1,
+        max_items=288,
+        min_items=1,
     )
 
 
 class FRBCInstruction(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.Instruction"] = Field(default="FRBC.Instruction")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.Instruction", const=True)
     message_id: ID = Field(..., description="ID of this message")
     id: ID = Field(
         ...,
@@ -90,10 +97,11 @@ class FRBCInstruction(BaseModel):
 
 
 class FRBCLeakageBehaviour(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.LeakageBehaviour"] = Field(
-        default="FRBC.LeakageBehaviour"
-    )
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.LeakageBehaviour", const=True)
     message_id: ID = Field(..., description="ID of this message")
     valid_from: datetime = Field(
         ...,
@@ -102,14 +110,17 @@ class FRBCLeakageBehaviour(BaseModel):
     elements: List[FRBCLeakageBehaviourElement] = Field(
         ...,
         description="List of elements that model the leakage behaviour of the buffer. The fill_level_ranges of the elements must be contiguous.",
-        max_length=288,
-        min_length=1,
+        max_items=288,
+        min_items=1,
     )
 
 
 class FRBCStorageStatus(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.StorageStatus"] = Field(default="FRBC.StorageStatus")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.StorageStatus", const=True)
     message_id: ID = Field(..., description="ID of this message")
     present_fill_level: float = Field(
         ..., description="Present fill level of the Storage"
@@ -117,24 +128,28 @@ class FRBCStorageStatus(BaseModel):
 
 
 class FRBCSystemDescription(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.SystemDescription"] = Field(
-        default="FRBC.SystemDescription"
-    )
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.SystemDescription", const=True)
     message_id: ID = Field(..., description="ID of this message")
     valid_from: datetime = Field(
         ...,
         description="Moment this FRBC.SystemDescription starts to be valid. If the system description is immediately valid, the DateTimeStamp should be now or in the past.",
     )
     actuators: List[FRBCActuatorDescription] = Field(
-        ..., description="Details of all Actuators.", max_length=10, min_length=1
+        ..., description="Details of all Actuators.", max_items=10, min_items=1
     )
     storage: FRBCStorageDescription = Field(..., description="Details of the storage.")
 
 
 class FRBCTimerStatus(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.TimerStatus"] = Field(default="FRBC.TimerStatus")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.TimerStatus", const=True)
     message_id: ID = Field(..., description="ID of this message")
     timer_id: ID = Field(..., description="The ID of the timer this message refers to")
     actuator_id: ID = Field(
@@ -147,8 +162,11 @@ class FRBCTimerStatus(BaseModel):
 
 
 class FRBCUsageForecast(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["FRBC.UsageForecast"] = Field(default="FRBC.UsageForecast")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("FRBC.UsageForecast", const=True)
     message_id: ID = Field(..., description="ID of this message")
     start_time: datetime = Field(
         ..., description="Time at which the FRBC.UsageForecast starts."
@@ -156,6 +174,6 @@ class FRBCUsageForecast(BaseModel):
     elements: List[FRBCUsageForecastElement] = Field(
         ...,
         description="Further elements that model the profile. There shall be at least one element. Elements must be placed in chronological order.",
-        max_length=288,
-        min_length=1,
+        max_items=288,
+        min_items=1,
     )
diff --git a/src/flexmeasures_client/s2/python_s2_protocol/FRBC/schemas.py b/src/flexmeasures_client/s2/python_s2_protocol/FRBC/schemas.py
index da1d596f..2b1affef 100644
--- a/src/flexmeasures_client/s2/python_s2_protocol/FRBC/schemas.py
+++ b/src/flexmeasures_client/s2/python_s2_protocol/FRBC/schemas.py
@@ -4,7 +4,7 @@
 
 from typing import List, Optional
 
-from pydantic import BaseModel, ConfigDict, Extra, Field
+from pydantic import BaseModel, Extra, Field
 
 from flexmeasures_client.s2.python_s2_protocol.common.schemas import (
     ID,
@@ -18,7 +18,10 @@
 
 
 class FRBCFillLevelTargetProfileElement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     duration: Duration = Field(..., description="The duration of the element.")
     fill_level_range: NumberRange = Field(
         ...,
@@ -27,7 +30,10 @@ class FRBCFillLevelTargetProfileElement(BaseModel):
 
 
 class FRBCLeakageBehaviourElement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     fill_level_range: NumberRange = Field(
         ...,
         description="The fill level range for which this FRBC.LeakageBehaviourElement applies. The start of the range must be less than the end of the range.",
@@ -39,7 +45,10 @@ class FRBCLeakageBehaviourElement(BaseModel):
 
