diff --git a/infra/build-server/lib.sh b/infra/build-server/lib.sh
index 9eb8757f7..ef05dffbf 100755
--- a/infra/build-server/lib.sh
+++ b/infra/build-server/lib.sh
@@ -45,7 +45,10 @@ bs_rsync() { rsync -e "$BS_SSH_CMD" "$@"; }
 # a different compiler gives different bytes and, across a minor version, different lints and errors
 # the pin (main, 7 October 2026): rust-toolchain.toml at the worktree root (and the fork's own copy) names the channel; the Mac's
 # rustc AS RESOLVED IN THE CRATE DIR (rustup reads the file), the box's rustc and the pin must agree, else the build is refused
-bs_pin() { sed -n 's/^channel *= *"\([^"]*\)".*/\1/p' "${1:-$BS_WT_ROOT}/rust-toolchain.toml" 2>/dev/null | head -1; }
+# a tree without the file (an older release branch) gives an empty pin, never a failed pipeline: under pipefail the sed status would
+# become the assignment's status and set -e would end the caller silently (7 Oct 2026, 03:36 UTC: the b3c228fa builds under the
+# 0.3.16 app tree 5d118f58 died right after the pairing line)
+bs_pin() { local f="${1:-$BS_WT_ROOT}/rust-toolchain.toml"; [ -f "$f" ] || { echo ""; return 0; }; { sed -n 's/^channel *= *"\([^"]*\)".*/\1/p' "$f" 2>/dev/null || true; } | head -1; }
 bs_toolchain_check() {
   local mac box pin
   pin=$(bs_pin "$BS_WT_ROOT"); [ -n "$pin" ] || pin=$(bs_pin "$BS_TOP")