There is a line in almost every Python web tutorial that nobody explains: uvicorn main:app --host 0.0.0.0 --port 8000 I copied it for weeks without thinking about it. Then I deployed the same application three times — to a local VM, to a production server, and into a container — and the correct value was different every time. Twice it was 0.0.0.0 . Once, in the place that mattered most, it was not. That gap is worth writing about, because the setting itself is trivia...