Frequently Asked Questions 

Project Information 

How do I get help? 

How do I report a bug? 

How can I help? 


My last command didn’t work but I can’t see an explanation. Why? 

sbt 1.0.0 by default suppresses most stack traces and debugging information. It has the nice side effect of giving you less noise on screen, but as a newcomer it can leave you lost for explanation. To see the previous output of a command at a higher verbosity, type last <task> where <task> is the task that failed or that you want to view detailed output for. For example, if you find that your update fails to load all the dependencies as you expect you can enter:

> last update

and it will display the full output from the last run of the update command.

How do I disable ansi codes in the output? 

Sometimes sbt doesn’t detect that ansi codes aren’t supported and you get output that looks like:

[0m[ [0minfo [0m]  [0mSet current project to root

or ansi codes are supported but you want to disable colored output. To completely disable ansi codes, set the sbt.log.format system property to false. For example,

How can I start a Scala interpreter (REPL) with sbt project configuration (dependencies, etc.)? 

In sbt’s shell run console.

Build definitions 

What are the :=, +=, and ++= methods? 

These are methods on keys used to construct a Setting or a Task. The Getting Started Guide covers all these methods, see .sbt build definition, task graph, and appending values for example.

What is the % method? 

It’s used to create a ModuleID from strings, when specifying managed dependencies. Read the Getting Started Guide about library dependencies.

What is ModuleID, Project, …? 

To figure out an unknown type or method, have a look at the Getting Started Guide if you have not. Also try the index of commonly used methods, values, and types, the API Documentation and the hyperlinked sources.

How do I add files to a jar package? 

The files included in an artifact are configured by default by a task mappings that is scoped by the relevant package task. The mappings task returns a sequence Seq[(File,String)] of mappings from the file to include to the path within the jar. See mapping files for details on creating these mappings.

For example, to add generated sources to the packaged source artifact:

mappings in (Compile, packageSrc) ++= {
  import Path.{flat, relativeTo}
  val base = (sourceManaged in Compile).value
  val srcs = (managedSources in Compile).value
  srcs x (relativeTo(base) | flat)

This takes sources from the managedSources task and relativizes them against the managedSource base directory, falling back to a flattened mapping. If a source generation task doesn’t write the sources to the managedSource directory, the mapping function would have to be adjusted to try relativizing against additional directories or something more appropriate for the generator.

How can I generate source code or resources? 

See Generating Files.

How can a task avoid redoing work if the input files are unchanged? 

There is basic support for only doing work when input files have changed or when the outputs haven’t been generated yet. This support is primitive and subject to change.

The relevant methods are two overloaded methods called FileFunction.cached. Each requires a directory in which to store cached data. Sample usage is:

// define a task that takes some inputs
//   and generates files in an output directory
myTask := {
  // wraps a function taskImpl in an uptodate check
  //   taskImpl takes the input files, the output directory,
  //   generates the output files and returns the set of generated files
  val cachedFun = FileFunction.cached(cacheDirectory.value / "my-task") { (in: Set[File]) =>
    taskImpl(in, target.value) : Set[File]
  // Applies the cached function to the inputs files

There are two additional arguments for the first parameter list that allow the file tracking style to be explicitly specified. By default, the input tracking style is FilesInfo.lastModified, based on a file’s last modified time, and the output tracking style is FilesInfo.exists, based only on whether the file exists. The other available style is FilesInfo.hash, which tracks a file based on a hash of its contents. See the FilesInfo API for details.

A more advanced version of FileFunction.cached passes a data structure of type ChangeReport describing the changes to input and output files since the last evaluation. This version of cached also expects the set of files generated as output to be the result of the evaluated function.

Extending sbt 

How can I add a new configuration? 

The following example demonstrates adding a new set of compilation settings and tasks to a new configuration called samples. The sources for this configuration go in src/samples/scala/. Unspecified settings delegate to those defined for the compile configuration. For example, if scalacOptions are not overridden for samples, the options for the main sources are used.

