diff --git a/OpenNest.Engine/NestEngineBase.cs b/OpenNest.Engine/NestEngineBase.cs
index 29c8eb6..c8d81b1 100644
--- a/OpenNest.Engine/NestEngineBase.cs
+++ b/OpenNest.Engine/NestEngineBase.cs
@@ -177,7 +177,7 @@ namespace OpenNest
 
         // --- Protected utilities ---
 
-        protected static void ReportProgress(
+        internal static void ReportProgress(
             IProgress<NestProgress> progress,
             NestPhase phase,
             int plateNumber,
diff --git a/OpenNest.Engine/PairFiller.cs b/OpenNest.Engine/PairFiller.cs
new file mode 100644
index 0000000..b448c96
--- /dev/null
+++ b/OpenNest.Engine/PairFiller.cs
@@ -0,0 +1,126 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading;
+using OpenNest.Engine.BestFit;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest
+{
+    /// <summary>
+    /// Fills a work area using interlocking part pairs from BestFitCache.
+    /// Extracted from DefaultNestEngine.FillWithPairs.
+    /// </summary>
+    public class PairFiller
+    {
+        private readonly Size plateSize;
+        private readonly double partSpacing;
+
+        public PairFiller(Size plateSize, double partSpacing)
+        {
+            this.plateSize = plateSize;
+            this.partSpacing = partSpacing;
+        }
+
+        public List<Part> Fill(NestItem item, Box workArea,
+            int plateNumber = 0,
+            CancellationToken token = default,
+            IProgress<NestProgress> progress = null)
+        {
+            var bestFits = BestFitCache.GetOrCompute(
+                item.Drawing, plateSize.Width, plateSize.Length, partSpacing);
+
+            var candidates = SelectPairCandidates(bestFits, workArea);
+            Debug.WriteLine($"[PairFiller] Total: {bestFits.Count}, Kept: {bestFits.Count(r => r.Keep)}, Trying: {candidates.Count}");
+            Debug.WriteLine($"[PairFiller] Plate: {plateSize.Width:F2}x{plateSize.Length:F2}, WorkArea: {workArea.Width:F2}x{workArea.Length:F2}");
+
+            List<Part> best = null;
+            var bestScore = default(FillScore);
+            var sinceImproved = 0;
+
+            try
+            {
+                for (var i = 0; i < candidates.Count; i++)
+                {
+                    token.ThrowIfCancellationRequested();
+
+                    var result = candidates[i];
+                    var pairParts = result.BuildParts(item.Drawing);
+                    var angles = result.HullAngles;
+                    var engine = new FillLinear(workArea, partSpacing);
+                    var filled = DefaultNestEngine.FillPattern(engine, pairParts, angles, workArea);
+
+                    if (filled != null && filled.Count > 0)
+                    {
+                        var score = FillScore.Compute(filled, workArea);
+                        if (best == null || score > bestScore)
+                        {
+                            best = filled;
+                            bestScore = score;
+                            sinceImproved = 0;
+                        }
+                        else
+                        {
+                            sinceImproved++;
+                        }
+                    }
+                    else
+                    {
+                        sinceImproved++;
+                    }
+
+                    NestEngineBase.ReportProgress(progress, NestPhase.Pairs, plateNumber, best, workArea,
+                        $"Pairs: {i + 1}/{candidates.Count} candidates, best = {bestScore.Count} parts");
+
+                    // Early exit: stop if we've tried enough candidates without improvement.
+                    if (i >= 9 && sinceImproved >= 10)
+                    {
+                        Debug.WriteLine($"[PairFiller] Early exit at {i + 1}/{candidates.Count} — no improvement in last {sinceImproved} candidates");
+                        break;
+                    }
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                Debug.WriteLine("[PairFiller] Cancelled mid-phase, using results so far");
+            }
+
+            Debug.WriteLine($"[PairFiller] Best pair result: {bestScore.Count} parts, density={bestScore.Density:P1}");
+            return best ?? new List<Part>();
+        }
+
+        private List<BestFitResult> SelectPairCandidates(List<BestFitResult> bestFits, Box workArea)
+        {
+            var kept = bestFits.Where(r => r.Keep).ToList();
+            var top = kept.Take(50).ToList();
+
+            var workShortSide = System.Math.Min(workArea.Width, workArea.Length);
+            var plateShortSide = System.Math.Min(plateSize.Width, plateSize.Length);
+
+            if (workShortSide < plateShortSide * 0.5)
+            {
+                var stripCandidates = bestFits
+                    .Where(r => r.ShortestSide <= workShortSide + Tolerance.Epsilon
+                             && r.Utilization >= 0.3)
+                    .OrderByDescending(r => r.Utilization);
+
+                var existing = new HashSet<BestFitResult>(top);
+
+                foreach (var r in stripCandidates)
+                {
+                    if (top.Count >= 100)
+                        break;
+
+                    if (existing.Add(r))
+                        top.Add(r);
+                }
+
+                Debug.WriteLine($"[PairFiller] Strip mode: {top.Count} candidates (shortSide <= {workShortSide:F1})");
+            }
+
+            return top;
+        }
+    }
+}
diff --git a/OpenNest.Tests/PairFillerTests.cs b/OpenNest.Tests/PairFillerTests.cs
new file mode 100644
index 0000000..d2199b7
--- /dev/null
+++ b/OpenNest.Tests/PairFillerTests.cs
@@ -0,0 +1,63 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class PairFillerTests
+{
+    private static Drawing MakeRectDrawing(double w, double h)
+    {
+        var pgm = new OpenNest.CNC.Program();
+        pgm.Codes.Add(new OpenNest.CNC.RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(w, 0)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(w, h)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, h)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, 0)));
+        return new Drawing("rect", pgm);
+    }
+
+    [Fact]
+    public void Fill_ReturnsPartsForSimpleDrawing()
+    {
+        var plateSize = new Size(120, 60);
+        var filler = new PairFiller(plateSize, 0.5);
+        var item = new NestItem { Drawing = MakeRectDrawing(20, 10) };
+        var workArea = new Box(0, 0, 120, 60);
+
+        var parts = filler.Fill(item, workArea);
+
+        Assert.NotNull(parts);
+        // Pair filling may or may not find interlocking pairs for rectangles,
+        // but should return a non-null list.
+    }
+
+    [Fact]
+    public void Fill_EmptyResult_WhenPartTooLarge()
+    {
+        var plateSize = new Size(10, 10);
+        var filler = new PairFiller(plateSize, 0.5);
+        var item = new NestItem { Drawing = MakeRectDrawing(20, 20) };
+        var workArea = new Box(0, 0, 10, 10);
+
+        var parts = filler.Fill(item, workArea);
+
+        Assert.NotNull(parts);
+        Assert.Empty(parts);
+    }
+
+    [Fact]
+    public void Fill_RespectsCancellation()
+    {
+        var cts = new System.Threading.CancellationTokenSource();
+        cts.Cancel();
+
+        var plateSize = new Size(120, 60);
+        var filler = new PairFiller(plateSize, 0.5);
+        var item = new NestItem { Drawing = MakeRectDrawing(20, 10) };
+        var workArea = new Box(0, 0, 120, 60);
+
+        var parts = filler.Fill(item, workArea, token: cts.Token);
+
+        // Should return empty or partial — not throw
+        Assert.NotNull(parts);
+    }
+}