diff --git a/src/AcDream.App/Diagnostics/FrameProfiler.cs b/src/AcDream.App/Diagnostics/FrameProfiler.cs
new file mode 100644
index 00000000..dd01f9a1
--- /dev/null
+++ b/src/AcDream.App/Diagnostics/FrameProfiler.cs
@@ -0,0 +1,197 @@
+using System;
+using System.Diagnostics;
+using System.Globalization;
+using System.Text;
+using AcDream.Core.Rendering;
+using Silk.NET.OpenGL;
+
+namespace AcDream.App.Diagnostics;
+
+/// <summary>Stage indices for per-frame CPU attribution.</summary>
+public enum FrameStage
+{
+    /// <summary>Whole OnUpdate body (simulation + streaming apply).</summary>
+    Update = 0,
+    /// <summary>WbMeshAdapter.Tick — staged mesh/texture GPU upload drain.</summary>
+    Upload = 1,
+    /// <summary>ImGui Render (dev overlay).</summary>
+    ImGui = 2,
+}
+
+/// <summary>
+/// MP0 (2026-07-05) — the permanent honest frame profiler. One
+/// <c>FrameBoundary</c> call at the top of <c>GameWindow.OnRender</c>
+/// measures CPU frame time as the delta between consecutive boundaries
+/// (captures the FULL frame including present), brackets the frame in a
+/// GPU <c>TimeElapsed</c> query (via <see cref="GpuFrameTimer"/>), and
+/// samples per-frame allocated bytes + GC collection counts. Stage scopes
+/// (<see cref="BeginStage"/>) attribute CPU time to Update / Upload /
+/// ImGui. Emits one <c>[frame-prof]</c> line every ~5 s while
+/// <see cref="RenderingDiagnostics.FrameProfEnabled"/> is true; costs one
+/// bool check per frame when off.
+///
+/// <para>Permanent apparatus — every MP-track gate reads it; do not strip.
+/// Whole-frame GPU timing self-disables under <c>ACDREAM_WB_DIAG=1</c>
+/// (nested TimeElapsed is illegal GL; see GpuFrameTimer).</para>
+/// Spec: docs/superpowers/specs/2026-07-05-modern-pipeline-design.md §5.
+/// </summary>
+public sealed class FrameProfiler : IDisposable
+{
+    private const int WindowCapacity = 2048;              // ~12 s at 165 fps
+    private const long ReportIntervalTicks = 5 * TimeSpan.TicksPerSecond;
+    private static readonly int StageCount = Enum.GetValues<FrameStage>().Length;
+
+    private readonly FrameStatsBuffer _cpuUs = new(WindowCapacity);
+    private readonly FrameStatsBuffer _gpuUs = new(WindowCapacity);
+    private readonly FrameStatsBuffer _allocBytes = new(WindowCapacity);
+    private readonly FrameStatsBuffer[] _stageUs;
+    private readonly long[] _stageAccumTicks;
+    private readonly bool _wbDiagActive =
+        Environment.GetEnvironmentVariable("ACDREAM_WB_DIAG") == "1";
+
+    private GpuFrameTimer? _gpuTimer;
+    private long _lastBoundaryTimestamp;
+    private long _lastAllocBytes;
+    private long _lastReportTicks;
+    private int _gc0Base, _gc1Base, _gc2Base;
+    private int _framesInWindow;
+    private int _ownerThreadId;
+    private bool _threadWarned;
+    private bool _wbDiagNoticePrinted;
+    private bool _wasEnabled;
+
+    public FrameProfiler()
+    {
+        _stageUs = new FrameStatsBuffer[StageCount];
+        for (int i = 0; i < StageCount; i++) _stageUs[i] = new FrameStatsBuffer(WindowCapacity);
+        _stageAccumTicks = new long[StageCount];
+    }
+
+    /// <summary>
+    /// Call as the FIRST statement of <c>GameWindow.OnRender</c>.
