[Bug]: Renderer crashes with TypeError: Failed to fetch in fetchMessages when background subagent completes #27292

Description

OpenCode GUI (Electron renderer) crashes with TypeError: Failed to fetch when a background subagent task completes and the parent session tries to fetch updated messages. The crash kills the entire session — no recovery possible without restarting OpenCode.

Environment

• OpenCode: 1.14.48
• OmO (oh-my-openagent): 4.1.1
• OS: Windows 11
• Model: claude-opus-4-6

Stack trace

TypeError: Failed to fetch
    at fetch (oc://renderer/assets/main-Cb1WDiRx.js:114735:45)
    at request (oc://renderer/assets/main-Cb1WDiRx.js:71162:24)
    at async retry (oc://renderer/assets/main-Cb1WDiRx.js:75706:14)
    at async fetchMessages (oc://renderer/assets/main-Cb1WDiRx.js:77478:24)
    at async loadMessages (oc://renderer/assets/main-Cb1WDiRx.js:77502:7)
    at async Promise.all (index 1)
    at async oc://renderer/assets/main-Cb1WDiRx.js:77660:13

Reproduction

1. Start OpenCode TUI/GUI
2. In a session, spawn a background task:

task(subagent_type="explore", load_skills=[], run_in_background=true,
     prompt="Return exactly: TEST")

3. Wait for <system-reminder> notification that task completed
4. Renderer crashes with the above stack trace
5. Session is lost

Observations

• The crash happens in fetchMessages → loadMessages — the renderer tries to fetch messages from the completed child session but the fetch fails
• Backend logs (~/.local/share/opencode/log/) do NOT contain TypeError or Failed to fetch — this is purely renderer-side
• The crash is intermittent but frequent (observed multiple times in one session)
• Backend background task infrastructure works correctly — the task completes, result is available via API. The crash is in the UI layer consuming the completion event

Possibly related

• [Bug] SESSION END WHEN WAIT FOR BACKGROUND TASK #26062 — SESSION END WHEN WAIT FOR BACKGROUND TASK (parent session abort)
• [Bug] task tool causes parent session abort — both parallel and sequential dispatch #23404 — Task tool causes parent session abort
• TaskTool returns no diagnostic context when subagent result text is empty or missing #24447 — TaskTool returns no diagnostic context when subagent result text is empty

Suggested investigation

The fetchMessages call in the renderer likely races with the session lifecycle — the child session may be cleaned up or rotated before the renderer's loadMessages call resolves. A guard or retry with session-existence check before fetch could prevent the crash.