diff --git a/OpenNest.Engine.Tests/Jobs/PlateFillerContractTests.cs b/OpenNest.Engine.Tests/Jobs/PlateFillerContractTests.cs
new file mode 100644
index 0000000..a93f944
--- /dev/null
+++ b/OpenNest.Engine.Tests/Jobs/PlateFillerContractTests.cs
@@ -0,0 +1,75 @@
+using OpenNest.Engine.Jobs.Placement.Fillers;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Tests.Jobs;
+
+public class PlateFillerContractTests
+{
+    [Fact]
+    public void NestProgressReporter_Report_ClonesPartsAndPreservesReportFields()
+    {
+        var source = new Part(
+            new Drawing("part", TestDrawingFactory.Rectangle(10, 20)),
+            new Vector(3, 5)
+        );
+        source.Rotate(0.5);
+        var progress = new CapturingProgress();
+        var workArea = new Box(1, 2, 30, 40);
+
+        NestProgressReporter.Report(
+            progress,
+            new ProgressReport
+            {
+                Phase = NestPhase.Pairs,
+                PlateNumber = 3,
+                Parts = new List<Part> { source },
+                WorkArea = workArea,
+                Description = "candidate preview",
+                IsOverallBest = true,
+            }
+        );
+
+        var reported = Assert.Single(progress.Reports);
+        Assert.Equal(NestPhase.Pairs, reported.Phase);
+        Assert.Equal(3, reported.PlateNumber);
+        Assert.Equal("candidate preview", reported.Description);
+        Assert.True(reported.IsOverallBest);
+        Assert.Equal(workArea.X, reported.ActiveWorkArea.X);
+        Assert.Equal(workArea.Y, reported.ActiveWorkArea.Y);
+        Assert.Equal(workArea.Width, reported.ActiveWorkArea.Width);
+        Assert.Equal(workArea.Length, reported.ActiveWorkArea.Length);
+
+        var preview = Assert.Single(reported.BestParts);
+        Assert.NotSame(source, preview);
+        Assert.Same(source.BaseDrawing, preview.BaseDrawing);
+        Assert.Equal(source.Location.X, preview.Location.X);
+        Assert.Equal(source.Location.Y, preview.Location.Y);
+        Assert.Equal(source.Rotation, preview.Rotation);
+    }
+
+    [Fact]
+    public void NestProgressReporter_Report_DoesNotForwardMissingProgressOrParts()
+    {
+        var progress = new CapturingProgress();
+        var part = new Part(new Drawing("part", TestDrawingFactory.Rectangle()));
+
+        NestProgressReporter.Report(
+            null,
+            new ProgressReport { Parts = new List<Part> { part } }
+        );
+        NestProgressReporter.Report(progress, new ProgressReport { Parts = null });
+        NestProgressReporter.Report(progress, new ProgressReport { Parts = new List<Part>() });
+
+        Assert.Empty(progress.Reports);
+    }
+
+    private sealed class CapturingProgress : IProgress<NestProgress>
+    {
+        public List<NestProgress> Reports { get; } = new();
+
+        public void Report(NestProgress value)
+        {
+            Reports.Add(value);
+        }
+    }
+}
diff --git a/OpenNest.Engine/Fill/IterativeShrinkFiller.cs b/OpenNest.Engine/Fill/IterativeShrinkFiller.cs
index ee8a267..9918e58 100644
--- a/OpenNest.Engine/Fill/IterativeShrinkFiller.cs
+++ b/OpenNest.Engine/Fill/IterativeShrinkFiller.cs
@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using OpenNest.Engine.Jobs.Placement.Fillers;
 using OpenNest.Geometry;
 
 namespace OpenNest.Engine.Fill
@@ -136,7 +137,7 @@ namespace OpenNest.Engine.Fill
                     var allParts = new List<Part>(placedSoFar.Count + best.Count);
                     allParts.AddRange(placedSoFar);
                     allParts.AddRange(best);
-                    NestEngineBase.ReportProgress(
+                    NestProgressReporter.Report(
                         progress,
                         new ProgressReport
                         {
diff --git a/OpenNest.Engine/Fill/ShrinkFiller.cs b/OpenNest.Engine/Fill/ShrinkFiller.cs
index 30e91d4..0e1d018 100644
--- a/OpenNest.Engine/Fill/ShrinkFiller.cs
+++ b/OpenNest.Engine/Fill/ShrinkFiller.cs
@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using OpenNest.Geometry;
+using OpenNest.Engine.Jobs.Placement.Fillers;
 using OpenNest.Engine.RectanglePacking;
 
 namespace OpenNest.Engine.Fill
@@ -89,7 +90,7 @@ namespace OpenNest.Engine.Fill
 
             var desc = $"Shrink {axis}: {bestParts.Count} parts, dim={dim:F1}";
 
