The `PHOENIX_HOST_ROOT_PATH` environment variable is only effective when Phoenix is deployed behind a reverse proxy that rewrites the path prefix; setting it in the Dockerfile alone does not make Phoenix serve at a subpath automatically. If you access Phoenix at a subpath (e.g., `/csg-innv-mcp-phoenix-ecs`) without a properly configured reverse proxy, static assets (like JS files) will fail to load, resulting in a blank or broken UI. This is a common issue and is documented in several support threads and issues—see <https://github.com/Arize-ai/phoenix/issues/5254|this GitHub issue> and <https://arize-ai.slack.com/archives/C04R3GXC8HK/p1721829771.966509|Slack discussion>.
To resolve this, you must deploy Phoenix behind a reverse proxy (such as NGINX, Traefik, or an AWS Application Load Balancer) that rewrites the incoming subpath to `/` and forwards requests to the Phoenix container, with `PHOENIX_HOST_ROOT_PATH` set to your subpath. Without this, Phoenix will not recognize or serve assets at the subpath, and the UI will not function correctly. Would you like more detail on how to configure the reverse proxy for this setup?
Sources:
- <https://github.com/Arize-ai/phoenix/issues/5254|GitHub Issue: Static files fail to load with ingress prefix route>
- <https://arize-ai.slack.com/archives/C04R3GXC8HK/p1721829771.966509|Slack: PHOENIX_HOST_ROOT_PATH and reverse proxy explanation>
