[BUG] Subagent task output not size-capped, single session filled 569GB on disk

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

A single Claude Code session consumed 569GB of disk space in /private/tmp/claude-501/<project>/<session-id>/tasks/ because subagent .output files are not size-capped. Three subagents each wrote ~190GB files (196GB, 187GB, 186GB) to disk in under 24 hours, nearly filling a 1TB SSD.

Root cause: subagents ran npm start on an Azure Functions project, which invokes func start. Without --worker-runtime specified, func prompts interactively: "Select a number for worker runtime: Choose option:". Subagents have no stdin/TTY, so func loops infinitely re-printing "Choose option:". The subagent output buffer has no size cap, so Claude Code wrote billions of repeated prompts to disk until the SSD was nearly full.

Impact: macOS reported 776GB as "System Data" in Storage settings. I had 2GB free when discovered. Content of output files confirmed to be the string "Choose option: " repeated until the file hit 210,664,984,575 bytes.

What Should Happen?

Subagent output should be capped at a reasonable size (e.g. 10-100MB). When the cap is hit, the subagent process should be terminated and reported as failed. Ideally, interactive-prompt detection would fail the subagent early when stdin is unavailable.

Error Messages/Logs

Subagent output files grow unbounded until disk is nearly full. No warning or cap is enforced.

Steps to Reproduce

1. Have a project with an Azure Functions backend where package.json has "start": "func start" (no --worker-runtime flag)
2. Open Claude Code in that project
3. Ask Claude to run or test the backend such that it spawns Task subagents to execute npm start
4. Subagents hang indefinitely on the interactive "Choose option:" prompt
5. Observe /private/tmp/claude-501/<project>/<session>/tasks/*.output files grow without bound

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v2.1.98

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_