diff --git a/OpenNest.Core/CNC/CuttingPlanning/CuttingPlanCommit.cs b/OpenNest.Core/CNC/CuttingPlanning/CuttingPlanCommit.cs
index 3a99505..739e88d 100644
--- a/OpenNest.Core/CNC/CuttingPlanning/CuttingPlanCommit.cs
+++ b/OpenNest.Core/CNC/CuttingPlanning/CuttingPlanCommit.cs
@@ -8,7 +8,7 @@ using OpenNest.Geometry;
 namespace OpenNest.CNC.CuttingPlanning;
 
 /// <summary>An owned planned program for one unlocked part; ownership passes to the part on Apply.</summary>
-public sealed class PlannedPartProgram
+internal sealed class PlannedPartProgram
 {
     public PlannedPartProgram(Part part, Program program, CuttingParameters parameters)
     {
@@ -23,7 +23,7 @@ public sealed class PlannedPartProgram
 }
 
 /// <summary>The verified order and planned programs for one plate, bound to its captured state.</summary>
-public sealed class PlateCuttingPlan
+internal sealed class PlateCuttingPlan
 {
     public PlateCuttingPlan(PlateCuttingState expected, IEnumerable<Part> order,
         IEnumerable<PlannedPartProgram> programs = null)
@@ -75,10 +75,12 @@ public sealed class CuttingCommitResult
 /// Installs verified cutting plans for a whole scope at once. Nothing is searched, emitted,
 /// rotated or regenerated here: inputs are validated and checked for freshness, bounds are
 /// staged, then order and programs are installed synchronously and published once per plate.
+/// Internal: it checks root program references only, so payloads must be owned copies of
+/// independently replayed proposals. CuttingPlanService.Apply is the public entry point.
 /// </summary>
-public static class CuttingPlanCommit
+internal static class CuttingPlanCommit
 {
-    public static CuttingCommitResult Apply(IEnumerable<PlateCuttingPlan> plans, CancellationToken token = default) =>
+    internal static CuttingCommitResult Apply(IEnumerable<PlateCuttingPlan> plans, CancellationToken token = default) =>
         Apply(plans, token, null);
 
     // beforeInstall is a test seam that runs inside the install boundary, before each program.
@@ -113,6 +115,7 @@ public static class CuttingPlanCommit
 
         var staged = new List<Staged>(scope.Length);
         var targets = new HashSet<Part>(ReferenceEqualityComparer.Instance);
+        var members = new HashSet<Part>(ReferenceEqualityComparer.Instance);
         var installed = new HashSet<Program>(ReferenceEqualityComparer.Instance);
         var live = new HashSet<Program>(scope.SelectMany(p => p.Expected.Order).Select(p => p.Program),
             ReferenceEqualityComparer.Instance);
@@ -128,11 +131,14 @@ public static class CuttingPlanCommit
             {
                 return Invalid(plate, "The planned order is not exactly the plate's current parts.");
             }
-            var members = new HashSet<Part>(order, ReferenceEqualityComparer.Instance);
+            // A part on two plates would let one plate's install change the other's verified program.
+            if (order.Any(part => !members.Add(part)))
+                return Invalid(plate, "A part appears on more than one plate in the commit scope.");
+            var onPlate = new HashSet<Part>(order, ReferenceEqualityComparer.Instance);
             var programs = new List<(Part, Program, Box, CuttingParameters)>();
             foreach (var planned in plan.Programs)
             {
-                if (planned?.Part == null || !members.Contains(planned.Part) || !targets.Add(planned.Part))
+                if (planned?.Part == null || !onPlate.Contains(planned.Part) || !targets.Add(planned.Part))
                     return Invalid(plate, "Planned programs must target distinct parts of their own plate.");
                 if (planned.Part.LeadInsLocked)
                     return Invalid(plate, "A locked part's program is retained exactly and cannot be replaced.");
diff --git a/OpenNest.Tests/CuttingPlanning/CuttingPlanCommitTests.cs b/OpenNest.Tests/CuttingPlanning/CuttingPlanCommitTests.cs
index 4325318..46e4f0d 100644
--- a/OpenNest.Tests/CuttingPlanning/CuttingPlanCommitTests.cs
+++ b/OpenNest.Tests/CuttingPlanning/CuttingPlanCommitTests.cs
@@ -179,6 +179,49 @@ public class CuttingPlanCommitTests
         Assert.False(ProgramContent.Equal(With("X"), With("x")));
     }
 
+    [Fact]
+    public void Apply_PartSharedByTwoPlates_IsRefusedWithoutChange()
+    {
+        var parameters = ExplicitContourTests.Parameters();
+        var clean = LeadPathValidationTests.Rectangle(0, 0, 2, 2);
+        var prepared = PreparedContours.Capture(clean, parameters);
+        var shared = new Part(new Drawing("same", clean), new Vector(5, 5));
+        Assert.True(shared.RestoreLeadInProgram(prepared.Emit([prepared.ClosestEntry(0, new Vector(3, 1))]), false));
+        var nest = new Nest();
+        var regenerated = nest.CreatePlate();
+        var fixedPlate = nest.CreatePlate();
+        regenerated.Parts.Add(shared);
+        fixedPlate.Parts.Add(shared);
+        var results = new[]
+        {
+            CuttingPlanService.Plan(new CuttingPlanRequest(regenerated, confirmedParameters: parameters)),
+            CuttingPlanService.Plan(new CuttingPlanRequest(fixedPlate))
+        };
+        Assert.All(results, r => Assert.Equal(CuttingPlanStatus.Ready, r.Status));
+        Assert.True(results[0].ProposedOrder[0].IsRegenerated);
+        var program = shared.Program;
+        var states = new[] { regenerated, fixedPlate }.Select(p => PlateCuttingState.Capture(p)).ToArray();
+
+        var commit = CuttingPlanService.Apply(results);
+
+        // Installing through one plate would silently replace the other plate's fixed program.
+        Assert.Equal(CuttingCommitStatus.InvalidInput, commit.Status);
+        Assert.Same(program, shared.Program);
+        Assert.All(states, s => Assert.True(s.IsCurrent(), s.Difference()));
+    }
+
+    [Fact]
+    public void CommitInstaller_IsOnlyReachableThroughTheVerifiedService()
+    {
+        // The installer validates root references only; owned, verified payloads come from
+        // CuttingPlanService.Apply. Public access would accept nested aliases of live programs.
+        Assert.False(typeof(CuttingPlanCommit).IsPublic);
+        Assert.False(typeof(PlateCuttingPlan).IsPublic);
+        Assert.False(typeof(PlannedPartProgram).IsPublic);
+        Assert.True(typeof(CuttingPlanService).GetMethod(nameof(CuttingPlanService.Apply),
+            [typeof(IEnumerable<CuttingPlanResult>), typeof(CancellationToken)])!.IsPublic);
+    }
+
     [Fact]
     public void Apply_LaterPlateStale_AppliesNothingAnywhere()
     {