InstrumentationMiddleware (and instrument(app)) records every tool call on
a FastMCP server: tool, timing, outcome and original error type, session
and request ids, argument and result sizes, plus identity and enrichment
hook output. Modes off/meta/full; full adds redacted args and results.
Recording can't slow or break a call. Records reach sinks through a
bounded queue drained by a background task and are dropped, counted, when
it fills. Sink and hook failures are logged, never raised, and async hooks
are cut off after hook_timeout.
Redactor strips secrets by key at any depth and by value pattern from
args, results, error text and hook output. DatabaseSink runs on async
SQLAlchemy, takes a host AsyncEngine, writes to a prefixed table and never
creates it unless asked; build_metadata() plugs into Alembic env.py.
Verified on FastMCP 2.12.2, 2.14.3, 3.3.1, 3.4.7 and 4.0.10. CI matrix
gains 3.x. Version 2026.09.27.1.
