What counts as evidence in a cover letter
Most cover letters restate the resume and list the job's own words back at it. Neither does any work. Here is what does.
The letter is not a summary of the resume
The reader already has the resume. A letter that restates it in prose costs them time and tells them nothing, which is why so many go unread.
What a letter adds is the part that does not fit on a resume: why this role specifically, what you would do with it, and anything about your situation that is better said than guessed at. Three paragraphs is usually enough.
Naming a technology is not evidence of using it
Compare two sentences.
I have experience with Kubernetes, PostgreSQL and AWS.
Three of the job's terms, and nothing a reader can do with them.
I led the migration of forty services to Kubernetes and took deploy time from forty-five minutes to eight.
One term, and it answers the question. The first sentence is faster to write, which is why letters are full of them. It is also the sentence a reader skips.
The letter checker makes exactly this distinction: a requirement counts as supported only when the sentence naming it also describes doing something. A sentence with more than four commas is treated as a list and gets no credit however many verbs are in it. The rule is crude, and it is crude in the direction of under-crediting you — which is the safer direction, because a letter that passes it is not relying on keyword presence.
Name the role and point at a line
Recruiters handling six openings look for the title. Leaving it out is a small, avoidable friction.
And "I read the posting" is proved by quoting something from it, not by asserting enthusiasm. One specific line — a responsibility, a problem they describe, a thing the team owns — does more than a paragraph about how excited you are.
Adjectives about yourself are unverifiable
Passionate. Detail-oriented. Hard-working. A team player. A self-starter. Anyone can type these, so a reader discounts all of them, including the ones that happen to be true of you.
Replace each with the thing you did that makes it true. "Detail-oriented" becomes "I caught a reconciliation error that had been running for eleven months." If nothing comes to mind for a particular adjective, that is useful information about the adjective.
Say the awkward thing plainly
A gap, a redundancy, a career change, a move to a new country: stating it in one sentence removes it as a question. Leaving it out does not make it invisible — it means somebody else decides what it means, usually less generously than you would.
"I was made redundant in March when the team was cut, and I have spent the time since retraining in X" is a complete answer. A conspicuous gap with no explanation is an open question sitting in the middle of your application.
Length, and why it is not arbitrary
Under about 180 words there is rarely room to evidence anything — you get a salutation, an assertion and a sign-off. Past about 400 the letter starts competing with the resume rather than introducing it, and a reader who has ten of these does not finish yours.
Between those, the constraint does useful work: it forces you to pick which two or three things matter.
What comes out of the file
A cover letter has the same extraction problems as a resume, for the same reasons. Text in a header, a text box or a second column renders perfectly and reads back scrambled or not at all.
The cover letter builder cannot emit any of those constructs — its document model has no word for them — and after it writes a Word file it reads the file back and compares it block by block with what went in. If anything is missing you are told not to send it. A tool that reports "exported successfully" without opening the file is telling you nothing.
Where a requirement is missing
Both tools here will show you which of the job's requirements your letter does not address. Neither will write the sentence.
Add it only if it is true. The gap you cover with something you cannot back up becomes the first question in the interview, and there is no good answer to it.