The body of the POST request MUST contain a single Activity (which MAY contain embedded objects), or a single non-Activity object which will be wrapped in a Create activity by the server.

Requirement

The body of the POST request MUST contain a single Activity (which MAY contain embedded objects), or a single non-Activity object which will be wrapped in a Create activity by the server.

Origin

The origin of this requirement is a quote from the ActivityPub specfiication:
prefixThe request MUST be authenticated with the credentials of the user to whom the outbox belongs.
The body of the POST request MUST contain a single Activity (which MAY contain embedded objects), or a single non-Activity object which will be wrapped in a Create activity by the server.
suffix Example 11: Submitting an Activity to the Outbox
https://www.w3.org/TR/activitypub/ , § 6

id

urn:uuid:4d1c969c-46ac-4e27-8ec9-7ff7ef25c47d

uuid

4d1c969c-46ac-4e27-8ec9-7ff7ef25c47d

Full JSON

{
  "id": "urn:uuid:4d1c969c-46ac-4e27-8ec9-7ff7ef25c47d",
  "type": "Behavior",
  "uuid": "4d1c969c-46ac-4e27-8ec9-7ff7ef25c47d",
  "content": "The body of the POST request MUST contain a single Activity (which MAY contain embedded objects), or a single non-Activity object which will be wrapped in a Create activity by the server.\n",
  "tag": [
    {
      "name": "ActivityPubClient",
      "id": "https://socialweb.coop/tag/ActivityPubClient"
    }
  ],
  "context": [
    "https://www.w3.org/TR/activitypub/",
    {
      "name": "Client to server interaction"
    },
    {
      "name": "clients posting Activities to an actor's outbox"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#x6-client-to-server-interactions",
      "branch": [
        6
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "prefix": "The request MUST be authenticated with the credentials of the user to whom the outbox belongs.\n",
      "exact": "The body of the POST request MUST contain a single Activity (which MAY contain embedded objects), or a single non-Activity object which will be wrapped in a Create activity by the server.\n",
      "suffix": "Example 11: Submitting an Activity to the Outbox\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}

Links