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`-v10` will print _a lot_ more information which might give a hint to what's failing

Finally had time to investigate this. Even `-v10` didn't give any clues which key was incorrect. Did a binary search by removing half the configuration at a time and found the offending key with a lot of trial&amp;error

Among  the several hundred keys under `aws:endpoints` that I need in my localstack setup, there was
`mobileanalytics: <http://localhost:4566>` which probably has been a real thing at some point (since I for sure didn't add any of these manually) but has since been removed

Pretty frustrating experience but now it works at least

Wait is that `aws:mobileanalytics` or just `mobileanalytics`?

Without `aws:` like all of the other ones under `aws:endpoints`, I guess the provider prefix is only mandatory on the top level keys

Ah I get you, a subkey of `aws:endpoints`. I'd suggest raising an issue about this at <https://github.com/pulumi/pulumi-aws/issues>.