Programming language: Scala
License: MIT License
Tags: Tools    
Latest version: v0.9.3

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Your shiny new Scala build tool! Confused by SBT? Frustrated by Maven? Perplexed by Gradle? Give Mill a try!

If you want to use Mill in your own projects, check out our documentation:

Here is some quick example, so that you can imagine how it looks:

import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"

object bar extends ScalaModule {
  def moduleDeps = Seq(foo)
  def scalaVersion = "2.13.1"

If you use Mill and like it, you will probably enjoy the following book by the Author:

Hands-on Scala has a Chapter 10: Static Build Pipelines dedicated to Mill, but rest of the book introduces other libraries and tools in a similar style. Hands-on Scala is a great way to level up your skills in Scala in general and Mill in particular

The remainder of this readme is developer-documentation targeted at people who wish to work on Mill's own codebase. The developer docs assume you have read through the user-facing documentation linked above. It's also worth spending a few minutes reading the following blog posts to get a sense of Mill's design & motivation:

How to build and test

Mill is built using Mill. To begin, first download & install Mill as described in the documentation above. As Mill is under active development, stable releases may not be able to build the current development branch of Mill. It is recommended to install the latest unstable release manually.

IntelliJ Setup

If you are using IntelliJ IDEA to edit Mill's Scala code, you can create the IntelliJ project files via:

./mill mill.scalalib.GenIdea/idea

Automated Tests

To run test suites:

./mill main.test
./mill scalalib.test
./mill scalajslib.test
./mill integration.test

Manual Testing

To manually test Mill on a small build, you can use the scratch folder:

./mill -i dev.run scratch -w resolve _

This runs the task resolve _ with your current checkout of Mill on the trivial build defined in scratch/build.sc. You can modify that build file to add additional modules, files, etc. and see how it behaves.

More generally, you can use:

./mill -i dev.run [target-dir] [...args]

To create run your current checkout of Mill in the given target-dir with the given args. This is useful e.g. to test a modified version of Mill on some other project's Mill build.

You can also create a launcher-script to let you run the current checkout of Mill without the bootstrap Mill process present:

./mill dev.launcher

This creates the out/dev/launcher/dest/run launcher script, which you can then use to run your current checkout of Mill where-ever you'd like. Note that this script relies on the compiled code already present in the Mill out/ folder, and thus isn't suitable for testing on Mill's own Mill build since you would be over-writing the compiled code at the same time as the launcher script is using it.

You can also run your current checkout of Mill on the build in your scratch/ folder without the bootstrap Mill process being present via:

./mill dev.launcher && (cd scratch && ../out/dev/launcher/dest/run -w show thingy)

Bootstrapping: Building Mill with your current checkout of Mill

To test bootstrapping of Mill's own Mill build using a version of Mill built from your checkout, you can run


This creates a standalone assembly at ~/mill-release you can use, which references jars published locally in your ~/.ivy2/local cache. You can then use this standalone assembly to build & re-build your current Mill checkout without worrying about stomping over compiled code that the assembly is using.

This assemby is design to work on bash, bash-like shells and Windows Cmd. If you have another default shell like zsh or fish, you probably need to invoke it with sh ~/mill-release or prepend the file with a proper shebang.


In case of troubles with caching and/or incremental compilation, you can always restart from scratch removing the out directory:

os.remove.all -rf out/

Project Layout

The Mill project is organized roughly as follows:

Core modules that are included in the main assembly

  • core, main, main.client, scalalib, scalajslib.

These are general lightweight and dependency-free: mostly configuration & wiring of a Mill build and without the heavy lifting.

Heavy lifting is delegated to the worker modules (described below), which the core modules resolve from Maven Central (or from the local filesystem in dev) and load into isolated classloaders.

Worker modules that are resolved from Maven Central

  • scalalib.worker, scalajslib.worker[0.6], scalajslib.worker[1.0]

These modules are where the heavy-lifting happens, and include heavy dependencies like the Scala compiler, Scala.js optimizer, etc.. Rather than being bundled in the main assembly & classpath, these are resolved separately from Maven Central (or from the local filesystem in dev) and kept in isolated classloaders.

This allows a single Mill build to use multiple versions of e.g. the Scala.js optimizer without classpath conflicts.

Contrib modules

  • contrib/bloop/, contrib/flyway/, contrib/scoverage/, etc.

