Convenience I think would be the main reason. I assume your problem with Plug’s design is that generally it is a bad idea to use errors to control flow. But since we generally want to respond with a user friendly error even when an exception is not expected, this kind of logic seems required here. And once it’s necessary it seems too convenient not to take advantage of in cases where the exception is not strictly speaking unexpected but does not and could not require any special logic other than building the response, because the alternative is needing separate and explicit handling for every single API requirement (like parameter shape/format).

How did you try that? Because the http request should receive whatever you send as response in the handler. It shouldn’t matter that the error is reraised (e.g. to get properly logged).

See Phoenix.ConnTest — Phoenix v1.7.7 for testing those.

While the latter is a viable option the former is not generally true. Exceptions are the only way to handle errors from LV right now (via Plug.ErrorHandler) and phoenix itself for a long time didn’t have action_fallback while already having means of handling exceptions.

Are you using Phoenix? My memory was that this works out of the box.

But maybe take a look at Plug.ErrorHandler — Plug v1.20.2

My understanding (albeit potentially flawed) is that there are three methods for handling errors, and two places for defining the error response in Phoenix:

1. Return an :error tuple in the controller and use the fallback controller, as @D4no0 mentioned. Unfortunately, this is not helpful for you, as you’re dealing with a raised Ecto.NoResultsError.
2. Handle the exception one level up in the router using Plug.ErrorHander, as @tfwright mentioned. However, I believe that this is more designed for handling side effects, as the error is reraised to be handled by point 3 below.
3. Use the error view. Plug wraps up all exceptions that occur during the request/response, and these are caught by Phoenix in the Endpoint, which then renders a special error view. You can customise this to return the JSON response you like - I think this is the best solution. See the documentation.

Phoenix does a bit more, but conceptionally it works exactly the same as Plug.ErrorHandler. It also reraises errors: phoenix/lib/phoenix/endpoint/render_errors.ex at 246ca7726e14ed94d3d878838cbd6add5a0df3e9 · phoenixframework/phoenix · GitHub