Messenger error 2018300: Another app is controlling this thread

Code 2018300 is the handover protocol speaking: "Message failed to send because another app is controlling this thread now." Messenger allows multiple apps on one Page, a bot platform, a live-chat tool, an ads integration, and the handover protocol arbitrates which one owns each conversation at any moment. Exactly one app is the thread owner and may send; everyone else is a secondary receiver who can observe (via standby webhooks) but not reply. Your send failed because your app is not the current owner of this thread.

The single most common real-world trigger has nothing to do with a second bot vendor: it is the Page's own human team replying from the Meta Business Suite inbox. The inbox participates in the protocol, and depending on the Page's app roles, a human reply can move thread control, after which the bot's sends fail with 2018300 and the bot appears "randomly broken" for exactly the conversations a human touched. The companion error 2018321 is the same ownership idea enforced for Messenger's built-in automation (the FAQ flow) rather than another app.

The protocol has an API surface for exactly this situation: pass_thread_control, take_thread_control (primary receiver only) and request_thread_control, plus webhook events announcing every transfer. A bot that records those events knows at all times whether it owns a thread, and never discovers ownership by failing to send.

{
  "error": {
    "message": "(#2018300) Message failed to send because another app is controlling this thread now.",
    "type": "OAuthException",
    "code": 2018300,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Decide the ownership topology before adding a second integration: one primary receiver, explicit pass/return flows, and standby processing for the non-owner. Test the human-then-bot sequence explicitly, reply from the Business Suite inbox, then confirm the bot recovers control, because that is the path users exercise daily. The handover-induced silent-bot symptom is covered in the Messenger troubleshooting guide, and the same protocol exists on Instagram (details here).