A founder agreement governs the founding team: who promised what to whom, before the company exists. A shareholders' agreement governs the shareholders of a company that already exists: how they hold, transfer and vote their shares.
Same family, different animals. And teams confuse them constantly, usually at the worst possible moment.
The scene
Two founders decide to formalize things. They search for "founders agreement template," download the first result, and sign it. Months later, a lawyer reads it and delivers the news: they signed a shareholders' agreement. It talks about share classes, drag-along rights, board composition. Their company does not exist yet. There are no shares, no board, no shareholders. They signed a detailed set of rules for a world they do not live in, and wrote down nothing about the world they actually live in: who owns the code, what happens if one of them leaves next month, how the equity promise is earned.
A document can be perfectly drafted and still be the wrong document.
The three differences that matter
Timing. The founder agreement belongs to the pre-incorporation phase, from the first serious conversation until the company is formed. The shareholders' agreement can only exist once the company does, because its entire subject matter, the shares, has to exist first.
Parties and object. A founder agreement binds individuals about their project: intended equity, vesting on the founder promise, contributions, roles, IP created before the company exists, what happens if someone walks away. A shareholders' agreement binds shareholders, often including investors, about the company: transfers of shares, preemption, tag-along and drag-along, governance, reserved matters.
The dispute it prevents. Most cofounder disputes are seeded in the pre-incorporation months, precisely the period a shareholders' agreement cannot reach. When one founder claims the other promised him thirty percent back in March, the shareholders' agreement signed in November has nothing to say about it. The founder agreement is the only document that was there.
One does not replace the other
This is the part that trips people up: you will eventually need both. The founder agreement is the bridge. At incorporation, its terms get translated into the company's real machinery: the equity split becomes issued shares on a cap table, the vesting schedule moves into stock purchase agreements, and the governance promises get mirrored into the shareholders' agreement and charter documents.
Skipping the founder agreement because "we'll do a shareholders' agreement later" means the most fragile period of the company, the one where everything is decided and nothing is written, stays ungoverned. Skipping the shareholders' agreement because "we already have a founder agreement" means running an incorporated company on a document written for a project.
One governs the promise. The other governs the shares. Confusing them means governing neither.