Nope, that didn’t work.

Hitting a non-existent endpoint with curl:

HTTP/1.1 404 Not Found
cache-control: max-age=0, private, must-revalidate
content-length: 1333
content-type: text/markdown; charset=utf-8
date: Mon, 12 Apr 2021 16:33:02 GMT
server: Cowboy

# Phoenix.Router.NoRouteError at POST /does_not_exist

Exception:

    ** (Phoenix.Router.NoRouteError) no route found for POST /does_not_exist (MyAppWeb.Router)
# <snip>

And it’s returning markdown text? Why?

I am still trying to understand how do I just intercept ALL errors and wrap them in keys in a resulting JSON object.

Sorry for not having given you a good solution

But did you try the Error view approach? I think that one should work

Because you are in development mode. Go to config/dev.exs and set debug_errors to false.