Discord error 220003: Webhooks can only create threads in forum channels

You executed a webhook with the thread_name parameter (which asks Discord to create a new thread and post the message as its starter) but the webhook's channel is not a forum or media channel. In regular text channels, webhooks can post into an existing thread using the thread_id query parameter, but they cannot create a thread from scratch. Thread creation via webhook is reserved for forum-style channels, where every post is a thread.

Integrations that were configured against a text channel and later had forum behavior bolted on hit this; so do bots that reuse one execute-webhook function for both channel kinds. The message itself is otherwise valid, so the fix is purely about which parameter you pass.

{
  "message": "Webhooks can only create threads in forum channels",
  "code": 220003
}

Branch your webhook sender on channel type at configuration time and store the resulting mode (post-to-channel, post-to-thread, create-forum-post) with the webhook record. Re-validate when a WEBHOOKS_UPDATE event fires.

Document clearly, for anyone who pastes a webhook URL into your bot, that forum posts require a forum-channel webhook; a channel-type check at paste time saves a support conversation later.

The webhook execution parameters differ just enough between channel kinds to deserve their own tests. The webhook capability table, including thread creation and posting rules, is on the Discord limits page, and the webhook debugging guide covers isolating parameter errors from transport errors.

Store the resolved channel type next to every saved webhook so the sender never has to guess at execution time.