The feature we built because of one email thread
Two companies, one delay, and four weeks of "I did tell you".
Everything in this product exists because something went wrong in a real business. The handshake exists because of a delay neither side could prove had been communicated.
The manufacturer had mentioned it, in a message, on a Thursday. The brand had not registered it, because it arrived in the middle of a thread about something else. Four weeks later there was a stockout, and then a conversation that helped nobody.
The fix is not a better notification. Notifications are read or not read, and either way there is no record of which. What was needed was for both companies to confirm the same specific fact — not "we discussed the delay" but "ships 18 August, not 16, because of a filler seal failure" — with both names against it.
Three decisions made it work. It is between organisations rather than people, so it survives someone leaving. There is no un-shaking, so it is a record rather than a negotiation. And each side has a window after which it auto-resolves, marked auto-acknowledged rather than agreed, so a silent partner can never block the other indefinitely while the difference stays visible.
It is the least technically interesting thing we have built and the one people react to.