diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 273f4d4..f8edcaa 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -18,8 +18,13 @@ public sealed class GameWindow : IDisposable
     private IWindow? _window;
     private GL? _gl;
     private IInputContext? _input;
-    private TerrainChunkRenderer? _terrain;
+    private TerrainModernRenderer? _terrain;
     private Shader? _shader;
+    /// <summary>Phase N.5b: terrain_modern.vert/.frag program. Owned by
+    /// <see cref="_terrain"/> at draw time but allocated + disposed here. Lives
+    /// in parallel with <see cref="_shader"/> (legacy terrain.vert/.frag) until
+    /// Task 9 deletes the legacy renderer.</summary>
+    private Shader? _terrainModernShader;
     private CameraController? _cameraController;
     private IMouse? _capturedMouse;
     private DatCollection? _dats;
@@ -68,6 +73,15 @@ public sealed class GameWindow : IDisposable
     private string _lastNearestObjLabel = "-";
     private bool   _lastColliding;
 
+    // Phase N.5b: CPU timing for [TERRAIN-DIAG] under ACDREAM_WB_DIAG=1
+    // (parallel diagnostic to [WB-DIAG] in WbDrawDispatcher — same env var
+    // gate so flipping one switch turns on both dispatcher rollups). Mirrors
+    // the rolling-256-sample buffer pattern from WbDrawDispatcher.
+    private readonly System.Diagnostics.Stopwatch _terrainCpuStopwatch = new();
+    private readonly long[] _terrainCpuSamples = new long[256];   // microseconds
+    private int  _terrainCpuSampleCursor;
+    private long _terrainLastDiagTick;
+
     // Phase A.1: streaming fields replacing the one-shot _entities list.
     private AcDream.App.Streaming.LandblockStreamer? _streamer;
     private AcDream.App.Streaming.GpuWorldState _worldState = new();
@@ -969,6 +983,13 @@ public sealed class GameWindow : IDisposable
             Path.Combine(shadersDir, "terrain.vert"),
             Path.Combine(shadersDir, "terrain.frag"));
 
+        // Phase N.5b: terrain_modern shader pair — bindless texture handles +
+        // glMultiDrawElementsIndirect dispatch path. Loaded in parallel with
+        // the legacy `_shader`; Task 9 will retire the legacy program.
+        _terrainModernShader = new Shader(_gl,
+            Path.Combine(shadersDir, "terrain_modern.vert"),
+            Path.Combine(shadersDir, "terrain_modern.frag"));
+
         // Phase G.1/G.2: shared scene-lighting UBO. Stays bound at
         // binding=1 for the lifetime of the process — every shader that
         // declares `layout(std140, binding = 1) uniform SceneLighting`
@@ -1385,10 +1406,44 @@ public sealed class GameWindow : IDisposable
         // TimeSync arrives.
         WorldTime.SyncFromServer(AcDream.Core.World.DerethDateTime.DayTicks / 16.0); // = 476.25 = Midsong (noon)
 
-        // Build the terrain atlas once from the Region dat.
-        var terrainAtlas = AcDream.App.Rendering.TerrainAtlas.Build(_gl, _dats);
+        // N.5: detect ARB_bindless_texture + ARB_shader_draw_parameters BEFORE
+        // building the terrain atlas / renderer — both consume BindlessSupport
+        // (atlas via Texture2DArray bindless handles, renderer for SSBO uploads).
+        // The modern path (SSBO + glMultiDrawElementsIndirect + bindless textures)
+        // is mandatory as of Phase N.5 — missing extensions throw at startup with
+        // a clear error so users can file a real bug report rather than silently
+        // falling back to a half-working renderer.
+        if (AcDream.App.Rendering.Wb.BindlessSupport.TryCreate(_gl, out var bindless))
+        {
+            if (bindless!.HasShaderDrawParameters(_gl))
+            {
+                _bindlessSupport = bindless;
+                Console.WriteLine("[N.5] modern path capabilities present (bindless + ARB_shader_draw_parameters)");
+            }
+            else
+            {
+                Console.WriteLine("[N.5] GL_ARB_shader_draw_parameters not present — modern path not available");
+            }
+        }
+        else
+        {
+            Console.WriteLine("[N.5] GL_ARB_bindless_texture not present — modern path not available");
+        }
 
-        _terrain = new TerrainChunkRenderer(_gl, _shader, terrainAtlas);
+        if (_bindlessSupport is null)
+        {
+            throw new NotSupportedException(
+                "acdream requires GL_ARB_bindless_texture + GL_ARB_shader_draw_parameters " +
+                "(GL 4.3+ with bindless support). Your GPU/driver does not expose these extensions. " +
+                "If this is unexpected, please file a bug report with your GPU vendor + driver version.");
+        }
+
+        // Build the terrain atlas once from the Region dat. Phase N.5b: the
+        // atlas exposes bindless handles for the modern terrain path, so
+        // BindlessSupport is threaded through.
+        var terrainAtlas = AcDream.App.Rendering.TerrainAtlas.Build(_gl, _dats, _bindlessSupport);
+
+        _terrain = new TerrainModernRenderer(_gl, _bindlessSupport, _terrainModernShader!, terrainAtlas);
 
         int centerX = (int)((centerLandblockId >> 24) & 0xFFu);
         int centerY = (int)((centerLandblockId >> 16) & 0xFFu);
@@ -1418,35 +1473,8 @@ public sealed class GameWindow : IDisposable
         _heightTable = heightTable;
         _surfaceCache = new Dictionary<uint, AcDream.Core.Terrain.SurfaceInfo>();
 
