diff --git a/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py b/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
index 8c52c7a521..516a4547a0 100644
--- a/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
+++ b/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
@@ -146,6 +146,10 @@ def decode_checkpoint_value(value: Any) -> Any:
                     cls = None
 
                 if cls is not None:
+                    # Verify the class actually supports the model protocol
+                    if not _class_supports_model_protocol(cls):
+                        logger.debug(f"Class {type_key} does not support model protocol; returning raw value")
+                        return decoded_payload
                     if strategy == "to_dict" and hasattr(cls, "from_dict"):
                         with contextlib.suppress(Exception):
                             return cls.from_dict(decoded_payload)
@@ -169,6 +173,10 @@ def decode_checkpoint_value(value: Any) -> Any:
                     if module is None:
                         module = importlib.import_module(module_name)
                     cls_dc: Any = getattr(module, class_name)
+                    # Verify the class is actually a dataclass type (not an instance)
+                    if not isinstance(cls_dc, type) or not is_dataclass(cls_dc):
+                        logger.debug(f"Class {type_key_dc} is not a dataclass type; returning raw value")
+                        return decoded_raw
                     constructed = _instantiate_checkpoint_dataclass(cls_dc, decoded_raw)
                     if constructed is not None:
                         return constructed
@@ -188,6 +196,22 @@ def decode_checkpoint_value(value: Any) -> Any:
     return value
 
 
+def _class_supports_model_protocol(cls: type[Any]) -> bool:
+    """Check if a class type supports the model serialization protocol.
+
+    Checks for pairs of serialization/deserialization methods:
+    - to_dict/from_dict
+    - to_json/from_json
+    """
+    has_to_dict = hasattr(cls, "to_dict") and callable(getattr(cls, "to_dict", None))
+    has_from_dict = hasattr(cls, "from_dict") and callable(getattr(cls, "from_dict", None))
+
+    has_to_json = hasattr(cls, "to_json") and callable(getattr(cls, "to_json", None))
+    has_from_json = hasattr(cls, "from_json") and callable(getattr(cls, "from_json", None))
+
+    return (has_to_dict and has_from_dict) or (has_to_json and has_from_json)
+
+
 def _supports_model_protocol(obj: object) -> bool:
     """Detect objects that expose dictionary serialization hooks."""
     try:
@@ -195,13 +219,7 @@ def _supports_model_protocol(obj: object) -> bool:
     except Exception:
         return False
 
-    has_to_dict = hasattr(obj, "to_dict") and callable(getattr(obj, "to_dict", None))  # type: ignore[arg-type]
-    has_from_dict = hasattr(obj_type, "from_dict") and callable(getattr(obj_type, "from_dict", None))
-
-    has_to_json = hasattr(obj, "to_json") and callable(getattr(obj, "to_json", None))  # type: ignore[arg-type]
-    has_from_json = hasattr(obj_type, "from_json") and callable(getattr(obj_type, "from_json", None))
-
-    return (has_to_dict and has_from_dict) or (has_to_json and has_from_json)
+    return _class_supports_model_protocol(obj_type)
 
 
 def _import_qualified_name(qualname: str) -> type[Any] | None:
diff --git a/python/packages/core/tests/workflow/test_checkpoint_decode.py b/python/packages/core/tests/workflow/test_checkpoint_decode.py
index b126eafacf..431c70cc3c 100644
--- a/python/packages/core/tests/workflow/test_checkpoint_decode.py
+++ b/python/packages/core/tests/workflow/test_checkpoint_decode.py
@@ -3,7 +3,10 @@
 from dataclasses import dataclass  # noqa: I001
 from typing import Any, cast
 
+
 from agent_framework._workflows._checkpoint_encoding import (
+    DATACLASS_MARKER,
+    MODEL_MARKER,
     decode_checkpoint_value,
     encode_checkpoint_value,
 )
@@ -126,3 +129,110 @@ def test_encode_decode_nested_structures() -> None:
     assert response.data == "first response"
     assert isinstance(response.original_request, SampleRequest)
     assert response.original_request.request_id == "req-1"
+
+
+def test_encode_allows_marker_key_without_value_key() -> None:
+    """Test that encoding a dict with only the marker key (no 'value') is allowed."""
