diff --git a/e2e/installer.bats b/e2e/installer.bats
index a8175d95b..a19bee88a 100644
--- a/e2e/installer.bats
+++ b/e2e/installer.bats
@@ -67,6 +67,15 @@ run_post_install_setup() {
   [[ "$output" != *"Basecamp CLI"* ]]  # banner would print if main ran
 }
 
+@test "install.sh runs main when piped to bash" {
+  # Keep PATH empty so main stops at its curl prerequisite without downloading
+  # anything or modifying the test environment.
+  run bash -c "cat '$INSTALL_SH' | PATH='$BATS_TEST_TMPDIR' '$BASH'"
+  [[ "$status" -ne 0 ]]
+  [[ "$output" == *"curl is required but not installed"* ]]
+  [[ "$output" != *"BASH_SOURCE[0]: unbound variable"* ]]
+}
+
 @test "new binary: post_install_setup dispatches to 'setup agents', never 'setup claude'" {
   run_post_install_setup
   [[ "$status" -eq 0 ]]
diff --git a/scripts/install.sh b/scripts/install.sh
index 4e915c19f..c03755439 100755
--- a/scripts/install.sh
+++ b/scripts/install.sh
@@ -542,6 +542,6 @@ post_install_setup() {
 # Guard so sourcing the script (e.g. from tests) doesn't run the installer.
 # The if-form is required: `[[ … ]] && main` returns 1 when sourced, which
 # trips `set -e` in the sourcing shell.
-if [[ "${BASH_SOURCE[0]}" == "$0" ]]; then
+if [[ "${BASH_SOURCE[0]:-$0}" == "$0" ]]; then
   main "$@"
 fi