Discord error 10003: Unknown channel

Discord could not find a channel with the ID you supplied, from the point of view of your bot. That last part matters: a channel that exists but is hidden from the bot by a permission overwrite, or that lives in a guild the bot was kicked from, looks exactly like a channel that never existed. Discord does not distinguish "does not exist" from "you cannot see it" in this code.

The ID itself is usually the problem. Snowflakes are 17-19 digit integers; if you stored one as a JavaScript number it has been silently rounded and will never match. Copying a channel ID from a different server, a deleted channel, or a thread that was archived and removed also produces 10003.

The overlap with 50001 Missing Access is worth pinning down, because the same misconfiguration can produce either. When a permission overwrite hides a channel from the bot, some routes return 403 with 50001 and others resolve the channel as unknown and return 404 with 10003; from the bot's perspective the channel has ceased to exist either way, so check overwrites before concluding the channel is gone. Threads add their own variant: an archived thread that is later deleted, or a thread addressed through its parent channel's ID, resolves as unknown too. And the most mundane version is configuration rot: a log channel or welcome channel stored in your database months ago, deleted by an admin since, now failing on every event.

{
  "message": "Unknown Channel",
  "code": 10003
}

Treat stored channel IDs as data that can go stale. Handle the CHANNEL_DELETE gateway event to remove them, and wrap every send in a handler that catches 10003, logs it once, and disables that destination rather than retrying forever. Use string types for all snowflakes in your schema.

Channel and thread caps, name rules and the per-guild channel budget are on the Discord limits page; if every channel suddenly looks unknown, check guild membership first as described in Discord bot not responding.