yagni

Apply YAGNI to: $ARGUMENTS

If no argument is given, apply it to the plan, design, or change under discussion.

The principle

Ron Jeffries (XP): "Always implement things when you actually need them, never when you just foresee that you will need them." Speculative functionality costs now (build, review, maintenance, cognitive load) against a benefit that usually never arrives, and when it does arrive, it arrives with better information than you have today.

Procedure

1. List every distinct element of the proposal: features, options, parameters, abstractions, config values, error paths, layers.
2. For each element, ask one question: is there a concrete, current requirement for this? Evidence means a user asking today, a failing case today, or a hard external constraint. "We might need it later", "it's more flexible", and "best practice" are not evidence.
3. Classify each element:
   • Keep: needed now, evidence named.
   • Delete: speculative, and rebuilding it later costs about the same as building it now.
   • Defer with a note: speculative, but a later retrofit would be genuinely expensive (data migrations, published API contracts, wire formats). Name the retrofit cost; this is the only class where foresight pays.
4. Output the trimmed proposal: what remains, what was cut, one line of justification per cut.

Calibration

• YAGNI removes speculative scope, not rigour: input validation at trust boundaries, error handling that prevents data loss, security, and accessibility stay regardless.
• YAGNI assumes you can add the feature later cheaply; that assumption holds only with tests and a codebase safe to refactor. If those are missing, say so rather than pretending deferral is free.
• One irreversible-decision exception per proposal is plausible; three "irreversible" exceptions means the analysis has gone soft.

Output format

## YAGNI: <proposal>

| Element | Verdict | Why |
|---------|---------|-----|
| ...     | Keep / Delete / Defer | one line |

### Trimmed version
<the proposal with only the Keep items>