/exec-ws SSE subscriptions publish loopback helper URLs for LAN viewers

[mwolson]

Summary

LAN or remote viewers can load the serve-sim UI through a host such as minipro24.lan, but the preview can switch back to loopback helper URLs after the UI subscribes to /api/events through /exec-ws.

Environment

• Observed with serve-sim@0.1.41
• Upstream main contains the same source behavior before the proposed fix
• macOS on Apple Silicon
• Xcode 26.5
• Browser running from another LAN machine or a remote browser preview

Steps to reproduce

1. Start serve-sim on all interfaces:

   npx --yes serve-sim@0.1.41 "iPhone 17" --port 3200 --host 0.0.0.0

2. Open the UI from another machine:

   http://minipro24.lan:3200
   

3. Observe that the initial HTML config can contain the LAN host.

4. Wait for the UI to subscribe to /api/events through /exec-ws.

Expected behavior

The /api/events preview config keeps helper URLs reachable by the remote viewer, for example:

http://minipro24.lan:3100/stream.mjpeg
ws://minipro24.lan:3100/ws

Actual behavior

The browser starts retrying loopback helper URLs, for example:

http://127.0.0.1:3100/stream.mjpeg?raw=1
ws://127.0.0.1:3100/ws

A direct request to http://minipro24.lan:3200/api/events returns the expected LAN host, so the issue appears specific to the /exec-ws SSE proxy path.

Root cause

packages/serve-sim/src/exec-ws.ts proxies allowed SSE subscriptions by making a loopback httpRequest to 127.0.0.1:<serverPort>, but it does not forward the viewer's original Host header.

That means /api/events sees a loopback request, so rewriteStateForRequestHost treats it as local and leaves the helper URLs on 127.0.0.1.

Related work

• Rewrite helper host to request hostname for remote viewers #64 fixed the original helper host rewriting for remote viewers.
• Add simulator-wide settings sidebar with WebSocket control channel #86 added the /exec-ws control channel and SSE side-channels.
• Support proxying #91 may overlap through broader proxy support, but it is a larger change.
• Bug: Online simulator shows grey overlay with “Connecting” after a few seconds #44 reports a similar "Connecting" symptom, but it predates /exec-ws and appears to have a different root cause.

Proposed fix

I have a small PR ready that forwards the WebSocket upgrade Host header into allowed SSE loopback requests and adds regression coverage for both LAN and loopback preview config URLs.