+    dict_with_marker_only = {
+        MODEL_MARKER: "some.module:FakeClass",
+        "other_key": "test",
+    }
+    encoded = encode_checkpoint_value(dict_with_marker_only)
+    assert MODEL_MARKER in encoded
+    assert "other_key" in encoded
+
+
+def test_encode_allows_value_key_without_marker_key() -> None:
+    """Test that encoding a dict with only 'value' key (no marker) is allowed."""
+    dict_with_value_only = {
+        "value": {"data": "test"},
+        "other_key": "test",
+    }
+    encoded = encode_checkpoint_value(dict_with_value_only)
+    assert "value" in encoded
+    assert "other_key" in encoded
+
+
+def test_encode_allows_marker_with_value_key() -> None:
+    """Test that encoding a dict with marker and 'value' keys is allowed.
+
+    This is allowed because legitimate encoded data may contain these keys,
+    and security is enforced at deserialization time by validating class types.
+    """
+    dict_with_both = {
+        MODEL_MARKER: "some.module:SomeClass",
+        "value": {"data": "test"},
+        "strategy": "to_dict",
+    }
+    encoded = encode_checkpoint_value(dict_with_both)
+    assert MODEL_MARKER in encoded
+    assert "value" in encoded
+
+
+class NotADataclass:
+    """A regular class that is not a dataclass."""
+
+    def __init__(self, value: str) -> None:
+        self.value = value
+
+    def get_value(self) -> str:
+        return self.value
+
+
+class NotAModel:
+    """A regular class that does not support the model protocol."""
+
+    def __init__(self, value: str) -> None:
+        self.value = value
+
+    def get_value(self) -> str:
+        return self.value
+
+
+def test_decode_rejects_non_dataclass_with_dataclass_marker() -> None:
+    """Test that decode returns raw value when marked class is not a dataclass."""
+    # Manually construct a payload that claims NotADataclass is a dataclass
+    fake_payload = {
+        DATACLASS_MARKER: f"{NotADataclass.__module__}:{NotADataclass.__name__}",
+        "value": {"value": "test_value"},
+    }
+
+    decoded = decode_checkpoint_value(fake_payload)
+
+    # Should return the raw decoded value, not an instance of NotADataclass
+    assert isinstance(decoded, dict)
+    assert decoded["value"] == "test_value"
+
+
+def test_decode_rejects_non_model_with_model_marker() -> None:
+    """Test that decode returns raw value when marked class doesn't support model protocol."""
+    # Manually construct a payload that claims NotAModel supports the model protocol
+    fake_payload = {
+        MODEL_MARKER: f"{NotAModel.__module__}:{NotAModel.__name__}",
+        "strategy": "to_dict",
+        "value": {"value": "test_value"},
+    }
+
+    decoded = decode_checkpoint_value(fake_payload)
+
+    # Should return the raw decoded value, not an instance of NotAModel
+    assert isinstance(decoded, dict)
+    assert decoded["value"] == "test_value"
+
+
+def test_encode_allows_nested_dict_with_marker_keys() -> None:
+    """Test that encoding allows nested dicts containing marker patterns.
+
+    Security is enforced at deserialization time, not serialization time,
+    so legitimate encoded data can contain markers at any nesting level.
+    """
+    nested_data = {
+        "outer": {
+            MODEL_MARKER: "some.module:SomeClass",
+            "value": {"data": "test"},
+        }
+    }
+
+    encoded = encode_checkpoint_value(nested_data)
+    assert "outer" in encoded
+    assert MODEL_MARKER in encoded["outer"]
diff --git a/python/packages/core/tests/workflow/test_checkpoint_encode.py b/python/packages/core/tests/workflow/test_checkpoint_encode.py
new file mode 100644
index 0000000000..3f4db1f864
--- /dev/null
+++ b/python/packages/core/tests/workflow/test_checkpoint_encode.py
@@ -0,0 +1,423 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+from dataclasses import dataclass
+from typing import Any
+
+from agent_framework._workflows._checkpoint_encoding import (
+    _CYCLE_SENTINEL,
+    DATACLASS_MARKER,
+    MODEL_MARKER,
+    encode_checkpoint_value,
+)
+
+
+@dataclass
+class SimpleDataclass:
+    """A simple dataclass for testing encoding."""
