Defining a dynamic input task with Def.inputTaskDyn 

Let’s suppose that there’s a task already that does the browser 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 {
        (Compile / run).toTask(" " + args.mkString(" ")).value
        openbrowser
      }
    }).evaluated,
    openbrowser := {
      println("open browser!")
    }
  )

build.sbt v2 

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

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

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

* Note that some tasks (ie. testOnly) will fail with trailing spaces, so a right trim (.replaceAll("\s+$", "")) of the string built for toTask might be needed to handle empty args.\

The Compile / actualRun’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 Compile / actualRun with the passed in argument, and then evaluate the openbrowser task.