diff --git a/src/controller/testsuite/test_repl_unit.c b/src/controller/testsuite/test_repl_unit.c
index 6562f48a..c69ca7c2 100644
--- a/src/controller/testsuite/test_repl_unit.c
+++ b/src/controller/testsuite/test_repl_unit.c
@@ -492,6 +492,46 @@ static void test_broadcast_send_failure_continues(void)
 #endif
 }
 
+/* ============================================================
+ * Task 5.3: Registration lifecycle — Req 1.1, 1.2, 1.3
+ *
+ * Comprehensive test covering all three lifecycle steps in sequence:
+ *   Step 1 (Req 1.1): Register ep → nstandbys==1, standbys[idx].ep==ep
+ *   Step 2 (Req 1.3): Re-register same ep → idempotent, nstandbys==1, same idx
+ *   Step 3 (Req 1.2): Deregister ep → nstandbys==0, standbys[idx].ep==NULL
+ * ============================================================ */
+static void test_repl_registration_lifecycle(void) {
+    replicator_t r;
+    int dummy_ep = 0;
+    repl_init(&r);
+
+    /* Step 1: Register ep — Req 1.1 */
+    int idx = repl_add_standby(&r, (void *)&dummy_ep, "sb1", 7001);
+    TEST_ASSERT_TRUE_MESSAGE(idx >= 0, "repl_add_standby must return a valid slot index");
+    TEST_ASSERT_EQUAL_INT_MESSAGE(1, r.nstandbys,
+        "nstandbys must be 1 after first registration (Req 1.1)");
+    TEST_ASSERT_EQUAL_PTR_MESSAGE((void *)&dummy_ep, r.standbys[idx].ep,
+        "standbys[idx].ep must equal registered ep (Req 1.1)");
+
+    /* Step 2: Re-register same ep — idempotent, Req 1.3 */
+    int idx2 = repl_add_standby(&r, (void *)&dummy_ep, "sb1", 7001);
+    TEST_ASSERT_TRUE_MESSAGE(idx2 >= 0, "duplicate repl_add_standby must return valid index");
+    TEST_ASSERT_EQUAL_INT_MESSAGE(1, r.nstandbys,
+        "nstandbys must remain 1 after duplicate registration (Req 1.3)");
+    TEST_ASSERT_EQUAL_INT_MESSAGE(idx, idx2,
+        "duplicate registration must return the same slot index (Req 1.3)");
+
+    /* Step 3: Deregister ep — Req 1.2 */
+    repl_remove_standby(&r, (void *)&dummy_ep);
+    TEST_ASSERT_EQUAL_INT_MESSAGE(0, r.nstandbys,
+        "nstandbys must be 0 after deregistration (Req 1.2)");
+    TEST_ASSERT_NULL_MESSAGE(r.standbys[idx].ep,
+        "standbys[idx].ep must be NULL after deregistration (Req 1.2)");
+
+    repl_fini(&r);
+    printf("test_repl_registration_lifecycle: PASS\n");
+}
+
 void test_repl_unit(void) {
     UNITY_BEGIN();
     RUN_TEST(test_repl_init_zeroes_nstandbys);
@@ -514,5 +554,7 @@ void test_repl_unit(void) {
     /* Task 4.1: WAL ACK reply handler — Req 4.3 */
     RUN_TEST(test_wal_ack_reply_updates_acked_lsn_and_signals);
     RUN_TEST(test_wal_nack_does_not_update_acked_lsn);
+    /* Task 5.3: registration lifecycle — Req 1.1, 1.2, 1.3 */
+    RUN_TEST(test_repl_registration_lifecycle);
     UNITY_END();
 }