Web UI: "Message not found: msg_xxx" crash after session reset/fork #36907

Description

When running opencode web and accessing through a browser, after resetting a session (fork/restart) and sending a new message, the web UI frequently crashes with an unhandled error:

Error: Message not found: msg_f61ae7f9b001US1iJ2z3b3Btl5
    at ixe (https://***/assets/index-ti7qIEGV.js:1009:58000)
    at o (https://***/assets/index-ti7qIEGV.js:1009:7544)
    at async Qi (https://***/assets/index-ti7qIEGV.js:1011:30116)
    at async j (https://***/assets/index-ti7qIEGV.js:1014:30464)
    at async ne (https://***/assets/index-ti7qIEGV.js:1014:33307)



────────────────────────────────────────
原因：
{
  "body": {
    "name": "NotFoundError",
    "data": {
      "message": "Message not found: msg_f61ae7f9b001US1iJ2z3b3Btl5"
    }
  },
  "status": 404
}

Refreshing the page or clicking the "Restart" (重启) button temporarily fix it until another session switch or reset triggers it again.

Root cause: packages/app/src/context/server-session.ts — fetchMessage() (line 485) calls client.session.message({ sessionID, messageID }) and when the server returns 404 (stale message ID cached in React state after session reset), the error is thrown uncaught. The call site at line 641 (fetchMessage(sessionID, parentID, ...)) has no try/catch, so it propagates to the React error boundary, crashing the UI.

The underlying issue is that the web app's message cache (server-session.ts) is not invalidated when a session is reset/forked, leaving stale msg_xxx references in the parent message chain.

Plugins

oh-my-openagent@latest, @tarquinen/opencode-dcp@latest

OpenCode version

1.17.20

Steps to reproduce

1. Start opencode web --hostname 0.0.0.0 --port 4096
2. Open the web UI in a browser (or via reverse proxy like nginx)
3. Create a session and send a few messages
4. Click the session reset button (重置会话)
5. Type a new message and send it
6. Switching to another session and switching back
7. Observe: "Message not found: msg_xxx" error crashes the UI
8. Refreshing the page or clicking the "Restart" button may temporarily fix it, but switching sessions triggers it again

Screenshot and/or share link

No response

Operating System

Ubuntu 22.04 (WSL2 on Windows)

Terminal

N/A (this is a web UI bug, reproduced via browser)