I would just use an env var

It sounds like process-compose could help you out?

I tried creating a simple example for your use case, although I realise it still doesnt offer what you are seeking; to be able to provide arbitrary app directory:

# process-compose.yaml

processes:

  frontend:
    working_dir: frontend
    command: pnpm run dev

  backend:
    working_dir: backend
    command: pnpm run dev

What's the correct way to do this? I've seen the advice "just use env variables" but I can't figure it out:

[tasks.build]
run = 'echo building image with tag {{arg(name="tag")}}'


[tasks.push]
depends = ["build ???"] # Can't use {{ env.* }} here
run = 'echo pushing image with tag {{arg(name="tag")}}'

I have the following use case: multiplatform C++ build with multiple build prefixes: build/linux and build/windows. So, something like this:

[tasks.configure]
usage = '''
arg "<platform>" help="Target platform" default="{{ os() }}"
'''
run = '''
meson setup build/{{usage.platform}}
'''

[tasks.build]
usage = '''
arg "<platform>" help="Target platform" default="{{ os() }}"
'''
depends = [
    { task = "configure", args = ["{{usage.platform}}"] },
]
run = '''
meson compile -C build/{{usage.platform}}
'''

So when i'm writing mise build it configures and builds for native, and with explicit argument: mise build linux it configures and builds for specified platform.

And then you can imagine if you have prefix variants with build config: Release, Debug, etc... you want to have more arguments that configures behavior for tasks

And argument

I would just use an env var

IMHO then why do you need usage anyway if you could use env? IMHO: DX. So when you have multiple switches and parameters for script s there is type checking, help, etc.

[env]
BUILD_PLATFORM = "{{env.BUILD_PLATFORM | default(value=os())}}"
BUILD_CONFIG = "{{env.BUILD_CONFIG | default(value='release'}}"

[tasks.configure]
run = '''
meson setup build/{{env.BUILD_PLATFORM}} --buildtype {{env.BUILD_CONFIG}} # Oops - bad platform, bad config, no type checking
'''

[tasks.build]
depends = [
    { task = "configure" },
]
run = '''
meson compile -C build/{{env.BUILD_PLATFORM}} # Oops, i forgor to unset BUILD_PLATFORM, so F me
'''

I think this is a fundamental problem with mise tasks and a big reason I can't use it as much as I want to. I want to use it for parallelizing simple commands under the umbrella of a parent task - all of the commands use the same argument(s). For example, I have a lint task that needs to call 5 different tasks, all of which take an optional 'folder' argument, in parallel - there is no way to do this without doing things that nullify the purpose of using the task system in the first place (and the niceties it entails such as prefixed output, timings, etc.) ie. mise run <task> <arg> ::: <task> <arg> ::: <task> <arg>. Task templates seem like a perfect fit but they don't work either because the tasks are provided no native way to access the parent task's arguments.