SDK not running

The MCUHex SDK server is not reachable.

SDK_NOT_RUNNING

Symptoms

  • The web app shows "SDK not running" and the device list stays empty.
  • Connecting fails instantly with a connection-refused style error.
  • Everything worked until you rebooted or closed a terminal window.

Likely causes

  • The MCUHex SDK (the local bridge between your browser and the debug probe) is not started.
  • The SDK crashed or its terminal/session was closed.
  • A firewall or security tool is blocking the local WebSocket port.

How to fix it

  1. Start the SDK

    Launch the MCUHex SDK on the same computer as your browser. If you installed it via the VS Code extension, use the extension’s start command; otherwise run the SDK from your terminal.

  2. Check it stayed up

    Watch the SDK output for a startup error (port already in use, missing Python/pyOCD dependency). A crash on startup looks identical to "never started" from the browser side.

  3. Allow the local port

    If a firewall prompt appeared and was denied, allow local (localhost) connections for the SDK. Corporate endpoint-security tools sometimes block localhost WebSockets.

  4. Refresh the page

    Once the SDK is running, refresh the MCUHex tab or click the connection indicator to retry.

Related errors