Invalidation issue related to 'assembly' naming causes dependent tasks to be rebuilt then they should not.

[strobe]

Recently we got strange issue with tasks graph invalidation in ScalaJS project which caused build to took more time that is should.

After some digging I was able to reduce this case to minimal reproducible example https://github.com/american-fable/demo-mill-assembly-invalidation-issue (see README.md it has instruction how to reproduce issue).

In short for unknown reason if task has name 'assembly' it causing to invalidate dependent tasks without any changes in those tasks but if different name is used for same task it won't invalidate those as it should be.

(tested on 1.0.6-jvm)

[lefou]

This is most likely because def assemply is already defined in JavaModule which is inherited via ScalaBaseJSModule -> ScalaJSModule -> ScalaModule -> JavaModule.

So you effectively overriding it, which means the other tasks in all the inherited modules that depend on assembly need to be re-wired to your assembly task. Mill is effectively creating a dedicated task instance for each inherited task definition, so that you can do things like calling super.assembly().

If your assembly task is not meant to enhance/replace JavaModule.assembly, choosing a distinct name it the right thing to do.

[strobe]

I updated example little bit to remove unnecessary calls to highlight usage pattern.

Anyway, I think while It has much sense technically but from Mill user perspective its very strange behavior because is hard to expect that small change in single module could lead to significant build process changes in unrelated base modules especially because you might not have visibility of that wiring like with 'scalablytyped' case (in that example is no direct relation between 'assembly' and scalablytyped plugin, but it's build process is affected).

[malcolmredheron]

@lefou: Does Strobe's comment above make sense? It seems extremely strange to me that changing the definition of module A can cause the recompilation of module B that A depends on, even if reusing then name assembly was ill-advised. It took a lot of work to figure out what was going on, and I'd love to spare others this surprise

[lefou]

I'm not sure what's being asked here. To clarify my own answer, the name clash with assembly might looks a bit surprising, since it can be defined without the override keyword. This is because Mill ships by default with IMHO an anti-feature: It disables via it's compiler plugin the need to use the override keyword to declare a def that is overriding another def in the type hierarchy. Personally, I don't like it and in my own projects I try to always use override, to get that little help from the Scala compiler. This saved me multiple times after a refactoring of the inherited traits. I don't think, this is up for debate in Mill, since many overrides I added over time in the Mill code base were removed at a later time. So I can only advise you to place these overrides whenever appropriate, to guide the reader (yourself included) and to let the compiler assist you.

[lefou]

You can always use the built-in capability of Mill like inspect or plan to get an idea why Mill is executing some specific tasks.

[malcolmredheron]

Thank you for the reply!

I still don't understand why editing a log line in one mill task, even if that task overrides something from a base class, causes unrelated tasks to be rebuilt. I can't tell whether this is a quirk (IMO) about mill that is obvious to you but not to me, or whether we haven't explained our surprise clearly :-)

I've updated the git repo to be clearer, but to recap:

• we have a module shared that contains all of the scalablyTyped stuff
• we have a second module, app, that depends on shared
• when we change a log line in app.assembly, shared.scalablyTyped is rebuilt
• when we rename app.assembly to app.assemblyX or anything else and update our other task to call assemblyX and again change that log line, shared.scalablyTyped is not rebuilt

I tried inspect and plan but I don't think that I understand how to use them to see how changes to the build file invalidate targets. We have also looked at out/mill-invalidation-tree.json. The results of this are in the readme in the git rep. I am unable to infer from them that the problem is that app.assembly is overriding a target from a base class and causing other modules to be rebuilt

I used every technique that I and chatgpt could come up with to try to understand this, but everything failed. It was only through a lot of experimentation that I was able to narrow down the problem

[lihaoyi]

If you can update to Mill 1.1.2, there's been some improvements to the mill-invalidation-tree.json file that should hopefully give moer insight into what is going on

[malcolmredheron]

I've updated the repo to use mill 1.1.2. I copied in new output too, but the only difference was the timing. The json file appears to be unchanged

[lihaoyi]

@malcolmredheron could you minimize it further

[lihaoyi]

e.g. remove the Nix stuff

[malcolmredheron]

