[Bugfix] Fix Mistral tool-parser regex for nested JSON (#20093)

Signed-off-by: mgoin <mgoin64@gmail.com>

tests/models/language/generation/test_mistral.py

@@ -10,6 +10,7 @@ import pytest
 from vllm.entrypoints.openai.tool_parsers.mistral_tool_parser import (
     MistralToolCall, MistralToolParser)
 from vllm.sampling_params import GuidedDecodingParams, SamplingParams
+from vllm.transformers_utils.tokenizer import MistralTokenizer
 
 from ...utils import check_logprobs_close
 
@@ -318,3 +319,53 @@ def test_mistral_guided_decoding(
                                 schema=SAMPLE_JSON_SCHEMA)
         except jsonschema.exceptions.ValidationError:
             pytest.fail("Generated response is not valid with JSON schema")
+
+
+def test_mistral_function_call_nested_json():
+    """Ensure that the function-name regex captures the entire outer-most
+    JSON block, including nested braces."""
+
+    # Create a minimal stub tokenizer that provides the few attributes the
+    # parser accesses (`version` and `get_vocab`).
+    class _StubMistralTokenizer(MistralTokenizer):
+        version = 11  # Satisfy the version check
+
+        def __init__(self):
+            pass
+
+        @staticmethod
+        def get_vocab():
+            # Provide the special TOOL_CALLS token expected by the parser.
+            return {"[TOOL_CALLS]": 0}
+
+    tokenizer = _StubMistralTokenizer()
+    parser = MistralToolParser(tokenizer)
+
+    # Craft a model output featuring nested JSON inside the arguments.
+    args_dict = {
+        "city": "Dallas",
+        "state": "TX",
+        "unit": "fahrenheit",
+        "sub_dict": {
+            "foo": "bar",
+            "inner": {
+                "x": 1,
+                "y": 2
+            }
+        },
+    }
+
+    model_output = (
+        f"{parser.bot_token}get_current_weather{json.dumps(args_dict)}")
+
+    parsed = parser.extract_tool_calls(model_output, None)
+
+    # Assertions: the tool call is detected and the full nested JSON is parsed
+    # without truncation.
+    assert parsed.tools_called
+
+    assert MistralToolCall.is_valid_id(parsed.tool_calls[0].id)
+    assert parsed.tool_calls[0].function.name == "get_current_weather"
+    assert json.loads(parsed.tool_calls[0].function.arguments) == args_dict
+    # No additional content outside the tool call should be returned.
+    assert parsed.content is None

vllm/entrypoints/openai/tool_parsers/mistral_tool_parser.py

@@ -77,8 +77,8 @@ class MistralToolParser(ToolParser):
         self.bot_token_id = self.vocab.get(self.bot_token)
         self.tool_call_regex = re.compile(r"\[{.*}\]", re.DOTALL)
         if _is_fn_name_regex_support(self.model_tokenizer):
-            self.fn_name_regex = re.compile(r'([a-zA-Z0-9_-]+)(\{.*?\})',
+            self.fn_name_regex = re.compile(
-                                            re.DOTALL)
+                r'([a-zA-Z0-9_-]+)(\{[\s\S]*?\})(?=\s*$|,|\s)', re.DOTALL)
         else:
             self.fn_name_regex = None