Decision logs: the cheapest institutional memory
Six months from now, nobody will remember why you chose what you chose. A one-line decision log saves you from relitigating it.
Teams make hundreds of decisions and record almost none of the reasoning. Months later, someone asks why a thing works the way it does, nobody remembers, and the debate reopens from scratch. A decision log is the cheap insurance against that.
The why disappears fastest
You can usually see what was decided — it's in the code, the process, the config. What evaporates is why: the constraints, the tradeoffs, the options rejected and the reasons. Without the why, future you can't tell a deliberate choice from an accident, and tends to undo good decisions.
Keep it lightweight
A decision log doesn't need ceremony. A short note per significant decision — what we chose, why, what we considered — is enough. The value isn't in the format; it's in having any record at all when the question inevitably comes back.
Record the why, not just the what. The what is visible. The why is what you'll wish you'd written down.