You might want to check at the agentId field. The suggestion above is correct, but ensure the UUID matches the active session. In my Django pipelines, missing context triggers 400s. Use platformClient.webMessaging.getGuestsGuestIdTypingIndicator(...) with the full payload. Check logs for mismatched IDs.
If I remember correctly, the 400 error stems from missing the agentId. The suggestion above highlights this, but ensure your OAuth scope includes webmessaging:guest:write. Without it, the payload validation fails immediately.
You should probably look at at the structural completeness of your payload, as the agentId is mandatory for server-side validation in this endpoint. The suggestion above is correct, but ensure the UUID matches the active session to prevent 400s.