These are modules that help integrate Mill with the wide variety of different tools and utilities available in the JVM ecosystem.

These modules are not as stringently reviewed as the main Mill core/worker codebase, and are primarily maintained by their individual contributors. These are maintained as part of the primary Mill Github repo for easy testing/updating as the core Mill APIs evolve, ensuring that they are always tested and passing against the corresponding version of Mill.



For details refer to milestone after 0.8.0 and the list of commits.

0.9.3 - 2020-11-26

(We also tagged 0.9.0, 0.9.1, and 0.9.2, but due to release difficulties, we ask you not to use them.)

  • Replace the built in @main method functionality with the MainArgs library
  • Note that the MainArgs replacement has some backwards incompatibilities: Short flags like -i can no longer be passed via --i, the @doc("") is now @arg(doc = ""), Seq[T] parameters are now passed via repeated --foo flags rather than comma-separated.
  • Add the ability to relocate/shade files in .assembly #947
  • Twirl enhancements #952
  • Add scalacPluginClasspath to Tests #956
  • Add toMap methods to BuildInfo #958
  • Bump coursier to version 2.0.0 #973
  • Make BSP support a first-class citizen #969
  • Omit the suffix in artifactName in cross modules #953
  • Allow test classes with constructor parameters #982
  • Proguard contrib module #972
  • Support Scala.js useECMAScript2015 option and ModuleKind.ESModule #1004
  • Support Scala.js incremental linking #1007

For details refer to milestone 0.9.3 and the list of commits.

0.8.0 - 2020-07-20

  • Bump external dependencies: uPickle 1.2.0, Ammonite 2.2.0, etc.
  • Use default coursier repos (#931)
  • Work around relative paths issue on windows (#936)
  • Support Scala.js versions >1.0.0 (#934)

For details refer to milestone 0.8.0 and the list of commits.

0.7.4 - 2020-07-03

  • new command line options --repl and --no-server, deprecated --interactive option
  • Support for Scala.js 1.1
  • Fixed missing source maps for Scala.js 1.0 and 1.1
  • Improved BSP contrib module

For details refer to milestone 0.7.4 and the list of commits.


For details refer to milestone 0.7.3 and the list of commits.

0.7.2 - 2020-05-19

For details refer to milestone 0.7.2 and the list of commits.

0.7.1 - 2020-05-17

For details refer to milestone 0.7.1 and the list of commits.

0.7.0 - 2020-05-15

  • Greatly improved parallel builds via -j <n>/--jobs <n>, with better scheduling and utilization of multiple cores
  • build.sc files now uses Scala 2.13.2
  • Avoid duplicate target resolution with mill resolve __
  • Add ability to pass GPG arguments to publish via --gpgArgs
  • -w/--watch now works for T.source targets

For details refer to milestone 0.7.0 and the list of commits.

0.6.3 - 2020-05-10

  • Finished incomplete support to publish extra artifacts to IVY repositories (publishLocal)
  • Improved Sonatype uploads
  • GenIdea: improvements for shared source dirs and skipped modules
  • ScoverageModule: Some refactorings to allow better customization
  • More robust classpath handling under Windows

For details refer to milestone 0.6.3 and the list of commits.

0.6.2 - 2020-04-22

  • Mill can now execute targets in parallel. This is experimental and need to be enabled with --jobs <n> option.
  • PublishModule: new publishM2Local to publish into local Maven repositories
  • PublishModule: enhanced publishLocal to specify to ivy repository location
  • Windows: Fixed windows launcher and more robust classpath handling
  • ScalaNativeModule: improved compiling and linking support
  • new contrib module VersionFile
  • Dependency: improved dependency update checker and expose results for programmatic use
  • ǹew contrib module Bintray
  • ǹew contrib module Artifactory
  • fixed testCached support in various modules
  • GenIdea: improvements, esp. related to source jars

For details refer to milestone 0.6.2 and the list of commits.

0.6.1 - 2020-02-24

  • Bugfix: Mill now no longer leaks open files (version bump to uPickle 1.0.0)
  • New --version option
  • Added Support for Scala.js 1.0.0+
  • Added Support for Scala Native 0.4.0-M2
  • JavaModule: Enhanced ivyDepsTree to optionally include compile-time and runtime-time dependencies
  • JavaModule: allSourceFiles no longer include Scala sources
  • JavaModule: assembly supports configurable separator when merging resources
  • ScoverageModule: respect unmanagedClasspath, added console reporter
  • ScalaPBModule: added more configuration options
  • Bloop: Fixed inconsistent working directory when executing tests via bloop (forces -Duser.dir when generating bloop config)

For details refer to milestone 0.6.1 and the list of commits.

