Request pool exhausted all endpoints

A Java service backed by a fixed 200-thread request pool starts returning 503s ('pool exhausted / task rejected') at 15:20; p99 on every endpoint balloons to the 30s timeout, even endpoints that don't touch the database. CPU sits at 20% and memory is flat, the box is nearly idle. A thread dump shows ~195 of 200 worker threads BLOCKED/parked in a synchronous call to one downstream recommendations API. That recommendations API got slow (its own incident) starting at 15:15. Throughput on your service has collapsed even though nothing about your service changed. Triage and mitigate.