Options specific to samples may be declared like:

scalacOptions in Samples += "-deprecation"

This uses the main options as base options because of +=. Use := to ignore the main options:

scalacOptions in Samples := "-deprecation" :: Nil

The example adds all of the usual compilation related settings and tasks to samples:


How do I add a test configuration? 

See the Additional test configurations section of Testing.

How can I create a custom run task, in addition to run? 

This answer is extracted from a mailing list discussion.

Read the Getting Started Guide up to custom settings for background.

A basic run task is created by:

lazy val myRunTask = taskKey[Unit]("A custom run task.")

// this can go either in a `build.sbt` or the settings member
//   of a Project in a full configuration
fullRunTask(myRunTask, Test, "foo.Foo", "arg1", "arg2")

If you want to be able to supply arguments on the command line, replace TaskKey with InputKey and fullRunTask with fullRunInputTask. The Test part can be replaced with another configuration, such as Compile, to use that configuration’s classpath.

This run task can be configured individually by specifying the task key in the scope. For example:

fork in myRunTask := true

javaOptions in myRunTask += "-Xmx6144m"

How should I express a dependency on an outside tool such as proguard? 

Tool dependencies are used to implement a task and are not needed by project source code. These dependencies can be declared in their own configuration and classpaths. These are the steps:

  1. Define a new configuration.
  2. Declare the tool dependencies in that configuration.
  3. Define a classpath that pulls the dependencies from the Update Report produced by update.
  4. Use the classpath to implement the task.

As an example, consider a proguard task. This task needs the ProGuard jars in order to run the tool. First, define and add the new configuration:

val ProguardConfig = config("proguard") hide

ivyConfigurations += ProguardConfig


// Add proguard as a dependency in the custom configuration.
//  This keeps it separate from project dependencies.
libraryDependencies +=
   "net.sf.proguard" % "proguard" % "4.4" %

// Extract the dependencies from the UpdateReport.
managedClasspath in proguard := {
    // these are the types of artifacts to include
    val artifactTypes: Set[String] = (classpathTypes in proguard).value
    Classpaths.managedJars(proguardConfig, artifactTypes, update.value)

// Use the dependencies in a task, typically by putting them
//  in a ClassLoader and reflectively calling an appropriate
//  method.
proguard := {
    val cp: Seq[File] = (managedClasspath in proguard).value
  // ... do something with , which includes proguard ...

Defining the intermediate classpath is optional, but it can be useful for debugging or if it needs to be used by multiple tasks. It is also possible to specify artifact types inline. This alternative proguard task would look like:

proguard := {
   val artifactTypes = Set("jar")
    val cp: Seq[File] =
      Classpaths.managedJars(proguardConfig, artifactTypes, update.value)
  // ... do something with , which includes proguard ...

How would I change sbt’s classpath dynamically? 

It is possible to register additional jars that will be placed on sbt’s classpath (since version 0.10.1). Through State, it is possible to obtain a xsbti.ComponentProvider, which manages application components. Components are groups of files in the ~/.sbt/boot/ directory and, in this case, the application is sbt. In addition to the base classpath, components in the “extra” component are included on sbt’s classpath.

(Note: the additional components on an application’s classpath are declared by the components property in the [main] section of the launcher configuration file

Because these components are added to the ~/.sbt/boot/ directory and ~/.sbt/boot/ may be read-only, this can fail. In this case, the user has generally intentionally set sbt up this way, so error recovery is not typically necessary (just a short error message explaining the situation.)

Example of dynamic classpath augmentation 

The following code can be used where a State => State is required, such as in the onLoad setting (described below) or in a command. It adds some files to the “extra” component and reloads sbt if they were not already added. Note that reloading will drop the user’s session state.

def augment(extra: Seq[File])(s: State): State = {
    // Get the component provider
  val cs: xsbti.ComponentProvider = s.configuration.provider.components()

    // Adds the files in 'extra' to the "extra" component
    //   under an exclusive machine-wide lock.
    //   The returned value is 'true' if files were actually copied and 'false'
    //   if the target files already exists (based on name only).
  val copied: Boolean = s.locked(cs.lockFile, cs.addToComponent("extra", extra.toArray))

    // If files were copied, reload so that we use the new classpath.
  if(copied) s.reload else s

How can I take action when the project is loaded or unloaded? 

