diff --git a/deploy/bootstrap.sh b/deploy/bootstrap.sh
index b2dbf99..eff72f3 100644
--- a/deploy/bootstrap.sh
+++ b/deploy/bootstrap.sh
@@ -2240,8 +2240,9 @@ prompt_install_mode() {
 
   # No override: ask on the controlling terminal. Under `curl | sudo bash` stdin
   # is the script, so we must read /dev/tty, not stdin. No tty (CI/cloud-init) →
-  # take the default.
-  if [ ! -r /dev/tty ]; then
+  # take the default. Open it to find out: /dev/tty is mode 0666 on every Linux host, so
+  # `-r` passes even when there is no controlling terminal and only the open fails.
+  if ! (: </dev/tty) 2>/dev/null; then
     INSTALL_MODE="$def"
     log "no terminal for a prompt; defaulting to a ${def} install (set FELIS_INSTALL_MODE=full or nano to override)"
     return 0
diff --git a/deploy/bootstrap_test.sh b/deploy/bootstrap_test.sh
index 0b462f3..d5af411 100644
--- a/deploy/bootstrap_test.sh
+++ b/deploy/bootstrap_test.sh
@@ -363,8 +363,9 @@ else
     "$(run_mode "$sdir/felis-nano.service" "$sdir/absent.done")"
   expect "a host with the full install re-runs as full" "MODE: full" \
     "$(run_mode "$sdir/felis-nano.service" "$sdir/bootstrap.done")"
-  expect "a fresh host defaults to full" "MODE: full" \
-    "$(run_mode "$sdir/absent.service" "$sdir/absent.done")"
+  out="$(run_mode "$sdir/absent.service" "$sdir/absent.done")"
+  expect "a fresh host defaults to full" "MODE: full" "$out"
+  expect "no controlling terminal takes the no-prompt path" "LOG: no terminal for a prompt" "$out"
   # felis setup goes on to need the control plane, so under it nano is refused, and the
   # nano-only default above must not apply either.
   expect "felis setup refuses FELIS_INSTALL_MODE=nano" "DIE: felis setup installs the full control plane" \