diff --git a/src/controller/ha.c b/src/controller/ha.c
index bff52132..82f88679 100644
--- a/src/controller/ha.c
+++ b/src/controller/ha.c
@@ -6739,6 +6739,34 @@ void ha_on_primary_ctrl_fd_rdhup(ha_state_t *ha, cluster_t *c)
 {
     if (!ha) return;
 
+    /* raft-ha-core-safety task 5.6: do NOT restart failover while an election is
+     * already in flight.  A won PreVote enters the real election (CTRL_ELECTING),
+     * or a PreVote round may be mid-decision (pre_vote_in_progress).  The dead
+     * primary's control fd keeps RDHUP'ing on a deterministic ~15s cadence — if
+     * each RDHUP re-triggers a fresh ha_begin_failover it PREEMPTS the in-progress
+     * real election before it can complete, bumping the term every cycle and
+     * keeping two surviving candidates dueling in lockstep (split vote, unbounded
+     * term escalation; research.md §"Task 5.5 fix VERIFIED").  Drop the RDHUP:
+     * close the dead control fd and let the in-progress round finish or time out —
+     * its election timeout is jittered, which desynchronizes the duel so one
+     * survivor wins.  A node NOT currently electing falls through to the normal
+     * 5.2 suppress / genuine-loss failover path below. */
+    {
+        ctrl_role_t cur = (ctrl_role_t)atomic_load_explicit(&ha->role,
+                                                            memory_order_acquire);
+        if (cur == CTRL_ELECTING || ha->raft.pre_vote_in_progress) {
+            if (ha->primary_ctrl_fd >= 0) {
+                close(ha->primary_ctrl_fd);
+                ha->primary_ctrl_fd = -1;
+            }
+            INFO("HA: primary_ctrl_fd RDHUP but an election is already in progress "
+                 "(role=%d, prevote=%d) — dropping the RDHUP, not restarting "
+                 "failover (task 5.6: don't preempt the in-progress election / "
+                 "churn the term)", (int)cur, ha->raft.pre_vote_in_progress);
+            return;
+        }
+    }
+
     /* Cancel the liveness poll task first (Req 5.4): prevents a concurrent
      * poll fn from triggering a second failover call after EPOLLRDHUP fires. */
     ha_disarm_heartbeat_timeout_vtask(ha);
diff --git a/src/controller/testsuite/test_ha_core_safety.c b/src/controller/testsuite/test_ha_core_safety.c
index 84d7e8bd..711c6c51 100644
--- a/src/controller/testsuite/test_ha_core_safety.c
+++ b/src/controller/testsuite/test_ha_core_safety.c
@@ -1738,6 +1738,79 @@ static void test_5_3_stale_candidate_denied_even_with_primary_gone(void)
     printf("test_5_3_stale_candidate_denied_even_with_primary_gone: PASS (pin)\n");
 }
 
+/* ═══════════════════════════════════════════════════════════════════════════
+ * TASK 5.6 — an RDHUP must not restart failover while an election is in progress
+ *
+ * Surfaced after 5.5 (research.md §"Task 5.5 fix VERIFIED"): with the freshest
+ * survivor now able to win a PreVote and ENTER the real election (CTRL_ELECTING),
+ * round 4 still stalls — the dead primary's control channel keeps RDHUP'ing on a
+ * deterministic ~15s cadence, and each RDHUP re-triggers ha_begin_failover → a
+ * fresh PreVote → real election → term++.  That preempts the in-progress real
+ * election before it can complete, churns the term unbounded (21→25→28→29…), and
+ * keeps two surviving candidates dueling in lockstep so the jittered election
+ * timeout never breaks the split vote.
+ *
+ *   (CORE RED) test_5_6_rdhup_does_not_restart_election_in_progress: a node that
+ *       already entered the real election (CTRL_ELECTING) must DROP an RDHUP —
+ *       not start a new PreVote failover.  FAILs on the baseline (the RDHUP path's
+ *       5.2 suppress only catches CTRL_STANDBY, so an ELECTING node falls through
+ *       to ha_begin_failover and restarts the round).
+ *
+ *   (PIN) test_5_6_genuine_loss_still_fails_over_when_not_electing: a CTRL_STANDBY
+ *       node with a genuinely-dead primary and NO election in progress must STILL
+ *       launch failover — the 5.6 guard only suppresses while already electing.
