diff --git a/durabletask/internal/shared.py b/durabletask/internal/shared.py
index 1872ad4..cda6bb3 100644
--- a/durabletask/internal/shared.py
+++ b/durabletask/internal/shared.py
@@ -103,15 +103,18 @@ def encode(self, obj: Any) -> str:
         return super().encode(obj)
 
     def default(self, obj):
-        if dataclasses.is_dataclass(obj):
+        if dataclasses.is_dataclass(obj) and not isinstance(obj, type):
             # Dataclasses are not serializable by default, so we convert them to a dict and mark them for
-            # automatic deserialization by the receiver
-            d = dataclasses.asdict(obj)  # type: ignore
+            # automatic deserialization by the receiver. We use a shallow field extraction instead of
+            # dataclasses.asdict() so that nested dataclass values are re-processed by the encoder
+            # individually (each receiving their own AUTO_SERIALIZED marker).
+            d = {f.name: getattr(obj, f.name) for f in dataclasses.fields(obj)}
             d[AUTO_SERIALIZED] = True
             return d
         elif isinstance(obj, SimpleNamespace):
-            # Most commonly used for serializing custom objects that were previously serialized using our encoder
-            d = vars(obj)
+            # Most commonly used for serializing custom objects that were previously serialized using our encoder.
+            # Copy the dict to avoid mutating the original object.
+            d = dict(vars(obj))
             d[AUTO_SERIALIZED] = True
             return d
         # This will typically raise a TypeError
diff --git a/tests/durabletask/test_json_serialization.py b/tests/durabletask/test_json_serialization.py
new file mode 100644
index 0000000..d7daa58
--- /dev/null
+++ b/tests/durabletask/test_json_serialization.py
@@ -0,0 +1,339 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import dataclasses
+from collections import namedtuple
+from types import SimpleNamespace
+
+import pytest
+
+from durabletask.internal.shared import (
+    AUTO_SERIALIZED,
+    from_json,
+    to_json,
+)
+
+
+# --- Dataclass fixtures ---
+
+@dataclasses.dataclass
+class SimpleData:
+    x: int
+    y: str
+
+
+@dataclasses.dataclass
+class InnerData:
+    value: int
+
+
+@dataclasses.dataclass
+class OuterData:
+    inner: InnerData
+    label: str
+
+
+@dataclasses.dataclass
+class DeeplyNested:
+    outer: OuterData
+    flag: bool
+
+
+# --- Namedtuple fixtures ---
+
+Point = namedtuple("Point", ["x", "y"])
+
+
+class TestDataclassSerialization:
+    """Tests for dataclass serialization/deserialization via to_json/from_json."""
+
+    def test_simple_dataclass_round_trip(self):
+        """A simple dataclass should serialize and deserialize to a SimpleNamespace."""
+        obj = SimpleData(x=1, y="hello")
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert result.x == 1
+        assert result.y == "hello"
+
+    def test_simple_dataclass_json_contains_auto_serialized_marker(self):
+        """The JSON output should contain the AUTO_SERIALIZED marker."""
+        obj = SimpleData(x=1, y="hello")
+        json_str = to_json(obj)
+
+        assert AUTO_SERIALIZED in json_str
+
+    def test_nested_dataclass_round_trip(self):
+        """Nested dataclasses should all deserialize to SimpleNamespace, not dict."""
+        obj = OuterData(inner=InnerData(value=42), label="test")
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert isinstance(result.inner, SimpleNamespace), (
+            "Inner dataclass should deserialize to SimpleNamespace, not dict"
+        )
+        assert result.inner.value == 42
+        assert result.label == "test"
+
+    def test_deeply_nested_dataclass_round_trip(self):
+        """Deeply nested dataclasses should all deserialize to SimpleNamespace."""
+        obj = DeeplyNested(
+            outer=OuterData(inner=InnerData(value=7), label="deep"),
+            flag=True,
+        )
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert isinstance(result.outer, SimpleNamespace)
+        assert isinstance(result.outer.inner, SimpleNamespace)
+        assert result.outer.inner.value == 7
+        assert result.outer.label == "deep"
+        assert result.flag is True
+
+    def test_dataclass_inside_dict(self):
+        """A dataclass value inside a dict should round-trip as SimpleNamespace."""
