diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index c51b508..20bebfd 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -598,6 +598,12 @@ static const OptionEntry OPTION_TABLE[] = {
      * charsets at startup (client_validation.c) and the full spec rides the
      * config frame so the receiver derives the wire charset symmetrically. */
     {"--iconv", NULL, OPT_STRING, offsetof(Config, iconv_spec)},
+    /* --protocol=NUM: rsync-compatible flag that forces the wire protocol
+     * version to the current value.  FastSync has exactly one wire format, so
+     * any value other than PROTOCOL_VERSION is rejected at validation, before
+     * any network I/O.  Client-only: the server does not negotiate, it just
+     * enforces an exact match. */
+    {"--protocol", NULL, OPT_STRING, offsetof(Config, version)},
     {"--delete-before", NULL, OPT_FLAG, offsetof(Config, delete_before)},
     {"--delete-during", "--del", OPT_FLAG, offsetof(Config, delete_during)},
     {"--delete-delay", NULL, OPT_FLAG, offsetof(Config, delete_delay)},
diff --git a/src/client/client_validation.c b/src/client/client_validation.c
index d42da2e..baa6fba 100644
--- a/src/client/client_validation.c
+++ b/src/client/client_validation.c
@@ -3,6 +3,7 @@
 #include "delay_updates.h"
 #include "log.h"
 #include "usage.h"
+#include <string.h>
 #include <stdio.h>
 
 /* Validate config after parsing. Returns true if valid. */
@@ -132,5 +133,15 @@ bool validate_config(const Config* config) {
                 "--iconv requires LOCAL[,REMOTE] charset names supported by iconv");
     return false;
   }
+  /* --protocol: FastSync has exactly one wire format, so the forced version
+     must equal the current PROTOCOL_VERSION exactly.  Rejected here, before any
+     network I/O, rather than letting the server hit its own mismatch check. */
+  if (strcmp(config->version, PROTOCOL_VERSION) != 0) {
+    log_message(LOG_LEVEL_ERROR,
+                "--protocol must be %s (FastSync supports only its current wire "
+                "protocol version and cannot speak an older or virtual one)",
+                PROTOCOL_VERSION);
+    return false;
+  }
   return true;
 }
diff --git a/src/client/usage.c b/src/client/usage.c
index 5814f93..3c6759b 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -41,6 +41,9 @@ void print_usage(void) {
   printf("                      converted before transmission and back on receipt; a\n");
   printf("                      name that cannot be represented in the target charset\n");
   printf("                      fails that transfer cleanly (rsync-compatible)\n");
+  printf("  --protocol=NUM       Force the wire protocol version (must equal the current\n");
+  printf("                       PROTOCOL_VERSION; FastSync cannot speak older/virtual\n");
+  printf("                       wire formats)\n");
   printf("  --delete            Delete files on receiver not in source\n");
   printf("                      (default timing: delete only after the whole\n");
   printf("                      transfer has succeeded)\n");