In six months, you won't remember why you built Feature X instead of Feature Y. Your decision log creates a paper trail of reasoning so you can learn from what worked (and what didn't).
Keep entries short. Three lines max.
This isn't a diary - it's evidence for future decisions.
Date: [MM/DD/YY]
Decision: [What we're building and why]
Evidence: [What user feedback or data informed this choice]
Date: 03/15/25
Decision: Building CSV upload fix instead of dashboard v2
Evidence: 4 users mentioned upload failures in feedback calls, dashboard only requested by 1 power user
Date: 03/22/25
Decision: Shipping basic progress bar, not real-time validation
Evidence: Users care about knowing upload is working, don't need cell-by-cell validation based on interviews