AI has genuinely changed how fast a SaaS product can be built. It has not changed which early decisions are expensive to reverse, and being fast at the wrong architecture just gets you to the rebuild sooner.
The first is tenancy. How you separate one customer's data from another is close to unchangeable once you have real users. Shared tables with a tenant column, schema per tenant, database per tenant — each has different cost, isolation and compliance characteristics. Generated code will pick one implicitly if nobody decides, and it usually picks the simplest.
The second is the permission model. Roles, resource ownership and inherited access need designing as a system. Products that grow permissions organically end up with authorisation logic scattered across dozens of endpoints, which is both a security risk and the reason adding a new role later takes a fortnight.
The third is the data model. Relationships, soft deletes, audit trails and historical accuracy — whether an invoice shows today's price or the price at the time it was issued. These are cheap now and enormously expensive after a year of production data.
So our order is deliberate: a human designs tenancy, permissions and the data model before anything is generated. Then AI builds fast on top of decisions that will hold. That first week of architecture is the least exciting part of the engagement and the one that determines whether year two is comfortable.