diff --git a/SAM/SAM.Geometry/Geometry/Planar/Classes/Triangle2D.cs b/SAM/SAM.Geometry/Geometry/Planar/Classes/Triangle2D.cs
index 52efba6a1..4537c098a 100644
--- a/SAM/SAM.Geometry/Geometry/Planar/Classes/Triangle2D.cs
+++ b/SAM/SAM.Geometry/Geometry/Planar/Classes/Triangle2D.cs
@@ -204,7 +204,7 @@ public bool On(Point2D point2D, double tolerance = 1E-09)
 
         public void Reverse()
         {
-            points.Reverse();
+            Array.Reverse(points);
         }
 
         public override JsonObject ToJsonObject()
diff --git a/SAM/SAM.Tests/Triangle2DReverseTests.cs b/SAM/SAM.Tests/Triangle2DReverseTests.cs
new file mode 100644
index 000000000..8fb94373c
--- /dev/null
+++ b/SAM/SAM.Tests/Triangle2DReverseTests.cs
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// Copyright (c) 2020–2026 Michal Dengusiak & Jakub Ziolkowski and contributors
+
+using System.Collections.Generic;
+using SAM.Geometry;
+using SAM.Geometry.Planar;
+using Xunit;
+
+namespace SAM.Tests
+{
+    /// <summary>
+    /// Triangle2D.Reverse() used to call the LINQ Enumerable.Reverse on its Point2D[] and discard the result, so it
+    /// never changed anything. Face.Reverse() reaches it through IReversible when an edge is a Triangle2D.
+    /// </summary>
+    public class Triangle2DReverseTests
+    {
+        private const double Tol = 1e-9;
+
+        private static readonly Point2D A = new Point2D(0, 0);
+        private static readonly Point2D B = new Point2D(4, 0);
+        private static readonly Point2D C = new Point2D(0, 3);
+
+        [Fact]
+        public void Reverse_ReversesStoredVertexOrder()
+        {
+            Triangle2D triangle2D = new Triangle2D(A, B, C);
+
+            triangle2D.Reverse();
+
+            List<Point2D> point2Ds = triangle2D.GetPoints();
+            AssertSame(C, point2Ds[0]);
+            AssertSame(B, point2Ds[1]);
+            AssertSame(A, point2Ds[2]);
+        }
+
+        [Fact]
+        public void Reverse_FlipsOrientation()
+        {
+            Triangle2D triangle2D = new Triangle2D(A, B, C);
+            Assert.Equal(Orientation.CounterClockwise, triangle2D.GetOrientation());
+
+            triangle2D.Reverse();
+
+            Assert.Equal(Orientation.Clockwise, triangle2D.GetOrientation());
+        }
+
+        [Fact]
+        public void Reverse_ThroughIReversible_ReversesStoredVertexOrder()
+        {
+            Triangle2D triangle2D = new Triangle2D(A, B, C);
+            IReversible reversible = triangle2D;
+
+            reversible.Reverse();
+
+            List<Point2D> point2Ds = triangle2D.GetPoints();
+            AssertSame(C, point2Ds[0]);
+            AssertSame(B, point2Ds[1]);
+            AssertSame(A, point2Ds[2]);
+        }
+
+        [Fact]
+        public void Reverse_ThroughFace2D_ReversesTriangleExternalEdge()
+        {
+            Face2D face2D = new Triangle2D(A, B, C);
+
+            face2D.Reverse();
+
+            Triangle2D externalEdge2D = Assert.IsType<Triangle2D>(face2D.ExternalEdge2D);
+            List<Point2D> point2Ds = externalEdge2D.GetPoints();
+            AssertSame(C, point2Ds[0]);
+            AssertSame(B, point2Ds[1]);
+            AssertSame(A, point2Ds[2]);
+            Assert.Equal(Orientation.Clockwise, externalEdge2D.GetOrientation());
+        }
+
+        [Fact]
+        public void Reverse_Twice_RestoresOriginalOrder()
+        {
+            Triangle2D triangle2D = new Triangle2D(A, B, C);
+
+            triangle2D.Reverse();
