{"error":"unknown","message":"data did not match any variant of untagged enum AnnouncableActivities"}

From my logs, it seems this is happening:

1. I make a valid ActivityPub request with the audience of a community (or cc a user).
2. Lemmy makes an ActivityPub request back at me at /.
3. My web site does not handle ActivityPub at /, and responds with a text/plain error message. This is default behavior in Drupal.
4. Lemmy rejects my request with a 400 response with the error message above.

I tested other instances by making the following get request to the root of each instance (thank you, Daniel Sternberg for maintaining curl):

curl -v -H "Accept: application/activity+json" -H "Content-Type: application/activity+json" <url>

• With the following results
  • Lemmy returns an Application Actor.
  • Misskey returns a 200 response in text/html.
  • Mastodon returns a 406 response in application/json.
  • NodeBB returns text/html.
  • GoToSocial returns a 406 response in application/json.
  • Mitra returns a 200 response in text/html.
  • Pixelfed returns a 200 response in text/html.
  • Peertube returns an empty 406 response.

My hypothesis is that Lemmy is rejecting the request because I return text/plain and/or it cannot handle a response with that content type.

To test that I am working on changing the default behavior. I will need to dynamically add a route based on system.site.page.front and return a 406 response in JSON. This is simple enough. If my hypothesis is correct, I can also make a merge request into drupal/activitypub module, which would have the same problem. If not, then I’ll need to go digging further.