Is it enough for you just not to use nix? If you already have java in your path you should be fine to ignore nix

…

On Tue, Feb 10 2026 at 18:29 , Li Haoyi < ***@***.*** > wrote: e.g. remove the Nix stuff — Reply to this email directly, view it on GitHub ( #6462 (reply in thread) ) , or unsubscribe ( https://github.com/notifications/unsubscribe-auth/AAAOIXLPCUTWPQLIVVKN6JD4LKHZJAVCNFSM6AAAAACUQTTSDWVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKNZWGM2DCMY ). You are receiving this because you were mentioned. Message ID: <com-lihaoyi/mill/repo-discussions/6462/comments/15763413 @ github. com>

[lihaoyi]

I can't reverse engineer your repro to figure out what I need or don't need, you need to give me clean minimal steps I can follow to reproduce the problem so I can fix it

[strobe]

only non standard step there is call of ./mill shared.npmInstall which required to install js dependencies from shared/package.json.

As mention in readme nix stuff is fully only optional, it was included to provide clear point of reference for 'nodejs' version been used.

[malcolmredheron]

Ok. That repo now uses mill 1.1.2 with whatever jvm mill chooses

[lihaoyi]

Can you cut down build.mill further until it only contains what is necessary to reproduce the problem?

[strobe]

probably not, I mean we have tried to do everything to minimize it. But essentially because problem reproduction depends on scalablyTyped usage, it should be some configuration related to it and some minimal js project should be in place to make it work.

[lefou]

To summarize,

1. You only see strange issues if you name your own task assembly.
2. Your assembly task overrides the existing assembly task and therefore changes the behavior of the module.

You need to find, where in your toolchain the originial assembly task is used. Maybe scalablyTyped is using it?

Beside of being curious what the underlying issue is, I think you should in general do not name your task assembly, since it is already defined, unless you want to re-define the original semantics of JavaModule.assembly for your use case.

[strobe]

Your assembly task overrides the existing assembly task and therefore changes the behavior of the module.

yes but it changes invalidation behavior of shared module, not that where assembly is defined

Maybe scalablyTyped is using it?

yes I assume that could be the case, but from user perspective who just wanna use such plugin is very unclear

you should in general do not name your task assembly

sure.

[malcolmredheron]

I think that we can simplify this more, Strobe. Let's sync up outside this thread

…

On Wed, Feb 11, 2026 at 6:16 AM, strobe < ***@***.*** > wrote: only non standard step there is call of./mill shared.npmInstall which required to install js dependencies from shared/package.json. As mention in readme nix stuff is fully only optional, it was included to provide clear point of reference for 'nodejs' version been used. — Reply to this email directly, view it on GitHub ( #6462 (reply in thread) ) , or unsubscribe ( https://github.com/notifications/unsubscribe-auth/AAAOIXITORXJ7YSYCPBPYC34LM2UDAVCNFSM6AAAAACUQTTSDWVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKNZXGAYDQMI ). You are receiving this because you were mentioned. Message ID: <com-lihaoyi/mill/repo-discussions/6462/comments/15770081 @ github. com>

[malcolmredheron]

Ok. This is minimal now. If you look in the readme, you'll see that there are actually two changes that avoid the surprising rebuild of scalablyTyped: * What we've been discussing so far — renaming app.assembly to app.assemblyX * The new one: comment out println(shared.hello) in app.assembly Thank you for all of your attention!

…

On Wed, Feb 11, 2026 at 11:07 AM, malcolm handley < ***@***.*** > wrote: I think that we can simplify this more, Strobe. Let's sync up outside this thread On Wed, Feb 11, 2026 at 6:16 AM, strobe < notifications@ github. com ( ***@***.*** ) > wrote: > > > only non standard step there is call of./mill shared.npmInstall which > required to install js dependencies from shared/package.json. > > > > As mention in readme nix stuff is fully only optional, it was included to > provide clear point of reference for 'nodejs' version been used. > > > > — > Reply to this email directly, view it on GitHub ( > #6462 (reply in thread) > ) , or unsubscribe ( > https://github.com/notifications/unsubscribe-auth/AAAOIXITORXJ7YSYCPBPYC34LM2UDAVCNFSM6AAAAACUQTTSDWVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKNZXGAYDQMI > ). > You are receiving this because you were mentioned. Message ID: <com-lihaoyi/mill/repo-discussions/6462/comments/15770081 > @ github. com> > > >

