diff --git a/main-command/src/test/scala/sbt/internal/ClassLoaderCacheTest.scala b/main-command/src/test/scala/sbt/internal/ClassLoaderCacheTest.scala
index 5a02df2f4..b29c37404 100644
--- a/main-command/src/test/scala/sbt/internal/ClassLoaderCacheTest.scala
+++ b/main-command/src/test/scala/sbt/internal/ClassLoaderCacheTest.scala
@@ -17,7 +17,7 @@ import sbt.internal.classpath.ClassLoaderCache
 import sbt.io.IO
 
 object ClassLoaderCacheTest {
-  implicit class CacheOps(val c: ClassLoaderCache) {
+  extension (c: ClassLoaderCache) {
     def get(classpath: Seq[File]): ClassLoader = c(classpath.toList)
   }
 }
diff --git a/main/src/main/scala/sbt/internal/Clean.scala b/main/src/main/scala/sbt/internal/Clean.scala
index b2a035cd9..48a0b5d48 100644
--- a/main/src/main/scala/sbt/internal/Clean.scala
+++ b/main/src/main/scala/sbt/internal/Clean.scala
@@ -153,8 +153,8 @@ private[sbt] object Clean {
       case _                => Nil
   end ToSeqPath
 
-  private implicit class ToSeqPathOps[T](val t: T) extends AnyVal {
-    def toSeqPath(implicit toSeqPath: ToSeqPath[T]): Seq[Path] = toSeqPath(t)
+  extension [T](t: T) {
+    private def toSeqPath(implicit toSeqPath: ToSeqPath[T]): Seq[Path] = toSeqPath(t)
   }
 
   @nowarn
diff --git a/main/src/main/scala/sbt/internal/Continuous.scala b/main/src/main/scala/sbt/internal/Continuous.scala
index fb0eecbe1..c6a43a5f6 100644
--- a/main/src/main/scala/sbt/internal/Continuous.scala
+++ b/main/src/main/scala/sbt/internal/Continuous.scala
@@ -1003,7 +1003,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
     key.get(deprecatedTriggeredMessage).map(Left(_)).getOrElse(Right(default))
   }
 
-  private implicit class ScopeOps(val scope: Scope) {
+  extension (scope: Scope) {
 
     /**
      * This shows the [[Scope]] in the format that a user would likely type it in a build
@@ -1014,7 +1014,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
      *
      * @return the pretty printed output.
      */
-    def show: String = {
+    private def show: String = {
       val mask = ScopeMask(
         config = scope.config.toOption.isDefined,
         task = scope.task.toOption.isDefined,
@@ -1035,7 +1035,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
     }
   }
 
-  private implicit class ScopedKeyOps(val scopedKey: ScopedKey[_]) extends AnyVal {
+  extension (scopedKey: ScopedKey[_]) {
 
     /**
      * Gets the value for a setting key scoped to the wrapped [[ScopedKey]]. If the task axis is not
@@ -1049,7 +1049,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
      * @return the optional value of the [[SettingKey]] if it is defined at the input
      *         [[ScopedKey]] instance's scope or task scope.
      */
-    def get[T](settingKey: SettingKey[T])(implicit extracted: Extracted): Option[T] = {
+    private def get[T](settingKey: SettingKey[T])(implicit extracted: Extracted): Option[T] = {
       lazy val taskScope = Project.fillTaskAxis(scopedKey).scope
       scopedKey.scope match {
         case scope if scope.task.toOption.isDefined =>
@@ -1071,7 +1071,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
      * @return the optional value of the [[SettingKey]] if it is defined at the input
      *         [[ScopedKey]] instance's scope or task scope.
      */
-    def get[T](taskKey: TaskKey[T])(implicit extracted: Extracted): Option[TaskKey[T]] = {
+    private def get[T](taskKey: TaskKey[T])(implicit extracted: Extracted): Option[TaskKey[T]] = {
       lazy val taskScope = Project.fillTaskAxis(scopedKey).scope
       scopedKey.scope match {
         case scope if scope.task.toOption.isDefined =>
@@ -1094,10 +1094,10 @@ private[sbt] object Continuous extends DeprecatedContinuous {
      *
      * @return the pretty printed output.
      */
-    def show: String = s"${scopedKey.scope.show} / ${scopedKey.key}"
+    private def show: String = s"${scopedKey.scope.show} / ${scopedKey.key}"
   }
 
