diff --git a/regress/decrqm-sync.sh b/regress/decrqm-sync.sh
new file mode 100644
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+++ b/regress/decrqm-sync.sh
@@ -0,0 +1,82 @@
+#!/bin/sh
+
+# Test DECRPM response for mode 2026 (synchronized output).
+#
+# DECRQM (ESC[?2026$p) should elicit DECRPM (ESC[?2026;Ps$y) where
+# Ps=1 when MODE_SYNC is active, Ps=2 when reset.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Ltest"
+$TMUX kill-server 2>/dev/null
+sleep 1
+
+TMP=$(mktemp)
+TMP2=$(mktemp)
+trap "rm -f $TMP $TMP2; $TMUX kill-server 2>/dev/null" 0 1 15
+
+$TMUX -f/dev/null new -d -x80 -y24 || exit 1
+sleep 1
+
+# Keep the session alive regardless of pane exits.
+$TMUX set -g remain-on-exit on
+
+exit_status=0
+
+# query_decrpm <outfile> [setup_seq]
+#   Spawn a pane that optionally sends setup_seq, then sends DECRQM for
+#   mode 2026 and captures the response into outfile in cat -v form.
+query_decrpm () {
+	_outfile=$1
+	_setup=$2
+
+	$TMUX respawnw -k -t:0 -- sh -c "
+		exec 2>/dev/null
+		stty raw -echo
+		${_setup:+printf '$_setup'; sleep 0.2}
+		printf '\033[?2026\$p'
+		dd bs=1 count=11 2>/dev/null | cat -v > $_outfile
+		sleep 0.2
+	" || exit 1
+	sleep 2
+}
+
+# ------------------------------------------------------------------
+# Test 1: mode 2026 should be reset by default (Ps=2)
+# ------------------------------------------------------------------
+query_decrpm "$TMP"
+
+actual=$(cat "$TMP")
+expected='^[[?2026;2$y'
+
+if [ "$actual" = "$expected" ]; then
+	if [ -n "$VERBOSE" ]; then
+		echo "[PASS] DECRQM 2026 (default/reset) -> $actual"
+	fi
+else
+	echo "[FAIL] DECRQM 2026 (default/reset): expected '$expected', got '$actual'"
+	exit_status=1
+fi
+
+# ------------------------------------------------------------------
+# Test 2: set mode 2026 (SM ?2026), then query (expect Ps=1)
+# ------------------------------------------------------------------
+query_decrpm "$TMP2" '\033[?2026h'
+
+actual=$(cat "$TMP2")
+expected='^[[?2026;1$y'
+
+if [ "$actual" = "$expected" ]; then
+	if [ -n "$VERBOSE" ]; then
+		echo "[PASS] DECRQM 2026 (set) -> $actual"
+	fi
+else
+	echo "[FAIL] DECRQM 2026 (set): expected '$expected', got '$actual'"
+	exit_status=1
+fi
+
+$TMUX kill-server 2>/dev/null
+
+exit $exit_status
\ No newline at end of file