The only info I’m aware of is this (perhaps you’ve already found this). Scroll down the post to the Slack bit:
Seems like Fly soft launched this feature. Are you running that CLI command from the folder the app’s fly.toml is in? If not I’d guess you’d need to provide the app name as a flag. If you are, does it need the type to list? Like fly checks handlers list --type slack? Total guess while waiting for someone from Fly to jump in.
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