Deliberately independent of the file sink: the mirror runs whether or not SHEETXL_TOOL_LOG
is set, because the two answer different questions. The file is the after-the-fact record a
benchmark submission ships; a mirror is a live view — the MCP server sends records to the client
UI's log channel (VS Code's SheetXL output channel), where "the agent just ran this and it did
the wrong thing" is a thing you watch, not a file you go find. Off by default (nothing is
mirrored until a surface asks), so the zero-telemetry posture is unchanged: this stays on the
machine, between two local processes.
The mirror sees the SAME enriched record the file would get, which is the point of hooking here rather than at the call sites — a tool added later reaches both sinks with nobody remembering. It is called synchronously and must not block; throwing is contained but pointless.
Registers a second destination for records, alongside the file.
nullremoves it.