You can say "this part isn't quite working yet" to someone's face and have it land as help. You lean in a little, your voice stays warm, you're clearly on their side, and they hear a colleague pointing at one fixable thing. Type those same words into Slack and something happens to them on the way. They arrive flat. Cold. The person on the other end reads them three times, hunting for the tone, and since there's no tone to find, they supply their own, which at work, under pressure, is usually the worst one. By the third read, "this part isn't quite working" has quietly become "you're not good at this."
That's the problem with feedback in writing. Everything that makes it feel kind in person, the warmth in your voice and the small look that says I respect you and this is minor, gets stripped off in transit. What's left is the verdict, naked. So if you want a Slack note to help instead of sting, you have to put back, on purpose, the cushioning the medium took away.
The first thing to rebuild is your intent. In person it's visible. In text it's invisible, so you have to say it. A note that opens with "this is close, just one thing" tells the reader, before they reach the criticism, that they aren't in trouble. You've framed it as a small adjustment instead of leaving them to guess whether you think the whole thing is a disaster. It costs you a sentence and it changes everything after it.
The second thing is to aim at the work, not the person. This is the line between a note that's useful and one that wounds. "The intro buries your main point" is an observation about a paragraph, and a paragraph can be moved. "This is confusing" is a verdict on their thinking, and there's nothing to do with it except feel bad. Specific and pointed at the work reads as respect, because it shows you engaged closely with what they made. Vague and pointed at the person reads as a shrug of disappointment, the cruelest note of all, because it can't even be acted on.
So here's the same feedback, cold and then warm:
this needs work
And:
This is really close. The one thing I'd change: your recommendation is at the very bottom, and I think it's the strongest part. Could we move it up so it's the first thing they see? Happy to take another look after.
Same critique. The second one names what's good and points at the exact fix, with the door left open. It reads like someone helping, because it is. The first reads like someone disappointed, even if the writer meant nothing of the kind.
The last thing, and people forget it constantly: pick the right room. A small correction is one thing in a private message and a different thing entirely in a public channel where a dozen people are watching. The moment feedback has an audience, it stops being a note and becomes a small public verdict, and the person receiving it will spend more energy managing their embarrassment than absorbing what you said. Praise in the channel. Correct in the DM. It's an old rule because it's a true one.
I've come to think feedback, done well, is one of the more respectful things you can offer a colleague. You took their work seriously enough to look closely and help it get better. But that respect only comes through if it survives the trip. Drop "this needs work" in a thread and the respect doesn't arrive, only the chill does, and they spend the afternoon wondering what you really think of them instead of fixing the intro.
Name what's good and name the one fix, kept pointed at the work and not the person. Then send it somewhere they don't have to be brave to read it.