Dispatch: background task error breaks all subsequent replies in main conversation

Description

When Dispatch attempts to output a long text (e.g., full contents of a CLAUDE.md file ~70 lines), the response fails and gets stuck as a background task with a SendUserMessage → No such tool available error.

The critical issue is that after this error, ALL subsequent replies in the Dispatch conversation are only visible via "Show background tasks" (バックグラウンドタスクを表示) and no longer appear in the main conversation view. This persists across app restarts and version upgrades.

Steps to Reproduce

1. Open Dispatch conversation (working normally, replies visible in main chat)
2. Ask Dispatch to output the full contents of a file (e.g., "Output all contents of CLAUDE.md here")
3. The long text response fails silently — no reply appears
4. Send additional messages — none of the replies appear in the main conversation
5. Replies are only visible by clicking the Dispatch menu → "Show background tasks"

Expected Behavior

• If a response fails due to length or tool errors, subsequent messages should still work normally
• Error should be recoverable without user intervention

Actual Behavior

• One failed long-text response permanently breaks the Dispatch conversation
• All subsequent replies only go to background tasks
• The broken state persists across app restart and version upgrade
• User has no indication of what went wrong

Workaround

Clicking "Clear background tasks" (バックグラウンドタスクをクリア) from the Dispatch menu resolved the issue and restored normal conversation behavior.

Environment

• Platform: Windows 11 Pro
• Claude Desktop (Cowork tab / Dispatch)
• Plan: Max

Additional Notes

• The underlying error visible in the background task was SendUserMessage returning No such tool available
• This is a poor user experience because: (1) the user cannot know in advance whether their request will produce output that's "too long", and (2) a single error silently breaks all future interactions with no clear way to diagnose or fix it