[Bugfix] Fix empty delta detection in Qwen3XMLToolParser streaming (#40090)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

tests/tool_parsers/test_qwen3xml_tool_parser.py

@@ -64,9 +64,6 @@ class TestQwen3xmlToolParser(ToolParserTests):
                 "test_empty_arguments": "Qwen3XML streaming has systematic issues",
                 "test_surrounding_text": "Qwen3XML streaming has systematic issues",
                 "test_escaped_strings": "Qwen3XML streaming has systematic issues",
-                "test_malformed_input": (
-                    "Qwen3XML parser is lenient with malformed input"
-                ),
                 "test_streaming_reconstruction": (
                     "Qwen3XML streaming reconstruction has known issues"
                 ),

vllm/tool_parsers/qwen3xml_tool_parser.py

@@ -1258,11 +1258,11 @@ class Qwen3XMLToolParser(ToolParser):
             return None
 
         # Parse the delta text and get the result
-        result = self.parser.parse_single_streaming_chunks(delta_text)
+        delta = self.parser.parse_single_streaming_chunks(delta_text)
 
         # Update tool call tracking arrays based on incremental parsing results
-        if result and result.tool_calls:
-            for tool_call in result.tool_calls:
+        if delta and delta.tool_calls:
+            for tool_call in delta.tool_calls:
                 if tool_call.function:
                     tool_index = (
                         tool_call.index
@@ -1292,4 +1292,7 @@ class Qwen3XMLToolParser(ToolParser):
                         self.streamed_args_for_tool[tool_index] += (
                             tool_call.function.arguments
                         )
-        return result
+        if delta.content is None and not delta.tool_calls and delta.reasoning is None:
+            # If no content and no tool calls, return None to indicate no update
+            return None
+        return delta