diff --git a/metropolis/node/abloader/spec/BUILD.bazel b/metropolis/node/abloader/spec/BUILD.bazel
new file mode 100644
index 0000000..3190c57
--- /dev/null
+++ b/metropolis/node/abloader/spec/BUILD.bazel
@@ -0,0 +1,44 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+load("@rules_proto//proto:defs.bzl", "proto_library")
+load("@rules_proto_grpc_buf//:defs.bzl", "buf_proto_lint_test")
+load("@rules_rust_prost//:defs.bzl", "rust_prost_library")
+
+buf_proto_lint_test(
+    name = "abloader_proto_lint_test",
+    except_rules = [
+        "PACKAGE_VERSION_SUFFIX",
+        "ENUM_ZERO_VALUE_SUFFIX",
+    ],
+    protos = [":abloader_proto"],
+    use_rules = [
+        "DEFAULT",
+        "COMMENTS",
+    ],
+)
+
+proto_library(
+    name = "abloader_proto",
+    srcs = ["abloader.proto"],
+    visibility = ["//visibility:public"],
+)
+
+rust_prost_library(
+    name = "abloader_proto_rs",
+    proto = ":abloader_proto",
+    visibility = ["//visibility:public"],
+)
+
+go_proto_library(
+    name = "abloader_go_proto",
+    importpath = "source.monogon.dev/metropolis/node/abloader/spec",
+    proto = ":abloader_proto",
+    visibility = ["//visibility:public"],
+)
+
+go_library(
+    name = "spec",
+    embed = [":abloader_go_proto"],
+    importpath = "source.monogon.dev/metropolis/node/abloader/spec",
+    visibility = ["//visibility:public"],
+)
diff --git a/metropolis/node/abloader/spec/abloader.proto b/metropolis/node/abloader/spec/abloader.proto
new file mode 100644
index 0000000..9704ce6
--- /dev/null
+++ b/metropolis/node/abloader/spec/abloader.proto
@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package metropolis.node.abloader.spec;
+
+enum Slot {
+    SLOT_NONE = 0;
+    SLOT_A = 1;
+    SLOT_B = 2;
+}
+
+// ABLoaderData contains data the A/B loader needs to chose which slot to boot
+// as well as for auto-rollback functionality.
+message ABLoaderData {
+    // The currently-active slot. This slot will be booted unless next_slot is
+    // set to a non-default value.
+    Slot active_slot = 1;
+    // The slot to boot next, but only once. Once the the slot has been selected
+    // to be booted by the loader, this value is reset before booting into that
+    // slot. If the OS boots successfully, it will update the active_slot to
+    // permanently boot from the new slot.
+    Slot next_slot = 2;
+}
diff --git a/metropolis/node/abloader/spec/gomod-generated-placeholder.go b/metropolis/node/abloader/spec/gomod-generated-placeholder.go
new file mode 100644
index 0000000..ca2145c
--- /dev/null
+++ b/metropolis/node/abloader/spec/gomod-generated-placeholder.go
@@ -0,0 +1,4 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package spec