the `» provider error detected (...)` excerpt was `chunk.substring(0, 500)`
— the head of whatever stderr buffer node delivered. on big writes that's
the front of an mcp tool-schema dump, not the matched error text. label
was correct (regex.test on the whole chunk), excerpt was misleading.
introduce findProviderErrorMatch(text) that returns { label, excerpt }
where excerpt is a windowed slice centered on the regex match index:
the matched line plus 1 line before and 2 lines after, hard-capped at
600 bytes. detectProviderError stays as a thin wrapper for label-only
callers. both opencode and claude harnesses log match.excerpt instead
of chunk.substring(0, 500).
regression tests cover the multi-line buffer case, surrounding-line
context, byte-cap fallback to matched-line-only, and head truncation
of a single oversize line.
Colin McDonnell
b9383bbcfd