diff --git a/aleph_message/models/__init__.py b/aleph_message/models/__init__.py
index 31fd060..32fdba7 100644
--- a/aleph_message/models/__init__.py
+++ b/aleph_message/models/__init__.py
@@ -7,7 +7,6 @@
 from pathlib import Path
 from typing import Any, Dict, List, Literal, Optional, Type, TypeVar, Union, cast
 
-from aleph_message.models.execution.volume import EphemeralVolumeSize, PersistentVolumeSizeMib
 from pydantic import BaseModel, Field, field_validator, ConfigDict
 from typing_extensions import TypeAlias
 
@@ -265,7 +264,10 @@ def convert_float_to_datetime(cls, v, values):
         assert isinstance(v, datetime.datetime)
         return v
 
-    model_config = ConfigDict(extra="forbid", exclude={"id_", "_id"})
+    model_config = ConfigDict(extra="forbid")
+
+    def custom_dump(self):
+        return self.model_dump(exclude={"id_", "_id"})
 
 
 class PostMessage(BaseMessage):
@@ -306,36 +308,38 @@ def cannot_be_forgotten(cls, v: Optional[List[str]], values) -> Optional[List[st
 
 class ProgramMessage(BaseMessage):
     type: Literal[MessageType.program]
-    content: Optional[ProgramContent] = None
+    content: ProgramContent
     forgotten_by: Optional[List[str]] = None
 
     @staticmethod
-    def normalize_content(content):
+    def normalize_content(content: Union[Dict[str, Any], Any]) -> Any:
         if not isinstance(content, dict):
             return content
 
-        normalized_content = {}
+        normalized_content: Dict[str, Any] = {}
+
         for key, value in content.items():
-            # Handle special cases such as ItemHash, Enum, list and dict
             if isinstance(value, ItemHash):
-                normalized_content[key] = str(value)  # ItemHash to string
+                normalized_content[key] = str(value)
             elif isinstance(value, Enum):
-                normalized_content[key] = value.value  # Enum to string
+                normalized_content[key] = value.value
             elif isinstance(value, list):
-                normalized_content[key] = [ProgramMessage.normalize_content(v) for v in value]
+                if key == "volumes" and all(isinstance(v, str) for v in value):
+                    normalized_content[key] = value
+                else:
+                    normalized_content[key] = [
+                        ProgramMessage.normalize_content(v) for v in value
+                    ]
             elif isinstance(value, dict):
-                # Special case for 'size_mib' and 'data'
-                if key == 'size_mib':
+                if key == "size_mib":
                     normalized_content[key] = list(value.values())[0]
                 else:
                     normalized_content[key] = ProgramMessage.normalize_content(value)
             else:
-                normalized_content[key] = value  # Keep the value as is
+                normalized_content[key] = value
 
         return normalized_content
 
-
-
     @field_validator("content")
     def check_content(cls, v, values):
         item_type = values.data.get("item_type")
@@ -343,7 +347,7 @@ def check_content(cls, v, values):
             item_content = json.loads(values.data.get("item_content"))
 
             # Normalizing content to fit the structure of item_content
-            normalized_content = cls.normalize_content(v.dict(exclude_none=True))
+            normalized_content = cls.normalize_content(v.model_dump(exclude_none=True))
 
             if normalized_content != item_content:
                 # Print les différences
@@ -354,9 +358,6 @@ def check_content(cls, v, values):
         return v
 
 
-
-
-
 class InstanceMessage(BaseMessage):
     type: Literal[MessageType.instance]
     content: InstanceContent
diff --git a/aleph_message/models/execution/instance.py b/aleph_message/models/execution/instance.py
index 4c54b25..3dc66d3 100644
--- a/aleph_message/models/execution/instance.py
+++ b/aleph_message/models/execution/instance.py
@@ -8,6 +8,7 @@
 from .abstract import BaseExecutableContent
 from .environment import InstanceEnvironment
 from .volume import ParentVolume, PersistentVolumeSizeMib, VolumePersistence
+from .base import Payment
 
 
 class RootfsVolume(HashableModel):
@@ -24,7 +25,7 @@ class RootfsVolume(HashableModel):
     size_mib: PersistentVolumeSizeMib
     forgotten_by: Optional[List[str]] = None
 
-    @field_validator('size_mib', mode="before")
+    @field_validator("size_mib", mode="before")
     def convert_size_mib(cls, v):
         if isinstance(v, int):
             return PersistentVolumeSizeMib(persistent_volume_size=v)
@@ -35,7 +36,7 @@ class InstanceContent(BaseExecutableContent):
     """Message content for scheduling a VM instance on the network."""
 
     metadata: Optional[dict] = None
-    payment: Optional[dict] = None
+    payment: Optional[Payment] = None
     environment: InstanceEnvironment = Field(
         description="Properties of the instance execution environment"
     )
diff --git a/aleph_message/models/execution/program.py b/aleph_message/models/execution/program.py
index ee3237c..9bb2228 100644
--- a/aleph_message/models/execution/program.py
+++ b/aleph_message/models/execution/program.py
@@ -7,7 +7,7 @@
 from ..abstract import HashableModel
 from ..item_hash import ItemHash
 from .abstract import BaseExecutableContent
-from .base import Encoding, Interface, MachineType
+from .base import Encoding, Interface, MachineType, Payment
 from .environment import FunctionTriggers
 
