diff --git a/RSYNC_COMPAT.md b/RSYNC_COMPAT.md
index 4b224a1..bb64fbb 100644
--- a/RSYNC_COMPAT.md
+++ b/RSYNC_COMPAT.md
@@ -59,7 +59,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--max-size=SIZE` | Skip files larger than SIZE | ✅ Implemented | `max_size` in scanner |
 | `--min-size=SIZE` | Skip files smaller than SIZE | ✅ Implemented | `min_size` in scanner |
 | `-I`, `--ignore-times` | Don't skip files matching size+time | ❌ Not Implemented | |
-| `--size-only` | Skip based on size only | ❌ Not Implemented | |
+| `--size-only` | Skip based on size only | ✅ Implemented | With `--incremental`, ignores mtime |
 | `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ❌ Not Implemented | |
 | `--existing` | Skip creating new files on receiver | ❌ Not Implemented | |
 | `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index 5fffe32..f359b6a 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -206,6 +206,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--remove-source-files", NULL, OPT_FLAG, offsetof(Config, remove_source_files)},
     {"--delete", NULL, OPT_FLAG, offsetof(Config, use_delete)},
     {"--incremental", NULL, OPT_FLAG, offsetof(Config, use_incremental)},
+    {"--size-only", NULL, OPT_FLAG, offsetof(Config, size_only)},
     {"--ignore-times", "-I", OPT_FLAG, offsetof(Config, ignore_times)},
     {"--delta", NULL, OPT_FLAG, offsetof(Config, use_delta)},
     {"--whole-file", "-W", OPT_FLAG, offsetof(Config, whole_file)},
diff --git a/src/client/usage.c b/src/client/usage.c
index 1d0aab3..079b871 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -30,6 +30,7 @@ void print_usage(void) {
   printf("  --max-size <n>      Skip files larger than n bytes\n");
   printf("  --min-size <n>      Skip files smaller than n bytes\n");
   printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  --size-only         Skip incremental files matching in size, ignoring mtime\n");
   printf("  -I, --ignore-times  Transfer files even when size and mtime match\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
   printf("  -W, --whole-file    Transfer changed files without delta processing\n");
diff --git a/src/server/receiver.c b/src/server/receiver.c
index a84cbc3..63d7d10 100644
--- a/src/server/receiver.c
+++ b/src/server/receiver.c
@@ -61,7 +61,7 @@ static bool receiver_process_batch(Config* config, int file_descriptor) {
     struct stat st;
     bool has_old = file_stat_secure(full_path, &st);
     bool match = !config->ignore_times && has_old && (unsigned long long)st.st_size == check_size &&
-                 (long long)st.st_mtime == check_mtime;
+                 (config->size_only || (long long)st.st_mtime == check_mtime);
     bool sent = send_status(file_descriptor, match ? STATUS_OK : STATUS_NEXT);
     free(full_path);
     free(check_path);
diff --git a/src/server/server.c b/src/server/server.c
index b1970af..7e0c2af 100644
--- a/src/server/server.c
+++ b/src/server/server.c
@@ -176,7 +176,7 @@ int receive_files(Config* config, int fd) {
         bool has_old = full_path && file_stat_secure(full_path, &st);
         bool match = !config->ignore_times && has_old &&
                      (unsigned long long)st.st_size == check_size &&
-                     (long long)st.st_mtime == check_mtime;
+                     (config->size_only || (long long)st.st_mtime == check_mtime);
         bool sent = send_status(fd, match ? STATUS_OK : STATUS_NEXT);
         free(full_path);
         free(check_path);
diff --git a/src/shared/config.c b/src/shared/config.c
index 47f529a..5ca0b97 100644
--- a/src/shared/config.c
+++ b/src/shared/config.c
@@ -36,6 +36,7 @@ static void config_set_defaults(Config* config) {
   config->min_size = 0;
   config->use_incremental = false;
   config->ignore_times = false;
+  config->size_only = false;
   config->use_delta = false;
   config->whole_file = false;
   config->delta_block_size = DELTA_BLOCK_SIZE_DEFAULT;
@@ -122,8 +123,8 @@ static bool validate_received_config(const Config* config) {
          valid_wire_bool(config->use_chunk_serialization) &&
          valid_wire_bool(config->use_compression) && valid_wire_bool(config->use_metadata) &&
          valid_wire_bool(config->use_sendfile) && valid_wire_bool(config->use_delete) &&
-         valid_wire_bool(config->use_incremental) && valid_wire_bool(config->use_delta) &&
-         valid_wire_bool(config->ignore_times) && valid_wire_bool(config->backup) &&
+         valid_wire_bool(config->use_incremental) && valid_wire_bool(config->size_only) &&
+         valid_wire_bool(config->ignore_times) && valid_wire_bool(config->use_delta) &&
          valid_wire_bool(config->follow_symlinks) && valid_wire_bool(config->copy_links) &&
          valid_wire_bool(config->safe_links) && valid_wire_bool(config->copy_unsafe_links) &&
          valid_wire_bool(config->preserve_hard_links) && valid_wire_bool(config->preserve_acls) &&
@@ -236,7 +237,8 @@ static bool send_core_fields(int fd, const Config* c) {
 
