[Bug] Subagent writes to predictable /tmp paths instead of isolated scratchpad; file-change reminder misleads on unintentional modifications

Bug Description
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Title: Background subagent wrote scratch to bare /tmp, collided with another process; file-change reminder then read as "conceal this"

What happened: In a multi-repo session, a background subagent wrote a working-file backup to the bare path /tmp/routed_parquet.py.bak instead of its assigned per-session scratchpad. On
this shared machine another process used the same generic path; a cp back from it briefly overwrote the agent's working file with unrelated content. The agent detected it, verified against
git (unaffected), restored, and disclosed it in its final report.

Two product issues this exposes:

1. Agent scratch hygiene: a subagent chose a predictable, world-writable /tmp/<generic-name> path despite being instructed to use its isolated scratchpad. Beyond collisions, predictable

names in world-writable dirs are the classic symlink/TOCTOU setup.

1. Misleading file-change reminder: the agent reported that the harness's external-modification reminder asserted the change was "intentional" and said not to tell the supervising agent —

which it read as an instruction to conceal corruption. I believe this is the standard file-change boilerplate ("this change was intentional… don't tell the user this, since they are
already aware") firing on an unintentional overwrite. If so, that boilerplate is actively misleading when the modification wasn't intentional, and could mask real state corruption.

Severity: low — integrity only (no data exposure), self-corrected, committed output verified clean. Filing because the misleading-reminder behavior seems generic and worth fixing.

Caveat: I could not verify the reminder claim myself (the stray file is gone; I'm instructed not to read subagent transcripts). The session transcript is the ground truth.

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Environment Info

• Platform: linux
• Terminal: gnome-terminal
• Version: 2.1.217
• Feedback ID: b25c0ff7-f91b-4f2f-961d-d3f87784f6c1

Errors

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