Soundcheck History

Every snapshot, run and comparison adds a line to .soundcheck/history.jsonl. rhythm soundcheck history shows them as a table with the site's median LCP (or TBT or TTFB) and its trend, so you can see a site getting slower over months, not only since the last snapshot.

rhythm soundcheck history

When               Kind        Name           Pages  Errors  Warnings  Verdict   LCP (median)
2026-10-25 21:52   snapshot    before-update      4       0         1  warnings  146 ms
2026-10-25 21:52   comparison  before-update      4       1         4  broke     24 ms

LCP  █▁  146 ms → 24 ms (-84%)

Column	Means
When	when the run started, local time
Kind	snapshot, run or comparison
Name	the snapshot's name; for a comparison, the snapshot it compared with
Errors, Warnings	the run's issues by level
Verdict	broke, changed or clean for a comparison; errors, warnings or clean otherwise
LCP (median)	the median over the pages that loaded

The last line is the trend: a sparkline of the runs shown, then the first and last value and the change. Under --plain the sparkline is left out and the arrow is text:

LCP  146 ms to 24 ms (-84%)

Flags

Flag	Description
--metric lcp|tbt|ttfb	The timing to show (default lcp).
--page <path>	That page's timing instead of the site's median. The history keeps the first 50 pages of each run.
--backfill	Add the snapshots and runs still in .soundcheck that the history doesn't have. The first history does this by itself and says on stderr Added 2 past results still in .soundcheck to the history.
--plain, --no-color	As for every command.

rhythm soundcheck: metric "cls": want lcp, tbt or ttfb

With no entries: No history yet: it grows with every snapshot, run and comparison.

Where the history shows up

• The HTML report shows the site's LCP trend under the stats once the history has two or more runs of the site: a sparkline and a line such as LCP 1.4 s to 2.1 s over the last 3 runs (the last 12 runs).
• The fleet page draws each site's trend in its LCP column. See Fleet and client reports.

Interrupted runs aren't added. A write that fails prints rhythm soundcheck: history: <why> and the command goes on. Each line is JSON (schema rhythm-soundcheck/history/v1) with the counts, the verdict, the medians and the first 50 pages' timings, so scripts can read it too. The file is in .soundcheck/ but, unlike snapshots and runs, isn't excluded from git; add it to .gitignore if you don't want to commit it.

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