[BugFix]: properly deserialize tool_calls iterator before processing by mistral-common when MistralTokenizer is used (#9951)

Signed-off-by: Guillaume Calmettes <gcalmettes@scaleway.com>

vllm/entrypoints/openai/serving_chat.py

@@ -127,6 +127,42 @@ class OpenAIServingChat(OpenAIServing):
                 return self.create_error_response(
                     "tool_choice = \"required\" is not supported!")
 
+            # NOTE: There is currently a bug in pydantic where attributes
+            # declared as iterables are replaced in in the instances by
+            # pydantic-core ValidatorIterator instance. In particular, this
+            # affects tool_calls defined in ChatCompletionAssistantMessageParam
+            # model:
+            # see:
+            #   - https://github.com/pydantic/pydantic/issues/9467
+            # As a result, tool_calls from assistant messages are never
+            # deserialized in the request object if the tool_calls iterator is
+            # not consumed. This affect messages passed to the MistralTokenizer
+            # since no chat template is applied and therefore the tools_calls
+            # iterator is not directly consumed.
+            # Issue is tracked on Pydantic side, with resolution planned for
+            # v2.11 release. In the meantime, the official workaround is to
+            # consume the iterator so the tool_calls are correctly deserialized
+            # in the OpenAI ChatCompletionAssistantMessageParam object
+            # https://github.com/pydantic/pydantic/issues/9467#issuecomment-2442097291 # noqa: E501
+            # Official Pydantic Issues:
+            #   - https://github.com/pydantic/pydantic/issues/9541
+            # TODO: remove when pydantic v2.11 is released
+            if isinstance(tokenizer, MistralTokenizer):
+                for i, message in enumerate(request.messages):
+                    if message.get("role") == 'assistant':
+                        tool_calls_validator = message.get(
+                            "tool_calls", ().__iter__())
+                        validated_tool_calls = []
+                        while True:
+                            try:
+                                tool_call = next(
+                                    tool_calls_validator)  # type: ignore
+                                validated_tool_calls.append(tool_call)
+                            except StopIteration:
+                                break
+                        request.messages[i][
+                            "tool_calls"] = validated_tool_calls
+
             if (request.tool_choice == "auto" and
                     not (self.enable_auto_tools and tool_parser is not None)
                     and not isinstance(tokenizer, MistralTokenizer)):