diff --git a/RSYNC_COMPAT.md b/RSYNC_COMPAT.md
index cf03c4e..52ba996 100644
--- a/RSYNC_COMPAT.md
+++ b/RSYNC_COMPAT.md
@@ -102,7 +102,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 |------|-------------------|-----------------|-------|
 | `--delete` | Delete extraneous files from dest | ✅ Implemented | `use_delete` config field |
 | `--delete-before` | Delete before transfer | ❌ Not Implemented | Removed because it had no effect |
-| `--delete-during` | Delete during transfer | ❌ Not Implemented | |
+| `--del`, `--delete-during` | Delete during transfer | ❌ Not Implemented | Both flags are recognized but rejected; delete timing is not implemented |
 | `--delete-delay` | Find deletions during, delete after | ❌ Not Implemented | |
 | `--delete-after` | Delete after transfer | ❌ Not Implemented | Removed because it had no effect |
 | `--delete-excluded` | Also delete excluded files | ❌ Not Implemented | Removed because it had no effect |
diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index de4b859..4caf6fb 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -387,6 +387,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--chmod", NULL, OPT_STRING, offsetof(Config, chmod_spec)},
     {"--dirs", "--old-dirs", OPT_UNSUPPORTED, 0},
     {"--old-d", NULL, OPT_UNSUPPORTED, 0},
+    {"--delete-during", "--del", OPT_UNSUPPORTED, 0},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},
@@ -520,12 +521,14 @@ static int apply_table_option(Config* config, const OptionEntry* entry, const ch
     *(unsigned long long*)field = v;
     return 0;
   }
-  case OPT_UNSUPPORTED:
-    log_message(LOG_LEVEL_ERROR,
-                "%s: directory-only transfer is not implemented; refusing to ignore option",
-                option_name);
+  case OPT_UNSUPPORTED: {
+    const char* reason = "directory-only transfer is not implemented";
+    if (strcmp(entry->name, "--delete-during") == 0)
+      reason = "delete-during is not implemented";
+    log_message(LOG_LEVEL_ERROR, "%s: %s; refusing to ignore option", option_name, reason);
     return -1;
   }
+  }
   return -1;
 }
 
diff --git a/src/client/usage.c b/src/client/usage.c
index 35116f7..3c49e2b 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -27,6 +27,7 @@ void print_usage(void) {
   printf("  --ignore-existing  Skip files that already exist on receiver\n");
   printf(
       "  --dirs, --old-dirs, --old-d  Transfer directories without recursing (not implemented)\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 7c631ad..16e65c8 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -574,6 +574,22 @@ static void test_parse_args_rejects_dirs_aliases() {
   }
 }
 
+/* --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[] = {"--silent",
@@ -1110,6 +1126,7 @@ void test_client_cli() {
   test_parse_args_rejects_invalid_max_alloc();
   test_parse_args_unknown_option();
   test_parse_args_rejects_dirs_aliases();
+  test_parse_args_delete_during_alias_unimplemented();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_quiet();
   test_parse_args_human_readable();