diff --git a/metropolis/node/core/update/BUILD.bazel b/metropolis/node/core/update/BUILD.bazel
index ed98d21..ff2faea 100644
--- a/metropolis/node/core/update/BUILD.bazel
+++ b/metropolis/node/core/update/BUILD.bazel
@@ -22,6 +22,7 @@
         "//osbase/oci/osimage",
         "//osbase/oci/registry",
         "@com_github_cenkalti_backoff_v4//:backoff",
+        "@com_github_opencontainers_image_spec//specs-go/v1:specs-go",
         "@org_golang_google_grpc//codes",
         "@org_golang_google_grpc//status",
         "@org_golang_google_protobuf//proto",
diff --git a/metropolis/node/core/update/e2e/BUILD.bazel b/metropolis/node/core/update/e2e/BUILD.bazel
index 0bc7652..51fc851 100644
--- a/metropolis/node/core/update/e2e/BUILD.bazel
+++ b/metropolis/node/core/update/e2e/BUILD.bazel
@@ -12,13 +12,13 @@
         "//metropolis/node/abloader",
         # For the two update tests
         "//metropolis/node/core/update/e2e/testos:testos_image_y",
-        "//metropolis/node/core/update/e2e/testos:testos_image_z",
+        "//metropolis/node/core/update/e2e/testos:testos_multiarch_z",
         "//build/toolchain/toolchain-bundle:qemu-kvm",
     ],
     x_defs = {
         "xImageXPath": "$(rlocationpath //metropolis/node/core/update/e2e/testos:testos_image_x )",
         "xImageYPath": "$(rlocationpath //metropolis/node/core/update/e2e/testos:testos_image_y )",
-        "xImageZPath": "$(rlocationpath //metropolis/node/core/update/e2e/testos:testos_image_z )",
+        "xImageZPath": "$(rlocationpath //metropolis/node/core/update/e2e/testos:testos_multiarch_z )",
         "xOvmfVarsPath": "$(rlocationpath //third_party/edk2:VARS.fd )",
         "xOvmfCodePath": "$(rlocationpath //third_party/edk2:CODE.fd )",
         "xAbloaderPath": "$(rlocationpath //metropolis/node/abloader )",
diff --git a/metropolis/node/core/update/e2e/testos/testos.bzl b/metropolis/node/core/update/e2e/testos/testos.bzl
index ab7d2f0..2057c95 100644
--- a/metropolis/node/core/update/e2e/testos/testos.bzl
+++ b/metropolis/node/core/update/e2e/testos/testos.bzl
@@ -2,6 +2,7 @@
 load("//osbase/build/genproductinfo:test.bzl", "test_product_info")
 load("//osbase/build/mkerofs:def.bzl", "erofs_image")
 load("//osbase/build/mkoci:def.bzl", "oci_os_image")
+load("//osbase/build/mkoci/index:def.bzl", "oci_index")
 load("//osbase/build/mkpayload:def.bzl", "efi_unified_kernel_image")
 load("//osbase/build/mkverity:def.bzl", "verity_image")
 
@@ -49,6 +50,17 @@
         visibility = ["//metropolis/node/core/update/e2e:__pkg__"],
     )
 
+    oci_index(
+        name = "testos_multiarch_" + variant,
+        src = ":testos_image_" + variant,
+        platforms = [
+            "//build/platforms:linux_x86_64",
+            "//build/platforms:linux_aarch64",
+        ],
+        tags = ["manual"],
+        visibility = ["//metropolis/node/core/update/e2e:__pkg__"],
+    )
+
     go_binary(
         name = "testos_" + variant,
         embed = [":testos_lib"],
diff --git a/metropolis/node/core/update/update.go b/metropolis/node/core/update/update.go
index 047e9f8..35f2240 100644
--- a/metropolis/node/core/update/update.go
+++ b/metropolis/node/core/update/update.go
@@ -15,11 +15,13 @@
 	"os"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strconv"
 	"strings"
 	"time"
 
 	"github.com/cenkalti/backoff/v4"
+	ocispecv1 "github.com/opencontainers/image-spec/specs-go/v1"
 	"golang.org/x/sys/unix"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -260,6 +262,30 @@
 	return nil
 }
 
+func selectArchitecture(ref oci.Ref, architecture string) (*oci.Image, error) {
+	switch ref := ref.(type) {
+	case *oci.Image:
+		return ref, nil
+	case *oci.Index:
+		var found *ocispecv1.Descriptor
+		for i := range ref.Manifest.Manifests {
+			descriptor := &ref.Manifest.Manifests[i]
+			if descriptor.Platform != nil && descriptor.Platform.Architecture == architecture {
+				if found != nil {
+					return nil, fmt.Errorf("invalid index, found multiple matching entries")
+				}
+				found = descriptor
+			}
+		}
+		if found == nil {
+			return nil, fmt.Errorf("no matching entry found in index for architecture %s", architecture)
+		}
+		return oci.AsImage(ref.Ref(found))
+	default:
+		return nil, fmt.Errorf("unknown manifest media type %q", ref.MediaType())
+	}
+}
+
 // InstallImage fetches the given image, installs it into the currently inactive
 // slot and sets that slot to boot next. If it doesn't return an error, a reboot
 // boots into the new slot.
@@ -290,7 +316,11 @@
 		Repository: imageRef.Repository,
 	}
 
-	image, err := oci.AsImage(client.Read(downloadCtx, imageRef.Tag, imageRef.Digest))
+	ref, err := client.Read(downloadCtx, imageRef.Tag, imageRef.Digest)
+	if err != nil {
+		return fmt.Errorf("failed to fetch OS image: %w", err)
+	}
+	image, err := selectArchitecture(ref, runtime.GOARCH)
 	if err != nil {
 		return fmt.Errorf("failed to fetch OS image: %w", err)
 	}