diff --git a/src/python_workflow_definition/models.py b/src/python_workflow_definition/models.py
index 4980cfa..8b25be8 100644
--- a/src/python_workflow_definition/models.py
+++ b/src/python_workflow_definition/models.py
@@ -1,5 +1,15 @@
 from pathlib import Path
-from typing import List, Union, Optional, Literal, Any, Annotated, Type, TypeVar
+from typing import (
+    List,
+    Union,
+    Optional,
+    Literal,
+    Any,
+    Annotated,
+    Type,
+    TypeVar,
+)
+from typing_extensions import TypeAliasType
 from pydantic import BaseModel, Field, field_validator, field_serializer
 from pydantic import ValidationError
 import json
@@ -19,6 +29,13 @@
 )
 
 
+JsonPrimitive = Union[str, int, float, bool, None]
+AllowableDefaults = TypeAliasType(
+    "AllowableDefaults",
+    "Union[JsonPrimitive, dict[str, AllowableDefaults], list[AllowableDefaults]]",
+)
+
+
 class PythonWorkflowDefinitionBaseNode(BaseModel):
     """Base model for all node types, containing common fields."""
 
@@ -33,7 +50,7 @@ class PythonWorkflowDefinitionInputNode(PythonWorkflowDefinitionBaseNode):
 
     type: Literal["input"]
     name: str
-    value: Optional[Any] = None
+    value: Optional[AllowableDefaults] = None
 
 
 class PythonWorkflowDefinitionOutputNode(PythonWorkflowDefinitionBaseNode):
diff --git a/tests/test_models.py b/tests/test_models.py
index 83f6066..d353f46 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -4,6 +4,7 @@
 from unittest import mock
 from pydantic import ValidationError
 from python_workflow_definition.models import (
+    JsonPrimitive,
     PythonWorkflowDefinitionInputNode,
     PythonWorkflowDefinitionOutputNode,
     PythonWorkflowDefinitionFunctionNode,
@@ -12,6 +13,11 @@
     INTERNAL_DEFAULT_HANDLE,
 )
 
+
+class _NoTrivialSerialization:
+    pass
+
+
 class TestModels(unittest.TestCase):
     def setUp(self):
         self.valid_workflow_dict = {
@@ -40,6 +46,50 @@ def test_input_node(self):
         )
         self.assertEqual(node_with_value.value, 42)
 
+    def test_input_node_valid_values(self):
+        good_values = (
+            1,
+            1.1,
+            "string",
+            True,
+            None,
+            [1, 2],
+            [["recursive", "tuple"], [True, False]],
+        )
+        for value in good_values:
+            with self.subTest(value=value):
+                model = PythonWorkflowDefinitionInputNode.model_validate(
+                    {
+                        "id": 0,
+                        "type": "input",
+                        "name": "x",
+                        "value": value,
+                    }
+                )
+                self.assertEqual(
+                    value,
+                    PythonWorkflowDefinitionInputNode.model_validate(
+                        model.model_dump(mode="json")
+                    ).value
+                )
+
+    def test_input_node_invalid_value_raises(self):
+        bad_values = (
+            {1: 2},
+            _NoTrivialSerialization(),
+        )
+        for value in bad_values:
+            with self.subTest(value=value):
+                with self.assertRaises(ValidationError):
+                    PythonWorkflowDefinitionInputNode.model_validate(
+                        {
+                            "id": 0,
+                            "type": "input",
+                            "name": "x",
+                            "value": value,
+                        }
+                    )
+
     def test_output_node(self):
         node = PythonWorkflowDefinitionOutputNode(id=1, type="output", name="test_output")
         self.assertEqual(node.id, 1)