A free macOS app called WhatCable identifies what a USB-C cable actually supports by reading e-marker data — electronic information embedded in modern cables — directly from the operating system, without needing extra hardware The Verge. The tool was built by developer Darryl Morley, runs on Apple Silicon Macs, and sits as a menu bar widget rather than a standalone application window The Verge.

Anyone who has rummaged through a drawer of visually identical USB-C cables looking for the one rated for 100W charging or 40Gbps data speed knows the core problem WhatCable solves. The app surfaces vendor ID, speed rating, power capacity, voltage limits, and whether a cable is active or passive — using only Apple's public APIs, with no root access or private system permissions needed The Verge. Beyond the cable's static specs, it also shows live data: the negotiated link speed, Thunderbolt speed, and the actual voltage and current flowing through each port right now The Verge.

The engineering behind this exploits how Apple Silicon Macs handle USB Power Delivery. Each machine has a dedicated port controller chip that negotiates power contracts with whatever is plugged in, and macOS records the e-marker responses it receives in the IOKit registry — Apple's kernel-level system for hardware driver data — as part of that process The Verge. Normally, that registry is accessed only by low-level system utilities. WhatCable's innovation is a clean read of data macOS was already collecting, exposed through documented public APIs rather than private or undocumented ones.

According to the project's GitHub documentation, the app cross-references three separate data sources: the Mac's USB-C port, the device or charger plugged in, and the cable connecting them GitHub. Cross-checking all three matters because a cable is only as capable as its weakest link: a 240W-rated cable connected to a 65W charger still negotiates at 65W. WhatCable's design makes that kind of mismatch visible rather than hidden.

The source code is public on GitHub at github.com/darrylmorley/whatcable, and the project site is at whatcable.uk The Verge. Morley is also developing an experimental Windows port, tracked separately at github.com/darrylmorley/whatcable-windows, though no release date has been announced GitHub.

This software approach differs sharply from how cable testing has historically worked. The Verge's Sean Hollister covered a physical hardware USB-C cable tester — priced at roughly $8 — about three years ago; that product was later discontinued The Verge. Older hardware testers relied on pass-through dongles with LED lights or small screens, adequate for checking a cable's static specs but unable to show what a live connection was actually negotiating.

Reading system-level power telemetry without private system permissions is a narrower technical achievement than it might seem. Apple has gradually tightened which system data third-party apps can access across recent macOS releases, and utilities that depend on undocumented registry access frequently break silently when the OS updates. Building WhatCable on documented, public interfaces rather than private hooks does follow the more defensible long-term strategy for developers, though whether this approach proves more durable across successive macOS versions will take time to demonstrate.

The cable identification problem itself stems from USB-C's versatility. USB-C is a single physical connector that handles USB 2.0 through USB4 and Thunderbolt 4/5, plus Power Delivery up to 240W under the latest Extended Power Range (EPR) spec. That universal design is exactly why you cannot tell two cables apart by looking at them: they can be identical and differ by ten times in bandwidth or power capacity. Free, native software that surfaces this information without extra hardware fits a broader pattern over the past decade: software tools have absorbed functions once served by standalone gadget categories. Phone diagnostic apps, network analysis tools, and battery health utilities have each displaced an earlier generation of physical testers.

For IT departments managing fleets of Apple Silicon machines, a free diagnostic tool with minimal system permissions could reduce reliance on physical cable testers for troubleshooting charging and display issues, though no enterprise adoption or endorsement has been reported to date.