@siteed/capture-helper

Generic window capture helper for agents, automation tools, and evidence pipelines — macOS (ScreenCaptureKit) and Linux/X11 (XComposite + ffmpeg).

On macOS, capture-helper is a small Swift CLI built on ScreenCaptureKit: it discovers windows, captures exact window targets, streams H.264 Annex B bytes, and records MP4 evidence with a native AVFoundation writer (no ffmpeg). On Linux it reproduces the same CLI/protocol over X11 using an XComposite grabber plus ffmpeg — see Linux support.

Status

Early standalone extraction from Farmslot's internal tools/capture-helper.

The CLI is intentionally generic. Product-specific concepts such as Farmslot slots, iOS simulator aliases, MetaMask runners, or recipe semantics belong in the caller.

Requirements

• macOS 13.0+ (Apple Silicon or Intel)
• Screen Recording permission for the terminal or parent app
• Optional: ffmpeg for external workflows that still want it; macOS record and record --framed session snapshots are native.
• Swift/Xcode is only required when building from source — the published npm package and Homebrew formula ship a prebuilt universal binary.

Linux support

On Linux the same CLI/protocol is implemented by a Node backend that drives a small C grabber (XComposite per-window capture) and ffmpeg for H.264 encoding. The capture-helper command auto-dispatches to it on Linux; commands, flags, the framed stream protocol, and JSON events are identical to macOS (see docs/protocol.md).

Requirements:

• X11 (Xorg) session. GNOME-on-Wayland per-window capture is portal-gated and not automatable — log into an Xorg session (e.g. "Ubuntu on Xorg", or set WaylandEnable=false in /etc/gdm3/custom.conf). A graphical desktop must be logged in (the GDM greeter has no app windows; use autologin for headless nodes).

• Captured apps must be X11 clients (Wayland-native surfaces are invisible to the X path). Force X11 where needed: GDK_BACKEND=x11, QT_QPA_PLATFORM=xcb, Electron --ozone-platform=x11.

• Build/runtime deps (ffmpeg is required on Linux):

  sudo apt install -y gcc ffmpeg libx11-dev libxcomposite-dev libxdamage-dev libxfixes-dev libxext-dev
  # or run the helper:
  bash scripts/setup-linux-node.sh

Hardware H.264 encoding is opt-in via --encoder h264_nvenc (or CAPTURE_HELPER_ENCODER=h264_nvenc); the default libx264 works everywhere. Verify readiness with capture-helper doctor --json. On Linux, id values are X11 window ids (XIDs).

Install

Use with npx without installing globally:

npx -y @siteed/capture-helper@latest doctor --json
npx -y @siteed/capture-helper@latest list --json

Install globally with npm (primary path used by Farmslot and MetaMask farm installers):

npm install -g @siteed/capture-helper
capture-helper doctor

Install with Homebrew (auto-taps deeeed/tap; no separate tap step):

brew install deeeed/tap/capture-helper
capture-helper doctor

doctor prints human-readable output by default. Use doctor --json when a script or installer needs machine-readable output (Farmslot and farm installers pass --json).

Download the native release binary directly:

curl -L https://github.com/deeeed/capture-helper/releases/latest/download/capture-helper-darwin-universal \
  -o /usr/local/bin/capture-helper
chmod +x /usr/local/bin/capture-helper
xattr -d com.apple.quarantine /usr/local/bin/capture-helper 2>/dev/null || true

If macOS Gatekeeper blocks a curl-downloaded binary, prefer npm install -g or brew install deeeed/tap/capture-helper — those paths do not attach quarantine. For a manual download, remove quarantine with xattr -d (above) or approve once in System Settings → Privacy & Security.

Build from source

swift build -c release
# or
npm run build:native

The npm build script copies the release binary to:

native/capture-helper

When installed as an npm package, postinstall verifies the bundled native binary (smoke test + SHA256 checksum). On macOS, if the prebuilt binary is missing or broken and Swift is available, it rebuilds from source; otherwise install fails loudly with the next command to run (brew install deeeed/tap/capture-helper or retry npm). On Linux it compiles the X11 grabber (native/x11-grabber) via gcc when possible. Set SITEED_CAPTURE_HELPER_SKIP_POSTINSTALL=1 to skip postinstall.