+
+    name: str
+    value: int
+
+
+@dataclass
+class NestedDataclass:
+    """A dataclass with nested dataclass field."""
+
+    outer_name: str
+    inner: SimpleDataclass
+
+
+class ModelWithToDict:
+    """A class that implements to_dict/from_dict protocol."""
+
+    def __init__(self, data: str) -> None:
+        self.data = data
+
+    def to_dict(self) -> dict[str, Any]:
+        return {"data": self.data}
+
+    @classmethod
+    def from_dict(cls, d: dict[str, Any]) -> "ModelWithToDict":
+        return cls(data=d["data"])
+
+
+class ModelWithToJson:
+    """A class that implements to_json/from_json protocol."""
+
+    def __init__(self, data: str) -> None:
+        self.data = data
+
+    def to_json(self) -> str:
+        return f'{{"data": "{self.data}"}}'
+
+    @classmethod
+    def from_json(cls, json_str: str) -> "ModelWithToJson":
+        import json
+
+        d = json.loads(json_str)
+        return cls(data=d["data"])
+
+
+class UnknownObject:
+    """A class that doesn't support any serialization protocol."""
+
+    def __init__(self, value: str) -> None:
+        self.value = value
+
+    def __str__(self) -> str:
+        return f"UnknownObject({self.value})"
+
+
+# --- Tests for primitive encoding ---
+
+
+def test_encode_string() -> None:
+    """Test encoding a string value."""
+    result = encode_checkpoint_value("hello")
+    assert result == "hello"
+
+
+def test_encode_integer() -> None:
+    """Test encoding an integer value."""
+    result = encode_checkpoint_value(42)
+    assert result == 42
+
+
+def test_encode_float() -> None:
+    """Test encoding a float value."""
+    result = encode_checkpoint_value(3.14)
+    assert result == 3.14
+
+
+def test_encode_boolean_true() -> None:
+    """Test encoding a True boolean value."""
+    result = encode_checkpoint_value(True)
+    assert result is True
+
+
+def test_encode_boolean_false() -> None:
+    """Test encoding a False boolean value."""
+    result = encode_checkpoint_value(False)
+    assert result is False
+
+
+def test_encode_none() -> None:
+    """Test encoding a None value."""
+    result = encode_checkpoint_value(None)
+    assert result is None
+
+
+# --- Tests for collection encoding ---
+
+
+def test_encode_empty_dict() -> None:
+    """Test encoding an empty dictionary."""
+    result = encode_checkpoint_value({})
+    assert result == {}
+
+
+def test_encode_simple_dict() -> None:
+    """Test encoding a simple dictionary with primitive values."""
+    data = {"name": "test", "count": 5, "active": True}
+    result = encode_checkpoint_value(data)
+    assert result == {"name": "test", "count": 5, "active": True}
+
+
+def test_encode_dict_with_non_string_keys() -> None:
+    """Test encoding a dictionary with non-string keys (converted to strings)."""
+    data = {1: "one", 2: "two"}
+    result = encode_checkpoint_value(data)
+    assert result == {"1": "one", "2": "two"}
+
+
+def test_encode_empty_list() -> None:
+    """Test encoding an empty list."""
+    result = encode_checkpoint_value([])
+    assert result == []
+
+
+def test_encode_simple_list() -> None:
+    """Test encoding a simple list with primitive values."""
+    data = [1, 2, 3, "four"]
+    result = encode_checkpoint_value(data)
+    assert result == [1, 2, 3, "four"]
+
+
+def test_encode_tuple() -> None:
+    """Test encoding a tuple (converted to list)."""
+    data = (1, 2, 3)
+    result = encode_checkpoint_value(data)
+    assert result == [1, 2, 3]
+
+
+def test_encode_set() -> None:
+    """Test encoding a set (converted to list)."""
+    data = {1, 2, 3}
+    result = encode_checkpoint_value(data)
+    assert isinstance(result, list)
+    assert sorted(result) == [1, 2, 3]
+
+
+def test_encode_nested_dict() -> None:
+    """Test encoding a nested dictionary structure."""
+    data = {
+        "outer": {
+            "inner": {
+                "value": 42,
+            }
+        }
+    }
+    result = encode_checkpoint_value(data)
+    assert result == {"outer": {"inner": {"value": 42}}}
+
+
+def test_encode_list_of_dicts() -> None:
+    """Test encoding a list containing dictionaries."""
