> ## Documentation Index
> Fetch the complete documentation index at: https://www.plain.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# sendCustomerChat

> Send a chat message on behalf of a customer into an existing CHAT-channel thread. Use this in a
headless portal integration to forward messages written by the customer in your own UI into Plain.
Unlike `sendChat`, `threadId` is required and the caller must be authenticated as a machine user.
An optional `timestamp` (ISO 8601, must be in the past) allows backdating messages when backfilling.
Requires the `chat:create` permission and the headless-portal billing entitlement.