Nobody wants to talk about a cofounder leaving while you are still in the honeymoon phase, sketching the future on a whiteboard. So most teams do not. And then it happens anyway, and the conversation that should have taken five minutes at the start takes five painful weeks.
When a founder leaves early, almost everything comes down to one question: how much of their intended equity had they earned by the time they left?
Earned stays. Unearned does not.
If you agreed vesting, the departing founder keeps the portion of intended equity they earned up to their departure date. The unearned portion is not retained by the departing founder. Before incorporation, that forfeiture happens under the founder agreement you signed. After incorporation, a similar outcome has to be implemented through proper equity documents, which is exactly why agreeing it early matters.
Equity is only half of it. A clean departure also means handing back what belongs to the project: the IP that founder created, plus the assets and access they control, like repositories, domains and accounts. Those obligations should survive the departure, not leave with the person.
The goal of all this is not to punish anyone. It is to make sure one person leaving does not quietly take a third of the company, or the codebase, with them, and sink the people who stay.
Good agreements do not prevent hard moments. They make hard moments less chaotic, and a lot less personal.