Earlier this year, a Googler revealed that the “Check for update” button on Pixel and Nexus devices would actually let users pull down the latest OTA update from the server even if there is a limitation from Google on the rollout. However, the functionality only seemed to work for a short while as hundreds of Pixel and Nexus users tried pulling the latest Android 8.1 OTA update from Google by mashing the ‘Check for update’ button on their device to no avail.
Well, as it turns out, an API incompatibility between the Checkin code in Google Play Services and the System Update code broke the functionality of the ‘Check for update’ button. While Google has solved the bug internally, it would only be rolled out to everyone through a Play Services update in 2018.
UPDATE: a Google Play Services update after I posted this broke the feature again. (An API incompatibility between the System Update code and the Checkin code, if you’re interested in the details: bugs involving two separate moving parts are often the sneakiest.) This was fixed again internally, but late enough that it will be next year before this works again for non-Googlers. Sorry about that. We will also be improving the testing process that allowed this incompatibility to slip through.
So, if you are frantically pressing the ‘Check for update’ button on your Pixel or Nexus device to get the Android 8.1 update, it is better for you to stop since the button is not doing anything. Google has started a wider rollout of the update from earlier today so the update should land on your device within a day or two now.