diff --git a/bsp/worker/src/mill/bsp/worker/MillBuildServer.scala b/bsp/worker/src/mill/bsp/worker/MillBuildServer.scala
index f0e066aabc7..236308d5062 100644
--- a/bsp/worker/src/mill/bsp/worker/MillBuildServer.scala
+++ b/bsp/worker/src/mill/bsp/worker/MillBuildServer.scala
@@ -608,7 +608,7 @@ private class MillBuildServer(
               Strict.Agg(cleanTask),
               logger = new MillBspLogger(client, cleanTask.hashCode, ev.baseLogger)
             )
-            if (cleanResult.failing.keyCount > 0) (
+            if (cleanResult.failing.size > 0) (
               msg + s" Target ${compileTargetName} could not be cleaned. See message from mill: \n" +
                 (cleanResult.results(cleanTask) match {
                   case TaskResult(Result.Failure(msg, _), _) => msg
diff --git a/core/api/package.mill b/core/api/package.mill
new file mode 100644
index 00000000000..9454ac17ce9
--- /dev/null
+++ b/core/api/package.mill
@@ -0,0 +1,27 @@
+package build.core.api
+
+import mill.contrib.buildinfo.BuildInfo
+import mill._
+
+object `package` extends RootModule with build.MillStableScalaModule with BuildInfo {
+  def moduleDeps = Seq(build.core.client)
+  def buildInfoPackageName = "mill.api"
+  def buildInfoMembers = Seq(
+    BuildInfo.Value("millVersion", build.millVersion(), "Mill version."),
+    BuildInfo.Value("millDocUrl", build.Settings.docUrl, "Mill documentation url."),
+    BuildInfo.Value(
+      "millReportNewIssueUrl",
+      build.Settings.newIssueUrl,
+      "URL to create a new issue in Mills issue tracker."
+    )
+  )
+
+  def ivyDeps = Agg(
+    build.Deps.osLib,
+    build.Deps.mainargs,
+    build.Deps.upickle,
+    build.Deps.pprint,
+    build.Deps.fansi,
+    build.Deps.sbtTestInterface
+  )
+}
diff --git a/core/client/package.mill b/core/client/package.mill
new file mode 100644
index 00000000000..07438ced54e
--- /dev/null
+++ b/core/client/package.mill
@@ -0,0 +1,14 @@
+package build.core.client
+
+import mill._
+import mill.contrib.buildinfo.BuildInfo
+import mill.scalalib._
+
+object `package` extends RootModule with build.MillPublishJavaModule with BuildInfo {
+  def buildInfoPackageName = "mill.main.client"
+  def buildInfoMembers = Seq(BuildInfo.Value("millVersion", build.millVersion(), "Mill version."))
+
+  object test extends JavaTests with TestModule.Junit4 {
+    def ivyDeps = Agg(build.Deps.junitInterface, build.Deps.commonsIo)
+  }
+}
diff --git a/core/define/package.mill b/core/define/package.mill
new file mode 100644
index 00000000000..3182b4b9ab3
--- /dev/null
+++ b/core/define/package.mill
@@ -0,0 +1,26 @@
+package build.core.define
+
+import mill._
+import mill.scalalib._
+
+object `package` extends RootModule with build.MillStableScalaModule {
+  def moduleDeps = Seq(build.core.api)
+
+  def compileIvyDeps = Task {
+    if (scalaVersion().startsWith("3.")) Agg(build.Deps.scalaCompiler(scalaVersion()))
+    else Agg(build.Deps.scalaReflect(scalaVersion()))
+  }
+
+  def ivyDeps = Agg(
+    build.Deps.millModuledefs,
+    // TODO: somewhere sourcecode is included transitively,
+    // but we need the latest version to bring the macro improvements.
+    build.Deps.sourcecode,
+    // Necessary so we can share the JNA classes throughout the build process
+    build.Deps.jna,
+    build.Deps.jnaPlatform,
+    build.Deps.jarjarabrams,
+    build.Deps.mainargs,
+    build.Deps.scalaparse
+  )
+}
diff --git a/core/define/src/mill/define/BaseModule.scala b/core/define/src/mill/define/BaseModule.scala
index af1b5bc9db3..7c80876a387 100644
--- a/core/define/src/mill/define/BaseModule.scala
+++ b/core/define/src/mill/define/BaseModule.scala
@@ -1,7 +1,6 @@
 package mill.define
 
