diff --git a/metropolis/installer/test/testos/BUILD b/metropolis/installer/test/testos/BUILD
new file mode 100644
index 0000000..b264527
--- /dev/null
+++ b/metropolis/installer/test/testos/BUILD
@@ -0,0 +1,43 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+load("//metropolis/node/build:def.bzl", "erofs_image")
+load("//metropolis/node/build:efi.bzl", "efi_unified_kernel_image")
+load("@rules_pkg//:pkg.bzl", "pkg_zip")
+
+erofs_image(
+    name = "rootfs",
+    files = {
+        ":testos": "/init",
+    },
+)
+
+efi_unified_kernel_image(
+    name = "kernel_efi",
+    cmdline = "loglevel=0 console=ttyS0 root=PARTLABEL=METROPOLIS-SYSTEM rootfstype=erofs init=/init",
+    kernel = "//third_party/linux",
+)
+
+# An intermediary "bundle" format until we finalize the actual bundle format. This is NOT stable until migrated
+# to the actual bundle format.
+# TODO(lorenz): Replace this
+pkg_zip(
+    name = "testos_bundle",
+    srcs = [
+        ":kernel_efi",
+        ":rootfs",
+    ],
+    visibility = ["//metropolis/installer/test:__subpackages__"],
+)
+
+go_library(
+    name = "go_default_library",
+    srcs = ["main.go"],
+    importpath = "source.monogon.dev/metropolis/installer/test/testos",
+    visibility = ["//visibility:private"],
+    deps = ["@org_golang_x_sys//unix:go_default_library"],
+)
+
+go_binary(
+    name = "testos",
+    embed = [":go_default_library"],
+    visibility = ["//visibility:public"],
+)