diff --git a/tests/portfolio/test_report_store.py b/tests/portfolio/test_report_store.py
index 9f042a26..3aa9dc57 100644
--- a/tests/portfolio/test_report_store.py
+++ b/tests/portfolio/test_report_store.py
@@ -174,3 +174,96 @@ def test_read_json_raises_on_corrupt_file(report_store, tmp_reports):
     corrupt.write_text("not valid json{{{", encoding="utf-8")
     with pytest.raises(ReportStoreError):
         report_store._read_json(corrupt)
+
+
+# ---------------------------------------------------------------------------
+# _sanitize
+# ---------------------------------------------------------------------------
+
+
+class _FakeMessage:
+    """Minimal stand-in for a LangChain HumanMessage / AIMessage."""
+
+    def __init__(self, type_: str, content: str) -> None:
+        self.type = type_
+        self.content = content
+
+
+class _FakeMessageWithDict(_FakeMessage):
+    """Stand-in that also exposes a .dict() method like LangChain BaseMessage."""
+
+    def dict(self) -> dict:
+        return {"type": self.type, "content": self.content, "extra": "field"}
+
+
+def test_sanitize_primitives_passthrough():
+    """Primitive values must be returned unchanged."""
+    assert ReportStore._sanitize(None) is None
+    assert ReportStore._sanitize(True) is True
+    assert ReportStore._sanitize(42) == 42
+    assert ReportStore._sanitize(3.14) == 3.14
+    assert ReportStore._sanitize("hello") == "hello"
+
+
+def test_sanitize_plain_dict_passthrough():
+    """A plain JSON-safe dict must survive _sanitize unchanged."""
+    data = {"a": 1, "b": [2, 3], "c": {"d": "e"}}
+    assert ReportStore._sanitize(data) == data
+
+
+def test_sanitize_list_and_tuple():
+    """Lists and tuples of primitives must be returned as lists."""
+    assert ReportStore._sanitize([1, 2, 3]) == [1, 2, 3]
+    assert ReportStore._sanitize((1, "x")) == [1, "x"]
+
+
+def test_sanitize_message_without_dict_method():
+    """A message-like object without .dict() must be converted to type/content."""
+    msg = _FakeMessage("human", "hello world")
+    result = ReportStore._sanitize(msg)
+    assert result == {"type": "human", "content": "hello world"}
+
+
+def test_sanitize_message_with_dict_method():
+    """A message-like object with .dict() must be sanitized via that dict."""
+    msg = _FakeMessageWithDict("ai", "response text")
+    result = ReportStore._sanitize(msg)
+    assert result == {"type": "ai", "content": "response text", "extra": "field"}
+
+
+def test_sanitize_nested_messages_in_state():
+    """Messages nested inside a LangGraph-style state dict must be sanitized."""
+    msg = _FakeMessage("human", "buy signal")
+    state = {
+        "messages": [msg],
+        "investment_debate_state": {"history": [msg]},
+        "ticker": "AAPL",
+    }
+    result = ReportStore._sanitize(state)
+    assert result["ticker"] == "AAPL"
+    assert result["messages"] == [{"type": "human", "content": "buy signal"}]
+    debate = result["investment_debate_state"]["history"]
+    assert debate == [{"type": "human", "content": "buy signal"}]
+
+
+def test_sanitize_arbitrary_non_serializable_falls_back_to_str():
+    """An arbitrary non-serializable object must fall back to str()."""
+
+    class _Weird:
+        def __str__(self) -> str:
+            return "weird_value"
+
+    result = ReportStore._sanitize(_Weird())
+    assert result == "weird_value"
+
+
+def test_write_json_with_message_objects_does_not_raise(report_store, tmp_reports):
+    """_write_json must not raise when data contains message-like objects."""
+    msg = _FakeMessage("human", "test")
+    data = {"messages": [msg], "ticker": "TSLA"}
+    path = tmp_reports / "test_output.json"
+    written = report_store._write_json(path, data)
+    assert written.exists()
+    loaded = json.loads(written.read_text(encoding="utf-8"))
+    assert loaded["ticker"] == "TSLA"
+    assert loaded["messages"] == [{"type": "human", "content": "test"}]
diff --git a/tradingagents/portfolio/report_store.py b/tradingagents/portfolio/report_store.py
index 2d641693..2976ae65 100644
--- a/tradingagents/portfolio/report_store.py
+++ b/tradingagents/portfolio/report_store.py
@@ -64,6 +64,35 @@ class ReportStore:
         """
         return self._base_dir / "daily" / date / "portfolio"
 
+    @staticmethod
+    def _sanitize(obj: Any) -> Any:
+        """Recursively convert non-JSON-serializable objects to safe types.
+
+        Handles LangChain message objects (``HumanMessage``, ``AIMessage``,
+        etc.) that appear in LangGraph state dicts, as well as any other
+        arbitrary objects that are not natively JSON-serializable.
+        """
+        if obj is None or isinstance(obj, (bool, int, float, str)):
+            return obj
+        if isinstance(obj, dict):
+            return {k: ReportStore._sanitize(v) for k, v in obj.items()}
+        if isinstance(obj, (list, tuple)):
+            return [ReportStore._sanitize(item) for item in obj]
+        # LangChain BaseMessage objects expose .type and .content
+        if hasattr(obj, "type") and hasattr(obj, "content"):
+            try:
+                if hasattr(obj, "dict") and callable(obj.dict):
+                    return ReportStore._sanitize(obj.dict())
+            except Exception:
+                pass
+            return {"type": str(obj.type), "content": str(obj.content)}
+        # Generic fallback: try a serialization probe first
+        try:
+            json.dumps(obj)
+            return obj
+        except (TypeError, ValueError):
+            return str(obj)
+
     def _write_json(self, path: Path, data: dict[str, Any]) -> Path:
         """Write a dict to a JSON file, creating parent directories as needed.
 
@@ -79,7 +108,8 @@ class ReportStore:
         """
         try:
             path.parent.mkdir(parents=True, exist_ok=True)
-            path.write_text(json.dumps(data, indent=2), encoding="utf-8")
+            sanitized = self._sanitize(data)
+            path.write_text(json.dumps(sanitized, indent=2), encoding="utf-8")
             return path
         except OSError as exc:
             raise ReportStoreError(f"Failed to write {path}: {exc}") from exc