 import mill.api.PathRef
-import mill.internal.Watchable
 
 import scala.collection.mutable
 
diff --git a/core/define/src/mill/define/Cross.scala b/core/define/src/mill/define/Cross.scala
index 2546d4dd830..966ab9a2f3a 100644
--- a/core/define/src/mill/define/Cross.scala
+++ b/core/define/src/mill/define/Cross.scala
@@ -1,7 +1,6 @@
 package mill.define
 
 import mill.api.BuildScriptException
-import mill.internal.Lazy
 
 import scala.collection.mutable
 import scala.reflect.ClassTag
diff --git a/core/internal/src/mill/internal/Lazy.scala b/core/define/src/mill/define/Lazy.scala
similarity index 72%
rename from core/internal/src/mill/internal/Lazy.scala
rename to core/define/src/mill/define/Lazy.scala
index 262a01374bd..20f42fe4850 100644
--- a/core/internal/src/mill/internal/Lazy.scala
+++ b/core/define/src/mill/define/Lazy.scala
@@ -1,4 +1,4 @@
-package mill.internal
+package mill.define
 
 class Lazy[T](t: () => T) {
   lazy val value: T = t()
diff --git a/core/internal/src/mill/internal/Watchable.scala b/core/define/src/mill/define/Watchable.scala
similarity index 97%
rename from core/internal/src/mill/internal/Watchable.scala
rename to core/define/src/mill/define/Watchable.scala
index 8ba4c2553b5..23c4ffd5121 100644
--- a/core/internal/src/mill/internal/Watchable.scala
+++ b/core/define/src/mill/define/Watchable.scala
@@ -1,4 +1,4 @@
-package mill.internal
+package mill.define
 
 import mill.api.internal
 
diff --git a/core/exec/package.mill b/core/exec/package.mill
new file mode 100644
index 00000000000..29434f9892e
--- /dev/null
+++ b/core/exec/package.mill
@@ -0,0 +1,7 @@
+package build.core.exec
+
+import mill._
+
+object `package` extends RootModule with build.MillStableScalaModule {
+  def moduleDeps = Seq(build.core.define, build.core.internal)
+}
diff --git a/core/exec/src/mill/exec/EvalResults.scala b/core/exec/src/mill/exec/ExecResults.scala
similarity index 58%
rename from core/exec/src/mill/exec/EvalResults.scala
rename to core/exec/src/mill/exec/ExecResults.scala
index 6a256164aa8..64a59d5a663 100644
--- a/core/exec/src/mill/exec/EvalResults.scala
+++ b/core/exec/src/mill/exec/ExecResults.scala
@@ -3,13 +3,11 @@ package mill.exec
 import mill.api.*
 import mill.api.Strict.Agg
 import mill.define.*
-import mill.internal.MultiBiMap
 
 trait ExecResults {
   def rawValues: Seq[Result[Val]]
   def evaluated: Agg[Task[?]]
-  def transitive: Agg[Task[?]]
-  def failing: MultiBiMap[Task[?], Result.Failing[Val]]
-  def results: collection.Map[Task[?], TaskResult[Val]]
+  def failing: Map[Task[?], Seq[Result.Failing[Val]]]
+  def results: Map[Task[?], TaskResult[Val]]
   def values: Seq[Val] = rawValues.collect { case Result.Success(v) => v }
 }
diff --git a/core/exec/src/mill/exec/ExecutionCore.scala b/core/exec/src/mill/exec/ExecutionCore.scala
index c2b838cd956..70b34897652 100644
--- a/core/exec/src/mill/exec/ExecutionCore.scala
+++ b/core/exec/src/mill/exec/ExecutionCore.scala
@@ -250,8 +250,7 @@ private[mill] trait ExecutionCore extends GroupExecution {
       Strict.Agg.from(
         finishedOptsMap.values.flatMap(_.toSeq.flatMap(_.newEvaluated)).iterator.distinct
       ),
-      plan.transitive,
-      getFailing(plan.sortedGroups, results),
+      getFailing(plan.sortedGroups, results).items().map { case (k, v) => (k, v.toSeq) }.toMap,
       results.map { case (k, v) => (k, v.map(_._1)) }
     )
   }
@@ -275,8 +274,7 @@ private[mill] object ExecutionCore {
   case class Results(
       rawValues: Seq[Result[Val]],
       evaluated: Agg[Task[?]],
-      transitive: Agg[Task[?]],
-      failing: MultiBiMap[Task[?], Result.Failing[Val]],
+      failing: Map[Task[?], Seq[Result.Failing[Val]]],
       results: Map[Task[?], TaskResult[Val]]
   ) extends ExecResults
 }
diff --git a/core/internal/package.mill b/core/internal/package.mill
new file mode 100644
index 00000000000..4d53c90eb4f
--- /dev/null
+++ b/core/internal/package.mill
@@ -0,0 +1,7 @@
+package build.core.internal
+
+import mill._
+
+object `package` extends RootModule with build.MillPublishScalaModule {
+  def moduleDeps = Seq(build.core.api)
+}
diff --git a/core/package.mill b/core/package.mill
index 75c5174ba97..7000eb933e9 100644
--- a/core/package.mill
+++ b/core/package.mill
@@ -1,79 +1,8 @@
 package build.core
 // imports
 import mill._
-import mill.scalalib._
-import mill.contrib.buildinfo.BuildInfo
-import mill.T
-import mill.define.Cross
-import mill.scalalib.api.ZincWorkerUtil
 
