diff --git a/nemoguardrails/colang/v2_x/runtime/serialization.py b/nemoguardrails/colang/v2_x/runtime/serialization.py
index 095bfbe0b..cf5bdf660 100644
--- a/nemoguardrails/colang/v2_x/runtime/serialization.py
+++ b/nemoguardrails/colang/v2_x/runtime/serialization.py
@@ -86,13 +86,21 @@ def encode_to_dict(obj: Any, refs: Dict[int, Any]):
                 "value": {k: encode_to_dict(v, refs) for k, v in obj.items()},
             }
         elif is_dataclass(obj):
-            value = {
-                "__type": type(obj).__name__,
-                "value": {
-                    k: encode_to_dict(getattr(obj, k), refs)
-                    for k in obj.__dataclass_fields__.keys()
-                },
+            # Encode dataclasses. If it's a known class (present in name_to_class),
+            # keep its type tag so we can fully round-trip via json_to_state.
+            # Otherwise, fall back to a plain dict to avoid "Unknown d_type" on decode.
+            cls = type(obj)
+            encoded_fields = {
+                k: encode_to_dict(getattr(obj, k), refs)
+                for k in obj.__dataclass_fields__.keys()
             }
+
+            if cls.__name__ in name_to_class and name_to_class[cls.__name__] is cls:
+                value = {"__type": cls.__name__, "value": encoded_fields}
+            else:
+                # Unknown dataclass → JSON-friendly dict
+                value = {"__type": "dict", "value": encoded_fields}
+
         elif isinstance(obj, RailsConfig):
             value = {
                 "__type": "RailsConfig",
diff --git a/tests/v2_x/test_serialization_dataclass.py b/tests/v2_x/test_serialization_dataclass.py
new file mode 100644
index 000000000..25ce80213
--- /dev/null
+++ b/tests/v2_x/test_serialization_dataclass.py
@@ -0,0 +1,35 @@
+# SPDX-FileCopyrightText: Copyright (c) 2023 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+from dataclasses import dataclass
+
+from nemoguardrails.colang.v2_x.runtime.serialization import state_to_json
+
+
+@dataclass
+class Foo:
+    bar: str
+    baz: int
+
+
+def test_state_to_json_unknown_dataclass_encodes_as_dict():
+    js = state_to_json({"out": Foo("ok", 1)})
+    d = json.loads(js)
+    # We expect unknown dataclasses to be encoded as plain dicts
+    assert d["__type"] == "dict"
+    out = d["value"]["out"]
+    assert out["__type"] == "dict"
+    assert out["value"] == {"bar": "ok", "baz": 1}