Defining a dynamic input task with Def.inputTaskDyn 

Let’s suppose that there’s a task already that does the bowser opening called openbrowser because of a plugin. Here’s how we can sequence a task after an input tasks.

build.sbt v1 

lazy val runopen = inputKey[Unit]("run and then open the browser")
lazy val openbrowser = taskKey[Unit]("open the browser")

lazy val root = (project in file(".")).
  settings(
    runopen := (Def.inputTaskDyn {
      import sbt.complete.Parsers.spaceDelimited
      val args = spaceDelimited("<args>").parsed
      Def.taskDyn {
        (run in Compile).toTask(" " + args.mkString(" ")).value
        openbrowser
      }
    }).evaluated,
    openbrowser := {
      println("open browser!")
    }
  )

build.sbt v2 

Trying to rewire run in Compile is going to be complicated. Since the reference to the inner run in Compile is already inside the continuation task, simply rewiring runopen to run in Compile will create a cyclic reference. To break the cycle, we will introduce a clone of run in Compile called actualRun in Compile:

lazy val actualRun = inputKey[Unit]("The actual run task")
lazy val openbrowser = taskKey[Unit]("open the browser")

lazy val root = (project in file(".")).
  settings(
    run in Compile := (Def.inputTaskDyn {
      import sbt.complete.Parsers.spaceDelimited
      val args = spaceDelimited("<args>").parsed
      Def.taskDyn {
        (actualRun in Compile).toTask(" " + args.mkString(" ")).value
        openbrowser
      }
    }).evaluated,
    actualRun in Compile := Defaults.runTask(
      fullClasspath in Runtime,
      mainClass in (Compile, run),
      runner in (Compile, run)
    ).evaluated,
    openbrowser := {
      println("open browser!")
    }
  )

The actualRun in Compile’s implementation was copy-pasted from run task’s implementation in Defaults.scala.

Now we can call run foo from the shell and it will evaluate actualRun in Compile with the passed in argument, and then evaluate the openbrowser task.