-  private implicit class LoggerOps(val logger: Logger) extends AnyVal {
+  extension (logger: Logger) {
 
     /**
      * Creates a logger that adds a prefix to the messages that it logs. The motivation is so that
@@ -1106,7 +1106,7 @@ private[sbt] object Continuous extends DeprecatedContinuous {
      * @param prefix the string to prefix the message with
      * @return the wrapped Logger.
      */
-    def withPrefix(prefix: String): Logger = new Logger {
+    private def withPrefix(prefix: String): Logger = new Logger {
       override def trace(t: => Throwable): Unit = logger.trace(t)
 
       override def success(message: => String): Unit = logger.success(message)
diff --git a/main/src/main/scala/sbt/internal/WatchTransitiveDependencies.scala b/main/src/main/scala/sbt/internal/WatchTransitiveDependencies.scala
index 7729b4061..e4ad1fa25 100644
--- a/main/src/main/scala/sbt/internal/WatchTransitiveDependencies.scala
+++ b/main/src/main/scala/sbt/internal/WatchTransitiveDependencies.scala
@@ -25,8 +25,8 @@ import sbt.nio.file.Glob
 import scala.annotation.{ nowarn, tailrec }
 
 private[sbt] object WatchTransitiveDependencies {
-  private implicit class SourceOps(val source: Source) {
-    def toGlob: Glob = {
+  extension (source: Source) {
+    private def toGlob: Glob = {
       val filter = source.includeFilter -- source.excludeFilter
       Globs.apply(source.base.toPath, source.recursive, filter)
     }
diff --git a/main/src/main/scala/sbt/nio/FileStamp.scala b/main/src/main/scala/sbt/nio/FileStamp.scala
index 05f00d374..5ebba247b 100644
--- a/main/src/main/scala/sbt/nio/FileStamp.scala
+++ b/main/src/main/scala/sbt/nio/FileStamp.scala
@@ -248,8 +248,8 @@ object FileStamp {
       }
   }
 
-  private implicit class EitherOps(val e: Either[FileStamp, FileStamp]) extends AnyVal {
-    def value: Option[FileStamp] = if (e == null) None else Some(e.fold(identity, identity))
+  extension (e: Either[FileStamp, FileStamp]) {
+    private def value: Option[FileStamp] = if (e == null) None else Some(e.fold(identity, identity))
   }
 
   private[sbt] class Cache {
diff --git a/main/src/main/scala/sbt/nio/Watch.scala b/main/src/main/scala/sbt/nio/Watch.scala
index af23a081a..890967802 100644
--- a/main/src/main/scala/sbt/nio/Watch.scala
+++ b/main/src/main/scala/sbt/nio/Watch.scala
@@ -53,18 +53,18 @@ object Watch {
   private val formatter = DateTimeFormatter.ofPattern("yyyy-MMM-dd HH:mm:ss.SSS")
   private val timeZone = ZoneId.systemDefault
   private val timeZoneName = timeZone.getDisplayName(TextStyle.SHORT, Locale.getDefault)
-  private implicit class DurationOps(val d: Duration) extends AnyVal {
-    def finite: FiniteDuration = d match {
+  extension (d: Duration) {
+    private def finite: FiniteDuration = d match {
       case f: FiniteDuration => f
       case _                 => new FiniteDuration(Long.MaxValue, TimeUnit.MILLISECONDS)
     }
-    def toEpochString: String = {
+    private def toEpochString: String = {
       val zdt = ZonedDateTime.ofInstant(Instant.ofEpochMilli(d.toMillis), timeZone)
       s"${formatter.format(zdt)} $timeZoneName"
     }
   }
-  private[sbt] implicit class EventOps(val event: Event) extends AnyVal {
-    def toEpochString: String = event.occurredAt.toEpochString
+  extension (event: Event) {
+    private[sbt] def toEpochString: String = event.occurredAt.toEpochString
   }
   private[sbt] object Event {
     trait Impl { self: Event =>
@@ -434,8 +434,8 @@ object Watch {
   private[sbt] def aggregate(events: Seq[(Action, Event)]): Option[(Action, Event)] =
     if (events.isEmpty) None else Some(events.minBy(_._1))
 
-  private implicit class StringToExec(val s: String) extends AnyVal {
-    def toExec: Exec = Exec(s, None)
+  extension (s: String) {
+    private def toExec: Exec = Exec(s, None)
   }
 
   /**