Guest API /v2/conversations/messaging/guest/send returning 401 when using internal token

Trying to build a custom messaging interface that bypasses the standard Messenger widget. I need to send and receive messages programmatically using the Guest API endpoints.

I have a valid OAuth token generated via the internal client credentials flow (which works fine for other /api/v2 calls). When I hit the guest endpoint, it immediately fails with a 401 Unauthorized.

POST /api/v2/conversations/messaging/guest/send

Here’s the payload:

{
 "externalId": "guest-12345",
 "from": {
 "address": "guest-12345"
 },
 "text": {
 "content": "Hello from custom UI"
 },
 "routing": {
 "queueId": "abc-123-xyz"
 }
}

The docs say this endpoint is for “guest” interactions, but it seems to require the token to be associated with a specific user context or perhaps the guest API expects a different auth mechanism like the x-guest-token header instead of the standard Authorization: Bearer.

I’ve tried swapping the header, but that just gives a 400 Bad Request. Is there a specific way to bind the OAuth token to a guest session before calling the send endpoint? Or am I missing a step in creating the guest conversation first?

No error details in the response body, just the 401.