diff --git a/ollama/_types.py b/ollama/_types.py
index b9bb53cb..de602173 100644
--- a/ollama/_types.py
+++ b/ollama/_types.py
@@ -390,8 +390,9 @@ def serialize_model(self, nxt):
     output = nxt(self)
     if output.get('tools'):
       for tool in output['tools']:
-        if 'function' in tool and 'parameters' in tool['function'] and 'defs' in tool['function']['parameters']:
-          tool['function']['parameters']['$defs'] = tool['function']['parameters'].pop('defs')
+        parameters = (tool.get('function') or {}).get('parameters')
+        if parameters and 'defs' in parameters:
+          parameters['$defs'] = parameters.pop('defs')
     return output
 
   messages: Optional[Sequence[Union[Mapping[str, Any], Message]]] = None
diff --git a/tests/test_type_serialization.py b/tests/test_type_serialization.py
index 02a69e95..405cbefa 100644
--- a/tests/test_type_serialization.py
+++ b/tests/test_type_serialization.py
@@ -4,7 +4,7 @@
 
 import pytest
 
-from ollama._types import CreateRequest, Image
+from ollama._types import ChatRequest, CreateRequest, Image, Tool
 
 
 def test_image_serialization_bytes():
@@ -105,3 +105,40 @@ def test_create_request_serialization_license_list():
   request = CreateRequest(model='test-model', license=['MIT', 'Apache-2.0'])
   serialized = request.model_dump()
   assert serialized['license'] == ['MIT', 'Apache-2.0']
+
+
+def test_chat_request_serialization_tool_without_parameters():
+  request = ChatRequest(model='test-model', tools=[Tool(function=Tool.Function(name='func'))])
+  serialized = request.model_dump()
+  assert serialized['tools'][0]['function']['name'] == 'func'
+
+  serialized_json = request.model_dump_json()
+  assert 'func' in serialized_json
+
+
+def test_chat_request_serialization_tool_without_function():
+  request = ChatRequest(model='test-model', tools=[Tool()])
+  serialized = request.model_dump()
+  assert serialized['tools'][0]['type'] == 'function'
+
+  request.model_dump_json()
+
+
+def test_chat_request_serialization_tool_defs_alias():
+  request = ChatRequest(
+    model='test-model',
+    tools=[
+      Tool(
+        function=Tool.Function(
+          name='func',
+          parameters=Tool.Function.Parameters(
+            **{'$defs': {'Nested': {'type': 'object'}}},
+            properties={'arg': Tool.Function.Parameters.Property(type='string')},
+          ),
+        )
+      )
+    ],
+  )
+  serialized = request.model_dump()
+  assert serialized['tools'][0]['function']['parameters']['$defs'] == {'Nested': {'type': 'object'}}
+  assert 'defs' not in serialized['tools'][0]['function']['parameters']