NFC/NFD, narrow no-break space (U+202F), and curly quotes render
identically in a terminal — a model retyping a visually-correct path
gets 'file not found' and can never discover the byte mismatch on its
own. On not-found, canonicalize the requested name and compare against
directory entries; exactly ONE equivalent spelling reads transparently
with an explanatory note. Zero or several matches (homoglyph twins)
fall through — never guess between collisions.
Also: difflib.SequenceMatcher >=0.8 fallback in _suggest_similar_files
catches near-miss typos (AGENT.md -> AGENTS.md) that substring scoring
misses entirely.
Measured (file-only arm, 3 reps, control=guard-only vs feature):
unicode task qwen3.8-max 31k->16k tok (-48%), turns 6.7->3.7;
opus-4.8 57k->33k tok (-42%), turns 8.3->5.0; accuracy held 1.00.
near-miss: opus mildly better, qwen flat, no regressions.
read_file on a workspace FIFO/socket blocked until the exec timeout —
the existing device guard is name-based (/dev/*, /proc/*) and cannot
see an arbitrary special file. Add _special_file_kind(): one os.stat
on the resolved path, refusing FIFO/socket/char/block devices with a
plain note ('no read was attempted') instead of hanging. Host-visible
filesystems only; regular files, dirs, and missing paths unchanged.
Also adds evals/readtool/: an A/B harness that runs the real AIAgent
against hostile-file fixtures (huge lockfile, one-line bundle, FIFO,
NFD filenames, lying extensions) and measures accuracy, turns, tool
calls, and tokens. Measured for this guard (3 reps, file-only arm):
qwen3.8-max fifo task tokens 122k -> 26k (-79%), turns 9.3 -> 5.0;
opus-4.8 tokens 40k -> 23k; accuracy held 1.00 both arms.
Teknium