Tool calling with closed-source models is seamless. You pass a list of functions to the API, the model calls them, you get structured JSON back. The wire format is invisible to you. Then you move to open models and discover that tool calling depends on a wire format the engine has to understand. If the engine doesn’t support that model’s format yet, the output comes back garbled: reasoning tokens in arguments, malformed JSON, missing tool calls. Then you either wait, or write the parser yourself...