See How to take an action on startup.

Example of project load/unload hooks 

The following example maintains a count of the number of times a project has been loaded and prints that number:

  // the key for the current count
  val key = AttributeKey[Int]("loadCount")
  // the State transformer
  val f = (s: State) => {
    val previous = s get key getOrElse 0
    println("Project load count: " + previous)
    s.put(key, previous + 1)
  onLoad in Global := {
    val previous = (onLoad in Global).value
    f compose previous


On project load, “Reference to uninitialized setting“ 

Setting initializers are executed in order. If the initialization of a setting depends on other settings that has not been initialized, sbt will stop loading.

In this example, we try to append a library to libraryDependencies before it is initialized with an empty sequence.

object MyBuild extends Build {
  val root = Project(id = "root", base = file("."),
    settings = Seq(
      libraryDependencies += "commons-io" % "commons-io" % "1.4" % "test"

To correct this, include the IvyModule plugin settings, which includes libraryDependencies := Seq(). So, we just drop the explicit disabling.

object MyBuild extends Build {
  val root = Project(id = "root", base = file("."),
    settings = Seq(
      libraryDependencies += "commons-io" % "commons-io" % "1.4" % "test"

A more subtle variation of this error occurs when using scoped settings.

// error: Reference to uninitialized setting
settings = Seq(
  libraryDependencies += "commons-io" % "commons-io" % "1.2" % "test",
  fullClasspath := fullClasspath.value.filterNot("commons-io"))

This setting varies between the test and compile scopes. The solution is use the scoped setting, both as the input to the initializer, and the setting that we update.

fullClasspath in Compile := (fullClasspath in Compile).value.filterNot("commons-io"))

Dependency Management 

How do I resolve a checksum error? 

This error occurs when the published checksum, such as a sha1 or md5 hash, differs from the checksum computed for a downloaded artifact, such as a jar or pom.xml. An example of such an error is:

[warn]  problem while downloading module descriptor:
invalid sha1: expected=ad3fda4adc95eb0d061341228cc94845ddb9a6fe computed=0ce5d4a03b07c8b00ab60252e5cacdc708a4e6d8 (1070ms)

The invalid checksum should generally be reported to the repository owner (as was done for the above error). In the meantime, you can temporarily disable checking with the following setting:

checksums in update := Nil

See library management for details.

I’ve added a plugin, and now my cross-compilations fail! 

This problem crops up frequently. Plugins are only published for the Scala version that sbt uses (currently, 2.9.1). You can still use plugins during cross-compilation, because sbt only looks for a 2.9.1 version of the plugin.

… unless you specify the plugin in the wrong place!

A typical mistake is to put global plugin definitions in ~/.sbt/plugins.sbt. THIS IS WRONG. .sbt files in ~/.sbt are loaded for each build—that is, for each cross-compilation. So, if you build for Scala 2.9.0, sbt will try to find a version of the plugin that’s compiled for 2.9.0—and it usually won’t. That’s because it doesn’t know the dependency is a plugin.

To tell sbt that the dependency is an sbt plugin, make sure you define your global plugins in a .sbt file in ~/.sbt/plugins/. sbt knows that files in ~/.sbt/plugins are only to be used by sbt itself, not as part of the general build definition. If you define your plugins in a file under that directory, they won’t foul up your cross-compilations. Any file name ending in .sbt will do, but most people use ~/.sbt/plugins/build.sbt or ~/.sbt/plugins/plugins.sbt.