Commands

# Human default: list likely capturable windows
capture-helper
capture-helper -l

# Version / provenance
capture-helper version
capture-helper --version

# Environment readiness and permissions diagnostics (human-readable by default)
capture-helper doctor
capture-helper doctor --json
# includes stable codes like screen_recording_denied, window_server_unavailable, ffmpeg_missing

# Request/open macOS Screen Recording permissions where possible
capture-helper permissions
capture-helper doctor --open-permissions --json

# List windows as a machine-readable JSON object
capture-helper list --json

# Human-readable table
capture-helper list --human
capture-helper -l
capture-helper list -h
capture-helper list --on-screen --capturable --human
capture-helper list --all --human

# Legacy JSON-lines listing
capture-helper --list-windows
capture-helper list --json-lines

# Capture a specific target as raw H.264 Annex B
capture-helper capture --window-id 12345 > /tmp/capture.h264
capture-helper capture --pid 12345 > /tmp/capture.h264
capture-helper capture --app-name Simulator --window-name "mm-1" > /tmp/capture.h264

# Legacy capture syntax remains supported
capture-helper --window-name "Simulator" > /tmp/capture.h264

# Framed multi-window stream with stdin control
capture-helper stream --framed --window-id 12345 > /tmp/windows.h264

# Record MP4 evidence (macOS: native AVFoundation, no ffmpeg; Linux: ffmpeg)
capture-helper record --window-id 12345 --duration 5 --output evidence.mp4 --open

# Record MP4 plus PNG proof frames from the same macOS native recording stream
{ sleep 1; echo "snapshot step-1.png"; echo stop; } | \
  capture-helper record --framed --window-id 12345 --output evidence.mp4

Resolve and snapshot

resolve lets agents debug target selection before starting video capture:

capture-helper resolve --app-name "Google Chrome" --window-name "MetaMask"

It returns the selected window, selector type, and all candidates considered for that selector.

snapshot captures a one-frame PNG using the same target selectors:

capture-helper snapshot --window-id 12345 --output screenshot.png

Target selectors

Prefer selectors in this order:

1. --window-id from capture-helper list --json for exact capture.
2. --pid when the caller owns the process tree and wants the largest suitable window.
3. --app-name + --window-name for human-friendly fallback matching.
4. --window-name alone only for ad hoc use.

npm wrapper

This package exposes a Node wrapper so JavaScript-based agents can call the native binary through a normal bin entry:

node bin/capture-helper.js doctor --json

The wrapper resolves the binary in this order:

1. SITEED_CAPTURE_HELPER_BIN
2. native/capture-helper
3. .build/release/capture-helper
4. /opt/homebrew/bin/capture-helper
5. /usr/local/bin/capture-helper

Output contract

• raw capture stdout: H.264 Annex B byte stream
  • 4-byte start codes: 00 00 00 01
  • SPS/PPS emitted before keyframes
  • baseline profile, no B-frames
• doctor: human-readable by default; --json for machine-readable output
• list / version: JSON on stdout by default
• streaming/capture diagnostics: JSON lines on stderr
• command failures: JSON error lines on stderr with stable code values such as target_required and window_not_found
• signal handling: SIGINT/SIGTERM perform cleanup for direct capture; record --duration stops automatically

See docs/protocol.md for the framed stream and event contract.

Permissions

Grant Screen Recording permission to the terminal app, IDE, or agent host that launches the helper:

System Settings → Privacy & Security → Screen & System Audio Recording

Open that pane from the CLI with:

capture-helper doctor --request-permission --open-permissions

macOS grants Screen Recording to the app that launched capture-helper. If a desktop UI or agent host is failing but Terminal passes doctor, enable the desktop UI or agent host in System Settings, then restart that app.

After granting permission, restart the launching app. Use this to check readiness:

capture-helper doctor --json

If you see Code=-3801 / screen_recording_denied, especially over SSH, see docs/troubleshooting.md.

Integration principle

Keep this tool generic:

• good: window IDs, PIDs, app names, window titles, capture formats, diagnostics
• bad: Farmslot slots, project resources, simulator naming conventions, MetaMask-specific selectors

Higher-level tools should resolve their domain objects to a concrete macOS window target, then call capture-helper.

Development

swift build -c release
swift test # includes subprocess CLI/error-shape tests
npm run build:native
npm run doctor
npm pack --dry-run

Release

See docs/release.md.

License

MIT