That JSONDecodeError usually means your client is talking to the UI instead of the API. If

PREFECT_API_URL

points at the root (e.g., http://127.0.0.1:4200) the server returns HTML, and

prefect deploy

tries to parse it as JSON and fails with “Expecting value: line 1 column 1”. For a self‑hosted Prefect 3 server, your API URL must include the /api suffix. For your local instance, set it to: - http://127.0.0.1:4200/api How to create a new profile and set its URL 1) Create and activate a profile:

prefect profile create local
prefect profile use local

2) Set the API URL for that profile:

prefect config set PREFECT_API_URL=<http://127.0.0.1:4200/api>

3) Validate / inspect:

prefect config view
prefect config validate

4) Retry:

prefect deploy

If you also have a remote self‑hosted instance, make a second profile:

prefect profile create remote
prefect profile use remote
prefect config set PREFECT_API_URL=http://<your-remote-host>:<port>/api

Quick health check - Hitting the root (/) returns HTML (the UI) — wrong for clients - The API should respond at /api; for example:

curl <http://127.0.0.1:4200/api/health>

This should return a small JSON/OK response. If you see HTML, your URL or reverse proxy routing is off. A couple of helpful commands - List/switch profiles:

prefect profile ls
prefect profile use <name>

If you share the current value of

PREFECT_API_URL

(

prefect config view | grep PREFECT_API_URL

) and your Prefect version (

prefect version

), I can double‑check compatibility and confirm you’re pointed at the right endpoint. Docs for reference: - Settings and profiles: Settings and profiles - Manage settings via CLI/env: Manage settings - Self-hosted server overview: Prefect Server - Deployments (prefect.yaml projects): Deployments with prefect.yaml