@@ -120,15 +120,23 @@ constexpr CAmount GetMaxCollateralAmount() { return GetCollateralAmount() * 4; }
 // Promotion/demotion constants (post-V24 feature)
 constexpr int PROMOTION_RATIO = 10;   // 10 smaller denomination coins = 1 larger denomination coin
 constexpr int GAP_DIVISOR = 5;        // Deficit gap required to trigger promotion/demotion, as 1/N of the goal
+/**
+ * Smallest gap that stops a conversion from making the opposite conversion worthwhile. A
+ * conversion needs the receiving deficit to exceed the other by more than the gap, and it moves
+ * the two deficits by at most PROMOTION_RATIO and 1, so reversing it right away would need
+ * 2 * gap + 1 <= PROMOTION_RATIO. Any gap with 2 * gap + 1 > PROMOTION_RATIO rules that out.
+ */
+constexpr int MIN_GAP_THRESHOLD = (PROMOTION_RATIO + 1) / 2;
 
 /**
  * How far behind the other denomination a denomination has to be before converting is worth it;
  * the gap is what keeps promotion and demotion from oscillating. It is a fraction of the goal
  * rather than a constant because the largest gap two denominations can show is the goal itself:
  * a constant of 10 was unsatisfiable at MIN_COINJOIN_DENOMS_GOAL (also 10) and silently
- * disabled the feature there. At the default goal of 50 this still yields 10.
+ * disabled the feature there. At the default goal of 50 this still yields 10; below a goal of 25
+ * it is floored at MIN_GAP_THRESHOLD.
  */
-constexpr int GetGapThreshold(int nGoal) { return std::max(nGoal / GAP_DIVISOR, 1); }
+constexpr int GetGapThreshold(int nGoal) { return std::max(nGoal / GAP_DIVISOR, MIN_GAP_THRESHOLD); }
 
 /**
  * Which side(s) of the session denomination a participant occupies. A standard entry mixes at
 
@@ -19,6 +19,7 @@
 #include <script/script.h>
 #include <streams.h>
 #include <test/util/setup_common.h>
+#include <tinyformat.h>
 #include <uint256.h>
 #include <util/check.h>
 #include <util/time.h>
@@ -1368,6 +1369,28 @@ BOOST_AUTO_TEST_CASE(gap_threshold_reachable_at_every_goal)
     BOOST_CHECK_EQUAL(CoinJoin::GetGapThreshold(DEFAULT_COINJOIN_DENOMS_GOAL), 10);
 }
 
+BOOST_AUTO_TEST_CASE(rebalance_never_immediately_reverses)
+{
+    // A conversion must never leave the wallet in a state where the opposite conversion fires,
+    // otherwise the wallet keeps paying for 10:1 and 1:10 conversions that undo each other
+    // (with a gap of 2, 10 smaller and 7 larger coins at goal 10 would promote to 0/8 and then
+    // demote straight back).
+    // Counts above the goal are included so the deficit cap is covered as well.
+    const int ratio = CoinJoin::PROMOTION_RATIO;
+    for (int goal = MIN_COINJOIN_DENOMS_GOAL; goal <= 200; ++goal) {
+        for (int smaller = 0; smaller <= 2 * goal; ++smaller) {
+            for (int larger = 0; larger <= 2 * goal; ++larger) {
+                if (TestShouldPromote(smaller, larger, goal) && TestShouldDemote(larger + 1, smaller - ratio, goal)) {
+                    BOOST_ERROR(strprintf("goal %d: promoting %d/%d is undone by a demotion", goal, smaller, larger));
+                }
+                if (TestShouldDemote(larger, smaller, goal) && TestShouldPromote(smaller + ratio, larger - 1, goal)) {
+                    BOOST_ERROR(strprintf("goal %d: demoting %d/%d is undone by a promotion", goal, larger, smaller));
+                }
+            }
+        }
+    }
+}
+
 BOOST_AUTO_TEST_CASE(should_promote_small_gap_false)
 {
     const int goal = 100;