Background Bash command reading other agents' output files can cause runaway disk usage (1.2TB)

Bug Report

Summary

A background Bash command that reads from other background agents' .output files can cause runaway disk usage. In my case, it grew to 1.2TB before I manually killed it and deleted the file.

Reproduction Steps

1. Launch multiple background agents (e.g., 6+ Agent tool invocations with run_in_background: true)
2. While those agents are running, launch a background Bash command that reads their output files:

``bash
# This was the actual command that caused the issue
for f in /private/tmp/claude-501/.../tasks/*.output; do
echo "=== $(basename $f) ==="
tail -3 "$f" 2>/dev/null || echo "(empty)"
done
``

1. The background Bash command's own output file grows unboundedly as it captures the streaming JSONL from the still-running agents.

Expected Behavior

• Background command output files should have a size cap or circuit breaker
• Or: tail on actively-growing files should not stream indefinitely into the output capture

Actual Behavior

• The output file at /private/tmp/claude-501/.../tasks/<id>.output grew to 1.2TB
• No automatic size limit or warning was applied
• Had to manually discover and delete the file

Environment

• Claude Code v2.1.71
• macOS Darwin 25.3.0
• Model: claude-opus-4-6

Suggested Mitigations

1. Output file size cap: Kill background commands if their output file exceeds a threshold (e.g., 100MB)
2. Warning on large output: Surface a warning when a background task's output exceeds a reasonable size
3. Prevent self-referential reads: Detect when a background command is reading from the same /tmp/claude-*/tasks/ directory where other tasks are writing

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