-            NestEngineBase.ReportProgress(
+            NestProgressReporter.Report(
                 progress,
                 new ProgressReport
                 {
diff --git a/OpenNest.Engine/Jobs/Placement/Fillers/NestProgressReporter.cs b/OpenNest.Engine/Jobs/Placement/Fillers/NestProgressReporter.cs
new file mode 100644
index 0000000..3be10c5
--- /dev/null
+++ b/OpenNest.Engine/Jobs/Placement/Fillers/NestProgressReporter.cs
@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace OpenNest.Engine.Jobs.Placement.Fillers;
+
+internal static class NestProgressReporter
+{
+    internal static void Report(IProgress<NestProgress> progress, ProgressReport report)
+    {
+        if (progress == null || report.Parts == null || report.Parts.Count == 0)
+            return;
+
+        var clonedParts = new List<Part>(report.Parts.Count);
+        foreach (var part in report.Parts)
+            clonedParts.Add((Part)part.Clone());
+
+        Debug.WriteLine(
+            $"[Progress] Phase={report.Phase}, Plate={report.PlateNumber}, "
+                + $"Parts={clonedParts.Count} | {report.Description}"
+        );
+
+        progress.Report(
+            new NestProgress
+            {
+                Phase = report.Phase,
+                PlateNumber = report.PlateNumber,
+                BestParts = clonedParts,
+                Description = report.Description,
+                ActiveWorkArea = report.WorkArea,
+                IsOverallBest = report.IsOverallBest,
+            }
+        );
+    }
+}
diff --git a/OpenNest.Engine/NestEngineBase.cs b/OpenNest.Engine/NestEngineBase.cs
index 101992b..ef0c53f 100644
--- a/OpenNest.Engine/NestEngineBase.cs
+++ b/OpenNest.Engine/NestEngineBase.cs
@@ -6,6 +6,7 @@ using System.Threading;
 using OpenNest.Engine;
 using OpenNest.Engine.BestFit;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Jobs.Placement.Fillers;
 using OpenNest.Engine.Strategies;
 using OpenNest.Geometry;
 using OpenNest.Math;
@@ -261,29 +262,7 @@ namespace OpenNest.Engine
 
         internal static void ReportProgress(IProgress<NestProgress> progress, ProgressReport report)
         {
-            if (progress == null || report.Parts == null || report.Parts.Count == 0)
-                return;
-
-            var clonedParts = new List<Part>(report.Parts.Count);
-            foreach (var part in report.Parts)
-                clonedParts.Add((Part)part.Clone());
-
-            Debug.WriteLine(
-                $"[Progress] Phase={report.Phase}, Plate={report.PlateNumber}, "
-                    + $"Parts={clonedParts.Count} | {report.Description}"
-            );
-
-            progress.Report(
-                new NestProgress
-                {
-                    Phase = report.Phase,
-                    PlateNumber = report.PlateNumber,
-                    BestParts = clonedParts,
-                    Description = report.Description,
-                    ActiveWorkArea = report.WorkArea,
-                    IsOverallBest = report.IsOverallBest,
-                }
-            );
+            NestProgressReporter.Report(progress, report);
         }
 
         protected string BuildProgressSummary()
diff --git a/OpenNest.Engine/Strategies/FillContext.cs b/OpenNest.Engine/Strategies/FillContext.cs
index ab56dac..3e4c0b2 100644
--- a/OpenNest.Engine/Strategies/FillContext.cs
+++ b/OpenNest.Engine/Strategies/FillContext.cs
@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Threading;
 using OpenNest.Engine;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Jobs.Placement.Fillers;
 using OpenNest.Geometry;
 
 namespace OpenNest.Engine.Strategies
@@ -50,7 +51,7 @@ namespace OpenNest.Engine.Strategies
                 WinnerPhase = ActivePhase;
             }
 
-            NestEngineBase.ReportProgress(
+            NestProgressReporter.Report(
                 Progress,
                 new ProgressReport
                 {