diff --git a/main-actions/src/main/scala/sbt/RawCompileLike.scala b/main-actions/src/main/scala/sbt/RawCompileLike.scala
index 59533a725..5f9404f9a 100644
--- a/main-actions/src/main/scala/sbt/RawCompileLike.scala
+++ b/main-actions/src/main/scala/sbt/RawCompileLike.scala
@@ -56,10 +56,9 @@ object RawCompileLike {
       type Inputs =
         FilesInfo[HashFileInfo] :+: FilesInfo[ModifiedFileInfo] :+: Seq[File] :+: File :+:
           Seq[String] :+: Int :+: HNil
-      val inputs
-          : Inputs = hash(sources.toSet ++ optionFiles(options, fileInputOpts)) :+: lastModified(
-        classpath.toSet
-      ) :+: classpath :+: outputDirectory :+: options :+: maxErrors :+: HNil
+      val inputs: Inputs = hash(sources.toSet ++ optionFiles(options, fileInputOpts)) :+:
+        FilesInfo(classpath.toSet.map(lastModified.fileOrDirectoryMax)) :+: classpath :+:
+        outputDirectory :+: options :+: maxErrors :+: HNil
       val cachedComp = inputChanged(cacheStoreFactory make "inputs") { (inChanged, in: Inputs) =>
         inputChanged(cacheStoreFactory make "output") {
           (outChanged, outputs: FilesInfo[PlainFileInfo]) =>
diff --git a/util-cache/src/main/scala/sbt/util/FileInfo.scala b/util-cache/src/main/scala/sbt/util/FileInfo.scala
index fde0ee4a9..b9d0e646d 100644
--- a/util-cache/src/main/scala/sbt/util/FileInfo.scala
+++ b/util-cache/src/main/scala/sbt/util/FileInfo.scala
@@ -13,6 +13,8 @@ import scala.util.control.NonFatal
 import sbt.io.{ Hash, IO }
 import sjsonnew.{ Builder, DeserializationException, JsonFormat, Unbuilder, deserializationError }
 import CacheImplicits.{ arrayFormat => _, _ }
+import sbt.nio.file._
+import sbt.nio.file.syntax._
 
 sealed trait FileInfo { def file: File }
 sealed trait HashFileInfo extends FileInfo {
@@ -209,6 +211,26 @@ object FileInfo {
       FileModified(file.getAbsoluteFile, IO.getModifiedTimeOrZero(file))
     def apply(file: File, lastModified: Long): ModifiedFileInfo =
       FileModified(file.getAbsoluteFile, lastModified)
+
+    /**
+     * Returns an instance of [[FileModified]] where, for any directory, the maximum last
+     * modified time taken from its contents is used rather than the last modified time of the
+     * directory itself. The specific motivation was to prevent the doc task from re-running when
+     * the modified time changed for a directory classpath but none of the classfiles had changed.
+     *
+     * @param file the file or directory
+     * @return the [[FileModified]]
+     */
+    private[sbt] def fileOrDirectoryMax(file: File): ModifiedFileInfo = {
+      val maxLastModified =
+        if (file.isDirectory) FileTreeView.default.list(file.toGlob / **).foldLeft(0L) {
+          case (max, (path, attributes)) =>
+            val lm = if (!attributes.isDirectory) IO.getModifiedTimeOrZero(path.toFile) else 0L
+            if (lm > max) lm else max
+        }
+        else IO.getModifiedTimeOrZero(file)
+      FileModified(file, maxLastModified)
+    }
   }
 
   object exists extends Style {