 object `package` extends RootModule with build.MillStableScalaModule {
-  def moduleDeps = Seq(define, exec, resolve)
-  object api extends build.MillStableScalaModule with BuildInfo {
-    def moduleDeps = Seq(client)
-    def buildInfoPackageName = "mill.api"
-    def buildInfoMembers = Seq(
-      BuildInfo.Value("millVersion", build.millVersion(), "Mill version."),
-      BuildInfo.Value("millDocUrl", build.Settings.docUrl, "Mill documentation url."),
-      BuildInfo.Value(
-        "millReportNewIssueUrl",
-        build.Settings.newIssueUrl,
-        "URL to create a new issue in Mills issue tracker."
-      )
-    )
-
-    def ivyDeps = Agg(
-      build.Deps.osLib,
-      build.Deps.mainargs,
-      build.Deps.upickle,
-      build.Deps.pprint,
-      build.Deps.fansi,
-      build.Deps.sbtTestInterface
-    )
-  }
-
-  object internal extends build.MillPublishScalaModule {
-    def moduleDeps = Seq(api)
-  }
-  object define extends build.MillStableScalaModule {
-    def moduleDeps = Seq(api, internal)
-    def compileIvyDeps = Task {
-      if (ZincWorkerUtil.isScala3(scalaVersion())) Agg(build.Deps.scalaCompiler(scalaVersion()))
-      else Agg(build.Deps.scalaReflect(scalaVersion()))
-    }
-    def ivyDeps = Agg(
-      build.Deps.millModuledefs,
-      // TODO: somewhere sourcecode is included transitively,
-      // but we need the latest version to bring the macro improvements.
-      build.Deps.sourcecode,
-      // Necessary so we can share the JNA classes throughout the build process
-      build.Deps.jna,
-      build.Deps.jnaPlatform,
-      build.Deps.jarjarabrams,
-      build.Deps.mainargs,
-      build.Deps.scalaparse
-    )
-  }
-
-  object exec extends build.MillStableScalaModule {
-    def moduleDeps = Seq(define)
-  }
-
-  object resolve extends build.MillStableScalaModule {
-    def moduleDeps = Seq(define)
-  }
-
-  object client extends build.MillPublishJavaModule with BuildInfo {
-    def buildInfoPackageName = "mill.main.client"
-    def buildInfoMembers = Seq(BuildInfo.Value("millVersion", build.millVersion(), "Mill version."))
-
-    object test extends JavaTests with TestModule.Junit4 {
-      def ivyDeps = Agg(build.Deps.junitInterface, build.Deps.commonsIo)
-    }
-  }
-
-  object server extends build.MillPublishScalaModule {
-    def moduleDeps = Seq(client, api)
-  }
+  def moduleDeps = Seq(build.core.define, build.core.exec, build.core.resolve)
 
