What prompts can and cannot change
A good prompt changes what the model writes about, how long the sentences are, and which stock formulations it avoids. Those are real improvements and they are visible to any reader.
What a prompt does not meaningfully change is the statistical signature detectors measure. Perplexity and burstiness are properties of the finished text as a whole, and constraining vocabulary or banning phrases moves them far less than the marketing around "undetectable prompts" suggests. The mechanism is explained here.
We sell a rewriter, so treat this as an interested party being straight with you: HumanFlow does not promise to beat any detector, and neither does any prompt on this page. What both can do is make a draft read more like a person wrote it, which is worth doing on its own terms.
The four moves that do most of the work
Ban the formula by name. Asking for "natural writing" produces nothing. Listing the exact phrases to avoid — "circling back", single-sentence paragraphs, the closing question — works immediately, because the instruction is checkable.
Constrain the evidence. Giving the model three sources and forbidding anything outside them removes its ability to reach for plausible general knowledge, which is where most vague sentences originate.
Ask for deletion, not rewriting. "Cut this to 70% without losing a claim" preserves your phrasing while removing padding. A general rewrite replaces your voice with the model's.
Give it permission to refuse. Several prompts here allow the model to say it has nothing to add. Most empty AI output exists because the model was required to produce something.
How to tell whether a prompt worked
Judge the output, not the prompt. Four checks take about a minute between them and catch most of what goes wrong.
Read it aloud. Machine-typical prose is hard to read aloud naturally because the sentences are all the same shape, so you run out of breath in the same place repeatedly. This is the fastest test and the one most people skip.
Look for the manufactured takeaway. If the last paragraph generalises from a specific into a lesson nobody asked for, the model added it. Cut it and check whether the piece is worse — it usually is not.
Count how many sentences you would defend. Not agree with; defend, to someone who pushed back. Frictionless drafts tend to score badly here, and the fix is usually to add the objection rather than to rewrite the prose.
Check whether anything is invented. Prompts that constrain evidence reduce fabrication; none eliminates it. Every statistic, quote and citation needs verifying against the source regardless of how the draft was produced.
Writing your own
Every prompt here is built from the same four moves, and once you see them you can stop collecting prompts and write the one your task needs.
Name what to avoid, specifically. Abstract instructions produce nothing — "write naturally", "sound human", "be engaging" are all unfalsifiable, so the model cannot tell whether it complied. "Do not use the words just, circling back, or touching base" is checkable, and checkable instructions get followed.
Constrain the inputs. Supplying the sources, the facts or the outline removes the model's room to reach for plausible general knowledge, which is where both vagueness and invention come from.
Restrict the operation. "Only delete", "only merge sentences", "do not introduce vocabulary I have not used" keep your voice intact. An unrestricted rewrite returns the model's voice every time.
Permit refusal. Adding "if there is nothing to add, say so" removes the pressure to produce, which is the source of most empty output. It is the single most underused line in prompting.
Using these honestly
Where AI use is permitted and disclosed, these prompts help you get a better draft faster. Where it is banned, no prompt makes it acceptable — and the outline-only prompt exists precisely because it produces a use you can describe in one sentence to an instructor.
Once you have a draft, the AI word finder shows which phrases survived, and the sentence rhythm tool shows whether the lengths still vary.