localhost was IPv6 on one end

I was building a flow that starts a remote dev box for each GitHub issue. The box runs a coding agent and the Astro dev server, and my Mac reaches the preview through an SSH tunnel:

ssh -fN -L 30032:localhost:4321 my-devbox

The preview didn’t load, even though Astro on the box said Local http://localhost:4321/. Two requests on the box showed why:

curl -s -o /dev/null -w "v4 %{http_code}\n" http://127.0.0.1:4321/
curl -s -o /dev/null -w "v6 %{http_code}\n" "http://[::1]:4321/"

v4 000
v6 200

Vite, which runs Astro’s dev server, listens on localhost by default, and Node used the first address that name resolved to on that box, which was ::1. The tunnel’s localhost connected to 127.0.0.1. Both ends said “localhost” and meant different addresses.

Pointing the tunnel at the IPv6 address worked. The address has to be quoted in zsh, or zsh reads the brackets as a glob and fails with no matches found:

ssh -fN -L "30032:[::1]:4321" my-devbox

Later I started the dev server with an explicit host so it doesn’t depend on how localhost resolves. The clearest setup uses the same address at both ends, for example --host 127.0.0.1 on the server and 127.0.0.1 in the tunnel.