diff --git a/src/dynalab_core/__init__.py b/src/dynalab_core/__init__.py
index db8d46a..0b4c8c8 100644
--- a/src/dynalab_core/__init__.py
+++ b/src/dynalab_core/__init__.py
@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 
+from copy import copy
 from datetime import datetime, timezone
 import logging
 from pathlib import Path
@@ -225,11 +226,6 @@ class Core:
         return stopped
 
     def set_mode(self, mode: CoreMode) -> None:
-        if mode not in ("realtime", "playback"):
-            raise CoreError(
-                f"Unknown core mode {mode!r}; expected 'realtime' or 'playback'",
-                code="invalid_mode",
-            )
         if self._recording.is_set():
             raise CoreError(
                 "Cannot change core mode while a recording is active",
@@ -241,7 +237,7 @@ class Core:
             self._live_values = {}
 
     def get_mode(self) -> CoreMode:
-        return self._mode.value
+        return copy(self._mode.value)
 
     def load_playback_engine(self, input: Path | None = None) -> None:
         if self._mode.value != "playback":
@@ -255,11 +251,14 @@ class Core:
                     "Cannot load playback because no recording is available",
                     code="no_playback_data",
                 )
-            loaded_data = self._processing_buffer
+            loaded_data = copy(self._processing_buffer)
         else:
             loaded_data = DLPak().read(input)
         self._playback_engine.set_data(loaded_data)
 
+    def load_processing_buffer(self, input: Path | None = None) -> None:
+        pass
+
     # def unload_playback_engine
 
     def start_recording(self) -> None:
@@ -299,8 +298,7 @@ class Core:
         time.sleep(1)
         self._recording_buffer.normalize()
         self._recording_buffer.sort()
-        self._processing_buffer = DLPak()
-        self._processing_buffer.set_data(self._recording_buffer)
+        self._processing_buffer = DLPak().set_data(copy(self._recording_buffer))
         connector_signal_descriptors = (
             self._connector_registry.get_all_signal_descriptors()
         )
@@ -343,7 +341,7 @@ class Core:
             if signal is None and self._core_config.playback_mode == "reprocess":
                 signal = self._derive_registry.get_signal_descriptor(signal_id)
 
-        return signal
+        return copy(signal)
 
     def get_all_signal_descriptors(self) -> list[SignalDescriptor]:
         root_signals: list[SignalDescriptor] = []
@@ -358,7 +356,7 @@ class Core:
             or self._mode.value == "realtime"
         ):
             derive_signals = self._derive_registry.get_all_signal_descriptors()
-        return root_signals + derive_signals
+        return copy(root_signals + derive_signals)
 
     def get_live_value(self, signal_id: UUID) -> float | None:
         with self._live_values_lock:
@@ -391,7 +389,7 @@ class Core:
             )
 
         unit = self._derive_registry.register(dlunit)
-        return unit.uuid()
+        return copy(unit.uuid())
 
     def unbind_derive_unit(self, unit_id: UUID) -> None:
         self._derive_registry.unregister(unit_id)
diff --git a/src/dynalab_core/dlpak.py b/src/dynalab_core/dlpak.py
index a40efe2..23f206f 100644
--- a/src/dynalab_core/dlpak.py
+++ b/src/dynalab_core/dlpak.py
@@ -12,14 +12,17 @@ from pydantic import BaseModel, Field
 
 from dynalab_core.buffer import ValueBuffer
 from dynalab_core.errors import DLPakError
-from dynalab_core.protocols.packets.handshake import SignalDescriptor
+from dynalab_core.protocols.packets.handshake import (
+    JsonSignalDescriptor,
+    SignalDescriptor,
+)
 
 log = logging.getLogger(__name__)
 
 
 class RecordManifest(BaseModel):
     timestamp: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
-    signals: list[SignalDescriptor] = Field(default_factory=list)
+    signals: list[JsonSignalDescriptor] = Field(default_factory=list)
     comments: str = ""
 
 
@@ -45,11 +48,11 @@ class DLPak:
 
         data_signal_ids = self._data.get_signal_ids()
 
-        manifest_signals: list[SignalDescriptor] = []
+        manifest_signals: list[JsonSignalDescriptor] = []
 
         for signal in all_signals:
             if signal.id in data_signal_ids:
-                manifest_signals.append(signal)
+                manifest_signals.append(signal.to_json())
 
         self._manifest = RecordManifest(
             timestamp=timestamp, signals=manifest_signals, comments=""
diff --git a/src/dynalab_core/dlunit.py b/src/dynalab_core/dlunit.py
index 22ab478..4c5b27a 100644
--- a/src/dynalab_core/dlunit.py
+++ b/src/dynalab_core/dlunit.py
@@ -11,7 +11,10 @@ from zipfile import ZIP_DEFLATED, ZipFile
 from pydantic import BaseModel, Field
 
 from dynalab_core.errors import DLPakError, DLUnitError
-from dynalab_core.protocols.packets.handshake import SignalDescriptor
+from dynalab_core.protocols.packets.handshake import (
+    JsonSignalDescriptor,
+    SignalDescriptor,
+)
 
 
 log = logging.getLogger(__name__)
@@ -19,8 +22,8 @@ log = logging.getLogger(__name__)
 
 class DeriveManifest(BaseModel):
     unit_uuid: UUID
-    input_signals: list[SignalDescriptor] = Field(default_factory=list)
-    return_signal: SignalDescriptor
+    input_signals: list[JsonSignalDescriptor] = Field(default_factory=list)
+    return_signal: JsonSignalDescriptor
     comments: str = ""
 
 
@@ -42,10 +45,13 @@ class DLUnit:
         return_signal: SignalDescriptor,
     ) -> None:
 
+        json_input_signals = [signal.to_json() for signal in input_signals]
+        json_return_signal = return_signal.to_json()
+
         self._manifest = DeriveManifest(
             unit_uuid=unit_uuid,
-            input_signals=input_signals,
-            return_signal=return_signal,
+            input_signals=json_input_signals,
+            return_signal=json_return_signal,
         )
 
     def read(self, file: str | Path) -> Self:
diff --git a/src/dynalab_core/types.py b/src/dynalab_core/types.py
index 7812ffa..f4d563f 100644
--- a/src/dynalab_core/types.py
+++ b/src/dynalab_core/types.py
@@ -7,7 +7,7 @@ from dataclasses import dataclass
 from typing import Literal
 
 
-CoreMode = Literal["realtime", "playback"]
+CoreMode = Literal["realtime", "playback", "offline"]
 
 
 @dataclass