The question worth asking first
Before you detect anything, ask what you actually care about. It is almost never “did a tool touch this cover letter”. It is “can this person do the job”. Those are different questions, and a detector only pretends to answer the first.
Plenty of excellent candidates use AI to fix grammar or tighten a draft. That is not dishonesty, it is using a normal tool. Penalizing it filters for people who did not bother, not for the best hires.
Who an auto-reject quietly screens out
If a high AI score triggers automatic rejection, you have built a filter with a bias in it. False positives fall hardest on non-native English speakers, whose plain, even writing reads as machine-made.
So an auto-reject rule can silently drop qualified international candidates while looking neutral. That is a legal and ethical problem, not just a quality one.
The legal risk hiring managers miss
This is the part that turns a quality problem into a liability. In most places, employment law does not care whether you meant to discriminate. It cares about the effect. If a hiring filter rejects one protected group at a higher rate than others, that can be unlawful even if the rule looks perfectly neutral on paper. Lawyers call this disparate impact.
An AI-detector auto-reject is a textbook example. It flags plain, even writing, which is exactly what many non-native English speakers produce. The rule never mentions nationality or first language, yet it can screen those candidates out at a higher rate. You would be building a bias into your pipeline and keeping no good record of why anyone was rejected, since “the detector said so” is not a defensible reason.
Even setting fairness aside, that is a bad position to be in. A candidate who asks why they were rejected deserves a real answer, and “a tool guessed your cover letter was too smooth” is not one.
Using AI to apply is not a red flag
It is worth challenging the assumption underneath all of this. Why would you penalize a candidate for using AI to write a cleaner cover letter? In most modern jobs, using the available tools well is a skill, not a failing. A candidate who used a model to tighten their application and fix their grammar showed exactly the kind of practical judgment you probably want.
The thing you are hiring for is almost never “wrote this cover letter unassisted.” It is whether they can do the work. A detector answers the wrong question, and then answers it badly.
A saner way to use it
Signal, not gate
Let a score add a small note of context, not decide the outcome. It never rejects on its own.
Judge the substance
Look at experience, skills, and fit. A polished cover letter is not the thing you are hiring.
Test what matters
If you want to know how someone works, use a real task or interview, not the texture of their writing.
Where detection does earn its place
There is a fair use: a written test given as part of the process, where you have told candidates the work should be their own. There, a smooth, machine-like answer is worth a second look. Even then it starts a conversation, it does not end one.