In ActivityPub, these are all equivalent:

@reiver@mastodon.social That’s not true. Most of those are not correct AS2. Only ‘"type": “Banana"’ is correct.

Activity Streams says, The serialized JSON form of an Activity Streams 2.0 document MUST be consistent with what would be produced by the standard JSON-LD 1.0 Processing Algorithms and API [JSON-LD-API] Compaction Algorithm using, at least, the normative JSON-LD @context definition provided here.

So, assuming that you’ve defined some Banana extension, your full code might look like this:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "ext": "https://banana.example/",
      "Banana": {
        "@type": "@id",
        "@id": "ext:Banana"
      }
    }
  ],
  "type": "Banana"
}

If you park all that context data at a context URL, you can have a much slimmer payload:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://banana.example/"
  ],
  "type": "Banana"
}

If you don’t define the extra context stuff, you have to use the full URL for the type:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "https://banana.example/Banana"
}

I prefer the slim one with the context URL.

If you use a JSON-LD library for parsing your input, all the funky variants you’re sharing get canonicalized into one easy-to-use one. And if you use it for formatting your output, you’ll always produce nice, clean AS2.