+    /// </summary>
+    public void FrameBoundary(GL gl)
+    {
+        bool enabled = RenderingDiagnostics.FrameProfEnabled;
+        if (!enabled)
+        {
+            if (_wasEnabled) { _gpuTimer?.Stop(); _wasEnabled = false; _lastBoundaryTimestamp = 0; }
+            return;
+        }
+
+        if (_ownerThreadId == 0) _ownerThreadId = Environment.CurrentManagedThreadId;
+        else if (!_threadWarned && _ownerThreadId != Environment.CurrentManagedThreadId)
+        {
+            _threadWarned = true;
+            Console.WriteLine("[frame-prof] WARNING: frame boundary crossed threads; alloc counter is per-thread and now unreliable");
+        }
+
+        long now = Stopwatch.GetTimestamp();
+        long allocNow = GC.GetAllocatedBytesForCurrentThread();
+
+        if (!_wasEnabled)
+        {
+            // First enabled frame (startup or runtime toggle-on): establish
+            // baselines, emit nothing.
+            _wasEnabled = true;
+            _lastReportTicks = DateTime.UtcNow.Ticks;
+            _gc0Base = GC.CollectionCount(0); _gc1Base = GC.CollectionCount(1); _gc2Base = GC.CollectionCount(2);
+            if (_gpuTimer is null && !_wbDiagActive) _gpuTimer = new GpuFrameTimer(gl);
+            if (_wbDiagActive && !_wbDiagNoticePrinted)
+            {
+                _wbDiagNoticePrinted = true;
+                Console.WriteLine("[frame-prof] GPU frame timing OFF: ACDREAM_WB_DIAG=1 owns TimeElapsed queries (nested queries are illegal GL)");
+            }
+        }
+        else
+        {
+            long cpuUs = (now - _lastBoundaryTimestamp) * 1_000_000L / Stopwatch.Frequency;
+            _cpuUs.Push(cpuUs);
+            _allocBytes.Push(allocNow - _lastAllocBytes);
+            for (int i = 0; i < StageCount; i++)
+            {
+                _stageUs[i].Push(_stageAccumTicks[i] * 1_000_000L / Stopwatch.Frequency);
+                _stageAccumTicks[i] = 0;
+            }
+            _framesInWindow++;
+        }
+
+        _lastBoundaryTimestamp = now;
+        _lastAllocBytes = allocNow;
+
+        if (_gpuTimer?.FrameBoundary() is long gpuUs)
+            _gpuUs.Push(gpuUs);
+
+        long nowTicks = DateTime.UtcNow.Ticks;
+        if (nowTicks - _lastReportTicks >= ReportIntervalTicks && _framesInWindow > 0)
+        {
+            int gc0 = GC.CollectionCount(0) - _gc0Base;
+            int gc1 = GC.CollectionCount(1) - _gc1Base;
+            int gc2 = GC.CollectionCount(2) - _gc2Base;
+            Console.WriteLine(FormatReport(_framesInWindow, _cpuUs, _gpuUs,
+                gpuActive: _gpuTimer is not null, _allocBytes, gc0, gc1, gc2, _stageUs));
+            _lastReportTicks = nowTicks;
+            _gc0Base += gc0; _gc1Base += gc1; _gc2Base += gc2;
+            _framesInWindow = 0;
+            _cpuUs.Reset(); _gpuUs.Reset(); _allocBytes.Reset();
+            for (int i = 0; i < StageCount; i++) _stageUs[i].Reset();
+        }
+    }
+
+    /// <summary>
+    /// Attribute the enclosed CPU time to <paramref name="stage"/>.
+    /// Usage: <c>using var _ = profiler.BeginStage(FrameStage.Update);</c>.
+    /// Zero-cost (default scope) when the profiler is off.
