diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index 191cc07..81e30d5 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -361,6 +361,12 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
         log_message(LOG_LEVEL_ERROR, "--compress-level must be between 1 and 22");
         return -1;
       }
+    } else if (opt_is(argv[i], "--del", NULL)) {
+      fprintf(stderr, "--del is an alias for --delete-during, which is not implemented\n");
+      return -1;
+    } else if (opt_is(argv[i], "--delete-during", NULL)) {
+      fprintf(stderr, "--delete-during is not implemented\n");
+      return -1;
     } else if (argv[i][0] == '-') {
       fprintf(stderr, "Unknown option: %s\n", argv[i]);
       print_usage();
diff --git a/src/client/usage.c b/src/client/usage.c
index 2f800af..b5e905a 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -21,6 +21,7 @@ void print_usage(void) {
   printf("  -p <port>           SSH port (default: 22)\n");
   printf("  --progress          Show transfer progress\n");
   printf("  --delete            Delete files on receiver not in source\n");
+  printf("  --del               Alias for --delete-during (not implemented)\n");
   printf("  --exclude <pattern> Exclude files matching pattern\n");
   printf("  --include <pattern> Only include files matching pattern\n");
   printf("  --exclude-from <file> Read exclude patterns from file\n");
diff --git a/tests/test_client_cli.c b/tests/test_client_cli.c
index 656fd40..ef9400a 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -260,6 +260,22 @@ static void test_parse_args_unknown_option() {
   config_delete(cfg);
 }
 
+/* --del is recognized as the rsync alias, but its timing mode is not implemented. */
+static void test_parse_args_delete_during_alias_unimplemented() {
+  static const char* const options[] = {"--del", "--delete-during"};
+
+  for (size_t i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", (char*)options[i], "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+
+    EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), -1);
+    EXPECT_FALSE(cfg->use_delete);
+    config_delete(cfg);
+  }
+}
+
 /* Parsed-but-unimplemented options must fail instead of being silently accepted. */
 static void test_parse_args_rejects_unimplemented_options() {
   static const char* const options[] = {"-q",
@@ -358,6 +374,7 @@ void test_client_cli() {
   test_parse_args_invalid_compression_level();
   test_parse_args_valid_compression_level();
   test_parse_args_unknown_option();
+  test_parse_args_delete_during_alias_unimplemented();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
 }