400 blog posts missing a meta description is not 400 problems. It is one decision in one template, and the useful unit of work is the template. Every tool reports the 400.
A cause carries a confidence taken from coverage: what share of the pages under the pattern are affected, and how many there are. High confidence is stated as a fact; low confidence is phrased as a question everywhere it appears, because a confidently wrong cause is worse than no cause at all.
Single-page causes are valid and common. They are simply causes whose pattern is a literal URL.
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No card, and the free tier does not expire — the first verdict takes about six weeks to arrive.