0.6.0 - 2020-01-20

  • Support for METALS 0.8.0 in VSCode

For details refer to milestone 0.6.0 and the list of commits.

0.5.9 - 2020-01-14

  • Bump library versions again
  • Alias T.ctx.* functions to T.*: T.dest, T.log, etc.
  • Bump Mill's client-connect-to-server timeout, to reduce flakiness when the server is taking a moment to start up

For details refer to the list of commits.

Verison 0.5.8 has some binary compatibility issues in requests-scala/geny and should not be used.

0.5.7 - 2019-12-28

  • Bump library versions: Ammonite 2.0.1, uPickle 0.9.6, Scalatags 0.8.3, OS-Lib 0.6.2, Requests 0.4.7, Geny 0.4.2

For details refer to milestone 0.5.7 and the list of commits.

0.5.5 / 0.5.6 - 2019-12-20

(we skipped version 0.5.4 as we had some publishing issues)

  • Bump library versions: Ammonite 1.9.2, uPickle 0.9.0, Scalatags 0.8.2, OS-Lib 0.5.0, Requests 0.3.0, Geny 0.2.0, uTest 0.7.1
  • Fixed a long standing issue that output of sub-processes are only shown when -i option was used. Now, you will always seen output of sub-process.
  • Mill now properly restarts it's server after it's version has changed
  • PublishModule: added ability to publish into non-staging repositories
  • ScalaPBModule: added extra include path option

For details refer to milestone 0.5.5 and the list of commits.

0.5.3 - 2019-12-07

  • GenIdea/idea: improved support for generated sources and use/download sources in more cases
  • ScalaJS: improvements and support for ScalaJS 0.6.29+ and 1.0.1.RC1
  • Introduced new CoursierModule to use dependency management independent from a compiler
  • ScoverageModule: better handling of report directories
  • ScalaPBModule: more configuration options
  • various other fixes and improvements

For details refer to milestone 0.5.3 and the list of commits.

0.5.2 - 2019-10-17

  • TestModule: new testCachedtarget, which only re-runs tests after relevant changes
  • TestModule.test: fixed issue when stacktraces have no filename info
  • Dependency/updates: fixed issue with reading stale dependencies
  • GenIdea/idea: no longer shared output directories between mill and IntelliJ IDEA
  • support for Dotty >= 0.18.1
  • Fixed backwards compatibility of mill wrapper script
  • Mill now support the Build Server Protocol 2.0 (BSP) and can act as a build server
  • bloop: removed semanticDB dependency
  • Documentation updates

For details refer to milestone 0.5.2 and the list of commits.

0.5.1 - 2019-09-05

  • GenIdea: Bug fixes
  • GenIdea: Support for module specific extensions (Facets) and additional config files
  • Add ability to define JAR manifests
  • Dotty support: Updates and support for binary compiler bridges
  • Ivy: improved API to create optional dependendies
  • Interpolate $MILL_VERSION in ivy imports
  • Zinc: Fixed logger output
  • Scoverage: Upgrade to Scoverage 1.4.0
  • Flyway: Upgrade to Flyway 6.0.1
  • Bloop: Updated semanticDB version to 4.2.2
  • Documentation updates
  • Improved robustness in release/deployment process

For details refer to milestone 0.5.1 and the list of commits.


  • Mill now supports a ./mill bootstrap script, allowing a project to pin the version of Mill it requires, as well as letting contributors use ./mill ... to begin development without needing to install Mill beforehand.