 static bool send_delta_fields(int fd, const Config* c) {
   return send_int(fd, c->use_delete) && send_int(fd, c->use_incremental) &&
-         send_int(fd, c->ignore_times) && send_int(fd, c->use_delta && !c->whole_file) &&
+         send_int(fd, c->size_only) && send_int(fd, c->ignore_times) &&
+         send_int(fd, c->use_delta && !c->whole_file) &&
          send_n_data(fd, &c->delta_block_size, sizeof(c->delta_block_size)) &&
          send_n_data(fd, &c->delta_max_file_size, sizeof(unsigned long long));
 }
@@ -293,6 +295,8 @@ static bool receive_delta_fields(int fd, Config* c) {
     return false;
   if (!receive_wire_bool(fd, &c->use_incremental))
     return false;
+  if (!receive_wire_bool(fd, &c->size_only))
+    return false;
   if (!receive_wire_bool(fd, &c->ignore_times))
     return false;
   if (!receive_wire_bool(fd, &c->use_delta))
diff --git a/src/shared/config.h b/src/shared/config.h
index 541ca15..3b37d38 100644
--- a/src/shared/config.h
+++ b/src/shared/config.h
@@ -36,6 +36,7 @@ typedef struct Config {
   unsigned long long min_size;
   bool use_incremental;
   bool ignore_times;
+  bool size_only;
   bool use_delta;
   bool whole_file;
   uint32_t delta_block_size;
diff --git a/src/shared/file_receive.c b/src/shared/file_receive.c
index 6c44c55..4c900c2 100644
--- a/src/shared/file_receive.c
+++ b/src/shared/file_receive.c
@@ -387,7 +387,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
     match = (old_size == 0 || old_data) && old_checksum == check_checksum;
     free(old_data);
     old_data = NULL;
-  } else if (match) {
+  } else if (match && !config->size_only) {
     match = (long long)st.st_mtime == check_mtime;
   }
 
diff --git a/tests/integration/test_features.py b/tests/integration/test_features.py
index eeb287f..5ad99af 100644
--- a/tests/integration/test_features.py
+++ b/tests/integration/test_features.py
@@ -331,6 +331,26 @@ class TestIncremental:
         with open(received_file, "rb") as f:
             assert f.read() == b"hello world\n"
 
+    def test_size_only_skips_same_size_with_different_mtime(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        received_file = os.path.join(received, "small.txt")
+        with open(received_file, "wb") as f:
+            f.write(b"different!!\n")
+        os.utime(received_file, (time.time() - 3600, time.time() - 3600))
+
+        result, _ = run_client(
+            SOURCE_DIR, DEST_DIR,
+            flags=["-M", "--incremental", "--size-only"],
+            port=shared_server.port,
+        )
+        assert result.returncode == 0, f"Size-only sync failed: {result.stderr[:200]}"
+        with open(received_file, "rb") as f:
+            assert f.read() == b"different!!\n"
+
     def test_ignore_times_transfers_same_size_and_mtime(self, shared_server):
         clean_dir(DEST_DIR)
         result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
diff --git a/tests/test_client_cli.c b/tests/test_client_cli.c
index bcb0e94..23b5176 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -210,6 +210,20 @@ static void test_parse_args_valid_port() {
   config_delete(cfg);
 }
 
+/* Test parse_args with --size-only. */
+static void test_parse_args_size_only() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--size-only", "/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->size_only);
+  EXPECT_EQ_INT(positional_count, 2);
+
+  config_delete(cfg);
+}
+
 /* Test parse_args rejects port > 65535 */
 static void test_parse_args_invalid_port() {
   Config* cfg = config_create();
@@ -568,6 +582,7 @@ void test_client_cli() {
   test_parse_args_help();
   test_parse_args_version();
   test_parse_args_valid_port();
+  test_parse_args_size_only();
   test_parse_args_invalid_port();
   test_parse_args_non_numeric_port();
   test_parse_args_invalid_server_port();
diff --git a/tests/test_config.c b/tests/test_config.c
index 748e53a..2cea3d5 100644
--- a/tests/test_config.c
+++ b/tests/test_config.c
@@ -124,10 +124,10 @@ static void test_config_send_receive() {
   send_cfg->use_chunk_serialization = true;
   send_cfg->use_compression = true;
   send_cfg->use_metadata = true;
-  send_cfg->use_fsync = true;
   send_cfg->use_delta = true;
   send_cfg->whole_file = true;
   send_cfg->ignore_times = true;
+  send_cfg->size_only = true;
   send_cfg->compression_level = 5;
   send_cfg->chunk_size = 1024;
   send_cfg->eight_bit_output = true;
@@ -161,13 +161,13 @@ static void test_config_send_receive() {
         ok = false;
       if (!recv_cfg->use_chunk_serialization)
         ok = false;
-      if (!recv_cfg->ignore_times)
-        ok = false;
       if (recv_cfg->compression_level != 5)
         ok = false;
       if (recv_cfg->chunk_size != 1024)
         ok = false;
-      if (!recv_cfg->use_fsync)
+      if (!recv_cfg->size_only)
+        ok = false;
+      if (!recv_cfg->ignore_times)
         ok = false;
       if (!recv_cfg->eight_bit_output)
         ok = false;
@@ -200,11 +200,11 @@ static void test_config_send_receive() {
 }
 
 static void test_config_send_receive_version_mismatch() {
-  /* Create a config with a different protocol version */
+  /* A peer using the previous wire format must be rejected. */
   Config* cfg = config_create();
   EXPECT_NOT_NULL(cfg);
   free(cfg->version);
-  cfg->version = str_dup("0.0");
+  cfg->version = str_dup("2.2.0");
   cfg->send_directory = str_dup("/src");
   cfg->receive_root_directory = str_dup("/dst");