Being flaky
tl;dr:
Here’s a version that keeps your tone but tightens it up a bit:
When a flaky person says they’ll do something, don’t thank them.
With flaky people, the second they say “I’ll do it” and you say “thanks,” the loop feels closed. In their head, that already counts.
They get the little dopamine hit and move on.
So when they say they’ll do something, respond like you’ll believe it when it actually happens.
Something like: “OK, I’ll look forward to that.” Or: “That would be good.”
Keep it open. Leave the loop unfinished.
Then if they actually follow through, thank them then. Not before.
I can make it sharper, softer, or more ruthless.