+            triangle2D.Reverse();
+
+            List<Point2D> point2Ds = triangle2D.GetPoints();
+            AssertSame(A, point2Ds[0]);
+            AssertSame(B, point2Ds[1]);
+            AssertSame(C, point2Ds[2]);
+        }
+
+        [Fact]
+        public void Reverse_KeepsAreaAndPerimeter()
+        {
+            Triangle2D triangle2D = new Triangle2D(A, B, C);
+            double area = triangle2D.GetArea();
+            double perimeter = triangle2D.GetPerimeter();
+
+            triangle2D.Reverse();
+
+            Assert.Equal(area, triangle2D.GetArea(), 9);
+            Assert.Equal(perimeter, triangle2D.GetPerimeter(), 9);
+            Assert.Equal(6.0, triangle2D.GetArea(), 9);
+            Assert.Equal(12.0, triangle2D.GetPerimeter(), 9);
+        }
+
+        private static void AssertSame(Point2D expected, Point2D actual)
+        {
+            Assert.NotNull(actual);
+            Assert.True(expected.Distance(actual) < Tol, $"expected {expected.X},{expected.Y} but was {actual.X},{actual.Y}");
+        }
+    }
+}
diff --git a/documentation/Triangle2D-Reverse-Q4.md b/documentation/Triangle2D-Reverse-Q4.md
new file mode 100644
index 000000000..3c0f9df3a
--- /dev/null
+++ b/documentation/Triangle2D-Reverse-Q4.md
@@ -0,0 +1,91 @@
+# Triangle2D.Reverse no-op — Q4 (2026-11-01)
+
+Status: open PR SAM#229 from `fix/triangle2d-reverse-q4` into `sow/2026-Q4` (base `15d8c9097360ffd2580ef3b82239e70c3b646c94`),
+pilot LOOP-1. Marked Ready for review on GitHub on 2026-11-01 at 09:15 UTC under `michaldengusiak`. Not merged and not
+approved. Tested content SHA: `5d608857004b3ad30b6d2f9e6546af7f53999f0b`, which adds one test to the reviewed content
+`914be22451da79dcc9b0aedf0f02e70496781245`. This record-only commit follows it and changes only this file. The final
+head and GitHub checks are in the PR description.
+
+This delivery does not close F2, the tamper promise, teardown, continuity, S-09 or Phase 1a. Those are items of the
+agent-workflow pilot, unrelated to `Triangle2D`, tracked in `tasks/phase1a/HANDOVER.md` on the `SAM_Workspace` branch
+`phase1a/build`.
+
+## Problem
+
+`Triangle2D` keeps its vertices in a `Point2D[]`. `Reverse()` was `points.Reverse();`. With `using System.Linq;` in
+scope that binds to `Enumerable.Reverse`, which returns a new sequence; the result was discarded, so `Reverse()` never
+changed the triangle. It is the same defect SAM#228 fixed in `Triangle3D`.
+
+## Callers (correction to the earlier record)
+
+The SAM#228 record says `Triangle2D.Reverse()` has no callers. That is incomplete. `Face.Reverse()`
+(`SAM/SAM.Geometry/Geometry/Base/Classes/Face.cs:274`) calls `IReversible.Reverse()` on its external edge (`:277`) and
+on each internal edge (`:283`). `Triangle2D` implements `IReversible`, so a `Face` whose edge is a `Triangle2D` reaches
+this method. Such a `Face` is easy to build: the public implicit `Face2D(Triangle2D)` operator
+(`SAM/SAM.Geometry/Geometry/Planar/Classes/Face2D.cs:29`) makes one, and `Face(IClosed2D)` keeps a `Triangle2D` clone.
+A text search of this repository found no caller of `Face.Reverse()`, and sibling repositories were not searched. Before
+the fix such a `Face.Reverse()` silently left a triangle edge unchanged.
+
+## Change
+
+`SAM/SAM.Geometry/Geometry/Planar/Classes/Triangle2D.cs`: `points.Reverse();` → `Array.Reverse(points);` (in place; the
+stored vertex order is reversed). One line. Unchanged: public signatures, JSON schema, coordinates, tolerances.
