{
"name": "subject-fallback-groups-headerless",
"description": "Headerless replies with the same normalized subject are merged.",
"spec": {
"source": "JWZ",
"url": "https://www.jwz.org/doc/threading.html",
"behavior": "subject fallback joins messages with the same base subject when no headers exist"
},
"input": [
{
"id": "<subject-root@example>",
"message_id": "<subject-root@example>",
"in_reply_to": null,
"references": [],
"subject": "Hello",
"date": "2026-05-15T09:00:00Z"
},
{
"id": "<subject-reply@example>",
"message_id": "<subject-reply@example>",
"in_reply_to": null,
"references": [],
"subject": "Re: Hello",
"date": "2026-05-15T09:05:00Z"
},
{
"id": "<subject-localized@example>",
"message_id": "<subject-localized@example>",
"in_reply_to": null,
"references": [],
"subject": "AW: Hello",
"date": "2026-05-15T09:10:00Z"
}
],
"expected": {
"threads": [
{
"root": "<subject-root@example>",
"messages": [
"<subject-root@example>",
"<subject-reply@example>",
"<subject-localized@example>"
]
}
]
}
}