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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.
## 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))
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This isn't the most elegant mapping template ever, but it's what I used to get this payload transformed into JSON for an AWS Lambda function. Add this as an Integration Request Body Mapping Template in API Gateway, for the Content Type application/x-www-form-urlencoded.

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Holy crap. Wasted hours on this before I found this gist. Thank you.

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ghost commented Nov 24, 2017

Thanks, dude... really useful. Do you know why is that? I'm thinking about the reasons for this kind of decision from Slack's team.

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mjzuppe commented Mar 10, 2018

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.

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cgjerow commented May 18, 2018

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.

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StJohn3D commented Jul 7, 2018

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

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You are my savior! Thanks!

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Saved my day. Thanks.

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