 
 class FRBCOperationModeElement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     fill_level_range: NumberRange = Field(
         ...,
         description="The range of the fill level for which this FRBC.OperationModeElement applies. The start of the NumberRange shall be smaller than the end of the NumberRange.",
@@ -51,8 +60,8 @@ class FRBCOperationModeElement(BaseModel):
     power_ranges: List[PowerRange] = Field(
         ...,
         description="The power produced or consumed by this operation mode. The start of each PowerRange is associated with an operation_mode_factor of 0, the end is associated with an operation_mode_factor of 1. In the array there must be at least one PowerRange, and at most one PowerRange per CommodityQuantity.",
-        max_length=10,
-        min_length=1,
+        max_items=10,
+        min_items=1,
     )
     running_costs: Optional[NumberRange] = Field(
         None,
@@ -61,7 +70,10 @@ class FRBCOperationModeElement(BaseModel):
 
 
 class FRBCOperationMode(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     id: ID = Field(
         ...,
         description="ID of the FRBC.OperationMode. Must be unique in the scope of the FRBC.ActuatorDescription in which it is used.",
@@ -73,8 +85,8 @@ class FRBCOperationMode(BaseModel):
     elements: List[FRBCOperationModeElement] = Field(
         ...,
         description="List of FRBC.OperationModeElements, which describe the properties of this FRBC.OperationMode depending on the fill_level. The fill_level_ranges of the items in the Array must be contiguous.",
-        max_length=100,
-        min_length=1,
+        max_items=100,
+        min_items=1,
     )
     abnormal_condition_only: bool = Field(
         ...,
@@ -83,7 +95,10 @@ class FRBCOperationMode(BaseModel):
 
 
 class FRBCActuatorDescription(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     id: ID = Field(
         ...,
         description="ID of the Actuator. Must be unique in the scope of the Resource Manager, for at least the duration of the session between Resource Manager and CEM.",
@@ -93,33 +108,33 @@ class FRBCActuatorDescription(BaseModel):
         description="Human readable name/description for the actuator. This element is only intended for diagnostic purposes and not for HMI applications.",
     )
     supported_commodities: List[Commodity] = Field(
-        ...,
-        description="List of all supported Commodities.",
-        max_length=4,
-        min_length=1,
+        ..., description="List of all supported Commodities.", max_items=4, min_items=1
     )
     operation_modes: List[FRBCOperationMode] = Field(
         ...,
         description="Provided FRBC.OperationModes associated with this actuator",
-        max_length=100,
-        min_length=1,
+        max_items=100,
+        min_items=1,
     )
     transitions: List[Transition] = Field(
         ...,
         description="Possible transitions between FRBC.OperationModes associated with this actuator.",
-        max_length=1000,
-        min_length=0,
+        max_items=1000,
+        min_items=0,
     )
     timers: List[Timer] = Field(
         ...,
         description="List of Timers associated with this actuator",
-        max_length=1000,
-        min_length=0,
+        max_items=1000,
+        min_items=0,
     )
 
 
 class FRBCStorageDescription(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     diagnostic_label: Optional[str] = Field(
         None,
         description="Human readable name/description of the storage (e.g. hot water buffer or battery). This element is only intended for diagnostic purposes and not for HMI applications.",
@@ -147,7 +162,10 @@ class FRBCStorageDescription(BaseModel):
 
 
 class FRBCUsageForecastElement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     duration: Duration = Field(
         ..., description="Indicator for how long the given usage_rate is valid."
     )
diff --git a/src/flexmeasures_client/s2/python_s2_protocol/__init__.py b/src/flexmeasures_client/s2/python_s2_protocol/__init__.py
deleted file mode 100644
index e69de29b..00000000
diff --git a/src/flexmeasures_client/s2/python_s2_protocol/common/messages.py b/src/flexmeasures_client/s2/python_s2_protocol/common/messages.py
index 6a3ea045..ef99d14b 100644
--- a/src/flexmeasures_client/s2/python_s2_protocol/common/messages.py
+++ b/src/flexmeasures_client/s2/python_s2_protocol/common/messages.py
@@ -3,9 +3,9 @@
 from __future__ import annotations
 
 from datetime import datetime
-from typing import List, Literal, Optional
+from typing import List, Optional
 
