{
  "capability": "events",
  "name": "a module emits an event",
  "why": "The envelope is what two implementations can disagree about without either failing: a missing header, a header spelled differently, or a body nested where metadata belongs. None of those stop a mesh running; they stop it reacting.",
  "given": {
    "module": "shop",
    "node": "one",
    "key": "order.placed",
    "body": { "id": "a1", "total": 12 },
    "headers": {
      "x-event-id": "0123456789abcdef0123456789abcdef",
      "x-source": "shop",
      "x-node": "one",
      "x-time": "2026-09-26T12:00:00Z",
      "content-type": "application/json"
    }
  },
  "wire": {
    "subject": "mesh.mod.shop.event.order.placed",
    "requiredHeaders": ["x-event-id", "x-source", "x-node", "x-time", "content-type"],
    "optionalHeaders": ["x-causation-id", "x-schema"],
    "headerFormats": {
      "x-time": "RFC3339",
      "content-type": "application/json",
      "x-event-id": "^[0-9a-f]{32}$"
    },
    "payloadIs": "the body alone, not the envelope",
    "keyRecoveredFrom": "the subject, after the .event. token"
  },
  "refuses": [
    {
      "what": "an envelope nested in the payload",
      "why": "an implementation that publishes the whole envelope as the body passes all of its own tests and is unreadable to every other"
    },
    {
      "what": "a missing x-event-id",
      "why": "delivery is at-least-once and only the emitter can say which of two messages is a redelivery"
    },
    {
      "what": "an x-source that differs from the subject's module",
      "why": "the bus enforces the namespace, so a disagreement means the envelope is lying about its origin"
    }
  ]
}