+        obj = {"data": SimpleData(x=10, y="world")}
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, dict)
+        assert isinstance(result["data"], SimpleNamespace)
+        assert result["data"].x == 10
+        assert result["data"].y == "world"
+
+    def test_dataclass_inside_list(self):
+        """Dataclass items inside a list should round-trip as SimpleNamespace."""
+        items = [SimpleData(x=1, y="a"), SimpleData(x=2, y="b")]
+        json_str = to_json(items)
+        result = from_json(json_str)
+
+        assert isinstance(result, list)
+        assert len(result) == 2
+        for item in result:
+            assert isinstance(item, SimpleNamespace)
+        assert result[0].x == 1
+        assert result[1].y == "b"
+
+    def test_array_of_nested_dataclasses(self):
+        """An array of dataclasses with nested dataclass fields should fully round-trip."""
+        items = [
+            OuterData(inner=InnerData(value=1), label="first"),
+            OuterData(inner=InnerData(value=2), label="second"),
+        ]
+        json_str = to_json(items)
+        result = from_json(json_str)
+
+        assert isinstance(result, list)
+        assert len(result) == 2
+        for item in result:
+            assert isinstance(item, SimpleNamespace)
+            assert isinstance(item.inner, SimpleNamespace)
+        assert result[0].inner.value == 1
+        assert result[0].label == "first"
+        assert result[1].inner.value == 2
+        assert result[1].label == "second"
+
+    def test_nested_array_of_dataclasses(self):
+        """An array nested inside another array of dataclasses should round-trip."""
+        items = [
+            [SimpleData(x=1, y="a"), SimpleData(x=2, y="b")],
+            [SimpleData(x=3, y="c")],
+        ]
+        json_str = to_json(items)
+        result = from_json(json_str)
+
+        assert isinstance(result, list)
+        assert len(result) == 2
+        assert isinstance(result[0], list)
+        assert len(result[0]) == 2
+        assert isinstance(result[1], list)
+        assert len(result[1]) == 1
+        for sublist in result:
+            for item in sublist:
+                assert isinstance(item, SimpleNamespace)
+        assert result[0][0].x == 1
+        assert result[0][1].y == "b"
+        assert result[1][0].x == 3
+
+    def test_dict_with_nested_dataclass_values(self):
+        """Dict values that are nested dataclasses should fully round-trip."""
+        obj = {"item": OuterData(inner=InnerData(value=99), label="nested")}
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, dict)
+        assert isinstance(result["item"], SimpleNamespace)
+        assert isinstance(result["item"].inner, SimpleNamespace)
+        assert result["item"].inner.value == 99
+        assert result["item"].label == "nested"
+
+    def test_dict_with_multiple_dataclass_values(self):
+        """A dict with several dataclass values should all round-trip."""
+        obj = {
+            "a": SimpleData(x=1, y="one"),
+            "b": SimpleData(x=2, y="two"),
+        }
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, dict)
+        for key in ("a", "b"):
+            assert isinstance(result[key], SimpleNamespace)
+        assert result["a"].x == 1
+        assert result["b"].y == "two"
+
+    def test_dict_with_array_of_dataclasses(self):
+        """A dict whose value is a list of dataclasses should round-trip."""
+        obj = {"items": [SimpleData(x=1, y="a"), SimpleData(x=2, y="b")]}
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, dict)
+        assert isinstance(result["items"], list)
+        assert len(result["items"]) == 2
+        for item in result["items"]:
+            assert isinstance(item, SimpleNamespace)
+        assert result["items"][0].x == 1
+        assert result["items"][1].y == "b"
+
+    def test_dict_with_array_of_nested_dataclasses(self):
+        """A dict whose value is a list of nested dataclasses should fully round-trip."""