-from pydantic import BaseModel, ConfigDict, Extra, Field
+from pydantic import BaseModel, Extra, Field
 
 from flexmeasures_client.s2.python_s2_protocol.common.schemas import (
     ID,
@@ -25,8 +25,11 @@
 
 
 class Handshake(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["Handshake"] = Field(default="Handshake")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("Handshake", const=True)
     message_id: ID = Field(..., description="ID of this message")
     role: EnergyManagementRole = Field(
         ..., description="The role of the sender of this message"
@@ -34,13 +37,16 @@ class Handshake(BaseModel):
     supported_protocol_versions: Optional[List[str]] = Field(
         None,
         description="Protocol versions supported by the sender of this message. This field is mandatory for the RM, but optional for the CEM.",
-        min_length=1,
+        min_items=1,
     )
 
 
 class HandshakeResponse(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["HandshakeResponse"] = Field(default="HandshakeResponse")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("HandshakeResponse", const=True)
     message_id: ID = Field(..., description="ID of this message")
     selected_protocol_version: str = Field(
         ..., description="The protocol version the CEM selected for this session"
@@ -48,10 +54,11 @@ class HandshakeResponse(BaseModel):
 
 
 class InstructionStatusUpdate(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["InstructionStatusUpdate"] = Field(
-        default="InstructionStatusUpdate"
-    )
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("InstructionStatusUpdate", const=True)
     message_id: ID = Field(..., description="ID of this message")
     instruction_id: ID = Field(
         ..., description="ID of this instruction (as provided by the CEM) "
@@ -65,8 +72,11 @@ class InstructionStatusUpdate(BaseModel):
 
 
 class PowerForecast(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["PowerForecast"] = Field(default="PowerForecast")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("PowerForecast", const=True)
     message_id: ID = Field(..., description="ID of this message")
     start_time: datetime = Field(
         ..., description="Start time of time period that is covered by the profile."
@@ -74,14 +84,17 @@ class PowerForecast(BaseModel):
     elements: List[PowerForecastElement] = Field(
         ...,
         description="Elements of which this forecast consists. Contains at least one element. Elements must be placed in chronological order.",
-        max_length=288,
-        min_length=1,
+        max_items=288,
+        min_items=1,
     )
 
 
 class PowerMeasurement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["PowerMeasurement"] = Field(default="PowerMeasurement")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("PowerMeasurement", const=True)
     message_id: ID = Field(..., description="ID of this message")
     measurement_timestamp: datetime = Field(
         ..., description="Timestamp when PowerValues were measured."
@@ -89,14 +102,17 @@ class PowerMeasurement(BaseModel):
     values: List[PowerValue] = Field(
         ...,
         description="Array of measured PowerValues. Must contain at least one item and at most one item per ‘commodity_quantity’ (defined inside the PowerValue).",  # noqa: E501
-        max_length=10,
-        min_length=1,
+        max_items=10,
+        min_items=1,
     )
 
 
 class ReceptionStatus(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["ReceptionStatus"] = Field(default="ReceptionStatus")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("ReceptionStatus", const=True)
     subject_message_id: ID = Field(
         ..., description="The message this ReceptionStatus refers to"
     )
@@ -110,10 +126,11 @@ class ReceptionStatus(BaseModel):
 
 
 class ResourceManagerDetails(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["ResourceManagerDetails"] = Field(
-        default="ResourceManagerDetails"
-    )
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("ResourceManagerDetails", const=True)
     message_id: ID = Field(..., description="ID of this message")
     resource_id: ID = Field(
         ...,
@@ -123,8 +140,8 @@ class ResourceManagerDetails(BaseModel):
     roles: List[Role] = Field(
         ...,
         description="Each Resource Manager provides one or more energy Roles",
-        max_length=3,
-        min_length=1,
+        max_items=3,
+        min_items=1,
     )
     manufacturer: Optional[str] = Field(None, description="Name of Manufacturer")
     model: Optional[str] = Field(
@@ -145,8 +162,8 @@ class ResourceManagerDetails(BaseModel):
     available_control_types: List[ControlType] = Field(
         ...,
         description="The control types supported by this Resource Manager.",
-        max_length=5,
-        min_length=1,
+        max_items=5,
+        min_items=1,
     )
     currency: Optional[Currency] = Field(
         None,
@@ -159,14 +176,17 @@ class ResourceManagerDetails(BaseModel):
     provides_power_measurement_types: List[CommodityQuantity] = Field(
         ...,
         description="Array of all CommodityQuantities that this Resource Manager can provide measurements for. ",  # noqa: E501
-        max_length=10,
-        min_length=1,
+        max_items=10,
+        min_items=1,
     )
 
