AJAX Add to Cart doesn't work if admin-init has output?

That can be true, though it depends on the output.

What is the output, do you know?

In my case, they were PHP notices.

Notices will often cause ajax requests to break (for all plugins). The best way to fix that is to fix the cause of the notices.

I solved this very problem in my plugin by making each request return ‘ok’ when successful and then looking for that ‘ok’ in the javascript caller function. This way, whatever output there may be (even your own plugin legitimate one), as long as the ‘ok’ is found, no problems arise.

Unfortunately, even?though I expect all plugins to be notice free and all, they are not, so I believe it is better to design plugins to deal with this.

Unfortunately, even though I expect all plugins to be notice free and all, they are not, so I believe it is better to design plugins to deal with this.

Absolutely, and we do the best we can.