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Deleting the whole ancestor walk from validate_trusted_directory and checking only the immediate parent left the suite green, so the invariant the design rests on had no coverage: a writable ancestor lets an attacker swap a validated directory out from under the leaf. Reject a run whose destination grandparent is world-writable while the destination itself is pristine. The harness also rewrites the root script's fixed /usr/share paths by string substitution. A drifted string silently no-ops that rewrite and would point the simulation at the real system tree, so assert each substitution landed. Assert the scratch directory exists too: a failed mktemp left it empty and the suite carried on building paths from it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Co-Authored-By: Codex XHigh <noreply@openai.com>