[BUG] Subagents silently never run when disk space is low; no error or notification to the user

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When the machine is low on disk space and the root agent spawns subagents via the Agent tool (including isolation: "worktree" and background agents), the subagents silently never run. The main agent gets no error, the user gets no notification, and the session just sits there as if the subagents were working. Nothing in the UI indicates the spawn failed or why.

The only way I found out was by noticing the task list never progressed, then checking df -h and seeing the disk was nearly full. After freeing space, the same spawns worked immediately.

What Should Happen?

• Spawning a subagent should fail loudly. The Agent tool call should return an error (e.g. "failed to start subagent: no space left on device") so the root agent can report it and stop waiting.
• Ideally a visible warning in the terminal when the disk is below some threshold, since this also affects transcript writes, worktree creation, and task output files.
• At minimum, a subagent that never started should not be shown as pending/running indefinitely.

Error Messages/Logs

No error was surfaced anywhere in the session. Subagents just never produced a task notification.

Steps to Reproduce

1. Fill the disk so only a small amount is free (a few hundred MB or less).
2. Start claude and ask it to do something that spawns a subagent, e.g. "spawn a worktree subagent to fix X" or run /code-review in the background.
3. Observe: the Agent tool call appears to succeed, but the subagent never runs and no completion notification ever arrives. No error or warning is shown.
4. Free disk space and retry: the same spawn works.

Claude Model

Fable 5 (root agent), Opus subagents

Is this a regression?

Not sure

Claude Code Version

2.1.245

Platform

macOS

Operating System

macOS 26.6.2 (Darwin 25.6.0)

Terminal/Shell

iTerm2 / zsh

Additional Information

Related but distinct from #41737 (task output filling the disk). This report is about the failure mode once the disk is already low: subagent spawns fail silently instead of surfacing an error.