{
"name": "same-subject-header-threads-not-merged",
"description": "Header-backed threads with the same subject stay separate.",
"spec": {
"source": "JWZ",
"url": "https://www.jwz.org/doc/threading.html",
"behavior": "subject fallback does not merge distinct threads that already have reply headers"
},
"input": [
{
"id": "<same-a-root@example>",
"message_id": "<same-a-root@example>",
"in_reply_to": null,
"references": [],
"subject": "Topic",
"date": "2026-05-15T09:00:00Z"
},
{
"id": "<same-a-reply@example>",
"message_id": "<same-a-reply@example>",
"in_reply_to": "<same-a-root@example>",
"references": [
"<same-a-root@example>"
],
"subject": "Re: Topic",
"date": "2026-05-15T09:05:00Z"
},
{
"id": "<same-b-root@example>",
"message_id": "<same-b-root@example>",
"in_reply_to": null,
"references": [],
"subject": "Topic",
"date": "2026-05-15T10:00:00Z"
},
{
"id": "<same-b-reply@example>",
"message_id": "<same-b-reply@example>",
"in_reply_to": "<same-b-root@example>",
"references": [
"<same-b-root@example>"
],
"subject": "Re: Topic",
"date": "2026-05-15T10:05:00Z"
}
],
"expected": {
"threads": [
{
"root": "<same-a-root@example>",
"messages": [
"<same-a-root@example>",
"<same-a-reply@example>"
]
},
{
"root": "<same-b-root@example>",
"messages": [
"<same-b-root@example>",
"<same-b-reply@example>"
]
}
]
}
}