WhatCable

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

— 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)

— when something's plugged in, a banner identifies the bottleneck: 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

— 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

— 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

— vendor name and product type, decoded from the PD Discover Identity response Active transports — USB 2 / USB 3 / Thunderbolt / DisplayPort

— 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:

Service What it gives us AppleHPMInterfaceType10/11 Per-port state: connection, transports, plug orientation, e-marker presence IOPortFeaturePowerSource Full PDO list from the connected source, with the live "winning" PDO IOPortTransportComponentCCUSBPDSOP PD Discover Identity VDOs for SOP (port partner) and SOP' (cable e-marker)

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:

Configuration Result No .env Ad-hoc signed. Works locally; Gatekeeper warns on other Macs. .env with DEVELOPER_ID Developer ID signed + hardened runtime. .env with DEVELOPER_ID + NOTARY_PROFILE Full notarisation + stapled ticket. Gatekeeper-clean for everyone.

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.

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.

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.

— 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.

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?".