  • Support for a .mill-version file or MILL_VERSION environment variable for Overriding Mill Versions

  • Fix scoverage: inherit repositories from outer project #645


  • Improvements to IntelliJ project generation #616

  • Allow configuration of Scala.js' JsEnv #628


  • Fixes for scala native test suites without test frameworks #627

  • Fix publication of artifacts by increasing sonatype timeouts

  • Bug fixes for Scoverage integration #623


  • Publish compileIvyDeps as provided scope (535)

  • Added contrib modules to integrate Bloop, Flyway, Play Framework, Scoverage

  • Allow configuration of GPG key names when publishing (530)

  • Bump Ammonite version to 1.6.7, making Requests-Scala available to use in your build.sc

  • Support for Scala 2.13.0-RC2

  • ScalaFmt support now uses the version specified in .scalafmt.conf


  • Started to splitting out mill.api from mill.core

  • Avoid unnecessary dependency downloading by providing fetches per cache policy

  • Added detailed dependency download progress to the progress ticker

  • Fixed internal code generator to support large projects

  • Zinc worker: compiler bridge can be either pre-compiled or on-demand-compiled

  • Zinc worker: configurable scala library/compiler jar discovery

  • Zinc worker: configurable compiler cache supporting parallelism

  • Version bumps: ammonite 1.6.0, scala 2.12.8, zinc 1.2.5

  • Mill now by default fails fast, so in case a build tasks fails, it exits immediately

  • Added new -k/--keep-going commandline option to disable fail fast behaviour and continue build as long as possible in case of a failure


  • Bump uPickle to 0.7.1


  • Mill is now bundled with OS-Lib, providing a simpler way of dealing with filesystem APIs and subprocesses


  • Added new debug method to context logger, to log additional debug info into the task specific output dir (out/<task>/log)

  • Added --debug option to enable debug output to STDERR

  • Fix ScalaModule#docJar task when Scala minor versions differ 475


  • Automatically detect main class to make ScalaModule#assembly self-executable


  • Bump Ammonite to 1.3.2, Fastparse to 2.0.4

  • Sped up ScalaModule#docJar task by about 10x, greatly speeding up publishing

  • Add a flag JavaModule#skipIdea you can override to disable Intellij project generation #458

  • Allow sub-domains when publishing #441


  • mill inspect now displays out the doc-comment documentation for a task.

  • Avoid shutdown hook failures in tests #422

  • Ignore unreadable output files rather than crashing #423

  • Don't compile hidden files #428


  • Add visualizePlan command

  • Basic build-info plugin in mill-contrib-buildinfo

  • ScalaPB integration in mill-contrib-scalapblib

  • Fixes for Twirl support, now in mill-contrib-twirllib

  • Support for building Dotty projects #397

  • Allow customization of run/runBackground working directory via forkWorkingDir

  • Reduced executable size, improved incremental compilation in #414


  • Improve incremental compilation to work with transitive module dependencies

  • Speed up hot compilation performance by properly re-using classloaders

  • Speed up compilation time of build.sc files by removing duplicate macro generated routing code


  • Add .runBackground and .runMainBackground commands, to run something in the background without waiting for it to return. The process will keep running until it exits normally, or until the same .runBackground command is run a second time to spawn a new version of the process. Can be used with -w for auto-reloading of long-running servers.

  • Scala-Native support. Try it out!

  • Add --disable-ticker to reduce spam in CI

  • Fix propagation of --color flag


  • Fix resolution of scala-{library,compiler,reflect} in case of conflict

  • Allow configuration of JavaModule and ScalafmtModule scala workers

  • Allow hyphens in module and task names

  • Fix publishing of ScalaJS modules to properly handle upstream ScalaJS dependencies


  • Added the mill show visualize command, making it easy to visualize the relationships between various tasks and modules in your Mill build.

  • Improve Intellij support (351): better jump-to-definition for third-party libraries, no longer stomping over manual configuration, and better handling of import $ivy in your build file.

  • Support for un-signed publishing and cases where your GPG key has no passphrase (346)

  • Basic support for Twirl, Play Framework's templating language (271)

  • Better performance for streaming large amounts of stdout from Mill's daemon process.

  • Allow configuration of append/exclude rules in ScalaModule#assembly (309)


  • Preserve caches when transitioning between -i/--interactive and the fast client/server mode (329)

  • Keep Mill daemon running if you Ctrl-C during -w/--watch mode (327)

  • Allow mill version to run without a build file (328)

  • Make docJar (and thus publishing) robust against scratch files in the source directories (334) and work with Scala compiler options (336)

  • Allow passing Ammonite command-line options to the foo.repl command (333)

  • Add mill clean (315) to easily delete the Mill build caches for specific targets

  • Improve IntelliJ integration of MavenModules/SbtModules' test folders (298)

