{
"name": "missing-message-id-reply-to-present-parent",
"description": "A message without Message-ID can still be threaded under its In-Reply-To parent.",
"spec": {
"source": "RFC 5256",
"url": "https://www.rfc-editor.org/rfc/rfc5256",
"behavior": "assigned unique IDs still participate in parent-child links from references"
},
"input": [
{
"id": "parent",
"message_id": "<missing-child-parent@example>",
"in_reply_to": null,
"references": [],
"subject": "Parent",
"date": "2026-05-15T09:00:00Z"
},
{
"id": "missing-child",
"message_id": null,
"in_reply_to": "<missing-child-parent@example>",
"references": [],
"subject": "Re: Parent",
"date": "2026-05-15T09:05:00Z"
}
],
"expected": {
"threads": [
{
"root": "parent",
"messages": [
"parent",
"missing-child"
]
}
]
}
}