 
@@ -72,4 +72,4 @@ class ProgramContent(BaseExecutableContent):
 
     metadata: Optional[dict] = None
     authorized_keys: Optional[List[str]] = None
-    payment: Optional[dict] = None
+    payment: Optional[Payment] = None
diff --git a/aleph_message/models/execution/volume.py b/aleph_message/models/execution/volume.py
index adbfe3c..7e215e1 100644
--- a/aleph_message/models/execution/volume.py
+++ b/aleph_message/models/execution/volume.py
@@ -22,7 +22,7 @@ def is_read_only(self): ...
 
 
 class ImmutableVolume(AbstractVolume):
-    ref: ItemHash = None
+    ref: Optional[ItemHash] = None
     use_latest: bool = True
 
     def is_read_only(self):
@@ -30,9 +30,7 @@ def is_read_only(self):
 
 
 class EphemeralVolumeSize(BaseModel):
-    ephemeral_volume_size: int = Field(gt=0,
-                                       le=1000, #Limit to 1GiB
-                                       strict=True)
+    ephemeral_volume_size: int = Field(gt=-1, le=1000, strict=True)  # Limit to 1GiB
 
     def __hash__(self):
         return hash(self.ephemeral_volume_size)
@@ -40,9 +38,9 @@ def __hash__(self):
 
 class EphemeralVolume(AbstractVolume):
     ephemeral: Literal[True] = True
-    size_mib: EphemeralVolumeSize = 0
+    size_mib: EphemeralVolumeSize = EphemeralVolumeSize(ephemeral_volume_size=0)
 
-    @field_validator('size_mib', mode="before")
+    @field_validator("size_mib", mode="before")
     def convert_size_mib(cls, v):
         if isinstance(v, int):
             return EphemeralVolumeSize(ephemeral_volume_size=v)
@@ -67,9 +65,9 @@ class VolumePersistence(str, Enum):
 
 
 class PersistentVolumeSizeMib(BaseModel):
-    persistent_volume_size: int = Field(gt=0,
-                                        le=gigabyte_to_mebibyte(Gigabytes(100)), #Limit to 1GiB
-                                        strict=True)
+    persistent_volume_size: int = Field(
+        gt=-1, le=gigabyte_to_mebibyte(Gigabytes(100)), strict=True  # Limit to 1GiB
+    )
 
     def __hash__(self):
         return hash(self.persistent_volume_size)
@@ -77,11 +75,13 @@ def __hash__(self):
 
 class PersistentVolume(AbstractVolume):
     parent: Optional[ParentVolume] = None
-    persistence: VolumePersistence = None
+    persistence: Optional[VolumePersistence] = None
     name: Optional[str] = None
-    size_mib: PersistentVolumeSizeMib = 0
+    size_mib: Optional[PersistentVolumeSizeMib] = PersistentVolumeSizeMib(
+        persistent_volume_size=0
+    )
 
-    @field_validator('size_mib', mode="before")
+    @field_validator("size_mib", mode="before")
     def convert_size_mib(cls, v):
         if isinstance(v, int):
             return PersistentVolumeSizeMib(persistent_volume_size=v)
diff --git a/aleph_message/models/item_hash.py b/aleph_message/models/item_hash.py
index 7037a99..e4074c5 100644
--- a/aleph_message/models/item_hash.py
+++ b/aleph_message/models/item_hash.py
@@ -2,9 +2,10 @@
 from functools import lru_cache
 
 from ..exceptions import UnknownHashError
-from pydantic_core import CoreSchema, core_schema
+from pydantic_core import core_schema
 from pydantic import GetCoreSchemaHandler
 
+
 class ItemType(str, Enum):
     """Item storage options"""
 
@@ -46,14 +47,15 @@ def __new__(cls, value: str):
         return obj
 
     @classmethod
-    def __get_pydantic_core_schema__(cls, source, handler: GetCoreSchemaHandler) -> core_schema.CoreSchema:
+    def __get_pydantic_core_schema__(
+        cls, source, handler: GetCoreSchemaHandler
+    ) -> core_schema.CoreSchema:
         # This function validates the input after the initial type validation (as a string).
         # The returned value from this function will be used as the final validated value.
 
         # Return a string schema and add a post-validation function to convert to ItemHash
         return core_schema.no_info_after_validator_function(
-            cls.validate,
-            core_schema.str_schema()
+            cls.validate, core_schema.str_schema()
         )
 
     @classmethod
diff --git a/aleph_message/tests/test_types.py b/aleph_message/tests/test_types.py
index f0a2dbf..1e8c03f 100644
--- a/aleph_message/tests/test_types.py
+++ b/aleph_message/tests/test_types.py
@@ -1,7 +1,7 @@
 import copy
 
 import pytest
-from pydantic import BaseModel, ValidationError, field_validator
+from pydantic import BaseModel, ValidationError
 
 from aleph_message.exceptions import UnknownHashError
 from aleph_message.models import ItemHash, ItemType