hyperguild#115: a session reported "old_str is empty" with old_str that was never empty on their end. Server-side round-trip re-verified clean (fresh brain_edit call, real content, succeeds) — the code path itself isn't broken, but its error left no way to tell "client sent empty" apart from "something between the client and here lost the field" without a second investigation. The refusal now names the raw argument byte count. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>