diff -r 3fc6b1d6cbe9 -r c33567d61ba7 src/main.c
--- a/src/main.c	Thu May 28 13:58:24 2026 +0200
+++ b/src/main.c	Sun Jun 07 14:55:20 2026 +0200
@@ -731,6 +731,18 @@
                 }
             }
             break;
+        case NETCODE_THREEFOLD:
+            /* validate the claim */
+            if (check_threefold_repetition(gamestate)) {
+                /* auto-accept the claim */
+                gamestate->threefold = true;
+                net_send_code(opponent, NETCODE_ACCEPT);
+                return 1;
+            } else {
+                /* silently decline, do not add message to UI */
+                net_send_code(opponent, NETCODE_DECLINE);
+            }
+            break;
         case NETCODE_MOVE: {
             Move move;
             net_recieve_data(opponent, &move, sizeof(Move));
@@ -748,7 +760,29 @@
                 } else {
                     net_send_code(opponent, NETCODE_ACCEPT);
                 }
-                return 0;
+                /* check for threefold repetition */
+                /* Note: in our implementation it is an auto-draw claimed
+                 * by the player confronted with the position. By standard
+                 * chess rules, also the player who creates the position may
+                 * claim the draw. But since we do it automatically, it makes
+                 * no practical difference.
+                 */
+                if (check_threefold_repetition(gamestate)) {
+                    /* send this as a new package (basically as next move) */
+                    net_send_code(opponent, NETCODE_THREEFOLD);
+                    /* the protocol supports declining the claim  */
+                    uint8_t resp = net_recieve_code(opponent);
+                    if (resp == NETCODE_ACCEPT) {
+                        gamestate->threefold = true;
+                        return 1;
+                    } else {
+                        /* does not happen in our implementation */
+                        // TODO: somehow add a message to the UI
+                        return 0;
+                    }
+                } else {
+                    return 0;
+                }
             } else {
                 uint32_t reason = htonl(code);
                 net_send_data(opponent, NETCODE_DECLINE,