  • Avoid showing useless stack traces when foo.test result-reporting fails or foo.run fails

  • ScalaFmt support (308)

  • Allow ScalaModule#generatedSources to allow single files (previous you could only pass in directories)


  • Universal (combined batch/sh) script generation for launcher, assembly, and release (#264)

  • Windows client/server improvements (#262)

  • Windows repl support (note: MSYS2 subsystem/shell will be supported when jline3 v3.6.3 is released)

  • Fixed Java 9 support

  • Remove need for running publishAll using --interactive when on OSX and your GPG key has a passphrase

  • First-class support for JavaModules

  • Properly pass compiler plugins to Scaladoc (#282)

  • Support for ivy version-pinning via ivy"...".forceVersion()

  • Support for ivy excludes via ivy"...".exclude() (#254)

  • Make ivyDepsTree properly handle transitive dependencies (#226)

  • Fix handling of runtime-scoped ivy dependencies (#173)

  • Make environment variables available to Mill builds (#257)

  • Support ScalaCheck test runner (#286)

  • Support for using Typelevel Scala (#275)

  • If a module depends on multiple submodules with different versions of an ivy dependency, only one version is resolved (#273)


  • Support for non-interactive (client/server) mode on Windows.

  • More fixes for Java 9

  • Bumped the Mill daemon timeout from 1 minute to 5 minutes of inactivity before it shuts down.

  • Avoid leaking Node.js subprocesses when running ScalaJSModule tests

  • Passing command-line arguments with spaces in them to tests no longer parses wrongly

  • ScalaModule#repositories, scalacPluginIvyDeps, scalacOptions, javacOptions are now automatically propagated to Tests modules

  • ScalaJSModule linking errors no longer show a useless stack trace

  • ScalaModule#docJar now properly uses the compileClasspath rather than runClasspath

  • Bumped underlying Ammonite version to 1.1.0, which provides the improved Windows and Java 9 support


  • Fixes for non-interactive (client/server) mode on Java 9

  • Windows batch (.bat) generation for launcher, assembly, and release


  • Introduced the mill plan foo.bar command, which shows you what the execution plan of running the foo.bar task looks like without actually evaluating it.

  • Mill now generates an out/mill-profile.json file containing task-timings, to make it easier to see where your mill evaluation time is going

  • Introduced ScalaModule#ivyDepsTree command to show dependencies tree

  • Rename describe to inspect for consistency with SBT

  • mill resolve now prints results sorted alphabetically

  • Node.js configuration can be customised with ScalaJSModule#nodeJSConfig

  • Scala.js fullOpt now uses Google Closure Compiler after generating the optimized Javascript output

  • Scala.js now supports NoModule and CommonJSModule module kinds

  • Include compileIvyDeps when generating IntelliJ projects

  • Fixed invalid POM generation

  • Support for Java 9 (and 10)

  • Fixes for Windows support

  • Fixed test classes discovery by skipping interfaces

  • Include "optional" artifacts in dependency resolution if they exist

  • out/{module_name} now added as a content root in generated IntelliJ project


  • Speed up Mill client initialization by another 50-100ms

  • Speed up incremental assemblys in the common case where upstream dependencies do not change.

  • Make ScalaJSModule#run work with main-method discovery

  • Make ScalaWorkerModule user-defineable, so you can use your own custom coursier resolvers when resolving Mill's own jars

  • Simplify definitions of SCM strings

  • Make the build REPL explicitly require -i/--interactive to run

  • Log a message when Mill is initializing the Zinc compiler interface


  • Greatly reduced the overhead of evaluating Mill tasks, with a warm already-cached mill dev.launcher now taking ~450ms instead of ~1000ms

  • Mill now saves compiled build files in ~/.mill/ammonite, which is configurable via the --home CLI arg.

  • Fixed linking of multi-module Scala.js projects


  • Mill now keeps a long-lived work-daemon around in between commands; this should improve performance of things like compile which benefit from the warm JVM. You can use -i/--interactive for interactive consoles/REPLs and for running commands without the daemon

  • Implemented the ScalaModule#launcher target for easily creating command-line launchers you can run outside of Mill

  • ScalaModule#docJar no longer fails if you don't have scala-compiler on classpath

  • Support for multiple testFrameworks in a test module.


  • Fixes for foo.console
  • Enable Ammonite REPL integration via foo.repl


  • First public release