diff --git a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
index 0d2ef2fe..68b7a097 100755
--- a/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
+++ b/packages/mosaic/framework/tools/tmux/test-send-message-socket.sh
@@ -133,7 +133,8 @@ env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TWIN" -m "prefix trap" >/dev/null 2
 # PSEUDO session is not a hit, so the sender must fail target-not-found
 # (rc 1) instead of delivering into the prefix-named session.
 [ "$a3_rc" -eq 1 ] || fail "prefix false-hit: rc=$a3_rc want 1 (stderr: $(cat "$A3_ERR"))"
-if tmux capture-pane -t "=$PSEUDO:0.0" -p 2>/dev/null | grep -qF "prefix trap"; then
+a3_pane="$(tmux capture-pane -t "=$PSEUDO:0.0" -p 2>/dev/null)" || a3_pane=""
+if printf '%s' "$a3_pane" | grep -F "prefix trap" >/dev/null; then
   fail "delivery landed in the prefix-named session (false socket hit)"
 fi
 tmux kill-session -t "$PSEUDO" >/dev/null 2>&1 || true
@@ -147,7 +148,8 @@ A4_ERR=$(mktemp)
 a4_rc=0
 env -u MOSAIC_TMUX_SOCKET "$AGENT_SEND" -s "$TWIN:0.0" -m "compound trap" >/dev/null 2>"$A4_ERR" || a4_rc=$?
 [ "$a4_rc" -eq 1 ] || fail "compound prefix false-hit: rc=$a4_rc want 1 (stderr: $(cat "$A4_ERR"))"
-if tmux capture-pane -t "=$PSEUDO2:0.0" -p 2>/dev/null | grep -qF "compound trap"; then
+a4_pane="$(tmux capture-pane -t "=$PSEUDO2:0.0" -p 2>/dev/null)" || a4_pane=""
+if printf '%s' "$a4_pane" | grep -F "compound trap" >/dev/null; then
   fail "compound delivery landed in the prefix-named session"
 fi
 tmux kill-session -t "$PSEUDO2" >/dev/null 2>&1 || true