Bootloop of any kind or stuck boot process

Don't panic. None of the commands we advised you to use can damage or brick your watch permanently.
You can always go back to the fastboot bootloader menu, by using the manual finger combos for your watch.

• Verify that you used the correct image files

  Some devices like the Asus Zenwatch 2, the Fossil Gen4 models and the MTK6580 watches have more than one system image supplied, for different variants with slightly different hardware. Most watches have a graphical fastboot menu that shows the correct codename for the watch. Others need to be identified using the hardware differences noted above in the install section.

• Clean your watch and charger contacts

  Even in case the fastboot flash process succeeds with no errors, it can still silently fail due to connection problems caused by dirty contacts. Clean the pins and pads with isopropyl alcohol. Ideally using that old toothbrush you keep around for electronics cleaning or a microfiber cloth. Carefully sanding the contacts (e.g. with a nailfile) can help in case of obvious corrosion.

• Ensure a stable USB connection

  Put the watch and your computer on the same flat surface. Try to not move the cable connecting your computer and the watch during the whole flashing process. In case of worn or corroded contacts and pogo-pins that lost their springiness, it might be useful to attach the watch to its cradle or charging cable using duct tape or rubber bands.

• Repeat the flash process

  Some users have reported needing up to five flashing attempts for a successful result.
  There is a small chance that the image files were corrupted during download. To guard against this, it's recommended to verify the checksums. Download the SHA512SUMS file ( harmony inharmony ) and verify the checksums with sha512sum --check SHA512SUMS. The checksums for both files will be checked.

Boot gets stuck even after repeated reflash

Congratulations, you might have found a new issue for us to solve. While this should not happen with stable builds, the nightlies are advancing quickly, and are not tested on all watches. You are likely using a nightly build, as the installation pages currently link to those. You can try again and install a stable build when available.
To help us solve the problem, you could try to collect logs. It is possible that the watch does not show a UI but is already accessible using ssh ceres@192.168.2.15 or adb shell in the state the boot process stops. In case SSH connection works, try the following commands to write logs to local .txt files.

• ssh root@192.168.2.15 "journalctl" > journal-watch.txt
• ssh root@192.168.2.15 "/system/bin/logcat" > logcat-watch.txt

or, if your watch presents an ADB interface, use the following commands:

• adb shell journalctl > journal-watch.txt
• adb shell "/system/bin/logcat" > logcat-watch.txt

In case no connection is available, it is still interesting to see if the watch leaves any traces in your local dmesg. Try to spot USB related messages in sudo dmesg -w while the watch is connected and booting. Record the log using the sudo dmesg > dmesg-local.txt command.
You can share those logs in a new Github issue tagged with the watch codename. For support chat, you are invited to join our Matrix channel.

Watch only boots when powered via USB, but shuts off when booted while worn

This is a clear sign of a worn battery that will soon fail. Maybe you should replace it.
The boot process of AsteroidOS uses all cores of a watch to ensure a fast startup. This leads to power spikes during the boot process, which can completely shut down watches with worn out batteries. It has been observed that WearOS can still boot on watches with worn out batteries, but AsteroidOS cannot, depending on the progress of battery wear. However, AsteroidOS can usually still provide a useful 10-20 hours of uptime when the watch is worn once booted while powered up.
Batteries are relatively easy to replace on watches with screwed down backs. Glued back plates usually require creative solutions involving hair driers.

Watch successfully boots into the AsteroidOS user interface, but immediately reboots

This is a known behavior when the battery charge is very low. The watch usually recovers from this condition by simply boot cycling for some time. All watches also charge in fastboot or recovery mode. Letting the watch dwell on the fastboot or recovery menu screen for some time should fix the problem even faster.

Watch is not detected and all fastboot commands fail

When using Linux,

try prepending sudo to the commands to execute them with root privileges. If that works, you are missing UDEV rules for Android devices that would allow your user to execute the commands. Many package managers provide the android-udev-rules, which avoids the need to manually set up these UDEV rules. If the package is not available on your system, you can follow the instructions at the official android-udev-rules repository.

On Windows systems,

this usually happens due to driver issues. If you have installed an Android USB driver other than the official Google USB driver that we link to in the installation fields above, please remove it. Then reinstall the Google USB driver or update it by installing a possibly newer version.
We find that the Linux fastboot drivers are more reliable. It is very easy to create a bootable live Linux USB stick, for example, following these instructions at askubuntu.com, and then follow the above Linux installation commands after booting the live Linux USB stick.

• USB 3.0 ports

have caused all kinds of random and hard to track problems for many projects using ADB and fastboot. If your ADB and fastboot still can't connect after following the steps above, it might be worth switching to a USB 2.0 port. If your shiny new computer no longer has such a port, a cheap USB 2.0 HUB may help.

Fastboot command stops with error during the ~5 minute flashing process

Smartwatches are prone to lose a proper connection to their cradle over time due to sweat corroding the watch pins.
Clean the pins and pads with isopropyl alcohol and repeat the flashing process.
Try to ensure a stable USB connection by putting the watch and your computer on the same flat surface. Try to not move the cable connecting your computer and the watch during the whole flashing process. In case of worn or corroded contacts and pogo-pins that lost their springiness, it might be useful to attach the watch to its cradle or charging cable using duct tape or rubber bands.

Fastboot command displays "invalid sparse file format at header magic" when starting to flash the device

This is not a fatal error and can safely be ignored. Fastboot expects images to be in sparse format. When a raw image is supplied like in case of most of the AsteroidOS watches, fastboot converts it to sparse and shows above message.