diff --git a/internal/rosa/all.go b/internal/rosa/all.go
index 1a80a0a..edbc058 100644
--- a/internal/rosa/all.go
+++ b/internal/rosa/all.go
@@ -22,6 +22,7 @@ const (
 	Curl
 	Diffutils
 	Findutils
+	Flex
 	Fuse
 	GMP
 	GLib
@@ -129,6 +130,7 @@ func ResolveName(name string) (p PArtifact, ok bool) {
 		"curl":              Curl,
 		"diffutils":         Diffutils,
 		"findutils":         Findutils,
+		"flex":              Flex,
 		"fuse":              Fuse,
 		"gmp":               GMP,
 		"glib":              GLib,
diff --git a/internal/rosa/flex.go b/internal/rosa/flex.go
new file mode 100644
index 0000000..46d59fc
--- /dev/null
+++ b/internal/rosa/flex.go
@@ -0,0 +1,21 @@
+package rosa
+
+import (
+	"hakurei.app/internal/pkg"
+)
+
+func (t Toolchain) newFlex() pkg.Artifact {
+	const (
+		version  = "2.6.4"
+		checksum = "p9POjQU7VhgOf3x5iFro8fjhy0NOanvA7CTeuWS_veSNgCixIJshTrWVkc5XLZkB"
+	)
+	return t.NewViaMake("flex", version, pkg.NewHTTPGetTar(
+		nil, "https://github.com/westes/flex/releases/download/"+
+			"v"+version+"/flex-"+version+".tar.gz",
+		mustDecode(checksum),
+		pkg.TarGzip,
+	), nil,
+		t.Load(M4),
+	)
+}
+func init() { artifactsF[Flex] = Toolchain.newFlex }