diff --git a/src/controller/cluster.c b/src/controller/cluster.c
index d151243..076cab2 100644
--- a/src/controller/cluster.c
+++ b/src/controller/cluster.c
@@ -1139,6 +1139,10 @@ int cluster_init_phase2(cluster_t* c)
                                                       : "controller";
         persist_config_t pcfg    = persist_config_defaults();
 
+        /* --sync-timeout-ms override (0 = keep the persist default). */
+        if (c->myconf->sync_timeout_ms > 0)
+            pcfg.sync_timeout_ms = c->myconf->sync_timeout_ms;
+
         persist_set_workdir(&c->persist, workdir, name);
         rc = persist_init(&c->persist, &pcfg, c);
         if (rc != 0) {
diff --git a/src/controller/main.c b/src/controller/main.c
index 12e35c5..ab346fb 100644
--- a/src/controller/main.c
+++ b/src/controller/main.c
@@ -29,6 +29,7 @@ const char* argp_program_bug_address = BUG_EMAIL;
 #define OPT_CONF_STALE_TIMEOUT 258
 #define OPT_VCPUS              259
 #define OPT_CERTDIR            260
+#define OPT_SYNC_TIMEOUT       261
 
 struct arguments {
     char* myname;
@@ -40,6 +41,7 @@ struct arguments {
     long  conf_stale_timeout;
     int   vcpus;
     char* certdir;
+    uint32_t sync_timeout_ms;
 };
 
 static struct argp_option options[] = {
@@ -58,6 +60,8 @@ static struct argp_option options[] = {
       "Number of virtual CPUs for the task manager (must be >= 1)" },
     { "certdir",            OPT_CERTDIR,  "DIR", 0,
       "Directory containing xce.crt and xce.key (default: <workdir>/certs)" },
+    { "sync-timeout-ms",    OPT_SYNC_TIMEOUT, "MS", 0,
+      "Synchronous majority-replication commit barrier timeout in ms (0 = default)" },
     { 0 }
 };
 
@@ -100,6 +104,13 @@ static error_t parse_opt(int key, char* arg, struct argp_state* state)
     case OPT_CERTDIR:
         arguments->certdir = arg;
         break;
+    case OPT_SYNC_TIMEOUT: {
+        int v = atoi(arg);
+        if (v < 0)
+            argp_error(state, "--sync-timeout-ms must be >= 0 (got: %s)", arg);
+        arguments->sync_timeout_ms = (uint32_t)v;
+        break;
+    }
     default:
         return ARGP_ERR_UNKNOWN;
     }
@@ -151,6 +162,7 @@ int main(int argc, char* argv[])
     arguments.conf_keep_n        = 2;
     arguments.conf_stale_timeout = 86400;
     arguments.vcpus              = 0; /* 0 = not set; use cluster config default */
+    arguments.sync_timeout_ms    = 0; /* 0 = not set; use persist config default */
 
     argp_parse(&argp, argc, argv, 0, 0, &arguments);
 
@@ -199,6 +211,7 @@ int main(int argc, char* argv[])
     myconf.conf_keep_n       = arguments.conf_keep_n;
     myconf.conf_stale_timeout = (time_t)arguments.conf_stale_timeout;
     myconf.vcpus             = arguments.vcpus;
+    myconf.sync_timeout_ms   = arguments.sync_timeout_ms;
     memset(&myconf.peers_cfg, 0, sizeof(myconf.peers_cfg));
 
     if (arguments.peers_str) {
diff --git a/src/controller/testsuite/test_cluster_persist.c b/src/controller/testsuite/test_cluster_persist.c
index 801611c..678045a 100644
--- a/src/controller/testsuite/test_cluster_persist.c
+++ b/src/controller/testsuite/test_cluster_persist.c
@@ -60,6 +60,11 @@ static void test_persist_config_defaults_finished_retain_s(void)
 static void test_persist_config_defaults_sync_timeout_ms(void)
 {
     persist_config_t cfg = persist_config_defaults();
+    /* Default raised to 500 ms: realistic for durable sync majority
+     * replication (each standby fdatasyncs before ACK), still bounded for
+     * failure detection. Literal-pinned so a regression to the old 50 ms
+     * tight window fails here, not just on a live cluster. */
+    TEST_ASSERT_EQUAL_UINT32(500, cfg.sync_timeout_ms);
     TEST_ASSERT_EQUAL_UINT32(DEFAULT_SYNC_TIMEOUT_MS, cfg.sync_timeout_ms);
 }
 
diff --git a/src/include/config.h b/src/include/config.h
index 11eeb1a..babe0b9 100644
--- a/src/include/config.h
+++ b/src/include/config.h
@@ -90,7 +90,7 @@ typedef uint64_t jobid_t;
 #define DEFAULT_TRANSIENT_JOB_TIMEOUT_S 300
 #define DEFAULT_FINISHED_RETAIN_S       3600
 #define DEFAULT_WORKER_RETAIN_S         86400
-#define DEFAULT_SYNC_TIMEOUT_MS         50
+#define DEFAULT_SYNC_TIMEOUT_MS         500
 #define DEFAULT_HB_INTERVAL_MS          3000
 #define DEFAULT_MAX_WAL_RECORDS         10000
 
diff --git a/src/include/myconf.h b/src/include/myconf.h
index 934d287..3ca4604 100644
--- a/src/include/myconf.h
+++ b/src/include/myconf.h
@@ -35,6 +35,7 @@ typedef struct {
     time_t               conf_stale_timeout;    /* seconds before saved config flagged stale (default 86400) */
     int                  conf_stale;             /* 1 when saved config older than conf_stale_timeout */
     int                  vcpus;                 /* Req 4.3: --vcpus CLI value; 0 = use config default */
+    uint32_t             sync_timeout_ms;       /* --sync-timeout-ms; 0 = use persist default */
 } myconf_t;
 
 extern cluster_config_t* get_default_conf(void*);