[feedback] silent-side-effects: A missed twin-patch let a route write the real reports/pipeline_state.json during a green test;

Context. Solo-operator project using Claude Code (desktop app, Windows 11) with a lead session on Claude Fable 5.1 orchestrating Opus 5.5 / Sonnet subagents (worktree-isolated), a JSON task store, PreToolUse/SubagentStop hooks and a harvest-train commit flow. This finding was measured on or before 2026-09-22 and filed on 2026-09-28 at the operator's request; numbers are from tool output. No secrets, identities or private paths.

Class / model / severity. silent-side-effects / Sonnet (api lane) / HIGH

Finding. A missed twin-patch let a route write the real reports/pipeline_state.json during a green test; found only by hashing files before and after by hand.

Evidence (measured). review-halt-leaf-1 R2; board #1258 row 2 (the write fence).

Minimal reproduction. Run a test suite with a monkeypatch that misses one binding and diff the tree afterwards.

Ask. A write fence during tests (fail any test that changes a file outside its temp dir) as a default agent capability near real state.