diff --git a/ollama/_utils.py b/ollama/_utils.py
index 15f1cc0c..ce2aace1 100644
--- a/ollama/_utils.py
+++ b/ollama/_utils.py
@@ -73,9 +73,14 @@ def convert_function_to_tool(func: Callable) -> Tool:
       schema['required'].remove(k)
       types.discard('null')
 
+    # Emit a single type as a string and multiple types as a JSON-Schema array.
+    # A comma-joined string (e.g. 'integer, string') is parsed by the server as one
+    # invalid type name, silently dropping the constraint. Sorting also makes the
+    # output deterministic across runs since types is a set.
+    sorted_types = sorted(types)
     schema['properties'][k] = {
       'description': parsed_docstring[k],
-      'type': ', '.join(types),
+      'type': sorted_types if len(sorted_types) > 1 else ''.join(sorted_types),
     }
 
   tool = Tool(
diff --git a/tests/test_utils.py b/tests/test_utils.py
index cb9e0d4f..1c99ee0a 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -98,8 +98,10 @@ def all_types(
   if sys.version_info >= (3, 10):
     assert tool['function']['parameters']['properties']['z']['type'] == 'array'
     assert tool['function']['parameters']['properties']['w']['type'] == 'object'
-    assert {x.strip().strip("'") for x in tool['function']['parameters']['properties']['v']['type'].removeprefix('[').removesuffix(']').split(',')} == {'string', 'integer'}
-    assert tool['function']['parameters']['properties']['v']['type'] != 'null'
+    # A multi-type parameter must be a JSON array, not a comma-joined string, so the
+    # server parses each type instead of one invalid 'integer, string' type name.
+    assert isinstance(tool['function']['parameters']['properties']['v']['type'], list)
+    assert tool['function']['parameters']['properties']['v']['type'] == ['integer', 'string']
     assert tool['function']['parameters']['required'] == ['x', 'y', 'z', 'w']
   else:
     assert tool['function']['parameters']['properties']['z']['type'] == 'array'