diff --git a/cmd/packserver/main.go b/cmd/packserver/main.go
index c51ba00..a374a66 100644
--- a/cmd/packserver/main.go
+++ b/cmd/packserver/main.go
@@ -57,6 +57,8 @@ func main() {
 		"Tell client how long it can cache data for; 0 means no caching")
 	rootCmd.Flags().Duration("expiry404", 0,
 		"Tell client how long it can cache 404 responses for; 0 means default (inherit from --expiry, max. 60s); -1 means no caching")
+	rootCmd.Flags().Bool("immutable", false,
+		"Tell client that cached assets are immutable (will not be revalidated on page reload)")
 	rootCmd.Flags().String("fallback-404", "",
 		"Name of file to return if response would be 404 (spa.html or similar)")
 	rootCmd.Flags().String("frames", "sameorigin",
@@ -138,12 +140,21 @@ func run(c *cobra.Command, args []string) error {
 	// parse expiry time
 	//  NB: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
 	cacheControl200, cacheControl404 := "no-cache", "no-cache"
+	immutable, err := c.Flags().GetBool("immutable")
+	if err != nil {
+		return err
+	}
+	var immutableDirective string
+	if immutable {
+		immutableDirective = ", immutable"
+	}
+
 	expiry, err := c.Flags().GetDuration("expiry")
 	if err != nil {
 		return err
 	}
 	if expiry > 0 {
-		cacheControl200 = fmt.Sprintf("public, max-age=%d", expiry/1e9)
+		cacheControl200 = fmt.Sprintf("public, max-age=%d%s", expiry/1e9, immutableDirective)
 	}
 
 	expiry404, err := c.Flags().GetDuration("expiry404")