We write down why a choice was made, not just what was built — because the reasoning is the part that is expensive to lose and impossible to recover later.
Every codebase records what was built — the code is right there. Almost none record why. Why this database and not the obvious one; why this boundary sits here and not there; why a tempting shortcut was deliberately not taken. That reasoning lives in one person's head at the moment of the decision and evaporates within months. The cost shows up later, when someone — often the same person — looks at a strange-seeming choice, assumes it was an accident, fixes it, and re-introduces the exact problem the original decision was avoiding.So we write the decision down where the work lives: the constraint that forced it, the alternative we rejected, and the tradeoff we knowingly accepted. Not a heavy process — a few honest sentences at the point of choosing. The test for whether a decision is worth recording is simple: would a competent engineer, arriving fresh, be tempted to undo this? If yes, it gets a why. This is also what makes a small team safe to hand work to: a new contributor reads the reasoning and starts from where we are, instead of relitigating settled questions or stepping on landmines we already mapped.For a client, this is the difference between a system you are locked into and one you actually own. When the reasoning travels with the code, your team — or any team — can pick it up, understand the bets that were made, and change them deliberately when the constraints change. Undocumented cleverness is a liability dressed as sophistication; it strands you the moment its author leaves. Documented reasoning is the opposite: it is what lets the next person be as informed as the last.
In practice: the data layer uses Drizzle ORM rather than the more common Prisma — a choice that looks contrarian until you read the one-paragraph note beside it: the code runs on a V8 edge runtime, not Node, and Prisma's engine assumes Node. The note records the constraint, the rejected option, and the tradeoff — so the next maintainer inherits the reasoning, not a mystery they are tempted to correct.