Bench

Activate When

• /godmode:bench, "benchmark", "compare variants", "A/B metric"
• User has a metric and wants a statistically-honest comparison between 2-3 code states
• NOT for optimization loops — use optimize for that. bench measures, never modifies source.

Inputs

Ask once, cache for the session:

• metric_cmd — shell command printing ONE number to stdout (lower-is-better or higher-is-better, user declares direction).
• variants — list of 2-3 entries. Each variant is a record:
  • name — short label (e.g. main, terse_on, feature_branch)
  • prep — EITHER a git ref (git checkout <ref>) OR an inline shell command that puts the repo into the desired state (e.g. export GODMODE_TERSE=1, git checkout feature-branch)
  • teardown — optional shell command to undo prep (default: git checkout - for refs, unset VAR for env)
• baseline — name of the variant all deltas are computed against. Must match one variants[].name.
• N — runs per variant. Default 5. Minimum 3. Maximum 20.
• variance_threshold — stdev/mean ratio that flags a variant noisy. Default 0.05 (5%).

Workflow

1. Validate inputs — metric_cmd must emit a single number; baseline must match a variant; N >= 3.
2. Snapshot starting git state (HEAD sha, branch, dirty bit). Refuse to run if working tree is dirty.
3. FOR each variant in order: a. Run prep. Abort variant on non-zero exit, mark prep_failed. b. Run metric_cmd N times. Collect numbers into an array. c. Compute mean, median, stdev, cv = stdev/mean. d. IF cv > variance_threshold: retry the FULL N-run block up to 3 times (variance recovery). e. IF cv still > threshold after 3 retries: mark variant measurement_error, record best-effort numbers. f. Run teardown. Fail loud if teardown leaves repo dirty.
4. Restore starting git state exactly (same sha, same branch). Verify with git rev-parse HEAD.
5. Compute delta_pct for each variant vs. baseline: (variant.median - baseline.median) / baseline.median * 100.
6. Emit outputs (see Output Format).

Math

mean   = sum(runs) / N
median = sorted(runs)[N//2]        # for even N, average of middle two
stdev  = sqrt(sum((x - mean)^2) / (N - 1))
cv     = stdev / mean
delta% = (variant.median - baseline.median) / baseline.median * 100

All computed with awk — no python, no bc, no external deps.

Output Format

Print to stdout a markdown table:

| variant    | runs | mean    | median  | stdev  | delta%  | status   |
|------------|------|---------|---------|--------|---------|----------|
| main       |    5 | 124.40  | 124.00  |  2.10  |   0.00  | baseline |
| terse_on   |    5 |  98.60  |  99.00  |  1.80  | -20.16  | ok       |
| feature    |    5 | 131.20  | 130.00  | 12.40  |  +4.84  | noisy*   |

Append one row per variant to .godmode/bench-results.tsv:

timestamp	run_id	variant	N	mean	median	stdev	cv	delta_pct	status	metric_cmd	git_ref

run_id is a UUID or date +%s — groups all variants from a single invocation.

Write a one-paragraph summary to .godmode/bench-summary.md, overwriting any prior version:

# Bench <run_id> — <timestamp>
Ran `<metric_cmd>` N=<N> times across <k> variants. Baseline: <baseline>.
Best: <winner> at <median> (<delta>% vs baseline). Worst: <loser> at <median> (<delta>%).
Noisy: <list or "none">. All variants measured from clean HEAD <sha>. Reproduce: re-run
`/godmode:bench` with identical variants file.

Hard Rules

1. READ-ONLY to the codebase. The only files this skill may write are .godmode/bench-results.tsv and .godmode/bench-summary.md. Touching any source file = abort + fail loud.
2. Refuse to run on a dirty working tree. git status --porcelain must be empty.
3. Restore the exact starting HEAD sha after the last variant. Verify and abort if mismatch.
4. Never skip the variance check. cv > threshold ALWAYS triggers retry, even if the delta looks conclusive.
5. Never fabricate numbers. If a run produces no number, record NaN and count it against N; never silently drop.
6. Commit the results TSV after every run — one commit per run_id with message bench: <run_id> <k> variants.
7. No external deps beyond bash, awk, git, and standard Unix utilities. No python, no jq, no node.
8. metric_cmd runs with set -e; set -o pipefail. Non-zero exit = failed run, not a zero measurement.

Variance Recovery

FOR variant in variants:
    runs = collect(N)
    retries = 0
    WHILE cv(runs) > threshold AND retries < 3:
        runs = collect(N)           # full fresh block, NOT append
        retries += 1
    IF cv(runs) > threshold:
        status = "measurement_error"
    ELSE:
        status = "ok"

Each retry is a fresh N-run block; never mix runs from different retry attempts.

Keep / Discard Discipline

KEEP a variant's measurement if:
  - N valid numeric samples collected
  - cv <= variance_threshold (possibly after retries)
  - prep and teardown both exited 0
  - git state restored cleanly between variants

DISCARD a variant's measurement if:
  - prep failed (record prep_failed, do NOT retry)
  - >3 retries still noisy (record measurement_error, keep best-effort row, mark unreliable)
  - metric_cmd produced NaN for >=N/2 runs
  - teardown left repo dirty (abort entire run, unsafe to continue)

On DISCARD of a whole run: git reset --hard to the snapshotted starting sha. Summary notes
the abort reason. TSV still gets partial rows with status=aborted.

Stop Conditions

STOP when FIRST of:
  - all_measured: every variant has status in {ok, measurement_error, prep_failed}
  - variance_unrecoverable: >=ceil(k/2) variants are measurement_error (comparison is meaningless)
  - budget_exhausted: total metric_cmd invocations > N * k * 4 (N runs * k variants * 4 retries)
  - unsafe_state: teardown failed or HEAD drifted mid-run
On stop: always write summary.md and commit results.tsv, even on abort.

Success Criteria

1. bench-results.tsv has exactly one row per variant in the run (k rows per run_id).
2. Every row has a delta_pct value (0.00 for the baseline row).
3. No variant is marked measurement_error unless cv truly exceeded threshold after 3 retries.
4. bench-summary.md names the winner, loser, baseline, and flags any noisy variants.
5. git rev-parse HEAD after the run matches the snapshotted starting sha.

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Error Recovery

Failure	Action
Dirty working tree at start	Abort before first variant. Tell user to stash or commit.
metric_cmd non-numeric	Pipe through tail -1 | awk '{print $NF}'. If still non-numeric, record NaN.
prep fails for a variant	Mark prep_failed, skip runs, continue to next variant.
Variance unrecoverable after 3 retries	Mark measurement_error, keep row, emit warning in summary.
HEAD drift mid-run	Abort. git reset --hard <starting_sha>. Refuse to emit comparison.
teardown leaves repo dirty	Abort entire run. Do not proceed to next variant.

TSV Schema

.godmode/bench-results.tsv is append-only. Header written on first create:

timestamp	run_id	variant	N	mean	median	stdev	cv	delta_pct	status	metric_cmd	git_ref

status is one of: baseline, ok, measurement_error, prep_failed, aborted. Never rewrite history. One run_id groups k rows. Commit every run.