diff --git a/ollama/_utils.py b/ollama/_utils.py
index 15f1cc0c..925cb1c5 100644
--- a/ollama/_utils.py
+++ b/ollama/_utils.py
@@ -56,21 +56,28 @@ def _parse_docstring(doc_string: Union[str, None]) -> dict[str, str]:
 def convert_function_to_tool(func: Callable) -> Tool:
   doc_string_hash = str(hash(inspect.getdoc(func)))
   parsed_docstring = _parse_docstring(inspect.getdoc(func))
+  signature = inspect.signature(func)
+  # Parameters with a default value are optional and should not appear in `required`.
+  params_with_defaults = {k for k, v in signature.parameters.items() if v.default is not inspect.Parameter.empty}
   schema = type(
     func.__name__,
     (pydantic.BaseModel,),
     {
-      '__annotations__': {k: v.annotation if v.annotation != inspect._empty else str for k, v in inspect.signature(func).parameters.items()},
-      '__signature__': inspect.signature(func),
+      '__annotations__': {k: v.annotation if v.annotation != inspect._empty else str for k, v in signature.parameters.items()},
+      '__signature__': signature,
       '__doc__': parsed_docstring[doc_string_hash],
     },
   ).model_json_schema()
 
+  if params_with_defaults and schema.get('required'):
+    schema['required'] = [k for k in schema['required'] if k not in params_with_defaults]
+
   for k, v in schema.get('properties', {}).items():
     # If type is missing, the default is string
     types = {t.get('type', 'string') for t in v.get('anyOf')} if 'anyOf' in v else {v.get('type', 'string')}
     if 'null' in types:
-      schema['required'].remove(k)
+      if k in schema.get('required', []):
+        schema['required'].remove(k)
       types.discard('null')
 
     schema['properties'][k] = {
diff --git a/tests/test_utils.py b/tests/test_utils.py
index cb9e0d4f..83ece440 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -256,3 +256,48 @@ def func_with_parentheses_and_args(a: int, b: int):
   tool = convert_function_to_tool(func_with_parentheses_and_args).model_dump()
   assert tool['function']['parameters']['properties']['a']['description'] == 'First (:thing) number to add'
   assert tool['function']['parameters']['properties']['b']['description'] == 'Second number to add'
+
+
+def test_function_with_default_values_not_required():
+  """Parameters with default values should be optional, not required.
+
+  Regression test: previously a parameter like `units: str = "celsius"`
+  was incorrectly included in the `required` list even though Python
+  treats it as optional. See OpenAI tool schema spec.
+  """
+
+  def get_weather(city: str, units: str = 'celsius', detailed: bool = False) -> str:
+    """Get the current weather for a city.
+    Args:
+        city: The city to look up.
+        units: Temperature units (celsius or fahrenheit).
+        detailed: Whether to return a detailed report.
+    """
+    return f'{city}: 20 {units}'
+
+  tool = convert_function_to_tool(get_weather).model_dump()
+  required = tool['function']['parameters']['required']
+
+  # Only `city` (no default) should be required.
+  assert required == ['city'], f'expected [city], got {required}'
+
+  # All three params should still appear in properties with descriptions.
+  properties = tool['function']['parameters']['properties']
+  assert set(properties.keys()) == {'city', 'units', 'detailed'}
+  assert properties['units']['description'] == 'Temperature units (celsius or fahrenheit).'
+  assert properties['detailed']['type'] == 'boolean'
+
+
+def test_function_with_all_defaults_has_empty_required():
+  def all_optional(a: int = 1, b: str = 'x') -> str:
+    """All optional.
+    Args:
+        a: first
+        b: second
+    """
+    return f'{a}{b}'
+
+  tool = convert_function_to_tool(all_optional).model_dump()
+  required = tool['function']['parameters']['required']
+  # An empty required list is acceptable; some pydantic versions omit the key.
+  assert not required, f'expected no required params, got {required}'