What can this USB-C cable actually do?

A small macOS menu bar app that tells you, in plain English, what each USB-C cable plugged into your Mac can actually do, and why your Mac might be charging slowly.

USB-C hides a lot under one connector. Anything from a USB 2.0 charge-only cable to a 240W / 40 Gbps Thunderbolt 4 cable, all looking identical in your drawer. macOS already exposes the relevant info via IOKit; WhatCable surfaces it as a friendly menu bar popover.

Per port, in plain English:

• At-a-glance headline — Thunderbolt / USB4, USB device, Charging only, Slow USB / charge-only cable, Nothing connected
• Charging diagnostic — when something's plugged in, a banner identifies the bottleneck:
  • "Cable is limiting charging speed" (cable rated below the charger)
  • "Charging at 30W (charger can do up to 96W)" (Mac is asking for less, e.g. battery near full)
  • "Charging well at 96W" (everything matches)
• Cable e-marker info — the cable's actual speed (USB 2.0, 5 / 10 / 20 / 40 / 80 Gbps), current rating (3 A / 5 A → up to 60W / 100W / 240W), and the chip's vendor
• Charger PDO list — every voltage profile the charger advertises (5V / 9V / 12V / 15V / 20V…) with the currently negotiated profile highlighted in real time
• Connected device identity — vendor name and product type, decoded from the PD Discover Identity response
• Active transports — USB 2 / USB 3 / Thunderbolt / DisplayPort
• ⌥-click the menu bar icon (or flip the toggle in Settings) to reveal the underlying IOKit properties for engineers

Click the gear icon in the popover header to open Settings, where you can:

• Hide empty ports
• Launch at login
• Run as a regular Dock app instead of a menu bar icon
• Get notifications when cables are connected or disconnected

Right-click the menu bar icon for Refresh, a Keep window open toggle (handy for screenshots and demos), Check for Updates…, About, WhatCable on GitHub, and Quit.

Download the latest WhatCable.zip from the Releases page, unzip, and drag WhatCable.app to /Applications.

The app is universal (Apple silicon + Intel), signed with a Developer ID, and notarised by Apple — no Gatekeeper warnings.

Requires macOS 14 (Sonoma) or later.

WhatCable reads three families of IOKit services. No entitlements, no private APIs, no helper daemons:

Cable speed and power decoding follow the USB Power Delivery 3.x spec.

swift run WhatCable

Requires Swift 5.9 (Xcode 15+).

./scripts/build-app.sh

Produces a universal dist/WhatCable.app (arm64 + x86_64) and dist/WhatCable.zip.

Modes:

One-time setup for full notarisation:

# 1. Find your signing identity
security find-identity -v -p codesigning

# 2. Store notarytool credentials in the keychain
xcrun notarytool store-credentials "WhatCable-notary" \
    --apple-id "you@example.com" \
    --team-id "ABCDE12345" \
    --password "<app-specific-password>"   # generate at appleid.apple.com

# 3. Create your .env from the template
cp .env.example .env
# ...and fill in DEVELOPER_ID

• Cable e-marker info only appears for cables that carry one. Most USB-C cables under 60 W are unmarked. Any Thunderbolt / USB4 cable, any 5 A / 100 W+ cable, and most quality data cables will be e-marked.
• PD spec coverage: the decoder targets PD 3.0 / 3.1. PD 3.2 EPR variants may need tweaks once we see real data.
• Vendor name lookup is bundled but not exhaustive — common cable, charger, hub, dock, and storage vendors are recognised; others fall back to the hex VID.
• macOS only. iOS sandboxing makes USB-C e-marker access much harder.
• Not on the App Store. App Sandbox blocks the IOKit reads we depend on.

Issues and PRs welcome. The code is small and tries to stay readable — start at Sources/WhatCable/ContentView.swift for the UI, PortSummary.swift for the plain-English logic, or PDVDO.swift for the bit-twiddling.

Built by Darryl Morley.

Inspired by every time someone has asked "is this cable any good?".