+        obj = {
+            "records": [
+                OuterData(inner=InnerData(value=10), label="r1"),
+                OuterData(inner=InnerData(value=20), label="r2"),
+            ]
+        }
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, dict)
+        assert isinstance(result["records"], list)
+        for item in result["records"]:
+            assert isinstance(item, SimpleNamespace)
+            assert isinstance(item.inner, SimpleNamespace)
+        assert result["records"][0].inner.value == 10
+        assert result["records"][1].label == "r2"
+
+    def test_dataclass_with_list_of_dataclass_field(self):
+        """A dataclass containing a list-of-dataclass field should round-trip."""
+        @dataclasses.dataclass
+        class Container:
+            items: list
+
+        obj = Container(items=[InnerData(value=1), InnerData(value=2)])
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert isinstance(result.items, list)
+        assert len(result.items) == 2
+        for item in result.items:
+            assert isinstance(item, SimpleNamespace)
+        assert result.items[0].value == 1
+        assert result.items[1].value == 2
+
+    def test_dataclass_with_dict_of_dataclass_field(self):
+        """A dataclass containing a dict-of-dataclass field should round-trip."""
+        @dataclasses.dataclass
+        class Mapping:
+            entries: dict
+
+        obj = Mapping(entries={"a": InnerData(value=5), "b": InnerData(value=6)})
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert isinstance(result.entries, dict)
+        for val in result.entries.values():
+            assert isinstance(val, SimpleNamespace)
+        assert result.entries["a"].value == 5
+        assert result.entries["b"].value == 6
+
+
+class TestSimpleNamespaceSerialization:
+    """Tests for SimpleNamespace serialization."""
+
+    def test_simple_namespace_round_trip(self):
+        """SimpleNamespace should serialize and deserialize correctly."""
+        obj = SimpleNamespace(a=1, b="two")
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert result.a == 1
+        assert result.b == "two"
+
+    def test_simple_namespace_not_mutated(self):
+        """Serializing a SimpleNamespace should NOT mutate the original object."""
+        obj = SimpleNamespace(x=1, y=2)
+        original_attrs = set(vars(obj).keys())
+
+        to_json(obj)
+
+        current_attrs = set(vars(obj).keys())
+        assert current_attrs == original_attrs, (
+            f"Original SimpleNamespace was mutated: gained {current_attrs - original_attrs}"
+        )
+        assert not hasattr(obj, AUTO_SERIALIZED)
+
+    def test_nested_simple_namespace_round_trip(self):
+        """Nested SimpleNamespace should deserialize as SimpleNamespace."""
+        obj = SimpleNamespace(child=SimpleNamespace(val=99))
+        json_str = to_json(obj)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert isinstance(result.child, SimpleNamespace)
+        assert result.child.val == 99
+
+
+class TestNamedtupleSerialization:
+    """Tests for namedtuple serialization."""
+
+    def test_namedtuple_top_level_round_trip(self):
+        """A namedtuple at the top level should serialize with field names."""
+        p = Point(x=3, y=4)
+        json_str = to_json(p)
+        result = from_json(json_str)
+
+        assert isinstance(result, SimpleNamespace)
+        assert result.x == 3
+        assert result.y == 4
+
+
+class TestPrimitiveSerialization:
+    """Tests for primitive/basic type round-trips."""
+
+    @pytest.mark.parametrize("value", [
+        42,
+        3.14,
+        "hello",
+        True,
+        False,
+        None,
+        [1, 2, 3],
+        {"key": "value"},
+    ])
+    def test_primitive_round_trip(self, value):
+        """Primitive types should round-trip unchanged."""
+        json_str = to_json(value)
+        result = from_json(json_str)
+        assert result == value
+
+
+class TestDataclassNotMutated:
+    """Ensure serialization does not mutate dataclass inputs."""
+
+    def test_dataclass_not_mutated(self):
+        """Serializing a dataclass should not add attributes to the original."""
+        obj = SimpleData(x=1, y="test")
+        to_json(obj)
+
+        # dataclass fields should be unchanged
+        assert obj.x == 1
+        assert obj.y == "test"
+        assert not hasattr(obj, AUTO_SERIALIZED)