This free Mac app reveals the truth about mysterious USB-C cables

I showed you that nearly three years ago Great cable tester for $8 It quickly tells you if your USB-C cable is fast, slow, strong or weak. Unfortunately, this tool was discontinued, and I haven’t found anything as intuitive or inexpensive since. But if you have a Mac with Apple Silicon chips, you can simply download an even more impressive testing tool for free.

It’s called WhatCableIt works by reading data your Mac already collects about connected USB devices, data that Apple doesn’t normally pass to you. Simply click on the little widget located in the menu bar at the top of your Mac, and you can see every USB-C cable and device connected to your computer.

Here’s how creator Daryl Morley explained it to me:

Every Apple Silicon Mac has a port control chip that handles USB power delivery negotiations. When you connect a cable with an electronic tag, the port controller sends a “detect ID” message to the chip in the cable and gets back a structured message: vendor ID, speed rating, current rating, voltage limits, whether active or passive, and so on.

macOS writes this response to the IOKit log. WhatCable reads them using Apple’s public APIs. No root access, no special entitlements. The data is not hidden, Apple’s firmware negotiates and publishes the result. They don’t appear anywhere in the standard macOS tools. WhatCable reads what’s already there.

The electronic tag is a single source. WhatCable also reads from your Mac’s own hardware, the actual negotiated connection speed, the Thunderbolt link speed, and the live voltage and current at each port. The connected device tells us who it is, who made it, and what it supports. Put the three together—the cable, the device, and the Mac—and WhatCable can tell you not just what everything claims to support, but what’s actually happening on the connection right now, and which part is a bottleneck if something doesn’t work as expected.

Do you want to see it in reality? I took photos while testing some of my favorite cables this week. It’s not a perfect solution, as cables can lie about their capabilities, but WhatCable really helped me find a bad cable along the way.

When I plugged the short, lightweight Satechi cable you see above into two ports on my MacBook Pro, I got this:

I know from experience that this information is correct, meaning it is still a valuable cable. USB 2.0 at 480Mbps is very slow, but the cable self-reports and can charge at 100W, almost as fast as it can charge my Mac.

This is a little more useful information than an $8 tester can provide. It also shows that the cable only offers USB 2.0 speeds and probably offers 60W charging or better due to an electronic tag. But it’s not possible to read the electronic tag data to know that this cable supports 100W charging speeds.

I definitely get more than 60 watts when I connect a 140 watt battery to my Mac:

WhatCable can detect that I’m connected to a 100W charger too:

Now, let’s try one of them My 5 favorite USB-C cables However – Supercalla 10Gbps 100W cable with magnetic beads:

That’s weird: the cable’s label claims 10Gbps and 100W, but your Mac doesn’t handle it that way!

When I connect a 10Gbps SSD, I don’t get that speed with this wire:

This seems to be because my daily driver cable is finally starting to wear out. I think it’s time to retire this one!

Now let’s try the latest and greatest cable in the drawer in theory: the 240W 40Gbps USB4 cable.

Again, the electronic tag appears to validate these speeds, even if the Mac itself isn’t connected to that rate.

Once the drive is connected, WhatCable detects that your Mac has a 10Gbps link:

This is more like it: a 25GB transfer is measured in seconds instead of minutes:

Here’s a cable that arrived at my house a few days ago exclusively for 100W charging. I wouldn’t expect much more than 480Mbps USB 2.0 data; On Amazon’s website, the company only advertises USB 2.0 speeds:

But WhatCable says their electronic label advertises 10Gbps USB 3 data…could that be the case?

I’m afraid not: This cable’s electronic tag has written checks her body can’t cash. Minutes, not seconds, for the same 25GB transfer:

Here, our $8 test device did a better job, immediately detecting that the cable didn’t support SS (SuperSpeed, aka USB 3).

It offers 5A charging speeds, though:

Next, I thought about connecting a USB-A to USB-C magnetic accordion cable, which of course is only capable of USB 2.0 speeds of 480Mbps:

Oddly enough, my Mac insists it’s running at 10Gbps…while connected to my external battery. That seems wrong!

Last but not least, here’s the old cable that came with the LaCie drive I bought in 2019, the one I always turned to for stability and speed:

It’s reported as a 20Gbps Thunderbolt cable, although it says 10Gbps at the end. I don’t have one handy, but I’ll try it with a Thunderbolt drive to check!

Morley isn’t the first to realize that the MacBook can be a USB-C cable tester. USB connection information It is a similar paid application that arrived a year ago. But Morley’s version is free, and he told me it will “always be free at its core”, although you can pay £9.99 to get it. Pro version Which provides real-time power monitoring, diagnostics, and terminal display.

He has now also built a simpler version of the idea called waterport Which simply monitors what each USB-C port on your Mac is doing right now, including power, data, and video.

Morley told me he wouldn’t be able to build a version of WhatCable for Windows because “there’s too much hardware variability and the Windows APIs don’t expose what WhatCable needs,” and he says Android and iOS don’t provide enough low-level access.

“If anyone has an alternative solution, I’d love to hear about it,” he says.

But it already works on a Linux port, and the Mac version keeps getting updated. You can follow along with updates On his GitHub page.

Photos by Sean Hollister/The Verge