gpu-debug

name	gpu-debug
description	Interactive GPU frame debugging with RenderDoc. Capture frames from windowed apps, inspect draw calls, pipeline state, textures, shaders, and pixel history. Also supports rdc-cli for scripted inspection. For headless apps, use gpu-capture instead.
argument-hint	[capture.rdc | binary args...]
allowed-tools	Bash, Read, Glob, Grep

GPU Debug Skill

Interactive GPU frame debugging using RenderDoc and rdc-cli. Best for windowed apps (milo-viewer) where you need to visually inspect draw calls, pipeline state, textures, and shaders.

For headless apps (render-test), use the gpu-capture skill with GFXReconstruct instead — RenderDoc requires a swapchain (vkQueuePresentKHR) to trigger capture.

All Bash calls in this skill MUST use dangerouslyDisableSandbox: true (GPU/Vulkan access is blocked by the sandbox).

Arguments

$ARGUMENTS

Capture a Frame

With the wrapper script

# Capture a Milo venue scene
scripts/gpu/rdc_capture.sh -o /tmp/venue.rdc native/build/milo-viewer \
  ~/code/milohax/milo-engine-libs/harmonix-repos/milo-rnd-library/rb3/world/gen/venue.milo_xbox

# Capture a character scene with longer delay for loading
scripts/gpu/rdc_capture.sh -d 5 -o /tmp/char.rdc native/build/milo-viewer \
  ~/code/milohax/milo-engine-libs/harmonix-repos/milo-rnd-library/rb3/char/main/gen/main.milo_xbox

# Capture rb3-native full game (requires MILO_RENDER=1, needs windowed mode)
MILO_RENDER=1 scripts/gpu/rdc_capture.sh -d 10 -o /tmp/rb3.rdc native/build/rb3-native

Note: For headless capture (no window), use /gpu-capture (GFXReconstruct) instead. RenderDoc needs a swapchain.

With renderdoccmd directly

ENABLE_VULKAN_RENDERDOC_CAPTURE=1 \
  ../gpu/renderdoc/build/bin/renderdoccmd capture \
  --wait-for-exit \
  -c /tmp/frame.rdc \
  native/build/milo-viewer path/to/scene.milo_xbox

Script Options

Option	Description	Default
-o <path>	Output .rdc file	/tmp/gpu_capture.rdc
-d <seconds>	Delay before capture trigger	2
--no-wait	Don't wait for app exit	off

Inspect with renderdoccmd

The built-in CLI has basic inspection commands:

RDCCMD=../gpu/renderdoc/build/bin/renderdoccmd

# Get thumbnail from capture
$RDCCMD thumb /tmp/frame.rdc /tmp/thumb.jpg

# Replay with preview window (requires display)
$RDCCMD replay /tmp/frame.rdc

# Convert capture to another format
$RDCCMD convert /tmp/frame.rdc /tmp/frame.xml

# Print version info
$RDCCMD version

Inspect with rdc-cli (66 commands)

rdc-cli wraps the RenderDoc Python API into shell commands. It requires renderdoc.so built against the same Python version.

Setup

pip install rdc-cli
export RENDERDOC_PYTHON_PATH=../gpu/renderdoc/build/lib

# Verify
rdc doctor

If rdc doctor reports Python version mismatch, rebuild RenderDoc against your active Python or use a matching Python version.

Session Workflow

Every inspection follows open -> work -> close:

rdc open /tmp/frame.rdc          # Load capture
rdc info --json                   # Capture metadata
rdc draws --limit 20              # List draw calls
rdc close                         # Release GPU resources

Draw Call Inspection

rdc draws                         # List all draws
rdc draws --limit 50              # With limit
rdc draw 42                       # Detail for event 42
rdc events --type draw            # Filter to draw events

Pipeline State

rdc pipeline 42                   # Full pipeline at event 42
rdc pipeline 42 rs                # Rasterizer state
rdc pipeline 42 om                # Output merger (blend, depth)
rdc pipeline 42 vs                # Vertex shader stage
rdc pipeline 42 ps                # Pixel/fragment shader stage
rdc bindings 42                   # Resource bindings

Shader Inspection

rdc shader 42 vs --source         # Vertex shader source
rdc shader 42 ps --source         # Fragment shader source
rdc shader 42 vs --constants      # Uniform values
rdc shader 42 vs --reflect        # Input/output reflection
rdc shaders --json                # List all shaders
rdc search "main" --json          # Search shader source

Visual Export

rdc rt 42 -o rt.png               # Render target at event 42
rdc texture 15 -o tex.png         # Texture by resource ID
rdc thumbnail -o thumb.png        # Capture thumbnail
rdc mesh 42 -o mesh.obj           # Mesh data
rdc buffer 10 -o buf.bin          # Buffer data

Claude can view exported PNGs directly via the Read tool (multimodal).

Pixel Debugging

rdc pixel 256 256                 # Pixel history at (256,256)
rdc pixel 256 256 42              # At specific event
rdc pick-pixel 256 256 42         # Pick primitive at pixel
rdc debug pixel 42 256 256        # Step through pixel shader
rdc debug vertex 42 0             # Step through vertex shader

Shader Edit & Replay

# Export shader source
rdc shader 42 ps --source > shader.glsl

# Build modified shader
rdc shader-build shader_modified.glsl --encoding GLSL

# Replace and re-render
rdc shader-replace 42 ps --with 123

# Restore original
rdc shader-restore 42 ps

Frame Comparison

rdc diff /tmp/before.rdc /tmp/after.rdc --shortstat
rdc diff /tmp/before.rdc /tmp/after.rdc --draws --json
rdc diff /tmp/before.rdc /tmp/after.rdc --framebuffer --diff-output diff.png

Debugging Recipes

Object is invisible

1. Open capture, list draws — is the draw present?
2. Check rasterizer state — backface culling direction?
3. Check depth state — depth test/write enabled?
4. Check blend state — alpha zero?
5. Export vertex buffer — are positions correct?
6. Debug vertex shader — is clip position valid?

Colors are wrong

1. Export render target at the draw — what color is actually written?
2. Check descriptor bindings — correct texture bound?
3. Check blend state — unexpected blend mode?
4. Debug fragment shader — trace the computation
5. Check uniforms/constants — correct values?

Source Code

• RenderDoc: ../gpu/renderdoc/ (github.com/baldurk/renderdoc)
  • CLI source: ../gpu/renderdoc/renderdoccmd/
  • Capture layer: ../gpu/renderdoc/renderdoc/
• rdc-cli: github.com/BANANASJIM/rdc-cli

Building (if not already built)

# RenderDoc (CLI only, no Qt GUI)
cd ../gpu/renderdoc
cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_QRENDERDOC=OFF -Bbuild -H.
cmake --build build -j$(nproc)

# Register Vulkan layer (one-time)
../gpu/renderdoc/build/bin/renderdoccmd vulkanlayer --register --user

# rdc-cli
pip install rdc-cli