# X_INSTRUMENT_SLOT_FORBIDDEN

## Means

a package exports a single observer slot instead of emitting on the instrumentation bus

## Typical cause

a file in `packages/*/src` exports `set<X>Observer`, `set<X>Sink`, `set<X>Exporter` or a `recent<X>()` ring-buffer reader. A slot holds one observer: a second install replaces the first, and one installed only by `x dev` is a signal production never sees

## Fix

edit the file the finding names — delete the slot, emit with `instrument('<event>', payload)` (the event declared in `packages/core/src/instrument-events.ts`) and have each consumer `subscribe('<event>', handler)`; then `bun run scripts/instrument-slots.ts --json`

[CLI and verify](/error-codes#cli-and-verify)