diff --git a/ClumsyPilot/ParkrobTrajplanner/PathSmoothing/Algorithms/PiecewiseQuinticSmoother.cs b/ClumsyPilot/ParkrobTrajplanner/PathSmoothing/Algorithms/PiecewiseQuinticSmoother.cs
index aa31a0b..5407a0f 100644
--- a/ClumsyPilot/ParkrobTrajplanner/PathSmoothing/Algorithms/PiecewiseQuinticSmoother.cs
+++ b/ClumsyPilot/ParkrobTrajplanner/PathSmoothing/Algorithms/PiecewiseQuinticSmoother.cs
@@ -11,6 +11,8 @@ namespace MultiWheelC.TrajectoryPlanning.PathSmoothing.Algorithms;
 internal sealed class PiecewiseQuinticSmoother : IPathSmoother
 {
     private const int SamplesPerInterval = 8;
+    private const double DoubleMachineEpsilon = 2.2204460492503131e-16d;
+    private const double EndpointNormalizationUlps = 32d;
 
     /// <inheritdoc />
     public SmoothingMethod Method => SmoothingMethod.PiecewiseQuintic;
@@ -237,15 +239,26 @@ internal sealed class PiecewiseQuinticSmoother : IPathSmoother
         }
         knots.Add(first);
 
-        double targetArcLength = startArcLength + knotSpacingMeters;
-        while (targetArcLength < endArcLength)
+        double endpointTolerance = GetEndpointNormalizationTolerance(
+            startArcLength,
+            endArcLength,
+            knotSpacingMeters);
+        double previousArcLength = startArcLength;
+        for (long knotOrdinal = 1L; ; knotOrdinal++)
         {
             cancellationToken.ThrowIfCancellationRequested();
+            double targetArcLength = startArcLength + knotOrdinal * knotSpacingMeters;
             if (!NumericGuard.IsFinite(targetArcLength))
             {
                 reason = "五次 Hermite 内部结点弧长无效。";
                 return false;
             }
+            if (targetArcLength >= endArcLength - endpointTolerance) break;
+            if (targetArcLength <= previousArcLength)
+            {
+                reason = "五次 Hermite 内部结点无法在浮点弧长尺度上保持递增。";
+                return false;
+            }
             if (!PathReferenceInterpolator.TryInterpolateByArcLength(
                     anchors,
                     targetArcLength,
@@ -257,7 +270,7 @@ internal sealed class PiecewiseQuinticSmoother : IPathSmoother
                 return false;
             }
             knots.Add(knot);
-            targetArcLength += knotSpacingMeters;
+            previousArcLength = targetArcLength;
         }
 
         if (!TryCreateKnot(anchors[anchors.Count - 1], direction, effectiveStrength, out Knot last))
@@ -279,6 +292,17 @@ internal sealed class PiecewiseQuinticSmoother : IPathSmoother
         return true;
     }
 
+    private static double GetEndpointNormalizationTolerance(
+        double startArcLength,
+        double endArcLength,
+        double knotSpacingMeters)
+    {
+        double magnitude = Math.Max(
+            Math.Abs(startArcLength),
+            Math.Max(Math.Abs(endArcLength), Math.Abs(knotSpacingMeters)));
+        return EndpointNormalizationUlps * DoubleMachineEpsilon * magnitude;
+    }
+
     private static bool TryCreateKnot(
         SmoothingPoint2D reference,
         TravelDirection direction,
diff --git a/ClumsyPilot/tests/verify_path_smoothing_quintic.ps1 b/ClumsyPilot/tests/verify_path_smoothing_quintic.ps1
index 59f027d..3a0f210 100644
--- a/ClumsyPilot/tests/verify_path_smoothing_quintic.ps1
+++ b/ClumsyPilot/tests/verify_path_smoothing_quintic.ps1
@@ -297,6 +297,31 @@ Assert-Equal 'Failed' (Get-PropertyValue $shortCandidate 'Status').ToString() 'A
 Assert-True (-not (Get-PropertyValue $shortCandidate 'Succeeded')) 'A degenerate short quintic segment must not be executable.'
 Assert-Equal 0 (Get-PropertyValue $shortCandidate 'Segments').Count 'A terminal quintic degeneracy must publish no geometry.'
 
+# Repeated decimal addition must not create a spurious 1e-16 m terminal residual when the
+# requested spacing divides the local length exactly. A real 0.005 m remainder remains below
+# the 0.01 m minimum and must still fail terminally.
+$decimalMultipleSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 1.0 0.0 1.0 0.0))
+$decimalMultipleCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $decimalMultipleSource)) 0.0 0.10 0.01
+Assert-Equal 'Success' (Get-PropertyValue $decimalMultipleCandidate 'Status').ToString() 'Decimal-exact knot multiples must not become a terminal under-minimum residual failure.'
+$decimalMultipleOutput = @(Get-PropertyValue $decimalMultipleCandidate 'Segments')[0].Points
+Assert-Equal 81 $decimalMultipleOutput.Count 'A 1.0 m segment with 0.1 m spacing must create exactly ten valid quintic intervals.'
+Assert-Near 1.0 $decimalMultipleOutput[$decimalMultipleOutput.Count - 1].ArcLength 0.0 'Decimal-multiple knot normalization must retain the exact terminal arc length.'
+
+$meaningfulShortResidualSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 1.005 0.0 1.005 0.0))
+$meaningfulShortResidualCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $meaningfulShortResidualSource)) 0.0 0.10 0.01
+Assert-Equal 'Failed' (Get-PropertyValue $meaningfulShortResidualCandidate 'Status').ToString() 'A genuine 0.005 m terminal residual must remain a terminal spacing failure.'
+Assert-Equal 0 (Get-PropertyValue $meaningfulShortResidualCandidate 'Segments').Count 'A genuine under-minimum residual must publish no geometry.'
+
+$smallScaleResidualSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 0.000000000000105 0.0 0.000000000000105 0.0))
+$smallScaleResidualCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $smallScaleResidualSource)) 0.0 0.00000000000001 0.000000000000006
+Assert-Equal 'Failed' (Get-PropertyValue $smallScaleResidualCandidate 'Status').ToString() 'Endpoint normalization tolerance must scale with local arc magnitude and preserve a genuine small residual failure.'
+
 # Local-arc reference mapping, rather than sample index/global distance, must reject this unsafe nonuniform path.
 $nonuniformUnsafeSource = @(
     (New-Point 0.0 0.0 0.0 0.0 0.50),