diff --git a/scalalib/src/mill/scalalib/AssemblyModule.scala b/scalalib/src/mill/scalalib/AssemblyModule.scala
index 7ba19d81a54..6cddd9fb240 100644
--- a/scalalib/src/mill/scalalib/AssemblyModule.scala
+++ b/scalalib/src/mill/scalalib/AssemblyModule.scala
@@ -63,15 +63,44 @@ trait AssemblyModule extends mill.Module {
 
   private[mill] def assemblyRules0: Seq[Assembly.Rule] = Assembly.defaultRules
 
+  /**
+   * Upstream classfiles and resources from third-party libraries
+   * necessary to build an executable assembly
+   */
+  def upstreamIvyAssemblyClasspath: T[Agg[PathRef]] = Agg.empty[PathRef]
+
+  /**
+   * Upstream classfiles and resources from locally-built modules
+   * necessary to build an executable assembly, but without this module's contribution
+   */
+  def upstreamLocalAssemblyClasspath: T[Agg[PathRef]] = upstreamAssemblyClasspath()
+
   def upstreamAssemblyClasspath: T[Agg[PathRef]]
 
   def localClasspath: T[Seq[PathRef]]
 
+  /**
+   * Build the assembly for third-party dependencies separate from the current
+   * classpath
+   *
+   * This should allow much faster assembly creation in the common case where
+   * third-party dependencies do not change
+   */
+  def resolvedIvyAssembly: T[Assembly] = Task {
+    Assembly.create(
+      destJar = Task.dest / "out.jar",
+      inputPaths = upstreamIvyAssemblyClasspath().map(_.path),
+      manifest = manifest(),
+      assemblyRules = assemblyRules
+    )
+  }
+
   private[mill] def upstreamAssembly2_0: T[Assembly] = Task {
     Assembly.create(
       destJar = Task.dest / "out.jar",
-      inputPaths = upstreamAssemblyClasspath().map(_.path),
+      inputPaths = upstreamLocalAssemblyClasspath().map(_.path),
       manifest = manifest(),
+      base = Some(resolvedIvyAssembly().pathRef.path),
       assemblyRules = assemblyRules
     )
   }
@@ -112,7 +141,7 @@ trait AssemblyModule extends mill.Module {
     val problematicEntryCount = 65535
     if (
       prependScript.isDefined &&
-      (upstream.addedEntries + created.addedEntries) > problematicEntryCount
+      (resolvedIvyAssembly().addedEntries + upstream.addedEntries + created.addedEntries) > problematicEntryCount
     ) {
       Result.Failure(
         s"""The created assembly jar contains more than ${problematicEntryCount} ZIP entries.
diff --git a/scalalib/src/mill/scalalib/JavaModule.scala b/scalalib/src/mill/scalalib/JavaModule.scala
index aaff671bb89..441f335ec84 100644
--- a/scalalib/src/mill/scalalib/JavaModule.scala
+++ b/scalalib/src/mill/scalalib/JavaModule.scala
@@ -1015,6 +1015,14 @@ trait JavaModule
     )
   }
 
+  def upstreamIvyAssemblyClasspath: T[Agg[PathRef]] = Task {
+    resolvedRunIvyDeps()
+  }
+
+  def upstreamLocalAssemblyClasspath: T[Agg[PathRef]] = Task {
+    transitiveLocalClasspath()
+  }
+
   /**
    * All upstream classfiles and resources necessary to build and executable
    * assembly, but without this module's contribution