 }
diff --git a/core/resolve/package.mill b/core/resolve/package.mill
new file mode 100644
index 00000000000..8b140ae1aee
--- /dev/null
+++ b/core/resolve/package.mill
@@ -0,0 +1,7 @@
+package build.core.resolve
+
+import mill._
+
+object `package` extends RootModule with build.MillStableScalaModule {
+  def moduleDeps = Seq(build.core.define, build.core.internal)
+}
diff --git a/core/server/package.mill b/core/server/package.mill
new file mode 100644
index 00000000000..d81cbea276a
--- /dev/null
+++ b/core/server/package.mill
@@ -0,0 +1,7 @@
+package build.core.server
+
+import mill._
+
+object `package` extends RootModule with build.MillPublishScalaModule {
+  def moduleDeps = Seq(build.core.client, build.core.api)
+}
diff --git a/core/src/mill/eval/Evaluator.scala b/core/src/mill/eval/Evaluator.scala
index 6bf6ea6736d..a589ec2a368 100644
--- a/core/src/mill/eval/Evaluator.scala
+++ b/core/src/mill/eval/Evaluator.scala
@@ -14,7 +14,7 @@ import mill.exec.{
   ProfileLogger,
   TaskResult
 }
-import mill.internal.Watchable
+import mill.define.Watchable
 import mill.main.client.OutFiles
 import mill.main.client.OutFiles.*
 import mill.resolve.{Resolve, SelectMode}
@@ -183,7 +183,7 @@ final case class Evaluator private[mill] (
         }
 
         val errorStr = Evaluator.formatFailing(evaluated)
-        evaluated.failing.keyCount match {
+        evaluated.failing.size match {
           case 0 =>
             val nameAndJson = for (t <- selectedTargets.toSeq) yield {
               t match {
@@ -206,7 +206,7 @@ private[mill] object Evaluator {
   class EvalOrThrow(evaluator: Evaluator, exceptionFactory: ExecResults => Throwable) {
     def apply[T: ClassTag](task: Task[T]): T =
       evaluator.evaluate(Agg(task)) match {
-        case r if r.failing.items().nonEmpty =>
+        case r if r.failing.nonEmpty =>
           throw exceptionFactory(r)
         case r =>
           // Input is a single-item Agg, so we also expect a single-item result
@@ -216,7 +216,7 @@ private[mill] object Evaluator {
 
     def apply[T: ClassTag](tasks: Seq[Task[T]]): Seq[T] =
       evaluator.evaluate(tasks) match {
-        case r if r.failing.items().nonEmpty =>
+        case r if r.failing.nonEmpty =>
           throw exceptionFactory(r)
         case r => r.values.map(_.value).asInstanceOf[Seq[T]]
       }
@@ -238,7 +238,7 @@ private[mill] object Evaluator {
   val defaultEnv: Map[String, String] = System.getenv().asScala.toMap
 
   def formatFailing(evaluated: ExecResults): String = {
-    (for ((k, fs) <- evaluated.failing.items())
+    (for ((k, fs) <- evaluated.failing)
       yield {
         val fss = fs.map {
           case Result.Failure(t, _) => t
diff --git a/idea/src/mill/idea/GenIdeaImpl.scala b/idea/src/mill/idea/GenIdeaImpl.scala
index d0d198681bf..81612bf589d 100755
--- a/idea/src/mill/idea/GenIdeaImpl.scala
+++ b/idea/src/mill/idea/GenIdeaImpl.scala
@@ -238,7 +238,7 @@ case class GenIdeaImpl(
     val resolvedModules: Seq[ResolvedModule] = {
       resolveTasks.toSeq.flatMap { case (evaluator, tasks) =>
         evaluator.evaluate(tasks) match {
-          case r if r.failing.items().nonEmpty =>
+          case r if r.failing.nonEmpty =>
             throw GenIdeaException(
               s"Failure during resolving modules: ${Evaluator.formatFailing(r)}"
             )
diff --git a/main/init/test/src/mill/init/InitModuleTests.scala b/main/init/test/src/mill/init/InitModuleTests.scala
index 77b7b71cedc..ec71d2f5a32 100644
--- a/main/init/test/src/mill/init/InitModuleTests.scala
+++ b/main/init/test/src/mill/init/InitModuleTests.scala
@@ -28,7 +28,7 @@ object InitModuleTests extends TestSuite {
       test("no args") {
         val results = evaluator.evaluator.evaluate(Agg(initmodule.init(None)))
 
