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.

What it shows

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
• "Show technical details" toggle revealing the underlying IOKit properties for engineers

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

Install

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.

How it works

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.

Build from source

swift run WhatCable

Requires Swift 5.9 (Xcode 15+).

Build a distributable .app

./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

Caveats

• 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.

Contributing

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

Credits

Built by Darryl Morley.

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