[lihaoyi]

@malcolmredheron looks like the problem was you had a .mill-version file that's still on 1.0.6. Once you update to 1.1.2 you should see the full invalidation tree I copied below. Although there's probably stuff we can do to improve the dependency graph, but fundamentally given the conservative nature of Mill's code-signature analysis it probably looks like it's working as intended

lihaoyi mill$ cat scratch/demo-mill-assembly-invalidation-issue/out/mill-invalidation-tree.json
{
  "def build_.package_$app$#assembly$$anonfun$1$$anonfun$1(scala.collection.immutable.Seq,mill.api.TaskCtx)mill.api.daemon.Result": {
    "external mill.javalib.AssemblyModule": {
      "external mill.javalib.JavaModule": {
        "call mill.javalib.JavaModule.depManagement$(mill.javalib.JavaModule)mill.api.Task$Simple": {
          "def build_.package_$shared$scalablyTyped$#depManagement()mill.api.Task$Simple": {
            "shared.scalablyTyped.depManagement": {}
          },
          "def build_.package_$shared$#depManagement()mill.api.Task$Simple": {
            "shared.depManagement": {}
          }
        },
        "call mill.javalib.JavaModule.bomMvnDeps$(mill.javalib.JavaModule)mill.api.Task$Simple": {
          "def build_.package_$shared$#bomMvnDeps()mill.api.Task$Simple": {
            "shared.bomMvnDeps": {}
          },
          "def build_.package_$shared$scalablyTyped$#bomMvnDeps()mill.api.Task$Simple": {
            "shared.scalablyTyped.bomMvnDeps": {
              "shared.transitiveCoursierProjects": {}
            }
          }
        },
        "call mill.javalib.JavaModule.jdkCommandsJavaHome$(mill.javalib.JavaModule)mill.api.Task": {
          "def build_.package_$shared$#jdkCommandsJavaHome()mill.api.Task": {
            "external mill.util.JdkCommandsModule": {
              "external mill.util.MainRootModule": {
                "call mill.util.MainRootModule!<init>(mill.api.internal.RootModule$Info,sourcecode.Enclosing,sourcecode.Line,sourcecode.File)void": {
                  "def build_.package_#<init>()void": {
                    "call build_.package_!<init>()void": {
                      "def build_.package_$#<init>()void": {
                        "shared.artifactTypes": {
                          "shared.resolvedMvnDeps.super.javalib.JavaModule": {},
                          "shared.resolvedRunMvnDeps": {}
                        },
                        "shared.checkGradleModules": {
                          "shared.enablePluginScalacOptions": {},
                          "shared.scalaCompilerBridge": {},
                          "shared.scalaCompilerClasspath": {},
                          "shared.scalacPluginClasspath": {}
                        },
                        "shared.compileMvnDeps": {},
                        "shared.crossFullScalaJSVersion": {
                          "shared.artifactScalaJSVersion": {
                            "shared.platformSuffix": {}
                          }
                        },
                        "shared.generatedSources": {
                          "shared.allSources": {
                            "shared.allSourceFiles": {
                              "shared.zincIncrementalCompilation": {}
                            }
                          }
                        },
                        "shared.javacOptions": {
                          "shared.compile": {}
                        },
                        "shared.jsEnvConfig": {
                          "shared.scalaJSJsEnvMvnDeps": {}
                        },
                        "shared.jvmId": {
                          "shared.jvmVersion": {}
                        },
                        "shared.jvmIndexVersion": {
                          "shared.javaHome": {}
                        },
                        "shared.localRunClasspath.super.RunModule": {
                          "shared.localRunClasspath": {
                            "shared.localClasspath": {}
                          }
                        },
                        "shared.mainClass": {
                          "shared.finalMainClassOpt": {}
                        },
                        "shared.mandatoryJavacOptions": {},
                        "shared.mandatoryMvnDeps.super.javalib.JavaModule": {
                          "shared.mandatoryMvnDeps.super.scalalib.ScalaModule": {}
                        },
                        "shared.mandatoryScalacOptions.super.scalalib.ScalaModule": {