-        assert(results.failing.keyCount == 0)
+        assert(results.failing.size == 0)
 
         val Result.Success(Val(value)) = results.rawValues.head: @unchecked
         val consoleShown = outStream.toString
@@ -42,7 +42,7 @@ object InitModuleTests extends TestSuite {
       test("non existing example") {
         val nonExistingModuleId = "nonExistingExampleId"
         val results = evaluator.evaluator.evaluate(Agg(initmodule.init(Some(nonExistingModuleId))))
-        assert(results.failing.keyCount == 1)
+        assert(results.failing.size == 1)
         assert(errStream.toString.contains(initmodule.moduleNotExistMsg(nonExistingModuleId)))
       }
     }
diff --git a/main/src/mill/main/MainModule.scala b/main/src/mill/main/MainModule.scala
index b45b01a48e5..6a4acb2e4b7 100644
--- a/main/src/mill/main/MainModule.scala
+++ b/main/src/mill/main/MainModule.scala
@@ -8,7 +8,7 @@ import mill.moduledefs.Scaladoc
 import mill.resolve.SelectMode.Separated
 import mill.resolve.SelectMode
 import mill.util.Util
-import mill.internal.Watchable
+import mill.define.Watchable
 import pprint.{Renderer, Tree, Truncated}
 
 import java.util.concurrent.LinkedBlockingQueue
diff --git a/main/test/src/mill/main/MainModuleTests.scala b/main/test/src/mill/main/MainModuleTests.scala
index a7cdeb0f814..d8305368208 100644
--- a/main/test/src/mill/main/MainModuleTests.scala
+++ b/main/test/src/mill/main/MainModuleTests.scala
@@ -145,7 +145,7 @@ object MainModuleTests extends TestSuite {
         val res = eval.evaluator.evaluate(Agg(mainModule.inspect(eval.evaluator, "hello")))
         val Result.Success(Val(value: String)) = res.rawValues.head: @unchecked
         assert(
-          res.failing.keyCount == 0,
+          res.failing.size == 0,
           value.startsWith("hello("),
           value.contains("MainModuleTests.scala:")
         )
@@ -155,7 +155,7 @@ object MainModuleTests extends TestSuite {
           eval.evaluator.evaluate(Agg(mainModule.inspect(eval.evaluator, "hello", "hello2")))
         val Result.Success(Val(value: String)) = res.rawValues.head: @unchecked
         assert(
-          res.failing.keyCount == 0,
+          res.failing.size == 0,
           value.startsWith("hello("),
           value.contains("MainModuleTests.scala:"),
           value.contains("\n\nhello2(")
@@ -213,7 +213,7 @@ object MainModuleTests extends TestSuite {
         val results =
           evaluator.evaluator.evaluate(Agg(mainModule.show(evaluator.evaluator, "hello")))
 
-        assert(results.failing.keyCount == 0)
+        assert(results.failing.size == 0)
 
         val Result.Success(Val(value)) = results.rawValues.head: @unchecked
 
@@ -241,7 +241,7 @@ object MainModuleTests extends TestSuite {
             "hello2"
           )))
 
-        assert(results.failing.keyCount == 0)
+        assert(results.failing.size == 0)
 
         val Result.Success(Val(value)) = results.rawValues.head: @unchecked
 
@@ -295,7 +295,7 @@ object MainModuleTests extends TestSuite {
         val results =
           evaluator.evaluator.evaluate(Agg(mainModule.showNamed(evaluator.evaluator, "hello")))
 
-        assert(results.failing.keyCount == 0)
+        assert(results.failing.size == 0)
 
         val Result.Success(Val(value)) = results.rawValues.head: @unchecked
 
@@ -312,7 +312,7 @@ object MainModuleTests extends TestSuite {
             "hello2"
           )))
 
