Slack's Interactive Messages endpoint reports a Content-Type of `application/x-www-form-urlencoded` but responds with something like looks like `payload=%7B%22actions%22%3A%5B%7B%22...`, which is encoded JSON inside the payload parameter, which takes the API Gateway VTL templates for a loop.

slack-interactive-messages-api-gateway-mapping-template.vtl

## The `substring(8)` returns the input with everything after "payload=" which is the only parameter passed in,
## everything else is encoded JSON inside that parameter.
#set ($encodedJSON = $input.body.substring(8))
$util.urlDecode(${encodedJSON})

Thanks a million for posting this. I'd also add for anyone else struggling that I had to delete and rebuild the API with this template for it to work.

EDIT: I was making a novice mistake and hadn't deployed the API changes.

I added this mapping template but am still receiving the same error. First time using API Gateway + Lambda, so if there are any gotchas I could be missing in the setup, like needing to publish changes to the API Gateway or something please let me know.

Thanks a million. I had a body mapping I got from another site that was working for slash commands but wasn't working for interactions. Slack should really just send json

You are my savior! Thanks!

Saved my day. Thanks.