+    /// </summary>
+    public StageScope BeginStage(FrameStage stage)
+        => RenderingDiagnostics.FrameProfEnabled
+            ? new StageScope(this, stage, Stopwatch.GetTimestamp())
+            : default;
+
+    internal void EndStage(FrameStage stage, long startTimestamp)
+        => _stageAccumTicks[(int)stage] += Stopwatch.GetTimestamp() - startTimestamp;
+
+    /// <summary>Pure report formatter — unit-tested; invariant culture.</summary>
+    public static string FormatReport(
+        int frameCount,
+        FrameStatsBuffer cpu, FrameStatsBuffer gpu, bool gpuActive,
+        FrameStatsBuffer alloc, int gc0, int gc1, int gc2,
+        FrameStatsBuffer[] stages)
+    {
+        var ci = CultureInfo.InvariantCulture;
+        var sb = new StringBuilder(256);
+        sb.Append("[frame-prof] n=").Append(frameCount);
+        sb.AppendFormat(ci, " | cpu_ms p50={0:0.0} p95={1:0.0} p99={2:0.0} max={3:0.0}",
+            cpu.Percentile(0.50) / 1000.0, cpu.Percentile(0.95) / 1000.0,
+            cpu.Percentile(0.99) / 1000.0, cpu.Max() / 1000.0);
+        if (gpuActive)
+            sb.AppendFormat(ci, " | gpu_ms p50={0:0.0} p95={1:0.0}",
+                gpu.Percentile(0.50) / 1000.0, gpu.Percentile(0.95) / 1000.0);
+        else
+            sb.Append(" | gpu=off(wbdiag)");
+        sb.AppendFormat(ci, " | alloc_kb p50={0:0.0} max={1:0.0} gc={2}/{3}/{4}",
+            alloc.Percentile(0.50) / 1024.0, alloc.Max() / 1024.0, gc0, gc1, gc2);
+        string[] names = { "upd", "upl", "imgui" };
+        for (int i = 0; i < stages.Length && i < names.Length; i++)
+            sb.AppendFormat(ci, " | {0} p50={1:0.0} p95={2:0.0}",
+                names[i], stages[i].Percentile(0.50) / 1000.0, stages[i].Percentile(0.95) / 1000.0);
+        return sb.ToString();
+    }
+
+    public void Dispose() => _gpuTimer?.Dispose();
+}
+
+/// <summary>Disposable stage scope; default instance is a no-op.</summary>
+public readonly struct StageScope : IDisposable
+{
+    private readonly FrameProfiler? _owner;
+    private readonly FrameStage _stage;
+    private readonly long _start;
+
+    internal StageScope(FrameProfiler owner, FrameStage stage, long start)
+    {
+        _owner = owner; _stage = stage; _start = start;
+    }
+
+    public void Dispose() => _owner?.EndStage(_stage, _start);
+}
diff --git a/tests/AcDream.App.Tests/FrameProfilerReportTests.cs b/tests/AcDream.App.Tests/FrameProfilerReportTests.cs
new file mode 100644
index 00000000..e451f576
--- /dev/null
+++ b/tests/AcDream.App.Tests/FrameProfilerReportTests.cs
@@ -0,0 +1,48 @@
+using AcDream.App.Diagnostics;
+using Xunit;
+
+namespace AcDream.App.Tests;
+
+public class FrameProfilerReportTests
+{
+    [Fact]
+    public void FormatReport_IsInvariantAndComplete()
+    {
+        var cpu = new FrameStatsBuffer(16);
+        var gpu = new FrameStatsBuffer(16);
+        var alloc = new FrameStatsBuffer(16);
+        var stages = new[] { new FrameStatsBuffer(16), new FrameStatsBuffer(16), new FrameStatsBuffer(16) };
+        for (long i = 1; i <= 10; i++)
+        {
+            cpu.Push(i * 1000);      // 1..10 ms in µs
+            gpu.Push(i * 100);
+            alloc.Push(i * 1024);    // bytes
+            stages[0].Push(i * 200);
+            stages[1].Push(i * 50);
+            stages[2].Push(i * 10);
+        }
+
+        string line = FrameProfiler.FormatReport(
+            frameCount: 10, cpu: cpu, gpu: gpu, gpuActive: true,
+            alloc: alloc, gc0: 3, gc1: 1, gc2: 0, stages: stages);
+
+        Assert.StartsWith("[frame-prof]", line);
+        Assert.Contains("n=10", line);
+        Assert.Contains("cpu_ms p50=5.0 p95=10.0 p99=10.0 max=10.0", line);
+        Assert.Contains("gpu_ms p50=0.5", line);
+        Assert.Contains("alloc_kb p50=5.0 max=10.0", line);
+        Assert.Contains("gc=3/1/0", line);
+        Assert.Contains("upd p50=1.0", line);   // stage 0: 200µs·5 = 1.0 ms
+    }
+
+    [Fact]
+    public void FormatReport_GpuInactive_SaysWhy()
+    {
+        var empty = new FrameStatsBuffer(4);
+        string line = FrameProfiler.FormatReport(
+            frameCount: 0, cpu: empty, gpu: empty, gpuActive: false,
+            alloc: empty, gc0: 0, gc1: 0, gc2: 0,
+            stages: new[] { empty, empty, empty });
+        Assert.Contains("gpu=off(wbdiag)", line);
+    }
+}