[Bugfix][Gemma4] Fix infinite loop and array boundary issues in tool parser (#41991)

Signed-off-by: David Oy <david.oy@baseten.co>
Co-authored-by: Claude <noreply@anthropic.com>

tests/tool_parsers/test_gemma4_tool_parser.py

@@ -135,6 +135,11 @@ class TestParseGemma4Args:
         result = _parse_gemma4_args('name:<|"|>test<|"|>,flag:', partial=True)
         assert result == {"name": "test"}
 
+    @pytest.mark.timeout(5)
+    def test_malformed_partial_array(self):
+        result = _parse_gemma4_args(":[t:[]")
+        assert isinstance(result, dict)
+
 
 class TestParseGemma4Array:
     def test_string_array(self):
@@ -149,6 +154,16 @@ class TestParseGemma4Array:
         result = _parse_gemma4_array("42,true,3.14")
         assert result == [42, True, 3.14]
 
+    @pytest.mark.timeout(5)
+    def test_string_element_with_closing_bracket(self):
+        result = _parse_gemma4_array('[<|"|>a]b<|"|>,<|"|>c<|"|>],<|"|>tail<|"|>')
+        assert result == [["a]b", "c"], "tail"]
+
+    @pytest.mark.timeout(5)
+    def test_stray_closing_bracket(self):
+        result = _parse_gemma4_array("42,]trailing")
+        assert result == [42]
+
 
 # ---------------------------------------------------------------------------
 # Non-streaming extraction tests

vllm/tool_parsers/gemma4_tool_parser.py

@@ -204,6 +204,13 @@ def _parse_gemma4_args(args_str: str, *, partial: bool = False) -> dict:
                 # Value may be incomplete (e.g. partial boolean) —
                 # withhold to avoid type instability during streaming.
                 break
+            if i == val_start:
+                logger.warning(
+                    "Gemma4 args parser made no progress at position %d; "
+                    "aborting on malformed input.",
+                    i,
+                )
+                break
             result[key] = _parse_gemma4_value(args_str[val_start:i])
 
     return result
@@ -258,6 +265,11 @@ def _parse_gemma4_array(arr_str: str, *, partial: bool = False) -> list:
             sub_start = i + 1
             i += 1
             while i < n and depth > 0:
+                if arr_str[i:].startswith(STRING_DELIM):
+                    i += len(STRING_DELIM)
+                    nd = arr_str.find(STRING_DELIM, i)
+                    i = nd + len(STRING_DELIM) if nd != -1 else n
+                    continue
                 if arr_str[i] == "[":
                     depth += 1
                 elif arr_str[i] == "]":
@@ -275,6 +287,13 @@ def _parse_gemma4_array(arr_str: str, *, partial: bool = False) -> list:
                 i += 1
             if partial and i >= n:
                 break
+            if i == val_start:
+                logger.warning(
+                    "Gemma4 array parser made no progress at position %d; "
+                    "aborting on malformed input.",
+                    i,
+                )
+                break
             items.append(_parse_gemma4_value(arr_str[val_start:i]))
 
     return items