 
 class RevokeObject(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["RevokeObject"] = Field(default="RevokeObject")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("RevokeObject", const=True)
     message_id: ID = Field(..., description="ID of this message")
     object_type: RevokableObjects = Field(
         ..., description="The type of object that needs to be revoked"
@@ -175,8 +195,11 @@ class RevokeObject(BaseModel):
 
 
 class SelectControlType(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["SelectControlType"] = Field(default="SelectControlType")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("SelectControlType", const=True)
     message_id: ID = Field(..., description="ID of this message")
     control_type: ControlType = Field(
         ...,
@@ -185,8 +208,11 @@ class SelectControlType(BaseModel):
 
 
 class SessionRequest(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
-    message_type: Literal["SessionRequest"] = Field(default="SessionRequest")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
+    message_type: str = Field("SessionRequest", const=True)
     message_id: ID = Field(..., description="ID of this message")
     request: SessionRequestType = Field(..., description="The type of request")
     diagnostic_label: Optional[str] = Field(
diff --git a/src/flexmeasures_client/s2/python_s2_protocol/common/schemas.py b/src/flexmeasures_client/s2/python_s2_protocol/common/schemas.py
index fd80049c..b0ad00bb 100644
--- a/src/flexmeasures_client/s2/python_s2_protocol/common/schemas.py
+++ b/src/flexmeasures_client/s2/python_s2_protocol/common/schemas.py
@@ -5,32 +5,32 @@
 from enum import Enum
 from typing import List, Optional
 
-from pydantic import BaseModel, ConfigDict, Extra, Field, RootModel, conint
+from pydantic import BaseModel, Extra, Field, conint, constr
 
 
-class ID(RootModel):
+class ID(BaseModel):
     class Config:
         validate_assignment = True
 
-    root: str = Field(
-        ...,
-        description="An identifier expressed as a UUID",
-        title="ID",
-        pattern=r"[a-zA-Z0-9\-_:]{2,64}",
+    __root__: constr(regex=r"[a-zA-Z0-9\-_:]{2,64}") = Field(
+        ..., description="An identifier expressed as a UUID", title="ID"
     )
 
 
-class Duration(RootModel):
+class Duration(BaseModel):
     class Config:
         validate_assignment = True
 
-    root: conint(ge=0) = Field(
+    __root__: conint(ge=0) = Field(
         ..., description="Duration in milliseconds", title="Duration"
     )
 
 
 class NumberRange(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     start_of_range: float = Field(
         ..., description="Number that defines the start of the range"
     )
@@ -60,7 +60,10 @@ class CommodityQuantity(Enum):
 
 
 class Timer(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     id: ID = Field(
         ...,
         description="ID of the Timer. Must be unique in the scope of the OMBC.SystemDescription, FRBC.ActuatorDescription or DDBC.ActuatorDescription in which it is used.",
@@ -76,7 +79,10 @@ class Timer(BaseModel):
 
 
 class Transition(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     id: ID = Field(
         ...,
         description="ID of the Transition. Must be unique in the scope of the OMBC.SystemDescription, FRBC.ActuatorDescription or DDBC.ActuatorDescription in which it is used.",
@@ -93,14 +99,14 @@ class Transition(BaseModel):
     start_timers: List[ID] = Field(
         ...,
         description="List of IDs of Timers that will be (re)started when this transition is initiated",
-        max_length=1000,
-        min_length=0,
+        max_items=1000,
+        min_items=0,
     )
     blocking_timers: List[ID] = Field(
         ...,
         description="List of IDs of Timers that block this Transition from initiating while at least one of these Timers is not yet finished",
-        max_length=1000,
-        min_length=0,
+        max_items=1000,
+        min_items=0,
     )
     transition_costs: Optional[float] = Field(
         None,
@@ -117,7 +123,10 @@ class Transition(BaseModel):
 
 
 class PowerRange(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     start_of_range: float = Field(
         ..., description="Power value that defines the start of the range."
     )
@@ -145,7 +154,10 @@ class InstructionStatus(Enum):
 
 
 class PowerForecastValue(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     value_upper_limit: Optional[float] = Field(
         None,
         description="The upper boundary of the range with 100\xa0% certainty the power value is in it",
@@ -177,18 +189,24 @@ class PowerForecastValue(BaseModel):
 
 
 class PowerForecastElement(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     duration: Duration = Field(..., description="Duration of the PowerForecastElement")
     power_values: List[PowerForecastValue] = Field(
         ...,
         description="The values of power that are expected for the given period of time. There shall be at least one PowerForecastValue, and at most one PowerForecastValue per CommodityQuantity.",
-        max_length=10,
-        min_length=1,
+        max_items=10,
+        min_items=1,
     )
 
 
 class PowerValue(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     commodity_quantity: CommodityQuantity = Field(
         ..., description="The power quantity the value refers to"
     )
@@ -327,7 +345,10 @@ class RoleType(Enum):
 
 
 class Role(BaseModel):
-    model_config = ConfigDict(validate_assigment=True, extra="forbid")
+    class Config:
+        extra = Extra.forbid
+        validate_assignment = True
+
     role: RoleType = Field(
         ..., description="Role type of the Resource Manager for the given commodity"
     )
diff --git a/src/flexmeasures_client/s2/script/websockets_client.py b/src/flexmeasures_client/s2/script/websockets_client.py
index 95643fed..cf3f0cc3 100644
--- a/src/flexmeasures_client/s2/script/websockets_client.py
+++ b/src/flexmeasures_client/s2/script/websockets_client.py
@@ -45,7 +45,7 @@ async def main_s2():
 
             print("SENDING: HANDSHAKE")
 
-            await ws.send_json(message.model_dump_json())
+            await ws.send_json(message.json())
 
             response = await ws.receive()
 
@@ -59,7 +59,7 @@ async def main_s2():
                 roles=[
                     Role(role=RoleType.ENERGY_STORAGE, commodity=Commodity.ELECTRICITY)
                 ],
-                instruction_processing_delay=Duration(root=1.0),
+                instruction_processing_delay=Duration(__root__=1.0),
                 available_control_types=[
                     ControlType.FILL_RATE_BASED_CONTROL,
                     ControlType.NO_SELECTION,
@@ -151,7 +151,7 @@ async def main_s2():
                 valid_from=valid_from,
                 actuators=[actuator],
                 storage=storage,
-            ).model_dump_json()
+            ).json()
 
             print("SENDING: FRBC.SystemDescription")
 
diff --git a/src/flexmeasures_client/s2/utils.py b/src/flexmeasures_client/s2/utils.py
index 3559a22e..29e69a61 100644
--- a/src/flexmeasures_client/s2/utils.py
+++ b/src/flexmeasures_client/s2/utils.py
@@ -57,9 +57,9 @@ def get_message_id(message: pydantic.BaseModel) -> str:
     ReceptionStatus.
     """
     if hasattr(message, "message_id"):
-        return message.message_id.root
+        return message.message_id.__root__
     elif hasattr(message, "subject_message_id"):
-        return message.subject_message_id.root
+        return message.subject_message_id.__root__
 
 
 def get_reception_status(
@@ -71,5 +71,5 @@ def get_reception_status(
     `subject_message`. By default, the status ReceptionStatusValues.OK is sent.
     """
     return ReceptionStatus(
-        subject_message_id=subject_message.message_id.root, status=status
+        subject_message_id=subject_message.message_id.__root__, status=status
     )
diff --git a/tests/test_s2_coordinator.py b/tests/test_s2_coordinator.py
index 0792552c..8026482d 100644
--- a/tests/test_s2_coordinator.py
+++ b/tests/test_s2_coordinator.py
@@ -73,7 +73,7 @@ async def test_cem():  # TODO: move into different test functions
         message_id=get_unique_id(),
         resource_id=get_unique_id(),
         roles=[Role(role=RoleType.ENERGY_STORAGE, commodity=Commodity.ELECTRICITY)],
-        instruction_processing_delay=Duration(root=1.0),
+        instruction_processing_delay=Duration(__root__=1.0),
         available_control_types=[
             ControlType.FILL_RATE_BASED_CONTROL,
             ControlType.NO_SELECTION,
@@ -170,7 +170,7 @@ async def test_cem():  # TODO: move into different test functions
     assert (
         cem._control_types_handlers[
             ControlType.FILL_RATE_BASED_CONTROL
-        ]._system_description_history[system_description_message.message_id.root]
+        ]._system_description_history[system_description_message.message_id.__root__]
         == system_description_message
     ), (
         "the FRBC.SystemDescription message should be stored"