Addlightness bench

/addlightness-bench -- before/after performance benchmark

Time two runnable commands head-to-head and report whether the difference is real -- not just noise. This is the measurement half of addlightness; it does not trim code (/addlightness) and does not measure static weight (/addlightness-review).

Scope

• Benchmark ONLY. No edits, no weight metrics, no refactoring.
• Input is two commands you can actually run; output is a timing comparison with a significance verdict.
• If the user has not trimmed anything yet, point them at /addlightness. If they want code-weight numbers (LOC/complexity), point them at /addlightness-review.

Inputs

The user supplies two runnable commands: a before command and an after command. Read trailing args / the request as exactly that pair.

• If they hand you two snapshot files instead of commands, ask for (or infer) the command that runs each -- e.g. node old.js vs node new.js, python3 before.py vs python3 after.py.
• Both commands must do the same work on the same input -- otherwise the comparison is meaningless. State the assumption if you have to guess.
• Quote each command so the harness receives it as one argument.

How to run

Call the benchmark harness once:

"${CLAUDE_PLUGIN_ROOT}/scripts/benchmark.sh" \
  --runs 10 --warmup 3 \
  --before '<before-command>' \
  --after  '<after-command>'

It uses hyperfine when present and falls back to a date+awk timing loop when it is not (this plugin assumes neither hyperfine nor any other profiler is installed, so expect the fallback). It prints one JSON line -- parse that, do not eyeball stdout. The emitted keys are exactly: before_ms, after_ms, pct_change (negative = after faster), faster (bool), welch_t, significant_at_95 (bool), runs, warmup, and tool. The harness does not compute median/p95/stddev -- do not expect or report those, even under hyperfine.

Reporting

Report a compact table, then a one-line verdict:

metric	value
before mean (ms)	before_ms
after mean (ms)	after_ms
% change	pct_change
welch t	welch_t
significant at 95%	significant_at_95

• % change (pct_change) -- negative means faster (after took less time). State it as "X% faster" / "X% slower" so the sign is unambiguous.
• Significance verdict -- gate every claim on the emitted significant_at_95 bool. The harness flags significance via a Welch t-test against a df-aware two-tailed 95% Welch critical value (emitted as t_crit_95; ~2.1-2.3 at the default N=10), NOT a fixed 1.96 — never recompute the verdict yourself.
  • If significant_at_95 is true -> report the speedup/regression as real.
  • If false -> say so plainly: "No statistically significant difference -- the observed delta is within run-to-run noise." Do not dress up a noise-level delta as a win. A faster-looking mean with significant_at_95 false is not a result.

Controls

For the numbers to mean anything:

• Same machine, same conditions. Run before/after back-to-back / interleaved, not at different times of day.
• Identical N and warmup for both commands; discard warmup runs from the stats.
• Quiesce background load -- close heavy apps, no concurrent builds; CPU contention dwarfs small deltas.
• Beware subprocess startup variance. Benchmarking node x.js / python3 x.py includes interpreter startup, which has large jitter. If the stddev is on the order of the mean difference, the signal is swamped -- recommend more runs (25-30+) and/or moving the measured work in-process rather than per-invocation.
• No thermal/load drift. Don't compare a run from an hour ago against a fresh one -- rerun both together.

What NOT to do

• Never report a single-run number as a benchmark. One run is an anecdote.
• Never claim an improvement without passing the significance gate. No gate, no win.
• Never compare runs taken hours apart or on different machines -- thermal state, background load, and CPU governor drift invalidate the comparison.
• Never silently swap the commands' work. If before and after don't compute the same result, a timing delta is meaningless -- flag it, don't report it.
• Don't trim or edit code here -- that's /addlightness.