Thanks for the details — that specific error means the UI is trying to call the API at 0.0.0.0, which is only a bind address inside the container and not reachable from your browser. What’s happening - When

PREFECT_UI_API_URL

is not set, the UI builds its API URL from

PREFECT_SERVER_API_HOST

and

PREFECT_SERVER_API_PORT

. - With

PREFECT_SERVER_API_HOST=0.0.0.0

, the UI ends up trying to hit

<http://0.0.0.0:4200/api>

from your browser, which fails. Fix - Set

PREFECT_UI_API_URL

to an address your browser can reach: - If you’re running Docker locally and visiting from the same machine:

<http://localhost:4200/api>

- If you’re on a remote VM:

http://<your-hostname-or-ip>:4200/api

Update your docker-compose for the server container like this:

services:
  prefect-server:
    image: prefecthq/prefect:3-latest
    environment:
      PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>
      PREFECT_SERVER_API_HOST: 0.0.0.0
      PREFECT_UI_API_URL: <http://localhost:4200/api>   # or http://<host>:4200/api if remote
      PREFECT_MESSAGING_BROKER: prefect_redis.messaging
      PREFECT_MESSAGING_CACHE: prefect_redis.messaging
      PREFECT_REDIS_MESSAGING_HOST: redis
      PREFECT_REDIS_MESSAGING_PORT: 6379
      PREFECT_REDIS_MESSAGING_DB: 0
    command: prefect server start --no-services
    ports:
      - "4200:4200"

Notes - Workers inside the compose network should still use the internal URL, e.g.

PREFECT_API_URL=<http://prefect-server:4200/api>

. - Make sure port mapping

4200:4200

is present and you’re visiting http://localhost:4200 (or the remote host). Helpful references - Self-hosted with Docker Compose (source for the official example): Docs: Docker Compose - UI setting that determines this behavior: prefect.settings.server.ui If this doesn’t fix it, can you share: - Whether you’re running locally or on a remote VM - The

prefect-server

section of your compose file - Any logs from the

prefect-server

container right after startup I’ll help you get it sorted quickly.

Thanks for sharing the compose and logs — that helps. What’s going on - Your browser is opening http://localhost:4200, but the UI still tries to call the API at http://0.0.0.0:4200/api because the UI’s API target is derived from the server’s bind host when

PREFECT_UI_API_URL

is not explicitly set. - The log lines showing 0.0.0.0 are expected (that’s the bind address inside the container), but the UI needs a browser-reachable URL. Fix - Add

PREFECT_UI_API_URL

to the

prefect-server

service so the UI knows to call localhost:

prefect-server:
  image: prefecthq/prefect:3-latest
  depends_on:
    postgres:
      condition: service_healthy
    redis:
      condition: service_healthy
  environment:
    PREFECT_API_DATABASE_CONNECTION_URL: <postgresql+asyncpg://prefect:prefect@postgres:5432/prefect>
    PREFECT_SERVER_API_HOST: 0.0.0.0
    PREFECT_UI_API_URL: <http://localhost:4200/api>
    PREFECT_MESSAGING_BROKER: prefect_redis.messaging
    PREFECT_MESSAGING_CACHE: prefect_redis.messaging
    PREFECT_REDIS_MESSAGING_HOST: redis
    PREFECT_REDIS_MESSAGING_PORT: 6379
    PREFECT_REDIS_MESSAGING_DB: 0
  command: prefect server start --no-services
  ports:
    - "4200:4200"

Then recreate the container:

docker compose up -d --force-recreate

And hard-refresh the UI (Shift+Reload or Ctrl/Cmd+Shift+R). Quick checks - Open http://localhost:4200/docs — if that loads, the API is up. - In DevTools > Network, the UI calls should now hit http://localhost:4200/api/... (not 0.0.0.0). References - Official Docker Compose guide: Docs: Docker Compose - UI setting used to determine the API URL: prefect.settings.server.ui If you still see the error after this, please share: - A fresh log snippet from the

prefect-server

container after restart - Screenshot or text of the failing UI Network request URL in DevTools - Confirmation that you’re visiting http://localhost:4200 (no proxies in between)