That error usually means your worker is connecting to a self‑hosted Prefect Server that requires basic auth for the event stream, but the worker isn’t sending matching credentials. Quick checklist (Prefect 3.x): 1) On the server: did you set

PREFECT_SERVER_API_AUTH_STRING

(e.g.,

admin:pass

)? If yes, the worker must use the same value via

PREFECT_API_AUTH_STRING

. 2) On the worker: - Point to your server API (must include

/api

) - Provide the matching auth string - Make sure you’re not setting a

PREFECT_API_KEY

(that’s only for Prefect Cloud) Example worker env:

export PREFECT_API_URL="http://<your-server-host>:4200/api"
export PREFECT_API_AUTH_STRING="admin:pass"   # must match server's PREFECT_SERVER_API_AUTH_STRING
unset PREFECT_API_KEY                         # ensure Cloud key is not set
prefect worker start --pool <your-work-pool>

If you’re behind a reverse proxy (nginx, etc.), ensure WebSocket upgrades are allowed and headers aren’t stripped: - Forward `Upgrade`/`Connection` headers - Preserve WebSocket subprotocols (Prefect uses one during handshake) Debugging tips: - Enable verbose logs:

export PREFECT_DEBUG_MODE=1

and retry the worker - Verify server health:

curl -I http://<your-server-host>:4200/api/health

- Confirm env on each side: - Server:

echo $PREFECT_SERVER_API_AUTH_STRING

- Worker:

echo $PREFECT_API_URL

and

echo $PREFECT_API_AUTH_STRING

If this doesn’t resolve it, can you share: -

prefect version

- How you start the server (and whether

PREFECT_SERVER_API_AUTH_STRING

is set) - The

PREFECT_API_URL

you’re using on the worker - Any reverse proxy in front of the server Helpful docs: - Prefect settings - Work pools and workers - Self‑hosting Prefect Server