then in prod, you would have to set

ENV=prod

when launching

{
    "env": "${build.environment}",
    // app.json...
}

is this?

I’m not sure if we can/should do that kind of detection since it could be misleading in some cases. Some could run locally a fatjar or from gradle in production. What I can do is to create a page in the documentation website with suggestions about setting this. Maybe we can set the environment in the HOCON file and allow to grab it from an environment variable, while providing a default. For example:

ktor {
    environment = dev
    environment = ${?ENV}
}

With a simple extension method we can get this value and decide or even create several extension properties that tries things like dev or prod. What do you think?