                          "shared.mandatoryScalacOptions": {}
                        },
                        "shared.moduleKind": {},
                        "shared.moduleSplitStyle": {},
                        "shared.mvnDeps": {},
                        "shared.repositories": {},
                        "shared.resolvedDepsWarnNonPlatform": {},
                        "shared.runClasspath.super.RunModule": {
                          "shared.runClasspath": {}
                        },
                        "shared.runMvnDeps": {},
                        "shared.scalaJSExperimentalUseWebAssembly": {},
                        "shared.scalaJSImportMap": {
                          "shared.scalaJSLinkerClasspath": {
                            "shared.scalaJSToolsClasspath": {}
                          }
                        },
                        "shared.scalaJSMinify": {},
                        "shared.scalaJSOptimizer": {},
                        "shared.scalaJSOutputPatterns": {},
                        "shared.scalaJSSourceMap": {},
                        "shared.scalaJSVersion": {
                          "shared.esFeatures": {},
                          "shared.mandatoryMvnDeps": {},
                          "shared.scalaJSBinaryVersion": {},
                          "shared.scalaJSWorkerVersion": {
                            "shared.scalaJSWorkerClasspath": {}
                          },
                          "shared.scalacPluginMvnDeps": {}
                        },
                        "shared.scalaVersion": {
                          "shared.resolvedMvnDeps": {
                            "shared.compileClasspath": {}
                          },
                          "shared.scalaLibraryMvnDeps": {},
                          "shared.scalaLibraryMvnDeps.super.scalalib.ScalaModule": {},
                          "shared.scalaOrganization": {},
                          "shared.zincAuxiliaryClassFileExtensions.super.scalalib.ScalaModule": {
                            "shared.zincAuxiliaryClassFileExtensions": {}
                          }
                        },
                        "shared.scalablyTyped.artifactTypes": {
                          "shared.scalablyTyped.resolvedMvnDeps.super.javalib.JavaModule": {}
                        },
                        "shared.scalablyTyped.checkGradleModules": {
                          "shared.scalablyTyped.enablePluginScalacOptions": {},
                          "shared.scalablyTyped.scalaCompilerBridge": {},
                          "shared.scalablyTyped.scalaCompilerClasspath": {},
                          "shared.scalablyTyped.scalacPluginClasspath": {}
                        },
                        "shared.scalablyTyped.compileMvnDeps": {},
                        "shared.scalablyTyped.crossFullScalaJSVersion": {
                          "shared.scalablyTyped.artifactScalaJSVersion": {
                            "shared.scalablyTyped.platformSuffix": {}
                          }
                        },
                        "shared.scalablyTyped.generatedSources.super.mill.javalib.JavaModule": {
                          "shared.scalablyTyped.generatedSources": {
                            "shared.scalablyTyped.allSources": {
                              "shared.scalablyTyped.allSourceFiles": {
                                "shared.scalablyTyped.zincIncrementalCompilation": {}
                              }
                            }
                          }
                        },
                        "shared.scalablyTyped.javacOptions": {
                          "shared.scalablyTyped.compile": {
                            "shared.upstreamCompileOutput": {}
                          }
                        },
                        "shared.scalablyTyped.jvmId": {
                          "shared.scalablyTyped.jvmVersion": {}
                        },
                        "shared.scalablyTyped.jvmIndexVersion": {
                          "shared.scalablyTyped.javaHome": {}
                        },
                        "shared.scalablyTyped.localRunClasspath.super.RunModule": {
                          "shared.scalablyTyped.localRunClasspath": {
                            "shared.scalablyTyped.localClasspath": {
                              "shared.transitiveLocalClasspath": {}
                            }
                          }
                        },
                        "shared.scalablyTyped.mandatoryJavacOptions": {},
