diff --git a/src/art/trajectories/__init__.py b/src/art/trajectories/__init__.py
index 04a9ab825..b28078caf 100644
--- a/src/art/trajectories/__init__.py
+++ b/src/art/trajectories/__init__.py
@@ -309,6 +309,21 @@ class MessagesExchange(_Exchange):
     request: _Preserved[MessagesRequest]
     response: AnthropicMessage
 
+    @pydantic.field_serializer("response")
+    def serialize_response(
+        self, response: AnthropicMessage, info: pydantic.SerializationInfo
+    ) -> dict[str, Any]:
+        # Provider omissions and explicit nulls must survive compact/full replay.
+        return response.model_dump(
+            mode=info.mode,
+            include=info.include,
+            exclude=info.exclude,
+            context=info.context,
+            by_alias=info.by_alias,
+            exclude_unset=True,
+            exclude_none=info.exclude_none,
+        )
+
     @pydantic.computed_field
     @property
     def model(self) -> str | None:
diff --git a/tests/unit/trajectories/test_messages_serialization.py b/tests/unit/trajectories/test_messages_serialization.py
new file mode 100644
index 000000000..4d07effff
--- /dev/null
+++ b/tests/unit/trajectories/test_messages_serialization.py
@@ -0,0 +1,123 @@
+from datetime import UTC, datetime
+import json
+from typing import Any, Literal
+
+from anthropic.types import Message
+import pytest
+
+from art.trajectories import (
+    MessagesExchange,
+    Trajectory,
+)
+
+
+def _message(stop_sequence: str) -> dict[str, Any]:
+    msg: dict[str, Any] = {
+        "id": "msg_01X9vhvwnz4Vzqz3X5V",
+        "type": "message",
+        "role": "assistant",
+        "model": "claude-3-5-sonnet-20241022",
+        "content": [
+            {
+                "type": "text",
+                "text": "Here is the response.",
+            }
+        ],
+        "stop_reason": "end_turn",
+        "usage": {
+            "input_tokens": 25,
+            "output_tokens": 12,
+        },
+    }
+    if stop_sequence != "absent":
+        msg["stop_sequence"] = None if stop_sequence == "null" else stop_sequence
+    return msg
+
+
+def _trajectory(raw: dict[str, Any]) -> Trajectory:
+    now = datetime.now(UTC)
+    trajectory = Trajectory()
+    trajectory.exchanges.messages.append(
+        MessagesExchange(
+            request={"model": "claude-3-5-sonnet-20241022", "messages": []},
+            response=Message.model_validate(raw),
+            start_time=now,
+            end_time=now,
+        )
+    )
+    return trajectory
+
+
+def _dump(
+    trajectory: Trajectory,
+    mode: Literal["python", "json", "json_string"],
+    **kwargs: Any,
+) -> dict[str, Any]:
+    if mode == "json_string":
+        return json.loads(trajectory.model_dump_json(**kwargs))
+    return trajectory.model_dump(mode=mode, **kwargs)
+
+
+@pytest.mark.parametrize("stop_sequence", ["absent", "null", "STOP"])
+@pytest.mark.parametrize("compact", [True, False])
+@pytest.mark.parametrize("mode", ["python", "json", "json_string"])
+def test_messages_round_trip_preserves_provider_fields(
+    stop_sequence: str, compact: bool, mode: Literal["python", "json", "json_string"]
+) -> None:
+    raw = _message(stop_sequence)
+    dumped = _dump(_trajectory(raw), mode, exclude_defaults=compact)
+    assert dumped["exchanges"]["messages"][0]["response"] == raw
+
+    restored = Trajectory.model_validate(dumped)
+    replayed = restored.exchanges.messages[0].response.model_copy(deep=True)
+    assert replayed.model_dump(mode="json", exclude_unset=True) == raw
+
+
+@pytest.mark.parametrize("mode", ["python", "json", "json_string"])
+def test_messages_nested_include_and_exclude(
+    mode: Literal["python", "json", "json_string"],
+) -> None:
+    trajectory = _trajectory(_message("null"))
+    included = _dump(
+        trajectory,
+        mode,
+        include={
+            "exchanges": {
+                "messages": {
+                    0: {
+                        "response": {
+                            "id",
+                            "model",
+                            "stop_sequence",
+                        }
+                    }
+                }
+            }
+        },
+        exclude={
+            "exchanges": {"messages": {0: {"response": {"id"}}}}
+        },
+    )
+    assert included == {
+        "exchanges": {
+            "messages": [
+                {
+                    "response": {
+                        "model": "claude-3-5-sonnet-20241022",
+                        "stop_sequence": None,
+                    }
+                }
+            ]
+        }
+    }
+    excluded = _dump(trajectory, mode, exclude_none=True)
+    response = excluded["exchanges"]["messages"][0]["response"]
+    assert "stop_sequence" not in response
+
+
+def test_legacy_messages_tapes_keep_explicit_nulls() -> None:
+    raw = Message.model_validate(_message("absent")).model_dump(mode="json")
+    restored = Trajectory.model_validate_json(
+        _trajectory(raw).model_dump_json(exclude_defaults=False)
+    )
+    assert _dump(restored, "json")["exchanges"]["messages"][0]["response"] == raw