diff --git a/sync_word_test.py b/sync_word_test.py
new file mode 100644
index 0000000..e77438b
--- /dev/null
+++ b/sync_word_test.py
@@ -0,0 +1,104 @@
+#!/usr/bin/env python3
+"""Verify the dongle applies ``LoRaConfig.sync_word`` to the radio.
+
+Two roles. Run on TWO SEPARATE PHYSICAL DONGLES (not the same dongle
+shared via donglora-mux — a single chip can only hold one sync word at
+a time, and the mux delivers local TX back to other sessions as
+LOCAL_LOOPBACK events that bypass the chip's sync filter). Pass an
+explicit port path to each invocation so neither side discovers the
+mux socket:
+
+    # Dongle A — pin to one sync word, listen:
+    python examples/sync_word_test.py --role rx /dev/ttyACM0 [--sync 0x5464]
+
+    # Dongle B — rotate through several sync words, transmitting on each:
+    python examples/sync_word_test.py --role tx /dev/ttyACM1
+
+Expected behavior with the firmware fix:
+    RX hears ONLY the cycles whose sync word matches its own
+    (one packet per rotation pass through SYNC_WORDS), and packets
+    have realistic RSSI / SNR (not 0.0 / 0.0).
+
+Without the fix:
+    RX hears every transmission regardless of which sync word the TX
+    side configured — the firmware was silently ignoring the field
+    and leaving the SX126x on the boot-time default 0x1424.
+
+The default RX sync word (0x5464) is deliberately off the MeshCore
+(0x1424) and LoRaWAN-public (0x3444) values so the test isn't polluted
+by ambient mesh traffic. The RX role drops `LOCAL_LOOPBACK` events
+defensively so accidental mux sharing fails loud (zero packets) rather
+than printing misleading "all sync words received" output.
+"""
+
+import argparse
+import dataclasses
+import time
+
+import donglora as dl
+from _common import MESHCORE_US
+
+# SX126x register-encoded sync words. Each is (hi << 8) | lo where
+# [hi, lo] = convert_sync_word(byte) for the listed underlying byte:
+#   byte 0x12 → 0x1424   (LoRa "private" — MeshCore default)
+#   byte 0x34 → 0x3444   (LoRaWAN "public")
+#   byte 0x56 → 0x5464
+#   byte 0x78 → 0x7484
+#   byte 0xAB → 0xA4B4
+#   byte 0xCD → 0xC4D4
+SYNC_WORDS = [0x1424, 0x3444, 0x5464, 0x7484, 0xA4B4, 0xC4D4]
+TX_PAUSE_S = 1.5
+
+parser = argparse.ArgumentParser(description="Sync-word verification test")
+parser.add_argument("--role", choices=["tx", "rx"], required=True)
+parser.add_argument(
+    "--sync",
+    type=lambda x: int(x, 0),
+    default=0x5464,
+    help="(rx) sync word to pin to, e.g. 0x5464 (default; avoids MeshCore noise)",
+)
+parser.add_argument("port", nargs="?", help="Serial port (auto-detect if omitted)")
+args = parser.parse_args()
+
+if args.role == "tx":
+    with dl.connect(args.port, config=MESHCORE_US) as d:
+        print(
+            f"TX: rotating {len(SYNC_WORDS)} sync words, "
+            f"{TX_PAUSE_S}s between (Ctrl+C to stop)\n"
+        )
+        try:
+            cycle = 0
+            while True:
+                for sw in SYNC_WORDS:
+                    cfg = dataclasses.replace(MESHCORE_US, sync_word=sw)
+                    d.set_config(cfg)
+                    msg = f"cycle={cycle} sync=0x{sw:04X}"
+                    td = d.tx(msg.encode())
+                    print(
+                        f"  TX sync=0x{sw:04X}  {msg!r}  ({td.airtime_us} µs)"
+                    )
+                    time.sleep(TX_PAUSE_S)
+                cycle += 1
+        except KeyboardInterrupt:
+            print("\nDone.")
+else:
+    sw = args.sync
+    cfg = dataclasses.replace(MESHCORE_US, sync_word=sw)
+    with dl.connect(args.port, config=cfg) as d:
+        print(f"RX: pinned to sync=0x{sw:04X} (Ctrl+C to stop)")
+        print("Only matching-sync-word transmissions should arrive.\n")
+        try:
+            for pkt in d.rx():
+                if pkt.origin != dl.RxOrigin.OTA:
+                    # Drop mux-injected local-loopback events. They
+                    # arrive without going through the chip's sync
+                    # filter and would falsely "match" every sync
+                    # word the TX side rotates through.
+                    continue
+                text = pkt.data.decode("utf-8", errors="replace")
+                print(
+                    f"  RX: {text!r}  RSSI:{pkt.rssi_dbm:.1f}dBm  "
+                    f"SNR:{pkt.snr_db:.1f}dB"
+                )
+        except KeyboardInterrupt:
+            print("\nDone.")