WebGPU Metal Debug Kit

Debug Chrome WebGPU applications on macOS at two levels:

1. Browser level — Chrome DevTools MCP
2. GPU driver level — Xcode xctrace Metal System Trace

Environment

Metal Toolchain: !/usr/bin/xcrun xctrace list templates 2>/dev/null | grep -q "Metal System Trace" && echo "installed" || echo "NOT INSTALLED — run: xcodebuild -downloadComponent MetalToolchain"

Rules

1. ONE Chrome only. MCP manages Chrome. Never launch Chrome manually. Never run setup-metal-debug.sh.
2. Wait after navigate. After navigate_page, wait 2-3 seconds before calling evaluate_script.
3. `evaluate_script` over `take_screenshot`. Stats = few tokens. Screenshot = ~1MB tokens.
4. Metal trace: direct command only. Always use /usr/bin/xcrun xctrace directly. Never use bash scripts/capture-metal-trace.sh (PATH issues in Claude Code).

Workflow

The user provides a URL to their running WebGPU app. If they say "demo", run bash ${CLAUDE_SKILL_DIR}/../../demo/start.sh --no-open in the background. The script finds an available port automatically and prints Server ready: http://localhost:<port>/demo/. Parse the URL from the output and use it. The --no-open flag prevents opening a system browser (MCP Chrome will be used instead).

Step 1: Open the app

navigate_page → <user's URL>

Wait, then verify the debug helpers are loaded:

// evaluate_script
() => window.__gpu ? window.__gpu.stats() : 'not loaded'

If window.__gpu is not available, the app may not include webgpu-debug-helpers.js. Guide the user to add it.

Step 2: Browser-level debugging

window.__gpu.stats()          // { fps, avgMs, maxMs, frameGapMs }
window.__gpu.setDebugMode(0)  // normal
window.__gpu.setDebugMode(1)  // render path (blue=hit, dark=miss)
window.__gpu.setDebugMode(2)  // step heatmap (green=few, red=many)
window.__gpu.setDebugMode(3)  // depth (dark=near, bright=far)
window.__gpu.setDebugMode(4)  // normals (RGB)
window.__gpu.textures()       // tracked texture info
window.__gpu.timings()        // GPU timing records

Check list_console_messages for WGSL shader errors or WebGPU warnings.

Step 3: Metal-level profiling (when needed)

When frameGapMs >> avgMs, there may be a driver-level bottleneck.

GPU_PID=$(pgrep -f "Google Chrome.*--type=gpu-process" | head -1)
echo "GPU PID: $GPU_PID"
rm -rf /tmp/webgpu-metal-trace.trace
/usr/bin/xcrun xctrace record \
  --template 'Metal System Trace' \
  --attach "$GPU_PID" \
  --time-limit 5s \
  --output /tmp/webgpu-metal-trace.trace

Export and analyze:

/usr/bin/xcrun xctrace export --input /tmp/webgpu-metal-trace.trace --toc | grep schema

/usr/bin/xcrun xctrace export --input /tmp/webgpu-metal-trace.trace \
  --xpath '/trace-toc/run/data/table[@schema="metal-gpu-intervals"]'

Look for:

• Command buffer count per frame (high = Dawn overhead)
• Encoder durations (long = GPU bottleneck)
• Gaps between GPU executions (stalls)

Note: For full Metal encoder/command buffer detail, the user should configure MCP with --browser-url to connect to a Chrome launched with --disable-gpu-sandbox. See assets/mcp-settings-metal.json.

Diagnostic Reference