diff --git a/.kiro/specs/ha-repl-hardening/tasks.md b/.kiro/specs/ha-repl-hardening/tasks.md
index d3107ea3..fa650c69 100644
--- a/.kiro/specs/ha-repl-hardening/tasks.md
+++ b/.kiro/specs/ha-repl-hardening/tasks.md
@@ -155,7 +155,7 @@
   - _Requirements: 2.3_
   - _Boundary: Controller FSM (ha.c)_
 
-- [ ] 4.2 Replace stale-ctrlhello truncation with vote-derived safe commit point
+- [x] 4.2 Replace stale-ctrlhello truncation with vote-derived safe commit point
   - Remove `persist_truncate_wal_above(last_known_committed_lsn)` from the become-leader path
   - Replace with `persist_truncate_wal_above(safe_committed_lsn)` using the value from task 4.1
   - Set `committed_lsn = safe_committed_lsn` after truncation
diff --git a/src/controller/ha.c b/src/controller/ha.c
index 5a1b4f8c..274d06ab 100644
--- a/src/controller/ha.c
+++ b/src/controller/ha.c
@@ -468,6 +468,17 @@ static void raft_cb_become_leader(void *ctx, uint64_t term)
             if (c) {
                 persist_truncate_wal_above(&c->persist, safe_lsn);
             }
+            /* Task 4.2 (ha-repl-hardening): set committed_lsn = safe_lsn.
+             * persist_truncate_wal_above already sets c->persist.committed_lsn
+             * in the production path (c != NULL).  Also update
+             * ha->last_known_committed_lsn so the test-mode decision loop
+             * (XCE_TESTING path in standby heartbeat reply handler) and any
+             * code reading ha->last_known_committed_lsn see the safe commit
+             * point immediately after promotion.
+             * Design §Flow 0 step 5: committed_lsn = safe_committed_lsn.
+             * Req 2.3 (ha-repl-hardening). */
+            atomic_store_explicit(&ha->last_known_committed_lsn, safe_lsn,
+                                  memory_order_release);
             /* Update committed config tag baseline for the COMMITTING decision loop */
             atomic_store_explicit(&ha->committed_config_tag, safe_tag,
                                   memory_order_release);
diff --git a/src/controller/testsuite/test_ha_evaluate_committing.c b/src/controller/testsuite/test_ha_evaluate_committing.c
index daad35e7..f8af96f2 100644
--- a/src/controller/testsuite/test_ha_evaluate_committing.c
+++ b/src/controller/testsuite/test_ha_evaluate_committing.c
@@ -608,6 +608,110 @@ static void test_safe_committed_lsn_one_peer_denied(void)
 #endif
 }
 
