A read past EOF returned content '900|' (a phantom line-number prefix
that looks like a real line) and an empty file returned '1|' — both
ambiguous silence: indistinguishable, from inside the model, from a
broken tool, so it re-reads and widens windows. Name the dead end and
its recovery instead: 'offset 900 is beyond the end of the file (412
lines total). Retry with offset <= 412.' / 'File is empty (0 bytes).'
Notes, not errors — a fact about the file is not a failure.
Boundary pinned by test: offset == total_lines still reads (an
off-by-one in a resume hint is a silently corrupted read).
Measured (file-only arm, 3 reps, control vs feature): qwen3.8-max
-18% tokens, -26% tool calls, -17% turns across the two affected
tasks; opus-4.8 flat (within rep noise); accuracy held 1.00.
