Android 13 is slowly rolling out to Pixel phones, but here's something to consider when that update message finally pops up on your device: You can never go back.

Google is apparently changing the way Android updates are enforced on its latest devices. A new warning message on the Pixel Factory Image page says that the Pixel 6, 6 Pro, and 6a can never go back to older versions of Android once they update:

Warning: The Android 13 update for Pixel 6, Pixel 6 Pro, and the Pixel 6a contains a bootloader update that increments the anti-roll back version. After flashing an Android 13 build on these devices you will not be able to flash older Android 12 builds.

Anti-rollback was first introduced in Android 8 as a security feature. Google can patch all the exploits it wants, but security fixes are meaningless if an attacker can just roll back a device to a previous version that's full of security holes. Rollback protection works by recording the newest installed version into tamper-evident storage that persists across device wipes, and now the system knows if it's on an old version or not. Previously, this feature would just show a warning message on boot (and it looks like that will still happen on the Pixel 5 and lower), but now, Google plainly says of the Pixel 6, "You will not be able to flash older Android 12 builds."

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It's not clear why only the Pixel 6 is affected by this change. If you don't count Android 12L, this is the Pixel 6's first major OS update. The three phones listed are also the only three phones that use Google's first in-house SoC, the Google Tensor, so maybe the chip is flexing its muscles with new anti-downgrade capabilities.

This isn't a big deal for most consumers, but in previous Android versions, it was nice to have an escape hatch if Google came out with a particularly buggy first release. If you frequently try out different software builds, this change will presumably mean that you can't use any older third-party ROMs, either.

Update: We have a justification! As spotted by Esper's Mishaal Rahman, the Android web flash tool explains this downgrade prevention is to stop a bootloader vulnerability specific to Android 12.