-        assert(results.failing.keyCount == 0)
+        assert(results.failing.size == 0)
 
         val Result.Success(Val(value)) = results.rawValues.head: @unchecked
 
@@ -347,17 +347,17 @@ object MainModuleTests extends TestSuite {
 
       test("all") {
         val r1 = ev.evaluator.evaluate(Agg(cleanModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         checkExists(true)(os.sub / "foo")
 
         val r2 = ev.evaluator.evaluate(Agg(cleanModule.clean(ev.evaluator)))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         checkExists(false)(os.sub / "foo")
       }
 
       test("single-target") {
         val r1 = ev.evaluator.evaluate(Agg(cleanModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         checkExists(true)(
           os.sub / "foo/target.json",
           os.sub / "foo/target.dest/dummy.txt",
@@ -366,7 +366,7 @@ object MainModuleTests extends TestSuite {
         )
 
         val r2 = ev.evaluator.evaluate(Agg(cleanModule.clean(ev.evaluator, "foo.target")))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         checkExists(false)(
           os.sub / "foo/target.log",
           os.sub / "foo/target.json",
@@ -380,7 +380,7 @@ object MainModuleTests extends TestSuite {
 
       test("single-module") {
         val r1 = ev.evaluator.evaluate(Agg(cleanModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         checkExists(true)(
           os.sub / "foo/target.json",
           os.sub / "foo/target.dest/dummy.txt",
@@ -389,7 +389,7 @@ object MainModuleTests extends TestSuite {
         )
 
         val r2 = ev.evaluator.evaluate(Agg(cleanModule.clean(ev.evaluator, "bar")))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         checkExists(true)(
           os.sub / "foo/target.json",
           os.sub / "foo/target.dest/dummy.txt"
@@ -408,11 +408,11 @@ object MainModuleTests extends TestSuite {
         val ev = UnitTester(workerModule, null)
 
         val r1 = ev.evaluator.evaluate(Agg(workerModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         assert(workers.size == 5)
 
         val r2 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator)))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         assert(workers.isEmpty)
       }
 
@@ -422,19 +422,19 @@ object MainModuleTests extends TestSuite {
         val ev = UnitTester(workerModule, null)
 
         val r1 = ev.evaluator.evaluate(Agg(workerModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         assert(workers.size == 5)
 
         val r2 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "foo.theWorker")))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         assert(workers.size == 4)
 
         val r3 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "bar.theWorker")))
-        assert(r3.failing.keyCount == 0)
+        assert(r3.failing.size == 0)
         assert(workers.size == 3)
 
         val r4 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "bazz[1].theWorker")))
-        assert(r4.failing.keyCount == 0)
+        assert(r4.failing.size == 0)
         assert(workers.size == 2)
       }
 
@@ -444,25 +444,25 @@ object MainModuleTests extends TestSuite {
         val ev = UnitTester(workerModule, null)
 
         ev.evaluator.evaluate(Agg(workerModule.foo.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 1)
 
         val originalFooWorker = workers.head
 
         ev.evaluator.evaluate(Agg(workerModule.bar.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 2)
         assert(workers.exists(_ eq originalFooWorker))
 
         val originalBarWorker = workers.filter(_ ne originalFooWorker).head
 
         ev.evaluator.evaluate(Agg(workerModule.foo.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 2)
         assert(workers.exists(_ eq originalFooWorker))
 
         ev.evaluator.evaluate(Agg(workerModule.bar.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 2)
         assert(workers.exists(_ eq originalBarWorker))
 
@@ -472,7 +472,7 @@ object MainModuleTests extends TestSuite {
         os.remove(outDir / "foo/theWorker.json")
 
         ev.evaluator.evaluate(Agg(workerModule.foo.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 2)
         assert(!workers.exists(_ eq originalFooWorker))
         assert(originalFooWorker.closed)
@@ -481,7 +481,7 @@ object MainModuleTests extends TestSuite {
         os.remove(outDir / "bar/theWorker.json")
 
         ev.evaluator.evaluate(Agg(workerModule.bar.theWorker))
-          .ensuring(_.failing.keyCount == 0)
+          .ensuring(_.failing.size == 0)
         assert(workers.size == 2)
         assert(!workers.exists(_ eq originalBarWorker))
         assert(originalBarWorker.closed)
@@ -493,19 +493,19 @@ object MainModuleTests extends TestSuite {
         val ev = UnitTester(workerModule, null)
 
         val r1 = ev.evaluator.evaluate(Agg(workerModule.all))
-        assert(r1.failing.keyCount == 0)
+        assert(r1.failing.size == 0)
         assert(workers.size == 5)
 
         val r2 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "foo")))
-        assert(r2.failing.keyCount == 0)
+        assert(r2.failing.size == 0)
         assert(workers.size == 4)
 
