diff --git a/opendisplay_pipe.c b/opendisplay_pipe.c
index 720d15c..509418b 100644
--- a/opendisplay_pipe.c
+++ b/opendisplay_pipe.c
@@ -1090,6 +1090,17 @@ static void dispatch(uint8_t connection, uint16_t cmd, const uint8_t *payload, u
     case CMD_DIRECT_WRITE_END:
       handle_direct_write_end(connection, payload, payload_len);
       break;
+    case CMD_DIRECT_WRITE_PARTIAL_START:
+    case CMD_BUZZER_ACTIVATE: {
+      /* Not implemented on Silabs, but the client sends these and blocks
+       * waiting for a reply. Emit a NACK so it fails fast instead of timing
+       * out: {0xFF, cmd_low, err, 0x00} matches the client's parse_nack()
+       * (0x76 falls back to a full upload; 0x77 raises promptly). Error code
+       * 0x07 = "unsupported" (ERR_PARTIAL_UNSUPPORTED on the client). */
+      uint8_t nack[] = { 0xFFu, (uint8_t)(cmd & 0xFFu), 0x07u, 0x00u };
+      pipe_send(connection, nack, sizeof(nack));
+      break;
+    }
     default:
       printf("[OD] unknown cmd 0x%04X\r\n", (unsigned)cmd);
       break;
diff --git a/opendisplay_protocol.h b/opendisplay_protocol.h
index a0eb1ad..61a3c3e 100644
--- a/opendisplay_protocol.h
+++ b/opendisplay_protocol.h
@@ -13,6 +13,9 @@
 #define CMD_DIRECT_WRITE_END    0x0072u
 #define CMD_LED_ACTIVATE        0x0073u
 #define CMD_LED_STOP            0x0075u
+/* Client-visible but unimplemented on Silabs; dispatch NACKs them (see dispatch()) */
+#define CMD_DIRECT_WRITE_PARTIAL_START 0x0076u
+#define CMD_BUZZER_ACTIVATE            0x0077u
 /* NFC: sub byte0 0=read, 1=write (+rec_type,len16,payload), 0x10/0x11/0x12=chunked write; rec_type OD_NFC_REC_* */
 #define CMD_NFC_ENDPOINT        0x0082u
 #define CMD_REBOOT              0x000Fu