matrix-spec/event-schemas/schema/core-event-schema/state_event.json

{
  "type": "object",
  "title": "State Event",
  "description": "In addition to the Room Event fields, State Events have the following additional fields.",
  "allOf":[{
    "$ref": "room_event.json"
  }],
  "properties": {
    "state_key": {
      "type": "string",
      "description": "A unique key which defines the overwriting semantics for this piece of room state. This value is often a zero-length string. The presence of this key makes this event a State Event. The key MUST NOT start with '_'."
    },
    "prev_content": {
      "title": "EventContent",
      "type": "object",
      "description": "Optional. The previous ``content`` for this event. If there is no previous content, this key will be missing."
    }
  },
  "required": ["state_key"]
}