+ *
+ * Requirements: 4.1, 4.3, 6.1, 6.2, 6.3.
+ * ═══════════════════════════════════════════════════════════════════════════ */
+
+/* CORE RED — an RDHUP while CTRL_ELECTING must not restart failover. */
+static void test_5_6_rdhup_does_not_restart_election_in_progress(void)
+{
+    ha_state_t ha;
+    hcs5_make_standby(&ha);                 /* 3-node */
+    atomic_store_explicit(&ha.term, 6, memory_order_release);
+
+    /* The node already WON a PreVote and entered the real election. */
+    atomic_store_explicit(&ha.role, (int32_t)CTRL_ELECTING, memory_order_release);
+    ha.raft.pre_vote_in_progress = 0;
+    hcs5_mark_primary_dead(&ha);            /* primary genuinely gone (no 5.2 suppress) */
+
+    /* The dead primary's control channel RDHUPs (the ~15s cadence). */
+    ha_on_primary_ctrl_fd_rdhup(&ha, NULL);
+
+    TEST_ASSERT_EQUAL_INT_MESSAGE(0, ha.raft.pre_vote_in_progress,
+        "RED: an RDHUP while an election is already in progress must NOT start a "
+        "new PreVote failover — that preempts the in-progress real election and "
+        "churns the term, keeping two survivors dueling (task 5.6). Baseline's "
+        "5.2 suppress only catches CTRL_STANDBY, so an ELECTING node restarts.");
+    TEST_ASSERT_EQUAL_INT_MESSAGE((int32_t)CTRL_ELECTING,
+        (int32_t)atomic_load_explicit(&ha.role, memory_order_acquire),
+        "the in-progress election (CTRL_ELECTING) must be preserved across the RDHUP");
+
+    ha_fini(&ha);
+    printf("test_5_6_rdhup_does_not_restart_election_in_progress: done\n");
+}
+
+/* PIN — genuine loss with NO election in progress must still fail over. */
+static void test_5_6_genuine_loss_still_fails_over_when_not_electing(void)
+{
+    ha_state_t ha;
+    hcs5_make_standby(&ha);                 /* CTRL_STANDBY, 3-node */
+    atomic_store_explicit(&ha.term, 6, memory_order_release);
+    ha.raft.pre_vote_in_progress = 0;
+    hcs5_mark_primary_dead(&ha);            /* primary genuinely gone */
+
+    ha_on_primary_ctrl_fd_rdhup(&ha, NULL);
+
+    TEST_ASSERT_EQUAL_INT_MESSAGE(1, ha.raft.pre_vote_in_progress,
+        "PIN: a CTRL_STANDBY node with a dead primary and NO election in progress "
+        "must STILL launch a real failover — the 5.6 guard only suppresses an "
+        "RDHUP while ALREADY electing, never genuine loss (no over-suppression)");
+
+    ha_fini(&ha);
+    printf("test_5_6_genuine_loss_still_fails_over_when_not_electing: PASS (pin)\n");
+}
+
 /* ═══════════════════════════════════════════════════════════════════════════
  * SCAFFOLDING (PASS) — prove each shared seam is usable by later phases.
  * ═══════════════════════════════════════════════════════════════════════════ */
@@ -1903,6 +1976,12 @@ void test_ha_core_safety(void)
     RUN_TEST(test_5_3_catchup_survivor_grants_fresh_candidate_when_primary_gone);
     RUN_TEST(test_5_3_catchup_denies_while_live_primary_feeds);
     RUN_TEST(test_5_3_stale_candidate_denied_even_with_primary_gone);
+    /* Task 5.6: an RDHUP must not restart failover while an election is in
+     * progress (dueling-candidate / term-escalation stall):
+     *   CORE RED — CTRL_ELECTING + RDHUP must NOT start a new PreVote.
+     *   PIN      — genuine loss (STANDBY, not electing) still fails over. */
+    RUN_TEST(test_5_6_rdhup_does_not_restart_election_in_progress);
+    RUN_TEST(test_5_6_genuine_loss_still_fails_over_when_not_electing);
     /* Scaffolding: each shared seam is usable by 2.3 / 4.1 / 5.1. */
     RUN_TEST(test_seam_wal_construct_and_probe);
     RUN_TEST(test_seam_divergence_is_observable);