                        "shared.scalablyTyped.mandatoryMvnDeps.super.javalib.JavaModule": {
                          "shared.scalablyTyped.mandatoryMvnDeps.super.scalalib.ScalaModule": {}
                        },
                        "shared.scalablyTyped.mandatoryScalacOptions.super.scalalib.ScalaModule": {
                          "shared.scalablyTyped.mandatoryScalacOptions": {}
                        },
                        "shared.scalablyTyped.mvnDeps.super.mill.javalib.JavaModule": {
                          "shared.scalablyTyped.mvnDeps": {}
                        },
                        "shared.scalablyTyped.repositories": {
                          "shared.scalablyTyped.scalablytypedWorkerClasspath": {}
                        },
                        "shared.scalablyTyped.resolvedDepsWarnNonPlatform": {},
                        "shared.scalablyTyped.runMvnDeps": {},
                        "shared.scalablyTyped.scalaJSVersion": {
                          "shared.scalablyTyped.mandatoryMvnDeps": {},
                          "shared.scalablyTyped.scalaJSBinaryVersion": {},
                          "shared.scalablyTyped.scalacPluginMvnDeps": {}
                        },
                        "shared.scalablyTyped.scalaVersion": {
                          "shared.scalablyTyped.resolvedMvnDeps": {
                            "shared.scalablyTyped.compileClasspath": {}
                          },
                          "shared.scalablyTyped.scalaLibraryMvnDeps": {},
                          "shared.scalablyTyped.scalaLibraryMvnDeps.super.scalalib.ScalaModule": {},
                          "shared.scalablyTyped.scalaOrganization": {},
                          "shared.scalablyTyped.zincAuxiliaryClassFileExtensions.super.scalalib.ScalaModule": {
                            "shared.scalablyTyped.zincAuxiliaryClassFileExtensions": {}
                          }
                        },
                        "shared.scalablyTyped.scalablyTypedBasePath": {},
                        "shared.scalablyTyped.scalablyTypedFlavour": {},
                        "shared.scalablyTyped.scalablyTypedIgnoredLibs": {
                          "shared.scalablyTyped.scalablyTypedWantedLibs": {}
                        },
                        "shared.scalablyTyped.scalablyTypedIncludeDev": {},
                        "shared.scalablyTyped.scalablyTypedIncludePeer": {},
                        "shared.scalablyTyped.scalablyTypedUseScalaJsDomTypes": {},
                        "shared.scalablyTyped.scalacOptions": {
                          "shared.scalablyTyped.allScalacOptions": {}
                        },
                        "shared.scalablyTyped.scalacPluginMvnDeps.super.scalalib.ScalaModule": {},
                        "shared.scalablyTyped.transitiveCoursierProjects": {},
                        "shared.scalablyTyped.unmanagedClasspath": {
                          "shared.scalablyTyped.localCompileClasspath": {}
                        },
                        "shared.scalablyTyped.unmanagedClasspathExistenceCheck": {},
                        "shared.scalablyTyped.upstreamCompileOutput": {},
                        "shared.scalablyTyped.zincAuxiliaryClassFileExtensions.super.javalib.JavaModule": {},
                        "shared.scalacOptions": {
                          "shared.allScalacOptions": {}
                        },
                        "shared.scalacPluginMvnDeps.super.scalalib.ScalaModule": {},
                        "shared.unmanagedClasspath": {
                          "shared.localCompileClasspath": {}
                        },
                        "shared.unmanagedClasspathExistenceCheck": {},
                        "shared.zincAuxiliaryClassFileExtensions.super.javalib.JavaModule": {}
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "com.github.lolgab.mill.scalablytyped.ScalablyTypedWorkerApi/scalablyTypedWorker": {
    "shared.scalablyTyped.scalablyTypedImportTask": {},
    "shared.scalablyTyped.scalablyTypedOutputPackage": {}
  },
  "mill.javalib.JvmWorkerModule/internalWorkerClassLoader": {
    "mill.javalib.JvmWorkerModule/internalWorker": {}
  },
  "mill.javalib.classgraph.ClassgraphWorkerModule/classgraphWorkerClassloader": {
    "mill.javalib.classgraph.ClassgraphWorkerModule/classgraphWorker": {
      "shared.allLocalMainClasses": {}
    }
  },
  "mill.scalajslib.worker.ScalaJSWorkerExternalModule/scalaJSWorker": {
    "shared.fastLinkJS": {
      "app.assembly": {}
    }
  }
}