POST requests (eg. to the inbox) MUST be made with a Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams" and GET requests (see also 3.2 Retrieving objects) with an Accept header of application/ld+json; profile="https://www.w3.org/ns/activitystreams".

Requirement

POST requests (eg. to the inbox) MUST be made with a Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams" and GET requests (see also 3.2 Retrieving objects) with an Accept header of application/ld+json; profile="https://www.w3.org/ns/activitystreams".

Origin

The origin of this requirement is a quote from the ActivityPub specfiication:
prefixAn Activity sent over the network SHOULD have an id, unless it is intended to be transient (in which case it MAY omit the id). exact: |
POST requests (eg. to the inbox) MUST be made with a Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams" and GET requests (see also 3.2 Retrieving objects) with an Accept header of application/ld+json; profile="https://www.w3.org/ns/activitystreams".
suffix Servers SHOULD interpret a Content-Type or Accept header of application/activity+json as equivalent to application/ld+json; profile="https://www.w3.org/ns/activitystreams" for server-to-server interactions.
https://www.w3.org/TR/activitypub/ , § 7

id

urn:uuid:92af9475-24a2-4787-8c75-720d274bd2bc

uuid

92af9475-24a2-4787-8c75-720d274bd2bc

Full JSON

{
  "id": "urn:uuid:92af9475-24a2-4787-8c75-720d274bd2bc",
  "type": "Behavior",
  "uuid": "92af9475-24a2-4787-8c75-720d274bd2bc",
  "content": "POST requests (eg. to the inbox) MUST be made with a Content-Type of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\" and GET requests (see also 3.2 Retrieving objects) with an Accept header of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\".\n",
  "tag": [
    {
      "name": "ActivityPubClient",
      "id": "https://socialweb.coop/tag/ActivityPubClient"
    }
  ],
  "context": [
    "https://www.w3.org/TR/activitypub/",
    {
      "name": "Server to Server Interactions",
      "id": "https://www.w3.org/TR/activitypub/#server-to-server-interactions"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#server-to-server-interactions",
      "branch": [
        7
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "prefix": "An Activity sent over the network SHOULD have an id, unless it is intended to be transient (in which case it MAY omit the id).    exact: |\n",
      "exact": "POST requests (eg. to the inbox) MUST be made with a Content-Type of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\" and GET requests (see also 3.2 Retrieving objects) with an Accept header of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\".\n",
      "suffix": "Servers SHOULD interpret a Content-Type or Accept header of application/activity+json as equivalent to application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\" for server-to-server interactions.\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}

Links