diff --git a/src/main/java/puzzle/Main.java b/src/main/java/puzzle/Main.java
index b0fe7f3..bd8718e 100644
--- a/src/main/java/puzzle/Main.java
+++ b/src/main/java/puzzle/Main.java
@@ -357,7 +357,7 @@ public class Main {
    static PuzzleResult _attempt(Rng rng, Dict dict, Opts opts) {
       TOTAL_ATTEMPTS.incrementAndGet();
       val stack  = new int[STACK_SIZE];
-      var swe    = new SwedishGenerator(rng, stack);
+      var swe    = new SwedishGenerator(rng, stack, Clues.createEmpty());
       var mask   = swe.generateMask(opts.pop, opts.gens, Math.max(opts.pop, (int) Math.floor(opts.pop * 1.5)));
       var filled = fillMask(rng, extractSlots(mask), mask.toGrid(), dict.index());
       
diff --git a/src/main/java/puzzle/SwedishGenerator.java b/src/main/java/puzzle/SwedishGenerator.java
index c80f57b..1291f2f 100644
--- a/src/main/java/puzzle/SwedishGenerator.java
+++ b/src/main/java/puzzle/SwedishGenerator.java
@@ -41,7 +41,7 @@ import static java.nio.charset.StandardCharsets.*;
  *   java SwedishGenerator [--seed N] [--pop N] [--gens N] [--tries N] [--words word-list.txt]
  */
 @SuppressWarnings("ALL")
-public record SwedishGenerator(Rng rng, int[] stack) {
+public record SwedishGenerator(Rng rng, int[] stack, Clues cache) {
    
    record CandidateInfo(int[] indices, int count) { }
    
@@ -283,6 +283,15 @@ public record SwedishGenerator(Rng rng, int[] stack) {
          for (var l = lo; l != X; l &= l - 1) processSlot(this, visitor, Long.numberOfTrailingZeros(l));
          for (var h = hi; h != X; h &= h - 1) processSlot(this, visitor, 64 | Long.numberOfTrailingZeros(h));
       }
+      public Clues from(Clues best) {
+         lo  = best.lo;
+         hi  = best.hi;
+         vlo = best.vlo;
+         vhi = best.vhi;
+         rlo = best.rlo;
+         rhi = best.rhi;
+         return this;
+      }
    }
    
    static record Grid(byte[] g, long lo, long hi) {
@@ -605,8 +614,7 @@ public record SwedishGenerator(Rng rng, int[] stack) {
       }
       return g;
    }
-   Clues mutate(Clues clues) {
-      var c     = clues.deepCopyGrid();
+   Clues mutate(Clues c) {
       int ri;
       var bytes = MUTATE_RI[rng.randint(0, SIZE_MIN_1)];
       for (var k = 0; k < 4; k++) {
@@ -666,6 +674,7 @@ public record SwedishGenerator(Rng rng, int[] stack) {
       var fails = 0;
       
       while (fails < limit) {
+         cache.from(best);
          var cand = mutate(best);
          var f    = maskFitness(cand);
          if (f < bestF) {
@@ -673,6 +682,7 @@ public record SwedishGenerator(Rng rng, int[] stack) {
             bestF = f;
             fails = 0;
          } else {
+            best.from(cache);
             fails++;
          }
       }
diff --git a/src/test/java/puzzle/ExportFormatTest.java b/src/test/java/puzzle/ExportFormatTest.java
index ba023d4..772c56c 100644
--- a/src/test/java/puzzle/ExportFormatTest.java
+++ b/src/test/java/puzzle/ExportFormatTest.java
@@ -14,7 +14,6 @@ import puzzle.SwedishGenerator.Rng;
 
 import java.io.IOException;
 import java.nio.file.Paths;
-import java.util.HashMap;
 
 import static org.junit.jupiter.api.Assertions.*;
 import static puzzle.SwedishGenerator.*;
@@ -28,7 +27,7 @@ public class ExportFormatTest {
    
    @Test
    void testExportFormatFromFilled() {
-      var swe = new SwedishGenerator(new Rng(0), new int[STACK_SIZE]);
+      var swe = new SwedishGenerator(new Rng(0), new int[STACK_SIZE], Clues.createEmpty());
       
       val clues = Clues.createEmpty();
       // Place a RIGHT clue at (0,0)
@@ -87,7 +86,7 @@ public class ExportFormatTest {
    
    @Test
    void testExportFormatEmpty() {
-      var swe          = new SwedishGenerator(new Rng(0), new int[STACK_SIZE]);
+      var swe          = new SwedishGenerator(new Rng(0), new int[STACK_SIZE], Clues.createEmpty());
       var grid         = Grid.createEmpty();
       val clues        = Clues.createEmpty();
       var fillResult   = new FillResult(true, new Gridded(grid), new long[300], new FillStats(0, 0, 0, 0));
diff --git a/src/test/java/puzzle/SwedishGeneratorTest.java b/src/test/java/puzzle/SwedishGeneratorTest.java
index 63bf94f..b594285 100644
--- a/src/test/java/puzzle/SwedishGeneratorTest.java
+++ b/src/test/java/puzzle/SwedishGeneratorTest.java
@@ -255,8 +255,7 @@ public class SwedishGeneratorTest {
       // This should detect a slot starting at 0,1 with length 2 (0,1 and 0,2)
       var clues = Clues.createEmpty();
       clues.setClue(OFF_0_0, CLUE_RIGHT);
-      var grid  = clues.toGrid();
-      
+      var grid = clues.toGrid();
       
       var slots = extractSlots(clues);
       assertEquals(1, slots.length);
@@ -269,7 +268,7 @@ public class SwedishGeneratorTest {
    
    @Test
    void testMaskFitnessBasic() {
-      var gen  = new SwedishGenerator(new Rng(0), new int[STACK_SIZE]);
+      var gen  = new SwedishGenerator(new Rng(0), new int[STACK_SIZE], Clues.createEmpty());
       var grid = Clues.createEmpty();
       // Empty grid should have high penalty (no slots)
       var f1 = gen.maskFitness(grid);
@@ -284,12 +283,12 @@ public class SwedishGeneratorTest {
    @Test
    void testGeneticAlgorithmComponents() {
       var rng = new Rng(42);
-      var gen = new SwedishGenerator(rng, new int[STACK_SIZE]);
+      var gen = new SwedishGenerator(rng, new int[STACK_SIZE], Clues.createEmpty());
       
       var g1 = gen.randomMask();
       assertNotNull(g1);
       
-      var g2 = gen.mutate(g1);
+      var g2 = gen.mutate(g1.deepCopyGrid());
       assertNotNull(g2);
       assertNotSame(g1, g2);
       
@@ -410,7 +409,7 @@ public class SwedishGeneratorTest {
    
    @Test
    void testMaskFitnessDetailed() {
-      var gen  = new SwedishGenerator(new Rng(42), new int[STACK_SIZE]);
+      var gen  = new SwedishGenerator(new Rng(42), new int[STACK_SIZE], Clues.createEmpty());
       var grid = Clues.createEmpty();
       // Empty grid: huge penalty
       var fitEmpty = gen.maskFitness(grid);