-        // N.5: detect ARB_bindless_texture + ARB_shader_draw_parameters.
-        // The modern path (SSBO + glMultiDrawElementsIndirect + bindless textures)
-        // is mandatory as of Phase N.5 — missing extensions throw at startup with
-        // a clear error so users can file a real bug report rather than silently
-        // falling back to a half-working renderer.
-        if (AcDream.App.Rendering.Wb.BindlessSupport.TryCreate(_gl, out var bindless))
-        {
-            if (bindless!.HasShaderDrawParameters(_gl))
-            {
-                _bindlessSupport = bindless;
-                Console.WriteLine("[N.5] modern path capabilities present (bindless + ARB_shader_draw_parameters)");
-            }
-            else
-            {
-                Console.WriteLine("[N.5] GL_ARB_shader_draw_parameters not present — modern path not available");
-            }
-        }
-        else
-        {
-            Console.WriteLine("[N.5] GL_ARB_bindless_texture not present — modern path not available");
-        }
-
-        if (_bindlessSupport is null)
-        {
-            throw new NotSupportedException(
-                "acdream requires GL_ARB_bindless_texture + GL_ARB_shader_draw_parameters " +
-                "(GL 4.3+ with bindless support). Your GPU/driver does not expose these extensions. " +
-                "If this is unexpected, please file a bug report with your GPU vendor + driver version.");
-        }
+        // (Bindless detection moved above — must precede TerrainAtlas.Build /
+        // TerrainModernRenderer ctor so they can consume BindlessSupport.)
 
         // Mesh shader always loads (modern path is the only path).
         _meshShader = new Shader(_gl,
@@ -6314,7 +6342,15 @@ public sealed class GameWindow : IDisposable
                 goto SkipWorldGeometry;
             }
 
+            // Phase N.5b: wrap Draw in CPU stopwatch for [TERRAIN-DIAG] rollup
+            // (gated on ACDREAM_WB_DIAG=1, same env var as [WB-DIAG]). Stopwatch
+            // is cheap; only the periodic Console.WriteLine is gated.
+            _terrainCpuStopwatch.Restart();
             _terrain?.Draw(camera, frustum, neverCullLandblockId: playerLb);
+            _terrainCpuStopwatch.Stop();
+            _terrainCpuSamples[_terrainCpuSampleCursor] = (long)(_terrainCpuStopwatch.Elapsed.TotalMicroseconds);
+            _terrainCpuSampleCursor = (_terrainCpuSampleCursor + 1) % _terrainCpuSamples.Length;
+            MaybeFlushTerrainDiag();
 
             // Conditional depth clear: when camera is inside a building, clear
             // depth (not color) so interior geometry writes fresh Z values on top
@@ -8713,6 +8749,51 @@ public sealed class GameWindow : IDisposable
         }
     }
 
+    /// <summary>Phase N.5b: emits [TERRAIN-DIAG] once per ~5s under
+    /// ACDREAM_WB_DIAG=1. Mirrors <c>WbDrawDispatcher.MaybeFlushDiag</c>:
+    /// rolling 256-sample buffer of microseconds, median + p95 reported.
+    /// Sample buffer is NOT cleared on flush — it's a moving window so the
+    /// next 5s window already has 256 frames of recent history.</summary>
+    private void MaybeFlushTerrainDiag()
+    {
+        if (!string.Equals(Environment.GetEnvironmentVariable("ACDREAM_WB_DIAG"), "1", StringComparison.Ordinal))
+            return;
+
+        long now = Environment.TickCount64;
+        if (now - _terrainLastDiagTick <= 5000) return;
+
+        long cpuMedUs = TerrainDiagMedianMicros(_terrainCpuSamples);
+        long cpuP95Us = TerrainDiagPercentile95Micros(_terrainCpuSamples);
+        Console.WriteLine(
+            $"[TERRAIN-DIAG] cpu_ms={cpuMedUs / 1000.0:F2}/{cpuP95Us / 1000.0:F2}  " +
+            $"draws={_terrain?.VisibleSlots ?? 0}/frame  " +
+            $"visible={_terrain?.VisibleSlots ?? 0}  " +
+            $"loaded={_terrain?.LoadedSlots ?? 0}  " +
+            $"capacity={_terrain?.CapacitySlots ?? 0}");
+        _terrainLastDiagTick = now;
+    }
+
+    private static long TerrainDiagMedianMicros(long[] samples)
+    {
+        var copy = (long[])samples.Clone();
+        Array.Sort(copy);
+        int nz = 0;
+        foreach (var v in copy) if (v > 0) nz++;
+        if (nz == 0) return 0;
+        return copy[copy.Length - nz / 2];
+    }
+
+    private static long TerrainDiagPercentile95Micros(long[] samples)
+    {
+        var copy = (long[])samples.Clone();
+        Array.Sort(copy);
+        int nz = 0;
+        foreach (var v in copy) if (v > 0) nz++;
+        if (nz == 0) return 0;
+        int idx = copy.Length - 1 - (int)(nz * 0.05);
+        return copy[idx];
+    }
+
     private void OnClosing()
     {
         // Phase A.1: join the streamer worker thread before tearing down GL
@@ -8733,6 +8814,7 @@ public sealed class GameWindow : IDisposable
         _meshShader?.Dispose();
         _terrain?.Dispose();
         _shader?.Dispose();
+        _terrainModernShader?.Dispose();
         _sceneLightingUbo?.Dispose();
         _particleRenderer?.Dispose();
         _debugLines?.Dispose();