diff --git a/boards/xiao_nrf54l15_nrf54l15_cpuapp.conf b/boards/xiao_nrf54l15_nrf54l15_cpuapp.conf
index f22b2e0..bda3775 100644
--- a/boards/xiao_nrf54l15_nrf54l15_cpuapp.conf
+++ b/boards/xiao_nrf54l15_nrf54l15_cpuapp.conf
@@ -42,3 +42,11 @@ CONFIG_OPENTHREAD_DEFAULT_TX_POWER=8
 CONFIG_GPIO=y
 CONFIG_REGULATOR=y
 CONFIG_REGULATOR_FIXED=y
+
+# Give the spinel link a deeper receive queue.
+#
+# The default 2048 bytes is sized for a link with RTS/CTS, where the host stops
+# sending once the buffer fills. This one has no flow control, so anything that
+# does not fit is simply lost. 8192 covers a burst of full-size frames arriving
+# while the OpenThread thread is busy.
+CONFIG_OPENTHREAD_COPROCESSOR_UART_RING_BUFFER_SIZE=8192
diff --git a/boards/xiao_nrf54l15_nrf54l15_cpuapp.overlay b/boards/xiao_nrf54l15_nrf54l15_cpuapp.overlay
index aaff0b2..67298ec 100644
--- a/boards/xiao_nrf54l15_nrf54l15_cpuapp.overlay
+++ b/boards/xiao_nrf54l15_nrf54l15_cpuapp.overlay
@@ -14,7 +14,26 @@
  *
  * Unlike Nordic's nrf54l15dk overlay, hw-flow-control is NOT set: the XIAO's
  * pinctrl only defines TX and RX for uart20, and the CMSIS-DAP bridge does not
- * carry RTS/CTS. See docs/ for the baud rate this was validated at.
+ * carry RTS/CTS.
+ *
+ * That missing back-pressure is why the spinel link runs at 460800 rather than
+ * the 1000000 Nordic uses. Bandwidth was never the constraint -- 802.15.4 tops
+ * out around 250 kbps -- but without RTS/CTS a single late RX interrupt loses a
+ * byte outright, and a lost byte corrupts the HDLC frame around it. At 1 Mbaud
+ * the driver has 10 us to service each byte; the radio ISR can hold it off
+ * longer than that during a burst. 460800 widens the window to 21.7 us while
+ * still leaving roughly 4x headroom over peak Thread throughput.
+ *
+ * The symptom this fixes: otbr-agent runs for hours, then a traffic burst
+ * desynchronises the stream, the next transmit gets no response, and
+ * HandleRcpTimeout aborts the agent with exit code 6.
+ *
+ *   [W] P-RadioSpinel-: radio tx timeout
+ *   [C] P-RadioSpinel-: Failed to communicate with RCP - no response from RCP
+ *
+ * The host must agree on the rate. In the RadioURL on the border router host:
+ *
+ *   spinel+hdlc+uart:///dev/ttyACM0?uart-baudrate=460800
  */
 
 / {
@@ -27,7 +46,7 @@
 
 &uart20 {
 	status = "okay";
-	current-speed = <1000000>;
+	current-speed = <460800>;
 };
 
 &uart21 {