When DC/residential tiers have a single rotating endpoint, worker_count defaulted to 1 (one URL = one worker). PROXY_CONCURRENCY lets you set an explicit thread count (e.g. 100) for providers that handle concurrent connections on a single URL. Capped at MAX_PROXY_CONCURRENCY=200 to avoid overloading the endpoint. Falls back to len(tiers[0]) when unset (existing behaviour). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>