Gearslip/README.md

Gearslip

Bring any app to your car screen.

What this is

Gearslip is a phone-side Android Auto client. It speaks the Android Auto protocol directly to a car head unit over USB and acts as a Car App Library host: apps that ship an Android Auto car UI (navigation and media apps) hand Gearslip a template, and Gearslip renders it natively on the car screen, full quality, no phone-screen mirroring. A separate screen-sharing mode is included for apps that don't ship a car UI at all.

Built by reverse-engineering the protocol for interoperability - getting a non-Google client talking to a car head unit that only expects Google's own app.

Status

Real progress, with one hard limit that's worth knowing before you build on this:

• Templated app hosting works. All 16 template types the Car App Library defines are rendered, including a keyboard Gearslip draws itself for search and sign-in. Verified against several real navigation and media apps.
• Media playback works. Car-enabled media apps are browsable and controllable from the car UI, with lyrics and playback state, over the same host connection.
• The connection to a real head unit needs a certificate Google controls, and Gearslip doesn't ship one. A head unit only accepts a phone identity that chains to Google's Automotive Link certificate authority - something only Google issues. Without one, Gearslip still runs and renders everything correctly against a debug harness, but a real car will reject the connection outright. See docs/SPIKE_FINDINGS.md for what was tested, including one publicly-known leaked certificate that works on some older head units and is rejected by newer firmware and by Google's own reference tooling.
• No adb, root, or Shizuku is required to run it. Every feature is gated on an ordinary Android permission or an on-device consent dialog - nothing needs privileged access, on the phone or in the car.

Building

./gradlew :gearslip:assembleDebug

The APK lands at build/Gearslip-debug.apk.

Further reading

• docs/BUILDING_APPS.md - how to write apps Gearslip can render: templated apps, media apps, and the Gearslip app protocol.
• docs/SPIKE_FINDINGS.md - the certificate investigation in full: what was tried, what a real head unit does and doesn't accept, and why.
• docs/CUSTOM_HEADUNIT_SCOPING.md - the original scoping of what a custom head unit client would need.

License

AGPL-3.0 - see LICENSE. A separate commercial license is available for private use outside the AGPL's terms; see NOTICE.md.