> Okay you say, older clients will skip those. Yes, that is what I say. > But then someone wants to add threading, suddenly the entire format needs to change

> Okay you say, older clients will skip those. Yes, that is what I say. > But then someone wants to add threading, suddenly the entire format needs to change Does it? If older clients don't want to implement threading, they can just receive the message bodies in the order they were posted. The old API should continue to work regardless of the extra metadata which one provider might be storing about it. Ideally, the host with the new feature would also add an "In response to #123456789:" line at the top of each message, so that users of the older clients can construct the threads in their mind (assuming that these older clients have been listing the IDs of the messages), but I don't think anyone's arguing that a service is breaking interoperability by failing to add such an affordance.