Messenger error 100: Incorrect App ID

Code 100 with subcode 2018164 is terse even by Graph API standards: "Incorrect App ID." The app identity attached to your request does not match what the operation expects, the token was minted under one app while the request context references another, or an explicit app-ID parameter in the call names the wrong application.

Messenger integrations are unusually exposed to this because the Page-app relationship is load-bearing: webhook subscriptions, PSID scoping and thread control all bind to a specific app. Organizations accumulate apps, a prototype, a staging app, the production app, an agency's app from a previous vendor, and Pages end up connected to several. A token generated in the wrong dashboard tab, an environment variable pointing at the staging app ID, or a migration that moved the Page to a new app while jobs still run with the old identity will all land here.

The diagnosis is mechanical: introspect the failing token in the Access Token Debugger and read which app and Page it belongs to, then compare with the app ID your configuration believes it is using. Remember the knock-on effect explained on the 2018001 page: PSIDs are scoped to the app-Page pair, so "fixing" a mismatch by switching apps invalidates stored recipient IDs, a planned migration with ID re-matching, not a config flip.

{
  "error": {
    "message": "(#100) Incorrect App ID.",
    "type": "OAuthException",
    "code": 100,
    "error_subcode": 2018164,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Treat (app ID, app secret, Page token) as one atomic credential set, stored together, rotated together, never mixed across environments. A startup assertion that introspects the token and compares its app ID against config catches every variant of this error before traffic does. App-and-Page identity pitfalls, including the PSID consequences, are covered in the Messenger troubleshooting guide.