1. Adjectives don't hold; rules do

'Professional but friendly' is a mood, not an instruction, a dozen different styles fit it, so you get the model's default with a smile. Voice starts holding when the rules are concrete: 'short sentences; contractions are fine; no exclamation marks; one idea per paragraph; greet by first name.'

The test for a voice rule is the same as for any constraint: could you check an output against it and say pass or fail? 'Sound human' fails that test. 'Never use a word you wouldn't say aloud to a colleague' passes it, barely, but it gives the model an operation to perform.

2. Negative rules kill recurring tics

Every default voice has tics you will meet again and again: the opening pleasantry ('I hope this email finds you well'), the closing offer ('Let me know if you need anything else!'), hedges, exclamation marks, bullet avalanches. Adjectives will not remove them. Nothing about 'professional' forbids any of it.

Name and ban each one: 'Never open with a pleasantry, first sentence is the point. Never close by offering more help. No exclamation marks.' A five-line blacklist of your personal irritants routinely outperforms a paragraph about tone, because each line kills one failure you have actually seen.

3. A sample is the strongest voice signal

Rules can't fully spell out rhythm, vocabulary and how formal you actually are, but a sample carries all of it. 'Match the voice of this sample:' followed by 100-200 words of your real writing beats any description of you. It is the few-shot lesson applied to identity.

Pick the sample the way you'd pick an example: your writing on a good day, in the medium you're targeting, an email sample for email work. Then combine: rules make the voice enforceable, the sample makes it natural. Each covers what the other can't.

4. One voice, many mediums, written down once

Voice is not format. The same voice tightens into a three-line chat message, stretches into a report, and relaxes slightly in a note to a colleague. Keep the rules constant and name the medium: 'Same voice rules. Now as a chat reply, three lines max.' What changes is shape, not personality.

The remaining problem is repetition: retyping rules and sample into every new conversation is a tax, and the day you skip it, the default voice returns. The durable fix is a persistent style profile, rules plus sample stored once, applied everywhere you work. That is exactly the kind of stable identity a portable profile exists to carry.