FastAPI sync endpoints using face.echo_err (stderr) for logging produce no visible output in Render.com's log collector when running in uvicorn's threadpool workers. The nlog/echo_err calls execute without error but their output is silently lost — Render only captures uvicorn's own access log lines and Python's standard logging module output from threadpool context.
Use Python's standard logging.getLogger(__name__) instead of custom stderr-based logging (like face's echo_err) in any code that runs inside FastAPI sync endpoints (which execute in uvicorn's anyio threadpool). Standard logging integrates with uvicorn's log configuration and is reliably captured by Render's log collector. Reserve stderr-based logging for CLI/interactive contexts where it works correctly.