with_structured_output returns None when MALFORMED_FUNCTION_CALL happens without raising errors

[pokutuna]

When using with_structured_output(), occasionally the return value becomes None unexpectedly. This happens even with function calling that normally succeeds, where the Gemini API sometimes returns MALFORMED_FUNCTION_CALL responses. This silent failure occurs without any error indication, making it extremely difficult to diagnose the root cause during development.

Through extensive debugging of this intermittent issue, I confirmed that the Gemini API sometimes probabilistically returns MALFORMED_FUNCTION_CALL responses. The investigation revealed that these responses have the following structure:

{
  "content": "",
  "response_metadata": {
    "finish_reason": "MALFORMED_FUNCTION_CALL",
    "finish_message": "Malformed function call: print(default_api.Output(...",
    "tool_calls": [],
    "invalid_tool_calls": [],
    ...
  }
  ...
}

When with_structured_output() is used, PydanticToolsParser and JsonOutputKeyToolsParser are connected to the chain. These parsers return None or empty lists when tool_calls is empty, without detecting the underlying MALFORMED_FUNCTION_CALL error.

• langchain-ai/langchain@langcha - libs/core/langchain_core/output_parsers/openai_tools.py#L328
• langchain-ai/langchain@langcha - libs/core/langchain_core/output_parsers/openai_tools.py#L256-L258

When using with_structured_output(..., method="json_mode"), PydanticOutputParser and JsonOutputParser are used instead. These parsers properly throw exceptions when content parsing fails.

However, json_mode doesn't support some JSON Schema features, so developers often must use the default method despite its error handling issues.

Suggestion: Throw an error when MALFORMED_FUNCTION_CALL occurs instead of returning None

The implementation could be as simple as using a custom parser wrapper, for example:

class MalformedFunctionCallError(ChatGoogleGenerativeAIError):
    pass

class StructuredOutputParserWithErrorHandling:
    def __init__(self, parser):
        self.parser = parser

    def parse(self, ai_message):
        if (hasattr(ai_message, 'response_metadata') and
            ai_message.response_metadata.get('finish_reason') == 'MALFORMED_FUNCTION_CALL'):
            finish_message = ai_message.response_metadata.get('finish_message', 'Unknown error')
            raise MalformedFunctionCallError(finish_message)

        return self.parser.parse(ai_message)

While the implementation seems not difficult, it would affect structured output behavior overall, so I haven't prepared a Pull Request and would like to wait for discussion from maintainers.