The default voice is wrong for everyone
Left alone, output arrives over-formal, slightly padded and relentlessly upbeat — recognisable within a sentence and unlike how any real business writes. It is the single most common giveaway.
Adjectives don’t work
Asking for “friendly, professional and concise” produces a generic imitation of those words. Eight real messages you were happy to send produce your voice, because examples transmit a style that adjectives only describe.
Show it eight emails you actually sent. That works better than a page describing how you sound.
Specify what to remove
No opening pleasantries, no restating the question, no closing offer of further assistance, no exclamation marks. Deletions are more effective than instructions because they are checkable, which is why the prohibition list is the enforceable half of any voice guide.
Length is a tone decision
Verbosity reads as evasive and wastes the reader’s time. Setting a hard length target for routine answers does more for perceived quality than any amount of tone guidance.
Don’t make it a character
Personality that overreaches becomes grating on the tenth interaction and unbearable when someone is annoyed. A neutral, competent voice ages far better — and it must never imply a person who doesn’t exist.
Tone has to bend under pressure
Cheerful phrasing in front of an upset customer reads as tone-deaf. The system needs a plainer register for complaints, which is part of handling them properly rather than a stylistic nicety.