{
  "kind": "payload_envelope",
  "expect": "invalid",
  "expect_error": "empty sentinel string in field `payload.payload_kind`",
  "description": "empty payload_kind must be rejected — empty sentinels are never legal",
  "data": {
    "schema_version": "lifeloop.v0.2",
    "payload_id": "pay-1",
    "client_id": "ccd",
    "payload_kind": "",
    "format": "client-defined",
    "content_encoding": "utf8",
    "body": "x",
    "byte_size": 1,
    "acceptable_placements": [
      { "placement": "pre_prompt_frame", "requirement": "required" }
    ]
  }
}