         val r3 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "bar")))
-        assert(r3.failing.keyCount == 0)
+        assert(r3.failing.size == 0)
         assert(workers.size == 3)
 
         val r4 = ev.evaluator.evaluate(Agg(workerModule.clean(ev.evaluator, "bazz[1]")))
-        assert(r4.failing.keyCount == 0)
+        assert(r4.failing.size == 0)
         assert(workers.size == 2)
       }
     }
diff --git a/runner/src/mill/runner/MillBuildBootstrap.scala b/runner/src/mill/runner/MillBuildBootstrap.scala
index da2beee8add..393e42c7ce1 100644
--- a/runner/src/mill/runner/MillBuildBootstrap.scala
+++ b/runner/src/mill/runner/MillBuildBootstrap.scala
@@ -1,7 +1,7 @@
 package mill.runner
 
 import mill.internal.PrefixLogger
-import mill.internal.Watchable
+import mill.define.Watchable
 import mill.main.{BuildInfo, RootModule}
 import mill.main.client.CodeGenConstants.*
 import mill.api.{ColorLogger, PathRef, SystemStreams, Val, internal}
diff --git a/runner/src/mill/runner/RunnerState.scala b/runner/src/mill/runner/RunnerState.scala
index a81d7f0a968..7bb64588272 100644
--- a/runner/src/mill/runner/RunnerState.scala
+++ b/runner/src/mill/runner/RunnerState.scala
@@ -2,7 +2,7 @@ package mill.runner
 
 import mill.api.{PathRef, Val, internal}
 import mill.define.Segments
-import mill.internal.Watchable
+import mill.define.Watchable
 import upickle.default.{ReadWriter, macroRW}
 import mill.eval.Evaluator
 import mill.main.RootModule
diff --git a/runner/src/mill/runner/Watching.scala b/runner/src/mill/runner/Watching.scala
index bdc1d9ce31a..b3c59fd9337 100644
--- a/runner/src/mill/runner/Watching.scala
+++ b/runner/src/mill/runner/Watching.scala
@@ -1,7 +1,8 @@
 package mill.runner
 
 import mill.api.internal
-import mill.internal.{Colors, Watchable}
+import mill.internal.Colors
+import mill.define.Watchable
 import mill.api.SystemStreams
 
 import java.io.InputStream
diff --git a/scalalib/test/src/mill/scalalib/giter8/Giter8Tests.scala b/scalalib/test/src/mill/scalalib/giter8/Giter8Tests.scala
index 38f68cabc08..4edc0d23482 100644
--- a/scalalib/test/src/mill/scalalib/giter8/Giter8Tests.scala
+++ b/scalalib/test/src/mill/scalalib/giter8/Giter8Tests.scala
@@ -38,7 +38,7 @@ object Giter8Tests extends TestSuite {
         )
 
         assert(
-          res.failing.keyCount == 0,
+          res.failing.size == 0,
           res.values.size == 1,
           files.forall(f => os.exists(g8Module.millSourcePath / "hello" / f))
         )
diff --git a/testkit/src/mill/testkit/UnitTester.scala b/testkit/src/mill/testkit/UnitTester.scala
index bda88f35542..4a5d84d0473 100644
--- a/testkit/src/mill/testkit/UnitTester.scala
+++ b/testkit/src/mill/testkit/UnitTester.scala
@@ -133,7 +133,7 @@ class UnitTester(
   ): Either[Result.Failing[?], UnitTester.Result[Seq[?]]] = {
     val evaluated = evaluator.evaluate(tasks)
 
-    if (evaluated.failing.keyCount == 0) {
+    if (evaluated.failing.isEmpty) {
       Right(
         UnitTester.Result(
           evaluated.rawValues.map(_.asInstanceOf[Result.Success[Val]].value.value),
@@ -148,10 +148,8 @@ class UnitTester(
     } else {
       Left(
         evaluated
-          .failing
-          .lookupKey(evaluated.failing.keys().next)
-          .items
-          .next()
+          .failing(evaluated.failing.keys.head)
+          .head
           .asFailing
           .get
           .map(_.value)
@@ -170,7 +168,7 @@ class UnitTester(
 
     assert(
       cleaned == expectedRawValues,
-      res.failing.keyCount == expectedFailCount
+      res.failing.size == expectedFailCount
     )
 
   }