This is a model training fault. It is not emitting a stop sequence of the message format. Are you using “gpt-4.1-mini” perhaps?
You can add your own, using the stop parameter - oh, wait, no you can’t, because Responses doesn’t offer any token level control, and is thus unsuitable for a great many things, such as building production applications against OpenAI models. And it still wouldn’t work when inside a tool call, because you’d get parsing validation if stopping at and removing something like "}\n\n if using any API SDK methods or a strict function schema.