+    data = [{"a": 1}, {"b": 2}]
+    result = encode_checkpoint_value(data)
+    assert result == [{"a": 1}, {"b": 2}]
+
+
+# --- Tests for dataclass encoding ---
+
+
+def test_encode_simple_dataclass() -> None:
+    """Test encoding a simple dataclass."""
+    obj = SimpleDataclass(name="test", value=42)
+    result = encode_checkpoint_value(obj)
+
+    assert isinstance(result, dict)
+    assert DATACLASS_MARKER in result
+    assert "value" in result
+    assert result["value"] == {"name": "test", "value": 42}
+
+
+def test_encode_nested_dataclass() -> None:
+    """Test encoding a dataclass with nested dataclass fields."""
+    inner = SimpleDataclass(name="inner", value=10)
+    outer = NestedDataclass(outer_name="outer", inner=inner)
+    result = encode_checkpoint_value(outer)
+
+    assert isinstance(result, dict)
+    assert DATACLASS_MARKER in result
+    assert "value" in result
+
+    outer_value = result["value"]
+    assert outer_value["outer_name"] == "outer"
+    assert DATACLASS_MARKER in outer_value["inner"]
+
+
+def test_encode_list_of_dataclasses() -> None:
+    """Test encoding a list containing dataclass instances."""
+    data = [
+        SimpleDataclass(name="first", value=1),
+        SimpleDataclass(name="second", value=2),
+    ]
+    result = encode_checkpoint_value(data)
+
+    assert isinstance(result, list)
+    assert len(result) == 2
+    for item in result:
+        assert DATACLASS_MARKER in item
+
+
+def test_encode_dict_with_dataclass_values() -> None:
+    """Test encoding a dictionary with dataclass values."""
+    data = {
+        "item1": SimpleDataclass(name="first", value=1),
+        "item2": SimpleDataclass(name="second", value=2),
+    }
+    result = encode_checkpoint_value(data)
+
+    assert isinstance(result, dict)
+    assert DATACLASS_MARKER in result["item1"]
+    assert DATACLASS_MARKER in result["item2"]
+
+
+# --- Tests for model protocol encoding ---
+
+
+def test_encode_model_with_to_dict() -> None:
+    """Test encoding an object implementing to_dict/from_dict protocol."""
+    obj = ModelWithToDict(data="test_data")
+    result = encode_checkpoint_value(obj)
+
+    assert isinstance(result, dict)
+    assert MODEL_MARKER in result
+    assert result["strategy"] == "to_dict"
+    assert result["value"] == {"data": "test_data"}
+
+
+def test_encode_model_with_to_json() -> None:
+    """Test encoding an object implementing to_json/from_json protocol."""
+    obj = ModelWithToJson(data="test_data")
+    result = encode_checkpoint_value(obj)
+
+    assert isinstance(result, dict)
+    assert MODEL_MARKER in result
+    assert result["strategy"] == "to_json"
+    assert '"data": "test_data"' in result["value"]
+
+
+# --- Tests for unknown object encoding ---
+
+
+def test_encode_unknown_object_fallback_to_string() -> None:
+    """Test that unknown objects are encoded as strings."""
+    obj = UnknownObject(value="test")
+    result = encode_checkpoint_value(obj)
+
+    assert isinstance(result, str)
+    assert "UnknownObject" in result
+
+
+# --- Tests for cycle detection ---
+
+
+def test_encode_dict_with_self_reference() -> None:
+    """Test that dict self-references are detected and handled."""
+    data: dict[str, Any] = {"name": "test"}
+    data["self"] = data  # Create circular reference
+
+    result = encode_checkpoint_value(data)
+    assert result["name"] == "test"
+    assert result["self"] == _CYCLE_SENTINEL
+
+
+def test_encode_list_with_self_reference() -> None:
+    """Test that list self-references are detected and handled."""
+    data: list[Any] = [1, 2]
+    data.append(data)  # Create circular reference
+
+    result = encode_checkpoint_value(data)
+    assert result[0] == 1
+    assert result[1] == 2
+    assert result[2] == _CYCLE_SENTINEL
+
+
+# --- Tests for reserved keyword handling ---
+# Note: Security is enforced at deserialization time by validating class types,
+# not at serialization time. This allows legitimate encoded data to be re-encoded.
