When running the UI locally with yarn start (webpack-dev-server) pointed at a remote Taskcluster deployment (e.g., community-tc), the dev server crashes shortly after compilation with an unhandled ECONNRESET error:

node:events:486
      throw er; // Unhandled 'error' event
      ^

Error: read ECONNRESET
    at TCP.onStreamRead (node:internal/stream_base_commons:216:20)
Emitted 'error' event on Socket instance at:
    at emitErrorNT (node:internal/streams/destroy:170:8)
    at emitErrorCloseNT (node:internal/streams/destroy:129:3)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21)

Cause

The /subscription WebSocket proxy in ui/webpack.config.js:91-95 has no error handlers:

"/subscription": {
  ws: true,
  changeOrigin: true,
  target: proxyTarget.replace(/^http(s)?:/, "ws$1:"),
},

When the target rejects the WebSocket connection (e.g., community-tc returning InsufficientScopes for missing web:read-pulse scope), the TCP connection resets. The underlying http-proxy (v1.18.1, via http-proxy-middleware v0.19.1) emits an 'error' event on the socket with no listener attached, which crashes the Node.js process.

This is a known issue with webpack-dev-server 3.x on Node.js 10+.

Fix

Add onError and onProxyReqWs handlers to the /subscription proxy config:

"/subscription": {
  ws: true,
  changeOrigin: true,
  target: proxyTarget.replace(/^http(s)?:/, "ws$1:"),
  onError: function (err, req, res) {
    console.warn('[WS Proxy Error]', err.code, err.message);
  },
  onProxyReqWs: function (proxyReq, req, socket) {
    socket.on('error', function (err) {
      console.warn('[WS Socket Error]', err.code, err.message);
    });
  },
},

Notes

• This only affects local development (yarn start / webpack-dev-server). Production uses nginx which handles connection resets gracefully.
• The docker-compose dev mode is also unaffected since it proxies through nginx internally.
• Reproducible on macOS (arm64) with Node.js v24.x by running TASKCLUSTER_ROOT_URL=https://community-tc.services.mozilla.com yarn start in ui/.
• Related to Add an indication when the graphql websocket gets closed due to the lack of web:read-pulse scope #8070 -- the same missing-scope scenario triggers both the silent UI failure and this dev server crash.