Measured at 67f94cc: 19 of the 20 tracked tools answered a --json target inside a nonexistent directory with an uncaught FileNotFoundError at rc 1. Only queue_append_audit.py already refused (rc 2). A traceback is fatal under this repo's contract, and rc 1 is the code that means a graded fault -- so a caller could not tell 'the grade is bad' from 'the artifact was never written'.
Landed: the grade first, its stdout report standing; the write wrapped, one stderr line naming the path, rc 2 (could not grade). gate_blame.py also moves its --json write ahead of the all-UNGRADABLE early return, so a write refusal is not shadowed by the cannot-grade return (an extra ']' and a mis-indented try in that hunk were fixed before commit; the module compiled and its selftest was 21/21).
tools/write_probe.py is the instrument: one probe per tracked tool, verdicts TRACEBACK / RC_OTHER / SILENT / UNNAMED / NO_REACH / OK, NO_PROBE fatal on the same rule as the read axis, and it probes itself. It is leg 6 of verify.py, and selftest_sweep.py now carries both a read probe and a malformed probe for it.
Verification at this commit follows in the next commit with docs/TOOLS.md and the queue record.
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