[Bugfix][Parser] Fix Mistral tool parser for HF tokenizers (#39294)

Signed-off-by: thomasmaindron <thomasmaindron@users.noreply.github.com>
Co-authored-by: thomasmaindron <thomasmaindron@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Chauncey <chaunceyjiang@gmail.com>

vllm/tool_parsers/mistral_tool_parser.py

@@ -91,7 +91,12 @@ class MistralToolCall(ToolCall):
 
 
 def _is_pre_v11_tokeniser(model_tokenizer: TokenizerLike) -> bool:
-    return not (is_mistral_tokenizer(model_tokenizer) and model_tokenizer.version >= 11)
+    if is_mistral_tokenizer(model_tokenizer):
+        return model_tokenizer.version < 11
+    # For HF tokenizers, check if [ARGS] token exists in vocab
+    # which indicates a v11+ equivalent tokenizer
+    vocab: dict[str, int] = getattr(model_tokenizer, "get_vocab", lambda: {})()
+    return "[ARGS]" not in vocab
 
 
 @dataclass
@@ -139,7 +144,8 @@ class MistralToolParser(ToolParser):
         self.current_tool_name: str | None = None
         self.current_tool_mistral_id: str | None = None
         self.starting_new_tool = False
-        if _is_pre_v11_tokeniser(self.model_tokenizer):
+        self._is_pre_v11 = _is_pre_v11_tokeniser(self.model_tokenizer)
+        if self._is_pre_v11:
             self.parse_coro = ijson.parse_coro(
                 self.update_stream_state_pre_v11_tokenizer()
             )
@@ -147,7 +153,6 @@ class MistralToolParser(ToolParser):
         self.bot_token = "[TOOL_CALLS]"
         self.bot_token_id = self.vocab.get(self.bot_token)
         self.tool_call_regex = re.compile(r"\[{.*}\]", re.DOTALL)
-        self._is_pre_v11 = _is_pre_v11_tokeniser(self.model_tokenizer)
 
         if self.bot_token_id is None:
             raise RuntimeError(
@@ -470,6 +475,8 @@ class MistralToolParser(ToolParser):
                     raw_tool_call[end_name:],
                 )
 
+                # HF tokenizers may include [ARGS] in the text
+                tool_name = tool_name.replace("[ARGS]", "")
                 tool_calls.append({"name": tool_name, "arguments": args})
 
         # < v11: content[BOT] [{tool_call1},{tool_call2}]
@@ -558,7 +565,7 @@ class MistralToolParser(ToolParser):
         # if the tool call token IS in the tokens generated so far, that
         # means we're parsing as tool calls now
         try:
-            if _is_pre_v11_tokeniser(self.model_tokenizer):
+            if self._is_pre_v11:
                 return self._extract_tool_calls_streaming_pre_v11_tokenizer(
                     delta_text=delta_text,
                     delta_token_ids=delta_token_ids,
@@ -646,6 +653,8 @@ class MistralToolParser(ToolParser):
                 tool_id = MistralToolCall.generate_random_id()
                 delta_function_name = delta_text.split("{")[0]
                 self.current_tool_name += delta_function_name
+                # HF tokenizers may include [ARGS] in the text
+                self.current_tool_name = self.current_tool_name.replace("[ARGS]", "")
                 delta_text = delta_text[len(delta_function_name) :]
                 self.streaming_state = StreamingState.PARSING_ARGUMENTS
             else: