diff --git a/contrib/playlib/src/mill/playlib/RouteCompilerWorker.scala b/contrib/playlib/src/mill/playlib/RouteCompilerWorker.scala
index 58ec70f3cdb..d0c695bff66 100644
--- a/contrib/playlib/src/mill/playlib/RouteCompilerWorker.scala
+++ b/contrib/playlib/src/mill/playlib/RouteCompilerWorker.scala
@@ -5,7 +5,7 @@ import mill.playlib.api.{RouteCompilerType, RouteCompilerWorkerApi}
 import mill.scalalib.api.CompilationResult
 import mill.{Agg, T}
 
-private[playlib] class RouteCompilerWorker {
+private[playlib] class RouteCompilerWorker extends AutoCloseable {
 
   private var routeCompilerInstanceCache =
     Option.empty[(Long, mill.playlib.api.RouteCompilerWorkerApi)]
@@ -63,4 +63,8 @@ private[playlib] class RouteCompilerWorker {
       case err => Result.Failure(err)
     }
   }
+
+  override def close(): Unit = {
+    routeCompilerInstanceCache = None
+  }
 }
diff --git a/contrib/scalapblib/src/ScalaPBWorker.scala b/contrib/scalapblib/src/ScalaPBWorker.scala
index 223f69bb4ce..3d0525705be 100644
--- a/contrib/scalapblib/src/ScalaPBWorker.scala
+++ b/contrib/scalapblib/src/ScalaPBWorker.scala
@@ -9,7 +9,7 @@ import mill.api.PathRef
 import mill.T
 import mill.define.{Discover, ExternalModule, Worker}
 
-class ScalaPBWorker {
+class ScalaPBWorker extends AutoCloseable {
 
   private var scalaPBInstanceCache = Option.empty[(Long, ScalaPBWorkerApi)]
 
@@ -100,6 +100,10 @@ class ScalaPBWorker {
 
     mill.api.Result.Success(PathRef(dest))
   }
+
+  override def close(): Unit = {
+    scalaPBInstanceCache = None
+  }
 }
 
 trait ScalaPBWorkerApi {
diff --git a/contrib/scoverage/src/ScoverageReportWorker.scala b/contrib/scoverage/src/ScoverageReportWorker.scala
index 49b9aca2f18..a5236a3cfd8 100644
--- a/contrib/scoverage/src/ScoverageReportWorker.scala
+++ b/contrib/scoverage/src/ScoverageReportWorker.scala
@@ -5,7 +5,7 @@ import mill.api.{ClassLoader, Ctx}
 import mill.contrib.scoverage.api.ScoverageReportWorkerApi
 import mill.define.{Discover, ExternalModule, Worker}
 
-class ScoverageReportWorker {
+class ScoverageReportWorker extends AutoCloseable {
   private[this] var scoverageClCache = Option.empty[(Long, ClassLoader)]
 
   def bridge(classpath: Agg[os.Path])(implicit ctx: Ctx): ScoverageReportWorkerApi = {
@@ -31,6 +31,10 @@ class ScoverageReportWorker {
       .newInstance()
       .asInstanceOf[api.ScoverageReportWorkerApi]
   }
+
+  override def close(): Unit = {
+    scoverageClCache = None
+  }
 }
 
 object ScoverageReportWorker extends ExternalModule {
diff --git a/scalajslib/src/ScalaJSWorkerApi.scala b/scalajslib/src/ScalaJSWorkerApi.scala
index 3b1a4c51724..fd454f59c17 100644
--- a/scalajslib/src/ScalaJSWorkerApi.scala
+++ b/scalajslib/src/ScalaJSWorkerApi.scala
@@ -6,7 +6,7 @@ import mill.api.{Ctx, Result}
 import mill.define.Discover
 import mill.scalajslib.api._
 import mill.{Agg, T}
-class ScalaJSWorker {
+class ScalaJSWorker extends AutoCloseable {
   private var scalaInstanceCache = Option.empty[(Long, ScalaJSWorkerApi)]
 
   private def bridge(toolsClasspath: Agg[os.Path])(implicit ctx: Ctx.Home) = {
@@ -68,6 +68,9 @@ class ScalaJSWorker {
     bridge(toolsClasspath).getFramework(config, frameworkName, linkedFile, moduleKind)
   }
 
+  override def close(): Unit = {
+    scalaInstanceCache = None
+  }
 }
 
 object ScalaJSWorkerApi extends mill.define.ExternalModule {
diff --git a/scalalib/src/scalafmt/ScalafmtWorker.scala b/scalalib/src/scalafmt/ScalafmtWorker.scala
index 5fa9822c309..c1d61d8f839 100644
--- a/scalalib/src/scalafmt/ScalafmtWorker.scala
+++ b/scalalib/src/scalafmt/ScalafmtWorker.scala
@@ -14,7 +14,7 @@ object ScalafmtWorkerModule extends ExternalModule {
   lazy val millDiscover = Discover[this.type]
 }
 
-private[scalafmt] class ScalafmtWorker {
+private[scalafmt] class ScalafmtWorker extends AutoCloseable {
   private val reformatted: mutable.Map[os.Path, Int] = mutable.Map.empty
   private var configSig: Int = 0
 
@@ -94,4 +94,8 @@ private[scalafmt] class ScalafmtWorker {
     }
   }
 
+  override def close(): Unit = {
+    reformatted.clear()
+    configSig = 0
+  }
 }
diff --git a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
index e0b331886e5..4aff9d9a04a 100644
--- a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
+++ b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
@@ -27,7 +27,7 @@ class ZincWorkerImpl(
     compilerJarNameGrep: (Agg[os.Path], String) => os.Path,
     compilerCache: KeyedLockedCache[Compilers],
     compileToJar: Boolean
-) extends ZincWorkerApi {
+) extends ZincWorkerApi with AutoCloseable {
   private val ic = new sbt.internal.inc.IncrementalCompilerImpl()
   lazy val javaOnlyCompilers = {
     // Keep the classpath as written by the user
@@ -558,4 +558,8 @@ class ZincWorkerImpl(
       reporter.foreach(_.finish())
     }
   }
+
+  override def close(): Unit = {
+    classloaderCache.clear()
+  }
 }
diff --git a/scalanativelib/src/ScalaNativeWorkerApi.scala b/scalanativelib/src/ScalaNativeWorkerApi.scala
index 71a4fefd096..6cdb4813627 100644
--- a/scalanativelib/src/ScalaNativeWorkerApi.scala
+++ b/scalanativelib/src/ScalaNativeWorkerApi.scala
@@ -7,7 +7,7 @@ import mill.define.{Discover, Worker}
 import mill.{Agg, T}
 import mill.scalanativelib.api._
 
-class ScalaNativeWorker {
+class ScalaNativeWorker extends AutoCloseable {
   private var scalaInstanceCache = Option.empty[(Long, ScalaNativeWorkerApi)]
 
   def impl(toolsClasspath: Agg[os.Path])(implicit ctx: mill.api.Ctx.Home): ScalaNativeWorkerApi = {
@@ -37,6 +37,11 @@ class ScalaNativeWorker {
         }
     }
   }
+
+  override def close(): Unit = {
+    // drop instance
+    scalaInstanceCache = None
+  }
 }
 
 object ScalaNativeWorkerApi extends mill.define.ExternalModule {