[Bugfix] Fix Gemma4 tool parser converting bare `null` to string `"null"` (#39679)

Signed-off-by: KimuGenie <baby11686@naver.com>
Signed-off-by: Chauncey <chaunceyjiang@gmail.com>
Co-authored-by: Chauncey <chaunceyjiang@gmail.com>

tests/tool_parsers/test_gemma4_tool_parser.py

@@ -85,6 +85,14 @@ class TestParseGemma4Args:
         result = _parse_gemma4_args("flag:false")
         assert result == {"flag": False}
 
+    def test_null_value(self):
+        # Bare `null` must parse as None (Python), not the string "null".
+        # Without this, tool_choice=auto would emit `{"param": "null"}`
+        # instead of `{"param": null}` for nullable tool parameters.
+        result = _parse_gemma4_args("param:null")
+        assert result == {"param": None}
+        assert json.dumps(result) == '{"param": null}'
+
     def test_mixed_types(self):
         result = _parse_gemma4_args(
             'name:<|"|>test<|"|>,count:42,active:true,score:3.14'

vllm/tool_parsers/gemma4_tool_parser.py

@@ -66,6 +66,10 @@ def _parse_gemma4_value(value_str: str) -> object:
     if value_str == "false":
         return False
 
+    # Null
+    if value_str.lower() in ("null", "none", "nil"):
+        return None
+
     # Number (int or float)
     try:
         if "." in value_str: