diff --git a/src/AcDream.App/Rendering/IndoorCellStencilPipeline.cs b/src/AcDream.App/Rendering/IndoorCellStencilPipeline.cs
new file mode 100644
index 0000000..0f113d8
--- /dev/null
+++ b/src/AcDream.App/Rendering/IndoorCellStencilPipeline.cs
@@ -0,0 +1,249 @@
+// IndoorCellStencilPipeline.cs — Phase A8 indoor-cell visibility culling.
+//
+// Ports WorldBuilder's RenderInsideOut stencil mechanism for acdream's
+// modern GL pipeline. Closes issue #78 (outdoor stabs visible through
+// indoor walls) and the cellar-stairs artifact (outdoor terrain visible
+// inside cottage cellars).
+//
+// Algorithm (per WB VisibilityManager.cs:73-239):
+//   Step 1: stencil ref=1 marked everywhere portal polygons cover
+//           (color/depth writes off, depth=Always, CullFace off).
+//   Step 2: gl_FragDepth=1.0 punched into stencil=1 regions
+//           (color off, depth on, depth=Always).
+//   Step 3: (caller) draw indoor entities with stencil OFF.
+//   Step 4: (caller) draw terrain + outdoor entities with
+//           glStencilFunc(Equal, 1, 0x01), stencil read-only.
+//
+// Retail equivalent: PView::DrawCells at
+// docs/research/named-retail/acclient_2013_pseudo_c.txt:432709 uses
+// screen-space polygon-clip scissor instead of stencil. Same observable
+// behavior; the stencil approach matches modern GL pipeline conventions.
+
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using Silk.NET.OpenGL;
+
+namespace AcDream.App.Rendering;
+
+/// <summary>
+/// Pure-math triangle-fan generation from a list of cells with
+/// <see cref="LoadedCell.PortalPolygons"/>. Extracted as a static class so
+/// the vertex-list construction is unit-testable without a GL context.
+/// </summary>
+public static class PortalMeshBuilder
+{
+    /// <summary>
+    /// Builds a flat triangle-fan vertex array in world space from every
+    /// exit portal (<see cref="CellPortalInfo.OtherCellId"/> == 0xFFFF) on
+    /// every cell in <paramref name="cells"/>. Inner portals are skipped
+    /// — they don't open to outdoors, so stencil-marking them would let
+    /// outdoor geometry bleed into adjacent rooms (incorrect).
+    /// </summary>
+    public static Vector3[] BuildTriangles(IReadOnlyCollection<LoadedCell> cells)
+    {
+        // Pre-count to size the output exactly.
+        int triCount = 0;
+        foreach (var cell in cells)
+        {
+            for (int p = 0; p < cell.Portals.Count; p++)
+            {
+                if (cell.Portals[p].OtherCellId != 0xFFFF) continue;
+                if (p >= cell.PortalPolygons.Count) continue;
+                var poly = cell.PortalPolygons[p];
+                if (poly.Length < 3) continue;
+                triCount += (poly.Length - 2) * 3;
+            }
+        }
+
+        if (triCount == 0) return Array.Empty<Vector3>();
+
+        var output = new Vector3[triCount];
+        int outIdx = 0;
+        foreach (var cell in cells)
+        {
+            var xform = cell.WorldTransform;
+            for (int p = 0; p < cell.Portals.Count; p++)
+            {
+                if (cell.Portals[p].OtherCellId != 0xFFFF) continue;
+                if (p >= cell.PortalPolygons.Count) continue;
+                var poly = cell.PortalPolygons[p];
+                if (poly.Length < 3) continue;
+
+                // Triangle-fan from vertex 0.
+                var v0 = Vector3.Transform(poly[0], xform);
+                for (int i = 1; i < poly.Length - 1; i++)
+                {
+                    output[outIdx++] = v0;
+                    output[outIdx++] = Vector3.Transform(poly[i], xform);
+                    output[outIdx++] = Vector3.Transform(poly[i + 1], xform);
+                }
+            }
+        }
+
+        return output;
+    }
+}
+
+/// <summary>
+/// GL pipeline owner. Holds the portal stencil shader and a dynamic
+/// VBO/VAO for per-frame portal triangle uploads. Public methods are
+/// called in the per-frame render path when the camera is inside an
+/// EnvCell. When outside, this object is dormant — no GL work.
+/// </summary>
+public sealed unsafe class IndoorCellStencilPipeline : IDisposable
+{
+    private readonly GL _gl;
+    private readonly Shader _shader;
+    private readonly uint _vao;
+    private readonly uint _vbo;
+    private int _vboCapacityVerts;
+    private int _lastVertexCount;
+    private int _uViewProjectionLoc;
+    private int _uWriteFarDepthLoc;
+
+    public IndoorCellStencilPipeline(GL gl, string vertPath, string fragPath)
+    {
+        _gl = gl;
+        _shader = new Shader(gl, vertPath, fragPath);
+        _uViewProjectionLoc = _gl.GetUniformLocation(_shader.Program, "uViewProjection");
+        _uWriteFarDepthLoc  = _gl.GetUniformLocation(_shader.Program, "uWriteFarDepth");
+
+        _vao = _gl.GenVertexArray();
+        _vbo = _gl.GenBuffer();
+        AllocateVbo(1024);
+        ConfigureVao();
+    }
+
+    /// <summary>
+    /// Builds the per-frame portal triangle array from <paramref name="cells"/>
+    /// and uploads it to <see cref="_vbo"/>. Returns the vertex count
+    /// (0 means no exit portals — caller should skip stencil setup entirely).
+    /// </summary>
+    public int UploadPortalMesh(IReadOnlyCollection<LoadedCell> cells)
+    {
+        var verts = PortalMeshBuilder.BuildTriangles(cells);
+        _lastVertexCount = verts.Length;
+        if (_lastVertexCount == 0) return 0;
+
+        if (_lastVertexCount > _vboCapacityVerts)
+        {
+            _vboCapacityVerts = Math.Max(_lastVertexCount * 2, 1024);
+            AllocateVbo(_vboCapacityVerts);
+        }
+
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);
+        fixed (Vector3* p = verts)
+        {
+            _gl.BufferSubData(BufferTargetARB.ArrayBuffer, 0,
+                (nuint)(_lastVertexCount * sizeof(Vector3)), p);
+        }
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, 0);
+        return _lastVertexCount;
+    }
+
+    /// <summary>
+    /// Steps 1+2 of WB's RenderInsideOut: mark stencil ref=1 wherever
+    /// portal polygons cover, then write gl_FragDepth=1.0 into those
+    /// regions. Leaves the GL state set up for the caller to invoke
+    /// <see cref="EnableOutdoorPass"/>.
+    /// </summary>
+    public void MarkAndPunch(Matrix4x4 viewProjection)
+    {
+        if (_lastVertexCount == 0) return;
+
+        _gl.Enable(EnableCap.StencilTest);
+        _gl.ClearStencil(0);
+        _gl.Clear(ClearBufferMask.StencilBufferBit);
+
+        // Step 1: stencil mark.
+        _gl.ColorMask(false, false, false, false);
+        _gl.DepthMask(false);
+        _gl.DepthFunc(DepthFunction.Always);
+        _gl.Disable(EnableCap.CullFace);
+        _gl.Enable(EnableCap.DepthClamp);          // portal polys can be at any Z
+        _gl.StencilFunc(StencilFunction.Always, 1, 0xFFu);
+        _gl.StencilMask(0x01u);
+        _gl.StencilOp(StencilOp.Keep, StencilOp.Keep, StencilOp.Replace);
+
+        _shader.Use();
+        var vp = viewProjection;
+        _gl.UniformMatrix4(_uViewProjectionLoc, 1, false, (float*)&vp);
+        _gl.Uniform1(_uWriteFarDepthLoc, 0);
+
+        _gl.BindVertexArray(_vao);
+        _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)_lastVertexCount);
+
+        // Step 2: far-depth punch.
+        _gl.ColorMask(false, false, false, false);
+        _gl.DepthMask(true);
+        _gl.DepthFunc(DepthFunction.Always);
+        _gl.StencilFunc(StencilFunction.Equal, 1, 0x01u);
+        _gl.StencilMask(0x00u);                    // read-only
+        _gl.StencilOp(StencilOp.Keep, StencilOp.Keep, StencilOp.Keep);
+
+        _gl.Uniform1(_uWriteFarDepthLoc, 1);
+        _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)_lastVertexCount);
+
+        _gl.BindVertexArray(0);
+        _gl.Disable(EnableCap.DepthClamp);
+        _gl.Enable(EnableCap.CullFace);
+
+        // Leave a clean state for the indoor-entities pass: stencil
+        // disabled, color+depth on, depth=Less, no cull change.
+        _gl.ColorMask(true, true, true, true);
+        _gl.DepthFunc(DepthFunction.Less);
+        _gl.Disable(EnableCap.StencilTest);
+    }
+
+    /// <summary>
+    /// Step 4 of WB's RenderInsideOut: enable stencil read-only with
+    /// ref=1, so subsequent terrain + outdoor entity draws are gated
+    /// to portal silhouette regions only.
+    /// </summary>
+    public void EnableOutdoorPass()
+    {
+        if (_lastVertexCount == 0) return;
+        _gl.Enable(EnableCap.StencilTest);
+        _gl.StencilFunc(StencilFunction.Equal, 1, 0x01u);
+        _gl.StencilMask(0x00u);
+        _gl.StencilOp(StencilOp.Keep, StencilOp.Keep, StencilOp.Keep);
+    }
+
+    /// <summary>
+    /// Restores stencil-off state. Call after all outdoor passes complete
+    /// so subsequent rendering (particles, sky, UI) is unaffected.
+    /// </summary>
+    public void DisableStencil()
+    {
+        _gl.Disable(EnableCap.StencilTest);
+        _gl.StencilMask(0xFFu);   // restore default write mask
+    }
+
+    public void Dispose()
+    {
+        _shader.Dispose();
+        _gl.DeleteVertexArray(_vao);
+        _gl.DeleteBuffer(_vbo);
+    }
+
+    private void AllocateVbo(int capacityVerts)
+    {
+        _vboCapacityVerts = capacityVerts;
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);
+        _gl.BufferData(BufferTargetARB.ArrayBuffer,
+            (nuint)(capacityVerts * sizeof(Vector3)),
+            null, BufferUsageARB.DynamicDraw);
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, 0);
+    }
+
+    private void ConfigureVao()
+    {
+        _gl.BindVertexArray(_vao);
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);
+        _gl.EnableVertexAttribArray(0);
+        _gl.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, false,
+            (uint)sizeof(Vector3), (void*)0);
+        _gl.BindVertexArray(0);
+    }
+}
diff --git a/tests/AcDream.App.Tests/Rendering/IndoorCellStencilPipelineTests.cs b/tests/AcDream.App.Tests/Rendering/IndoorCellStencilPipelineTests.cs
new file mode 100644
index 0000000..2f5d2d5
--- /dev/null
+++ b/tests/AcDream.App.Tests/Rendering/IndoorCellStencilPipelineTests.cs
@@ -0,0 +1,151 @@
+// Phase A8 — portal mesh triangle-fan generation tests.
+//
+// Pure-math coverage of PortalMeshBuilder.BuildTriangles — the part of
+// IndoorCellStencilPipeline that converts a list of LoadedCell with
+// PortalPolygons + WorldTransform into a flat Vector3[] of triangles
+// in world space. The GL/upload portion is exercised at runtime only.
+
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class IndoorCellStencilPipelineTests
+{
+    [Fact]
+    public void BuildTriangles_NoCells_ReturnsEmpty()
+    {
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell>());
+        Assert.Empty(verts);
+    }
+
+    [Fact]
+    public void BuildTriangles_SkipsInnerPortals()
+    {
+        // Two portals on one cell: one exit (OtherCellId=0xFFFF, should be
+        // included), one inner (OtherCellId=0x0102, should be skipped).
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new()
+            {
+                new CellPortalInfo(0xFFFF, 100, 0),  // exit — included
+                new CellPortalInfo(0x0102, 101, 0),  // inner — skipped
+            },
+            ClipPlanes = new() { default, default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(1, 1, 0),
+                },
+                new[]
+                {
+                    new Vector3(10, 0, 0),
+                    new Vector3(11, 0, 0),
+                    new Vector3(11, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        // Only the exit polygon (3 verts → 1 triangle → 3 vertices).
+        Assert.Equal(3, verts.Length);
+        Assert.Equal(new Vector3(0, 0, 0), verts[0]);
+        Assert.Equal(new Vector3(1, 0, 0), verts[1]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[2]);
+    }
+
+    [Fact]
+    public void BuildTriangles_TriangulatesAsFan()
+    {
+        // 4-vertex quad → fan = 2 triangles → 6 vertices.
+        // Quad: (0,0,0), (1,0,0), (1,1,0), (0,1,0).
+        // Fan from vertex 0: (0,1,2), (0,2,3).
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new() { new CellPortalInfo(0xFFFF, 100, 0) },
+            ClipPlanes = new() { default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(1, 1, 0),
+                    new Vector3(0, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        Assert.Equal(6, verts.Length);
+        // Triangle 1: (0,1,2)
+        Assert.Equal(new Vector3(0, 0, 0), verts[0]);
+        Assert.Equal(new Vector3(1, 0, 0), verts[1]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[2]);
+        // Triangle 2: (0,2,3)
+        Assert.Equal(new Vector3(0, 0, 0), verts[3]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[4]);
+        Assert.Equal(new Vector3(0, 1, 0), verts[5]);
+    }
+
+    [Fact]
+    public void BuildTriangles_AppliesWorldTransform()
+    {
+        // Identity cell-local triangle, translated by WorldTransform.
+        var translate = Matrix4x4.CreateTranslation(new Vector3(100, 200, 300));
+        var cell = new LoadedCell
+        {
+            WorldTransform = translate,
+            Portals = new() { new CellPortalInfo(0xFFFF, 100, 0) },
+            ClipPlanes = new() { default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(0, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        Assert.Equal(3, verts.Length);
+        Assert.Equal(new Vector3(100, 200, 300), verts[0]);
+        Assert.Equal(new Vector3(101, 200, 300), verts[1]);
+        Assert.Equal(new Vector3(100, 201, 300), verts[2]);
+    }
+
+    [Fact]
+    public void BuildTriangles_SkipsEmptyOrDegeneratePolygons()
+    {
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new()
+            {
+                new CellPortalInfo(0xFFFF, 100, 0),
+                new CellPortalInfo(0xFFFF, 101, 0),
+            },
+            ClipPlanes = new() { default, default },
+            PortalPolygons = new()
+            {
+                System.Array.Empty<Vector3>(),                 // empty
+                new[] { new Vector3(0, 0, 0), new Vector3(1, 0, 0) },  // degenerate (2 verts)
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+        Assert.Empty(verts);
+    }
+}