Forum webhook: repeating a post with the same nonce returns 404 instead of the first post

Reproduction steps

Create a forum channel (type 15) with a bot token
Create a webhook in it
POST /webhooks/{id}/{token}?wait=true with {"content":"News","thread_name":"News","nonce":"probe-<timestamp>"}
Send step 3 again with the same body a few seconds later

Expected behaviour

The second request returns the first post and creates nothing new. The webhook docs say: "Repeating a nonce within five minutes returns the first message and creates no new one" (webhooks.mdx:144)

Actual behaviour

The first request returns 200 and creates the post. The second returns 404 {"code":"UNKNOWN_MESSAGE","message":"Message wasn't found."}. Tested on fluxer.com on 2026-11-03

Platform

API

Evidence

Why it happens, at commit 5f3fc0b1:

Each named forum post first creates a brand new thread (ThreadCreationService.ts:308)
The starter message is then sent into that new thread with the new thread as the expected channel (MessageSendService.ts:991)
The nonce cache remembers the first thread. Its channel doesn't match the new thread, so it throws UnknownMessageError (MessageOperationsHelpers.ts:70)
The new thread is cleaned up and the 404 goes back to the caller

Any bot or integration that safely retries a forum webhook post after a timeout or network error gets an error instead of the post it already made