diff --git a/java/hiddenapi_modular.go b/java/hiddenapi_modular.go
index 0193657..8a0d634 100644
--- a/java/hiddenapi_modular.go
+++ b/java/hiddenapi_modular.go
@@ -236,7 +236,7 @@
 //
 // The rule is initialized but not built so that the caller can modify it and select an appropriate
 // name.
-func buildRuleToGenerateHiddenAPIStubFlagsFile(ctx android.BuilderContext, name, desc string, outputPath android.WritablePath, bootDexJars android.Paths, input HiddenAPIFlagInput) {
+func buildRuleToGenerateHiddenAPIStubFlagsFile(ctx android.BuilderContext, name, desc string, outputPath android.WritablePath, bootDexJars android.Paths, input HiddenAPIFlagInput, moduleStubFlagsPaths android.Paths) {
 	// Singleton rule which applies hiddenapi on all boot class path dex files.
 	rule := android.NewRuleBuilder(pctx, ctx)
 
@@ -276,6 +276,18 @@
 	// Add the output path.
 	command.FlagWithOutput("--out-api-flags=", tempPath)
 
+	// If there are stub flag files that have been generated by fragments on which this depends then
+	// use them to validate the stub flag file generated by the rules created by this method.
+	if len(moduleStubFlagsPaths) > 0 {
+		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, moduleStubFlagsPaths)
+
+		// Add the file that indicates that the file generated by this is valid.
+		//
+		// This will cause the validation rule above to be run any time that the output of this rule
+		// changes but the validation will run in parallel with other rules that depend on this file.
+		command.Validation(validFile)
+	}
+
 	commitChangeForRestat(rule, tempPath, outputPath)
 
 	rule.Build(name, desc)
@@ -712,28 +724,6 @@
 	outputPath android.WritablePath, baseFlagsPath android.Path, annotationFlagPaths android.Paths,
 	flagFilesByCategory FlagFilesByCategory, allFlagsPaths android.Paths, generatedRemovedDexSignatures android.OptionalPath) {
 
-	// The file which is used to record that the flags file is valid.
-	var validFile android.WritablePath
-
-	// If there are flag files that have been generated by fragments on which this depends then use
-	// them to validate the flag file generated by the rules created by this method.
-	if len(allFlagsPaths) > 0 {
-		// The flags file generated by the rule created by this method needs to be validated to ensure
-		// that it is consistent with the flag files generated by the individual fragments.
-
-		validFile = pathForValidation(ctx, outputPath)
-
-		// Create a rule to validate the output from the following rule.
-		rule := android.NewRuleBuilder(pctx, ctx)
-		rule.Command().
-			BuiltTool("verify_overlaps").
-			Input(outputPath).
-			Inputs(allFlagsPaths).
-			// If validation passes then update the file that records that.
-			Text("&& touch").Output(validFile)
-		rule.Build(name+"Validation", desc+" validation")
-	}
-
 	// Create the rule that will generate the flag files.
 	tempPath := tempPathForRestat(ctx, outputPath)
 	rule := android.NewRuleBuilder(pctx, ctx)
@@ -759,7 +749,11 @@
 
 	commitChangeForRestat(rule, tempPath, outputPath)
 
-	if validFile != nil {
+	// If there are flag files that have been generated by fragments on which this depends then use
+	// them to validate the flag file generated by the rules created by this method.
+	if len(allFlagsPaths) > 0 {
+		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, allFlagsPaths)
+
 		// Add the file that indicates that the file generated by this is valid.
 		//
 		// This will cause the validation rule above to be run any time that the output of this rule
@@ -770,6 +764,25 @@
 	rule.Build(name, desc)
 }
 
+// buildRuleValidateOverlappingCsvFiles checks that the modular CSV files, i.e. the files generated
+// by the individual bootclasspath_fragment modules are subsets of the monolithic CSV file.
+func buildRuleValidateOverlappingCsvFiles(ctx android.BuilderContext, name string, desc string, monolithicFilePath android.WritablePath, modularFilePaths android.Paths) android.WritablePath {
+	// The file which is used to record that the flags file is valid.
+	validFile := pathForValidation(ctx, monolithicFilePath)
+
+	// Create a rule to validate the output from the following rule.
+	rule := android.NewRuleBuilder(pctx, ctx)
+	rule.Command().
+		BuiltTool("verify_overlaps").
+		Input(monolithicFilePath).
+		Inputs(modularFilePaths).
+		// If validation passes then update the file that records that.
+		Text("&& touch").Output(validFile)
+	rule.Build(name+"Validation", desc+" validation")
+
+	return validFile
+}
+
 // hiddenAPIRulesForBootclasspathFragment will generate all the flags for a fragment of the
 // bootclasspath and then encode the flags into the boot dex files.
 //
@@ -793,7 +806,7 @@
 
 	// Generate the stub-flags.csv.
 	stubFlagsCSV := android.PathForModuleOut(ctx, hiddenApiSubDir, "stub-flags.csv")
-	buildRuleToGenerateHiddenAPIStubFlagsFile(ctx, "modularHiddenAPIStubFlagsFile", "modular hiddenapi stub flags", stubFlagsCSV, bootDexInfoByModule.bootDexJars(), input)
+	buildRuleToGenerateHiddenAPIStubFlagsFile(ctx, "modularHiddenAPIStubFlagsFile", "modular hiddenapi stub flags", stubFlagsCSV, bootDexInfoByModule.bootDexJars(), input, nil)
 
 	// Extract the classes jars from the contents.
 	classesJars := extractClassesJarsFromModules(contents)