[Bugfix][Parser] Fix Mistral pre-v11 tool parser failing on trailing model output (#40531)

Signed-off-by: dougbtv <[email protected]>
Signed-off-by: Doug Smith <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Flora Feng <[email protected]>

tests/tool_parsers/test_mistral_tool_parser.py

@@ -24,7 +24,6 @@ from mistral_common.protocol.instruct.tool_calls import (
     ToolChoiceEnum as MistralToolChoiceEnum,
 )
 from partial_json_parser.core.options import Allow
-from pydantic import ValidationError
 
 from vllm.entrypoints.openai.chat_completion.protocol import (
     ChatCompletionRequest,
@@ -250,6 +249,7 @@ def test_extract_tool_calls_no_tools(parser_fixture, request):
         "argument_before_name_and_name_in_argument",
         "multiple_tools",
         "content_before_tool",
+        "trailing_data_after_json",
     ],
     argnames=["model_output", "expected_tool_calls", "expected_content"],
     argvalues=[
@@ -338,6 +338,24 @@ def test_extract_tool_calls_no_tools(parser_fixture, request):
             ],
             "Hello",
         ),
+        (
+            """[TOOL_CALLS] [{"name": "get_current_weather", "arguments":{"city": "Dallas", "state": "TX", "unit": "fahrenheit"}}]\nextra trailing data""",  # noqa: E501
+            [
+                ToolCall(
+                    function=FunctionCall(
+                        name="get_current_weather",
+                        arguments=json.dumps(
+                            {
+                                "city": "Dallas",
+                                "state": "TX",
+                                "unit": "fahrenheit",
+                            }
+                        ),
+                    )
+                )
+            ],
+            None,
+        ),
     ],
 )
 def test_extract_tool_calls_pre_v11_tokenizer(
@@ -366,19 +384,22 @@ def test_extract_tool_calls_pre_v11_multiple_bot_tokens_raises(
         )
 
 
-def test_extract_tool_calls_pre_v11_regex_fallback_raises(
+def test_extract_tool_calls_pre_v11_regex_fallback(
     mistral_pre_v11_tool_parser,
 ):
-    """The regex fallback path finds valid JSON but does not re-serialize
-    the `arguments` dict to a string, causing a Pydantic
-    `ValidationError` when constructing `FunctionCall`."""
+    """The regex fallback path finds valid JSON via regex when the primary
+    raw_decode fails on leading junk. It should re-serialize arguments
+    and return a valid tool call."""
     model_output = (
         '[TOOL_CALLS]  junk [{"name": "add", "arguments":{"a": 1, "b": 2}}] trail'
     )
-    with pytest.raises(ValidationError):
-        mistral_pre_v11_tool_parser.extract_tool_calls(
-            model_output, request=_DUMMY_REQUEST
-        )
+    result = mistral_pre_v11_tool_parser.extract_tool_calls(
+        model_output, request=_DUMMY_REQUEST
+    )
+    assert result.tools_called
+    assert len(result.tool_calls) == 1
+    assert result.tool_calls[0].function.name == "add"
+    assert result.tool_calls[0].function.arguments == json.dumps({"a": 1, "b": 2})
 
 
 def test_extract_tool_calls_pre_v11_regex_fallback_fails(
@@ -579,6 +600,7 @@ def _test_extract_tool_calls_streaming(
         "argument_before_name",
         "argument_before_name_and_name_in_argument",
         "multiple_tools",
+        "trailing_data_after_json",
     ],
     argnames=["model_output", "expected_tool_calls", "expected_content"],
     argvalues=[
@@ -668,6 +690,24 @@ def _test_extract_tool_calls_streaming(
             ],
             "",
         ),
+        (
+            """[TOOL_CALLS] [{"name": "get_current_weather", "arguments":{"city": "Dallas", "state": "TX", "unit": "fahrenheit"}}]\nextra trailing data""",  # noqa: E501
+            [
+                ToolCall(
+                    function=FunctionCall(
+                        name="get_current_weather",
+                        arguments=json.dumps(
+                            {
+                                "city": "Dallas",
+                                "state": "TX",
+                                "unit": "fahrenheit",
+                            }
+                        ),
+                    )
+                )
+            ],
+            "\nextra trailing data",
+        ),
     ],
 )
 def test_extract_tool_calls_streaming_pre_v11_tokenizer(

vllm/tool_parsers/mistral_tool_parser.py

@@ -479,21 +479,28 @@ class MistralToolParser(ToolParser):
                 )
             stringified_tool_calls = raw_tool_calls[0].strip()
             try:
-                tool_calls = json.loads(stringified_tool_calls)
+                # Use raw_decode to parse the first valid JSON value,
+                # ignoring trailing tokens the model may emit after
+                # the tool call array.
+                tool_calls, _ = json.JSONDecoder().raw_decode(stringified_tool_calls)
             except json.JSONDecodeError:
-                # use a regex to find the part corresponding to the tool call.
-                # NOTE: This use case should not happen if the model is trained
-                # correctly. It's an easy possible fix so it's included, but
-                # can be brittle for very complex / highly nested tool calls
                 try:
                     raw_tool_call = self.tool_call_regex.findall(
                         stringified_tool_calls
                     )[0]
                     tool_calls = json.loads(raw_tool_call)
+                    tool_calls = [
+                        {
+                            "name": tool_call["name"],
+                            "arguments": json.dumps(
+                                tool_call.get("arguments", {}),
+                                ensure_ascii=False,
+                            ),
+                        }
+                        for tool_call in tool_calls
+                    ]
                 except (IndexError, json.JSONDecodeError):
                     logger.exception("Error in extracting tool call from response.")
-                    # If raw decoding and decoding post regex rule fails, then just
-                    # return content.
                     return ExtractedToolCallInformation(
                         tools_called=False,
                         tool_calls=[],
@@ -504,7 +511,8 @@ class MistralToolParser(ToolParser):
                     {
                         "name": tool_call["name"],
                         "arguments": json.dumps(
-                            tool_call["arguments"], ensure_ascii=False
+                            tool_call.get("arguments", {}),
+                            ensure_ascii=False,
                         ),
                     }
                     for tool_call in tool_calls
@@ -515,7 +523,7 @@ class MistralToolParser(ToolParser):
                 type="function",
                 function=FunctionCall(
                     name=tool_call["name"],
-                    arguments=tool_call["arguments"],
+                    arguments=tool_call.get("arguments", "{}"),
                 ),
             )
             for tool_call in tool_calls