Guessing it reads some credentials out of environment variables. The client side example shows the server issuing a token so I guess it's only the server that has/needs the credentials.

Agreed. The demo highlights "extra" AWS code for authz + authn, but doesn't explain how f0 doesn't need it... Does the server need to set an ENV? You could argue that's no different then one AWS auth method.

This looks like an interesting product, but it's missing some key technical details to woo engineers -- both how it's done and how reliable the CDN+service is.

Yeah more or less the comments are correct, but let me clarify! FILE0 is looking for an environment variable, that contains your API key. This is how you authenticate yourself from your backend. When you want to use client uploads you can use f0.createToken('myfile.png') and send that token to the frontend, where you can also import f0 and use it like this: f0.useToken(myTokenFromBackend).set(blob) In the dashboard you will find a setup guide and code snippets with all this info.