+
+## Tests (`SAM/SAM.Tests/Triangle2DReverseTests.cs`, xUnit, SPDX header)
+
+- `Reverse_ReversesStoredVertexOrder`
+- `Reverse_FlipsOrientation` (CounterClockwise → Clockwise)
+- `Reverse_ThroughIReversible_ReversesStoredVertexOrder` (the route `Face.Reverse()` uses)
+- `Reverse_ThroughFace2D_ReversesTriangleExternalEdge` (a real `Face2D` built from a `Triangle2D`)
+- `Reverse_Twice_RestoresOriginalOrder`
+- `Reverse_KeepsAreaAndPerimeter` (6 and 12 for the 3-4-5 triangle)
+
+## Validation at `5d608857` (local, 2026-11-01)
+
+Developer run on Windows, Debug, outside MXC, after the `Face2D` test was added following the owner's PR review:
+
+| Check | Result |
+| --- | --- |
+| `dotnet test SAM/SAM.Tests/SAM.Tests.csproj -p:NuGetAudit=false` | **Full suite, no filter:** 3981 passed, 0 failed, 0 skipped |
+| Fail-first control: `dotnet test SAM/SAM.Tests/SAM.Tests.csproj -p:NuGetAudit=false --filter "FullyQualifiedName~Triangle2DReverseTests"` with `Array.Reverse(points);` temporarily put back to `points.Reverse();` | 4 of 6 failed: order, orientation, `IReversible` and `Face2D`. The twice-reversed and area/perimeter tests passed, as expected of a no-op |
+
+The controller test step, the Codex review and the acceptance gate were not repeated at `5d608857`; their results below
+apply to `914be224`, which has the same production code.
+
+## Validation at `914be224` (local, 2026-11-01; GitHub checks are in the PR description)
+
+| Check | Where | Result |
+| --- | --- | --- |
+| `dotnet build SAM/SAM.Tests/SAM.Tests.csproj -m:1 -nodeReuse:false -p:UseSharedCompilation=false -p:NuGetAudit=false` | Controller test step, isolated checkout, MXC profile G | exit 0, 20.5 s |
+| `dotnet test SAM/SAM.Tests/SAM.Tests.csproj --no-build -m:1 --filter "FullyQualifiedName!=SAM.Tests.PartOBaselineReferenceTests.ARenamedDesign_IsFoundByIdentity&FullyQualifiedName!=SAM.Tests.PartOBaselineReferenceTests.ARuntimeHint_FindsADesignTheLocatorCannot_AndIsNotPersisted"` (Debug) | Same | **Filtered run:** 3978 passed, 0 failed, 0 skipped, 196.6 s |
+| The two excluded tests, by name (Debug) | Scratch clone, outside MXC, extra check | Both passed |
+| Fail-first control: the new tests with the unfixed `Triangle2D.cs` | Scratch clone, outside MXC | `Reverse_ReversesStoredVertexOrder`, `Reverse_FlipsOrientation` and `Reverse_ThroughIReversible_ReversesStoredVertexOrder` failed. `Reverse_Twice_RestoresOriginalOrder` and `Reverse_KeepsAreaAndPerimeter` passed, as expected of a no-op |
+| `samctl spdx` | Local | Pass, 2 files |
+| Developer run, same commands | Developer worktree | 3978 passed, 0 failed, 0 skipped (informational) |
+| Independent review, Codex `gpt-6.1-sol` High, MXC profile G, at the tested SHA | Local | ACCEPT, round 1 of 3, no findings |
+| Controller acceptance gate | Local | Passed |
+
+The controller run is **filtered, not the full suite**: it excludes exactly `SAM.Tests.PartOBaselineReferenceTests.ARenamedDesign_IsFoundByIdentity`
+and `SAM.Tests.PartOBaselineReferenceTests.ARuntimeHint_FindsADesignTheLocatorCannot_AndIsNotPersisted` (owner direction 29,
+pilot LOOP-1 only: these two tests alone may be filtered from the controller run in this repository, and they must still
+pass by name in Release CI; they fail under MXC profile G on `File.Move` access denied and pass outside it).
+
+Machine-local evidence, not in this repository: `tasks/phase1a/evidence/loop-1/` on the `SAM_Workspace` branch
+`phase1a/build`, which is local only.
+
+## Unresolved
+
+- Release CI at the final head must show a `Passed` line for both excluded tests and all six new tests; the result is
+  in the PR description. A green build alone is insufficient.
+- `SAM.Core.Grasshopper.Tests` was not run (needs Rhino).
+- The controller test step runs Debug only. Release is covered by CI.
+- The `Face2D` test covers the external edge only; the internal-edge branch (`Face.cs:283`) has no test.
+
+## Compatibility
+
+No public API, JSON, tolerance, Grasshopper component, version, `PROJECT_PROGRESS.md` or CI change. Behaviour changes
+only for callers that reverse a `Triangle2D`, which now gets the reversed vertex order the method always promised.
+
+## Next step
+
+Owner: review the final head and give the exact approval line; merging needs it. After merge, add the `PROJECT_PROGRESS.md` closeout with the merge SHA.