+
+
+def test_encode_allows_dict_with_model_marker_and_value() -> None:
+    """Test that encoding a dict with MODEL_MARKER and 'value' is allowed.
+
+    Security is enforced at deserialization time, not serialization time.
+    """
+    data = {
+        MODEL_MARKER: "some.module:SomeClass",
+        "value": {"data": "test"},
+    }
+    result = encode_checkpoint_value(data)
+    assert MODEL_MARKER in result
+    assert "value" in result
+
+
+def test_encode_allows_dict_with_dataclass_marker_and_value() -> None:
+    """Test that encoding a dict with DATACLASS_MARKER and 'value' is allowed.
+
+    Security is enforced at deserialization time, not serialization time.
+    """
+    data = {
+        DATACLASS_MARKER: "some.module:SomeClass",
+        "value": {"field": "test"},
+    }
+    result = encode_checkpoint_value(data)
+    assert DATACLASS_MARKER in result
+    assert "value" in result
+
+
+def test_encode_allows_nested_dict_with_marker_keys() -> None:
+    """Test that encoding nested dict with marker keys is allowed.
+
+    Security is enforced at deserialization time, not serialization time.
+    """
+    nested_data = {
+        "outer": {
+            MODEL_MARKER: "some.module:SomeClass",
+            "value": {"data": "test"},
+        }
+    }
+    result = encode_checkpoint_value(nested_data)
+    assert "outer" in result
+    assert MODEL_MARKER in result["outer"]
+
+
+def test_encode_allows_marker_without_value() -> None:
+    """Test that a dict with marker key but without 'value' key is allowed."""
+    data = {
+        MODEL_MARKER: "some.module:SomeClass",
+        "other_key": "allowed",
+    }
+    result = encode_checkpoint_value(data)
+    assert MODEL_MARKER in result
+    assert result["other_key"] == "allowed"
+
+
+def test_encode_allows_value_without_marker() -> None:
+    """Test that a dict with 'value' key but without marker is allowed."""
+    data = {
+        "value": {"nested": "data"},
+        "other_key": "allowed",
+    }
+    result = encode_checkpoint_value(data)
+    assert "value" in result
+    assert result["other_key"] == "allowed"
+
+
+# --- Tests for max depth protection ---
+
+
+def test_encode_deep_nesting_triggers_max_depth() -> None:
+    """Test that very deep nesting triggers max depth protection."""
+    # Create a deeply nested structure (over 100 levels)
+    data: dict[str, Any] = {"level": 0}
+    current = data
+    for i in range(105):
+        current["nested"] = {"level": i + 1}
+        current = current["nested"]
+
+    result = encode_checkpoint_value(data)
+
+    # Navigate to find the max_depth sentinel
+    current_result = result
+    found_max_depth = False
+    for _ in range(110):
+        if isinstance(current_result, dict) and "nested" in current_result:
+            current_result = current_result["nested"]
+            if current_result == "<max_depth>":
+                found_max_depth = True
+                break
+        else:
+            break
+
+    assert found_max_depth, "Expected <max_depth> sentinel to be found in deeply nested structure"
+
+
+# --- Tests for mixed complex structures ---
+
+
+def test_encode_complex_mixed_structure() -> None:
+    """Test encoding a complex structure with mixed types."""
+    data = {
+        "string_value": "hello",
+        "int_value": 42,
+        "float_value": 3.14,
+        "bool_value": True,
+        "none_value": None,
+        "list_value": [1, 2, 3],
+        "nested_dict": {"a": 1, "b": 2},
+        "dataclass_value": SimpleDataclass(name="test", value=100),
+    }
+
+    result = encode_checkpoint_value(data)
+
+    assert result["string_value"] == "hello"
+    assert result["int_value"] == 42
+    assert result["float_value"] == 3.14
+    assert result["bool_value"] is True
+    assert result["none_value"] is None
+    assert result["list_value"] == [1, 2, 3]
+    assert result["nested_dict"] == {"a": 1, "b": 2}
+    assert DATACLASS_MARKER in result["dataclass_value"]