How do I use the Scala interpreter in my code? 

sbt runs tests in the same JVM as sbt itself and Scala classes are not in the same class loader as the application classes. Therefore, when using the Scala interpreter, it is important to set it up properly to avoid an error message like:

Failed to initialize compiler: class scala.runtime.VolatileBooleanRef not found.
** Note that as of 2.8 scala does not assume use of the java classpath.
** For the old behavior pass -usejavacp to scala, or if using a Settings
** object programmatically, settings.usejavacp.value = true.

The key is to initialize the Settings for the interpreter using embeddedDefaults. For example:

 val settings = new Settings
 val interpreter = new Interpreter(settings, ...)

Here, MyType is a representative class that should be included on the interpreter’s classpath and in its application class loader. For more background, see the original proposal that resulted in embeddedDefaults being added.

Similarly, use a representative class as the type argument when using the break and breakIf methods of ILoop, as in the following example:

def x(a: Int, b: Int) = {
  ILoop.breakIf[MyType](a != b, "a" -> a, "b" -> b )

0.7 to 0.10+ Migration 

How do I migrate from 0.7 to 0.10+? 

See the migration page first and then the following questions.

Where has 0.7’s lib_managed gone? 

By default, sbt 1.0.0 loads managed libraries from your ivy cache without copying them to a lib_managed directory. This fixes some bugs with the previous solution and keeps your project directory small. If you want to insulate your builds from the ivy cache being cleared, set retrieveManaged := true and the dependencies will be copied to lib_managed as a build-local cache (while avoiding the issues of lib_managed in 0.7.x).

This does mean that existing solutions for sharing libraries with your favoured IDE may not work. Refer to Community Plugins page for a list of currently available plugins for your IDE.

What are the commands I can use in 1.0.0 vs. 0.7? 

For a list of commands, run help. For details on a specific command, run help <command>. To view a list of tasks defined on the current project, run tasks. Alternatively, see the Running page in the Getting Started Guide for descriptions of common commands and tasks.

If in doubt start by just trying the old command as it may just work. The built in TAB completion will also assist you, so you can just press TAB at the beginning of a line and see what you get.

The following commands work pretty much as in 0.7 out of the box:


Why have the resolved dependencies in a multi-module project changed since 0.7? 

sbt 0.10 fixes a flaw in how dependencies get resolved in multi-module projects. This change ensures that only one version of a library appears on a classpath.

Use last update to view the debugging output for the last update run. Use show update to view a summary of files comprising managed classpaths.

My tests all run really fast but some are broken that weren’t in 0.7! 

Be aware that compilation and tests run in parallel by default in sbt 1.0.0. If your test code isn’t thread-safe then you may want to change this behaviour by adding one of the following to your build.sbt:

// Execute tests in the current project serially.
// Tests from other projects may still run concurrently.
parallelExecution in Test := false

// Execute everything serially (including compilation and tests)
parallelExecution := false

What happened to the web development and Web Start support since 0.7? 

Web application support was split out into a plugin. See the xsbt-web-plugin project.

For an early version of an xsbt Web Start plugin, visit the xsbt-webstart project.

How are inter-project dependencies different in 1.0.0 vs. 0.7? 

In 1.0.0, there are three types of project dependencies (classpath, execution, and configuration) and they are independently defined. These were combined in a single dependency type in 0.7.x. A declaration like:

lazy val a = project("a", "A")
lazy val b = project("b", "B", a)

meant that the B project had a classpath and execution dependency on A and A had a configuration dependency on B. Specifically, in 0.7.x:

  1. Classpath: Classpaths for A were available on the appropriate classpath for B.
  2. Execution: A task executed on B would be executed on A first.
  3. Configuration: For some settings, if they were not overridden in A, they would default to the value provided in B.

In 1.0.0, declare the specific type of dependency you want. Read about multi-project builds in the Getting Started Guide for details.

Where did class/object X go since 0.7? 

0.7 1.0.0
FileUtilities IO
Path class and object Path object, File, RichFile
PathFinder class Seq[File], PathFinder class, PathFinder object

Where can I find plugins for 1.0.0? 

See Community Plugins for a list of currently available plugins.


sbt Reference Manual
  1. Frequently Asked Questions