It looks like the value of the content type overrides the format. See

I'm not sure what the safest way is to reverse this? Could it literally just be reversed?

If this test shows the intended behaviour, I don't think we'll be able to override the output by just changing the order the api_format is looked up in.

If we did, it would break the behaviour where there is an explicit format set, but the implementation expects to be able to override it by changing the content type.

For example:

format :json

def action
  content_type "text/plain"
  "plain text"
end

would still output as JSON if we reversed the lookup to api_format = env[Grape::Env::API_FORMAT] || mime_types[headers[Grape::Http::Headers::CONTENT_TYPE]] because it would always pull out :json first.

I don't think there's any non-breaking way to override the behaviour of one of the content-type associated formatters at the moment on a per-request basis.

So it looks like a bug because we explicitly document You can override this process (content negotiation) explicitly by specifying env['api.format'] in the API itself. Since you already have a bunch of examples, turn them into specs in Grape? We can discuss how to fix it, flipping the order on that line should be a start. And it does look like it would be breaking either way, so maybe we should remove support for env['api.format'] that doesn't work, and find another way to do it.

Related, what's a real use-case where you want to do this? Is it that you want to produce actual JSON and leave the formatter alone? Why not rely on the formatter to do this work?

here's my use case:
I build JSON in the database and want to return it as is, no additional formatting is needed

Yeah. Our use case is similar. We have cached data that's already JSON that we'd like to just return vs having to parse and re-encode.

I'll take a look at starting a PR tomorrow with some options. 👍

The PR in #2505 fixes the issue but is not being finished. If anyone here wants to take it on, we would appreciate it!

@dblock you might want to check my PR #2506

@dblock I think we should add a feature to return unformatted data aka raw data. Lots of people already have already formatted JSON and there's no need to reformat it again obvisouly. Setting binary seems wrong to me and setting :json will still call the JSON formatter.

It's kind of a combination of content_type and setting raw: true

@ericproulx It's not a bad idea, let's see it. If I understand correctly you want something that would bypass a formatter, and it makes sense because the intent of all these existing methods is to set a content-type along with data (binary should be (is?) binary/octet-stream).