Each proof sentence says which line of your résumé it came from

The reason a generated cover letter is dangerous is not that it writes badly. It is that you cannot tell, three weeks later in a room, which of its confident sentences came from your career and which came from the generator's sense of what a good candidate sounds like.

The cover letter builder removes the question by attaching the source to every claim.

What it looks like on the page

Under Your letter the draft is a set of blocks, greeting, opening, experience, relevant skills, closing and sign-off, each editable, each with its own word count and a Details button. Open Details on the Experience block of our draft and it lists its five sentences, each marked traced to your evidence with a line number: line 9, line 15, line 10, line 11 and line 15 again, because two sentences came from the same role. Under them a receipt says it plainly: from lines 9, 15, 10 and 11 of your evidence, and every sentence here carries one of your own lines.

Where the number comes from

When the engine reads candidate sentences from your résumé it keeps each one with the index of the line it came from, and that index travels with the sentence into the letter. The builder is printing the engine's own source index rather than a label added afterwards, which is why it survives versioning: a saved version keeps its trace.

What it chose, and why

Our invented candidate's résumé, against an invented payments posting, gave the engine seven achievement lines to choose from, and it used six. The scoring is plain: a line scores for each skill it matches that the posting asked for, again if it carries a number, and a little for breadth. After the first pick it refuses a line that adds nothing the earlier picks did not already evidence, which is why the paragraphs do different jobs rather than saying the same thing twice.

Connectives are named too

Not every sentence in a letter is evidence. The greeting, the line naming the role and company, the close and the thank-you are connectives, and Details marks each as a pattern, for example pattern: close, with a note that it asserts nothing. The one sentence the engine writes about gaps is marked the same way: pattern: gaps. Between the traced sentences and the named patterns, every sentence in the letter is accounted for.

Editing does not break the trace

A traced paragraph is a starting point. You can rewrite it, and most people should: a bullet lifted from a résumé is compressed in a way that reads oddly in prose. What editing changes is the wording. What it does not change is that the paragraph began as your line, which is the thing you will want to remember in the room, and Details keeps an option to restore the wording built from your evidence.

Why the letter stays short

A letter that lists every achievement has stopped being a letter and become a second résumé, and the reader skims it the way they skim the first one. The refusal rule keeps it from doing that: after the first pick, a line has to add something the earlier picks did not already evidence, so near-identical achievements become one and the letter reaches for something different. Where the evidence runs out the letter comes back shorter, which is the behaviour described in a letter that only claims what you proved.

What tracing cannot do

It cannot check your résumé against reality. If the line was an exaggeration when you wrote it, the letter will carry the exaggeration faithfully, with a line number attached. It also cannot tell you whether the lines it chose are the ones this particular employer cares about; it ranks against the words in the posting, and postings are written badly. What it does is remove one entire category of unpleasant surprise, and leave the rest of the letter, including everything it refused to claim, visible before you send. The designs the finished letter can arrive on are shown whole, which is why the gallery shows whole letters.