diff --git a/README.md b/README.md
index fdbd9f3..be6fbc7 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,9 @@ replacement for every rsync feature or protocol mode.
 - Sparse-file handling does not yet preserve all holes correctly.
 - `--partial`, `--partial-dir`, `--append`, and `--append-verify` are not yet
   full rsync-style resumable transfers.
+- `--dirs` is not implemented. Its compatibility aliases `--old-dirs` and
+  `--old-d` are recognized but rejected explicitly rather than silently using
+  FastSync's recursive directory behavior.
 - Several rsync short options currently have FastSync-specific meanings. Do
   not assume every short option is interchangeable yet.
 
diff --git a/RSYNC_COMPAT.md b/RSYNC_COMPAT.md
index 4b236ec..8484928 100644
--- a/RSYNC_COMPAT.md
+++ b/RSYNC_COMPAT.md
@@ -71,7 +71,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-r`, `--recursive` | Recurse into directories | ✅ Implemented | Default behavior |
 | `-R`, `--relative` | Use relative path names | ❌ Not Implemented | Removed because it had no effect |
 | `--no-implied-dirs` | Don't send implied dirs with -R | ❌ Not Implemented | |
-| `-d`, `--dirs` | Transfer dirs without recursing | ❌ Not Implemented | |
+| `-d`, `--dirs`, `--old-dirs`, `--old-d` | Transfer dirs without recursing | ❌ Not Implemented | The aliases are recognized and rejected explicitly; they depend on the unimplemented `--dirs` behavior |
 | `--mkpath` | Create missing path components | ❌ Not Implemented | |
 
 ## 5. Transfer Modifications
diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index 191cc07..0de3a4f 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -125,6 +125,7 @@ typedef enum {
   OPT_POS_INT,
   OPT_NONNEG_INT,
   OPT_ULL,
+  OPT_UNSUPPORTED,
 } OptKind;
 
 typedef struct {
@@ -153,6 +154,8 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
     {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
     {"--checksum", NULL, OPT_FLAG, offsetof(Config, checksum)},
+    {"--dirs", "--old-dirs", OPT_UNSUPPORTED, 0},
+    {"--old-d", NULL, OPT_UNSUPPORTED, 0},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},
@@ -203,6 +206,11 @@ 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 requires --dirs, which is not implemented; refusing to ignore option",
+                entry->name);
+    return -1;
   }
   return -1;
 }
diff --git a/src/client/usage.c b/src/client/usage.c
index 2f800af..4689df9 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -21,6 +21,8 @@ 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(
+      "  --dirs, --old-dirs, --old-d  Transfer directories without recursing (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..d998a3e 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -260,6 +260,21 @@ static void test_parse_args_unknown_option() {
   config_delete(cfg);
 }
 
+/* Directory aliases require the not-yet-implemented --dirs behavior. */
+static void test_parse_args_rejects_dirs_aliases() {
+  static const char* const options[] = {"--dirs", "--old-dirs", "--old-d"};
+
+  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);
+    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 +373,7 @@ void test_client_cli() {
   test_parse_args_invalid_compression_level();
   test_parse_args_valid_compression_level();
   test_parse_args_unknown_option();
+  test_parse_args_rejects_dirs_aliases();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
 }