How to fix ESP32 TRANSPORT_WS: Sec-WebSocket-Accept not found

Problem:

Your ESP32 running a MQTT client is printing the following error messages:

E (285025) TRANSPORT_WS: Sec-WebSocket-Accept not found
E (285025) MQTT_CLIENT: Error transport connect

Solution

You’re using MQTT over websockets (ws:// or wss://) but on the given MQTT URL, no MQTT-over-websocket server is running.

This is often caused by using a wrong URL (possibly the URL is missing the path), but it might also be caused by a misconfiguraton of the server or the reverse proxy.

It’s often best to try using a software websocket client to test the correct settings.