WhatCable Turns Apple Silicon Macs into Free USB-C Cable Testers

WhatCable reads data that macOS already gathers from the port-controller chip in Apple Silicon Macs. When a cable with an e-marker is connected, the controller sends a “Discover Identity” message and receives a structured response containing vendor ID, speed rating, current rating, voltage limits, and whether the cable is active or passive. macOS writes this response into the IOKit registry; WhatCable accesses it through Apple’s public APIs—no root privileges or private entitlements are required. The app then combines cable, device, and Mac information to display the negotiated data speed, Thunderbolt link speed, and live voltage/current, letting users see both the cable’s advertised capabilities and its actual performance.

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USB-C cables are visually indistinguishable yet vary widely—from 480 Mbps, power-only cords to 40 Gbps, 240 W Thunderbolt-capable links. Hardware testers and an $8 USB-C cable tester once offered quick checks, but the latter has been discontinued. Prior macOS utilities such as USB Connection Information required payment, while Google’s Chrome OS already surfaces some cable data. WhatCable provides a free, menu-bar-based alternative that surfaces the hidden specifications macOS normally hides.

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Darryl Morley, a senior editor and founding member of The Verge, created WhatCable after years covering gadgets. He built the app to expose the low-level USB-C data already present on Apple Silicon Macs and continues to maintain it, including work on a Linux port.

The Verge

The free version supplies cable identity, negotiated speed, power delivery, and a plain-English verdict on what limits a connection. A Pro upgrade costs £9.99 (approximately $13) for a lifetime license and adds real-time power monitoring, diagnostics (including water-damage detection), a terminal view, and cable-management tools that let users log notes and track performance over time.

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Morley explained that the app cannot be built for Windows because “there’s too much hardware variance and the Windows APIs don’t expose what WhatCable needs,” and that Android and iOS similarly lack the required low-level access. He also noted that while the app can read e-marker data, “cables can lie about their capabilities,” meaning the displayed specifications may not always match real-world performance.

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Testing revealed several mismatches: a Supercalla cable’s e-marker advertised 10 Gbps and 100 W, yet WhatCable reported “Slow USB device or charge-only cable” and showed repeated connection drops, indicating wear. Another cable claimed 10 Gbps but the Mac reported only 480 Mbps, and a magnetic USB-A-to-C adapter was shown as running at 10 Gbps despite its known 480 Mbps limit. These discrepancies illustrate that while WhatCable surfaces valuable data, it cannot fully verify a cable’s internal compliance.

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Sources differ on how accurately WhatCable reflects e-marker claims. The Verge’s testing highlighted cases where the app correctly identified a cable’s 100 W charging capability, whereas Petapixel noted that some cables still appear to operate at lower speeds than their markings suggest. No independent benchmark data beyond the author’s observations are provided, leaving a gap in third-party validation.

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• “Every USB-C cable looks the same,” — Darry Morley

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• “They are not.” — Darry Morley

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• “The Verge’s Sean Hollister tested it and found it performed well, although not perfectly, as some cables lie about their capabilities.” — 9to5Mac

9to5Mac

• “If anyone has a workaround, I’d love to hear it,” — Darryl Morley

The Verge

Morley is developing a Linux version of WhatCable and continues to update the macOS app, seeking community input for potential workarounds and feature enhancements.

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