+/* ── Task 4.2 (ha-repl-hardening): committed_lsn set to safe_committed_lsn ──
+ *
+ * raft_cb_become_leader must set ha->last_known_committed_lsn to the value
+ * returned by ha_compute_safe_committed_lsn after WAL truncation.
+ *
+ * This ensures that in test mode (c == NULL) the decision loop reads the
+ * correct committed_lsn from ha->last_known_committed_lsn (see decision-loop
+ * XCE_TESTING path in ha.c).
+ *
+ * Requirement 2.3 (ha-repl-hardening): safe_committed_lsn is derived from
+ * voter replies; entries at or below it survive truncation; entries above are
+ * removed.
+ *
+ * The test uses raft_cb_become_leader_for_test (XCE_TESTING shim) which calls
+ * raft_cb_become_leader with ha->raft_c = NULL (no real cluster, no WAL I/O).
+ * The WAL-level truncation behaviour (file entries above safe_lsn deleted) is
+ * covered by test_persist_truncate.c; this test focuses on the committed_lsn
+ * field assignment that task 4.2 adds.
+ *
+ * Design ref: design.md §Flow 0 step 5: committed_lsn = safe_committed_lsn
+ */
+
+#ifdef XCE_TESTING
+extern void raft_cb_become_leader_for_test(ha_state_t *ha, uint64_t term);
+#endif
+
+/*
+ * Test E: raft_cb_become_leader sets ha->last_known_committed_lsn = safe_lsn.
+ *
+ * 3-node cluster (n_other=2, votes_granted=3):
+ *   votes[0]: my_lsn=200, peer_lsn=100, vote_granted=1
+ *   votes[1]: peer_lsn=150, vote_granted=1
+ *
+ * Sorted granted LSNs desc: [200 (self), 150 (peer1), 100 (peer0)]
+ * Majority threshold for 3 granted = ceil(3/2) = 2; element at index 1 = 150
+ * → safe_committed_lsn = 150
+ *
+ * After raft_cb_become_leader_for_test, ha->last_known_committed_lsn must == 150.
+ *
+ * Additionally verify via ha_compute_safe_committed_lsn that entry AT safe_lsn
+ * (lsn=150) is the boundary: safe_lsn=150 means truncate ABOVE 150 (entries
+ * 151+ removed); entry at 150 is preserved.
+ */
+static void test_become_leader_sets_committed_lsn_to_safe_lsn(void)
+{
+#ifdef XCE_TESTING
+    reset_counters();
+
+    ha_state_t ha;
+    ha_init(&ha, &g_cfg, 0);
+    ha.phase2_done = 1;  /* skip phase2 in this test */
+
+    /* Build elect_ctx with 2 peers; safe_lsn will be 150 */
+    ha_elect_ctx_t *ctx = calloc(1, sizeof(*ctx) + 2 * sizeof(ctx->votes[0]));
+    TEST_ASSERT_NOT_NULL(ctx);
+    ctx->n_peers       = 2;
+    ctx->votes_granted = 3;   /* self + 2 peers */
+    ctx->n_replied     = 2;
+    pthread_mutex_init(&ctx->lock, NULL);
+    sem_init(&ctx->done, 0, 0);
+
+    /* votes[0]: self data (my_lsn=200) + peer0 granted at lsn=100 */
+    ctx->votes[0].my_lsn      = 200;
+    ctx->votes[0].my_tag      = 5;
+    ctx->votes[0].vote_granted = 1;
+    ctx->votes[0].peer_lsn    = 100;
+    ctx->votes[0].peer_tag    = 5;
+
+    /* votes[1]: peer1 granted at lsn=150 */
+    ctx->votes[1].vote_granted = 1;
+    ctx->votes[1].peer_lsn    = 150;
+    ctx->votes[1].peer_tag    = 5;
+
+    /* Verify the safe_lsn computation independently */
+    uint64_t safe_lsn = 0, safe_tag = 0;
+    ha_compute_safe_committed_lsn(ctx, &safe_lsn, &safe_tag);
+    TEST_ASSERT_EQUAL_UINT64_MESSAGE(150, safe_lsn,
+        "ha_compute_safe_committed_lsn: voters [100,150]+self=200 → safe_lsn must be 150");
+
+    /* Wire elect_ctx into ha_state so raft_cb_become_leader can read it */
+    atomic_store_explicit(&ha.elect_ctx, (void *)ctx, memory_order_release);
+
+    /* Drive the leader promotion path (c=NULL → no WAL I/O, no phase2) */
+    raft_cb_become_leader_for_test(&ha, /*term=*/1);
+
+    /* Task 4.2: ha->last_known_committed_lsn must equal safe_lsn (150) */
+    uint64_t committed_lsn =
+        atomic_load_explicit(&ha.last_known_committed_lsn, memory_order_acquire);
+    TEST_ASSERT_EQUAL_UINT64_MESSAGE(150, committed_lsn,
+        "raft_cb_become_leader must set ha->last_known_committed_lsn = safe_lsn (150); "
+        "WAL entry AT safe_lsn=150 is preserved; entries above 150 are truncated");
+
+    sem_destroy(&ctx->done);
+    pthread_mutex_destroy(&ctx->lock);
+    free(ctx);
+    ha_fini(&ha);
+    clear_hook();
+    printf("test_become_leader_sets_committed_lsn_to_safe_lsn: PASS "
+           "(committed_lsn=%" PRIu64 ")\n", committed_lsn);
+#else
+    printf("test_become_leader_sets_committed_lsn_to_safe_lsn: SKIP (non-test build)\n");
+#endif
+}
+
 /* ── Suite entry point ───────────────────────────────────────────────────── */
 
 void test_ha_evaluate_committing(void)
@@ -627,4 +731,6 @@ void test_ha_evaluate_committing(void)
     RUN_TEST(test_safe_committed_lsn_two_peers_at_50_and_51);
     RUN_TEST(test_safe_committed_lsn_single_node);
     RUN_TEST(test_safe_committed_lsn_one_peer_denied);
+    /* Task 4.2 (ha-repl-hardening): committed_lsn set to safe_committed_lsn */
+    RUN_TEST(test_become_leader_sets_committed_lsn_to_safe_lsn);
 }