feat: add secluded args compatibility option

README.md

@@ -89,6 +89,7 @@ partial, alternate, and planned behavior.
 | `-a, --archive` | Archive mode: enables `-c -m -M` (no `-s`) |
 | `-m` | Multithreading mode |
 | `-s` | Chunk serialization (batch all files per chunk) |
+| `--secluded-args` | Rsync-compatible long form. SSH already sends transfer arguments through FastSync's protocol; `-s` remains chunk serialization. |
 | `-f, --sendfile` | Sendfile zero-copy. Incompatible with `-c` / `-s`. TCP only. |
 | `-M, --preserve` | Preserve supported file metadata (mode and mtime; ownership and atime are unsupported) |
 | `-n, --dry-run` | Scan and print what would be transferred |
@@ -359,6 +360,12 @@ particular, FastSync currently uses `-p` for SSH port, `-s` for chunk
 serialization, and `-S` for sparse handling. These meanings must be reconciled
 before FastSync can claim full rsync CLI compatibility.
 
+`--secluded-args` is supported as a long-form compatibility option. FastSync's
+SSH transport already launches only the remote server's `--stdio` command and
+sends transfer configuration through its protocol. The rsync short form `-s`
+is intentionally not aliased because it remains FastSync's chunk-serialization
+option.
+
 ## Client Options
 
 ### Selection and transfer

RSYNC_COMPAT.md

@@ -233,7 +233,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--protocol=NUM` | Force older protocol version | ❌ Not Implemented | |
 | `--iconv=CONVERT_SPEC` | Charset conversion | ❌ Not Implemented | |
 | `--checksum-seed=NUM` | Set checksum seed | ❌ Not Implemented | |
-| `-s`, `--secluded-args` | Use protocol to send args | ❌ Not Implemented | |
+| `--secluded-args` | Use protocol to send args | ✅ Implemented | SSH already sends transfer configuration through FastSync's protocol; `-s` remains chunk serialization because of the existing FastSync short-option meaning. |
 | `--no-OPTION` | Turn off implied option | ❌ Not Implemented | |
 
 ---

src/client/client_cli.c

@@ -146,6 +146,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--backup", NULL, OPT_FLAG, offsetof(Config, backup)},
     {"--stats", NULL, OPT_FLAG, offsetof(Config, stats)},
     {"--partial", NULL, OPT_FLAG, offsetof(Config, partial)},
+    {"--secluded-args", NULL, OPT_FLAG, offsetof(Config, use_secluded_args)},
     {"--links", "-l", OPT_FLAG, offsetof(Config, follow_symlinks)},
     {"--copy-links", NULL, OPT_FLAG, offsetof(Config, copy_links)},
     {"--safe-links", NULL, OPT_FLAG, offsetof(Config, safe_links)},

src/client/usage.c

@@ -35,6 +35,7 @@ void print_usage(void) {
          DELTA_MAX_FILE_SIZE);
   printf("  -m                  Enable multithreading\n");
   printf("  -s                  Enable chunk serialization\n");
+  printf("  --secluded-args    Send transfer arguments through the protocol (SSH)\n");
   printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  -M, --preserve      Preserve file metadata\n");

src/shared/config.c

@@ -82,6 +82,7 @@ static void config_set_defaults(Config* config) {
   config->relative = false;
   config->rsh_command = NULL;
   config->rsync_path = NULL;
+  config->use_secluded_args = false;
   config->temp_dir = NULL;
   config->compare_dest = NULL;
   config->copy_dest = NULL;

src/shared/config.h

@@ -96,6 +96,7 @@ typedef struct Config {
   // Issue #130: Remote shell/connection options
   char* rsh_command;
   char* rsync_path;
+  bool use_secluded_args;
   char* temp_dir;
   char* compare_dest;
   char* copy_dest;

tests/test_client_cli.c

@@ -339,6 +339,31 @@ static void test_parse_args_archive() {
   config_delete(cfg);
 }
 
+/* --secluded-args is the rsync-compatible long form; -s keeps its FastSync meaning. */
+static void test_parse_args_secluded_args() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--secluded-args", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->use_secluded_args);
+  EXPECT_FALSE(cfg->use_chunk_serialization);
+  config_delete(cfg);
+}
+
+static void test_parse_args_short_s_remains_chunk_serialization() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-s", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->use_chunk_serialization);
+  EXPECT_FALSE(cfg->use_secluded_args);
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -360,4 +385,6 @@ void test_client_cli() {
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
+  test_parse_args_secluded_args();
+  test_parse_args_short_s_remains_chunk_serialization();
 }