diff --git a/packages/mosaic/framework/tools/tmux/send-message.sh b/packages/mosaic/framework/tools/tmux/send-message.sh
index 88ffe3ba..56144d54 100755
--- a/packages/mosaic/framework/tools/tmux/send-message.sh
+++ b/packages/mosaic/framework/tools/tmux/send-message.sh
@@ -122,9 +122,16 @@ locate_input_box() {
   glyph_line=$(printf '%s\n' "$pane" | grep -E '❯|^>|│ >' | tail -1)
   if [ -n "$glyph_line" ]; then printf '%s\n' "$glyph_line"; return 0; fi
   rule_lines=$(printf '%s\n' "$pane" | grep -nE '^[[:space:]]*─{4,}[[:space:]]*$' | cut -d: -f1 | tail -2)
-  [ "$(printf '%s\n' "$rule_lines" | grep -c .)" -eq 2 ] || return 1
-  top=$(printf '%s\n' "$rule_lines" | head -1)
-  bottom=$(printf '%s\n' "$rule_lines" | tail -1)
+  [ -n "$rule_lines" ] || return 1
+  # Split the (at most two) captured line numbers with parameter expansion. Not
+  # `head -1`: piping into an early-exiting consumer SIGPIPEs the producer, which
+  # under `set -euo pipefail` aborts the caller with rc=141 and no output. The
+  # scripts/pipefail-early-exit.test.mjs guard reds on that shape, correctly.
+  # With one rule captured both halves resolve to the same value and the
+  # ordering test below rejects it, which is the answer we want anyway.
+  top=${rule_lines%%$'\n'*}
+  bottom=${rule_lines##*$'\n'}
+  [ "$top" != "$bottom" ] || return 1
   [ "$bottom" -gt "$top" ] || return 1
   # An empty range (adjacent rules) prints nothing and still returns 0: found,
   # empty, which is the delivered shape.