diff --git a/.github/workflows/cd.yml b/.github/workflows/cd.yml
index 743da54..21d757b 100644
--- a/.github/workflows/cd.yml
+++ b/.github/workflows/cd.yml
@@ -16,45 +16,46 @@ concurrency:
   group: cd-${{ github.ref }}
   cancel-in-progress: ${{ !startsWith(github.ref, 'refs/tags/v') }}
 
+env:
+  REGISTRY: ghcr.io
+  # Lowercase, hardcoded: GHCR image names must be lowercase, and
+  # github.repository would be `sessatakuma/API-tools` (mixed case).
+  # Must match deploy/compose.yml's api-tools image in jpcorrect-backend.
+  IMAGE_NAME: sessatakuma/api-tools
+
 jobs:
   tests:
     name: Validate source
     uses: ./.github/workflows/tests.yml
 
-  build-and-push:
-    name: Build and push image to GHCR
+  build:
+    name: Build and smoke-test ${{ matrix.platform }}
     needs: [tests]
-    runs-on: ubuntu-latest
-    env:
-      REGISTRY: ghcr.io
-      # Lowercase, hardcoded: GHCR image names must be lowercase, and
-      # github.repository would be `sessatakuma/API-tools` (mixed case).
-      # Must match deploy/compose.yml's api-tools image in jpcorrect-backend.
-      IMAGE_NAME: sessatakuma/api-tools
+    # Each arch builds on a runner of its own architecture, so no QEMU is
+    # involved. That matters because pyopenjtalk ships source-only on PyPI
+    # (see Dockerfile:9-13) and compiles its bundled OpenJTalk/HTS-engine C++
+    # during `uv sync` — CPU-bound work that ran roughly an order of magnitude
+    # slower under emulation (~+12 min per cold arm64 build). Building
+    # natively also gives each arch a single-platform image it can `load` and
+    # actually run, so the arm64 image that ships to the Ampere deploy host is
+    # smoke-tested rather than published sight-unseen.
+    runs-on: ${{ matrix.runner }}
+    strategy:
+      # Let both arches report: a failure on one shouldn't hide the other's
+      # result, and `merge` gates publication on both succeeding anyway.
+      fail-fast: false
+      matrix:
+        include:
+          - runner: ubuntu-latest
+            platform: linux/amd64
+            arch: amd64
+          - runner: ubuntu-24.04-arm  # free for public repos
+            platform: linux/arm64
+            arch: arm64
     steps:
-      - name: Wait for earlier CD runs before publishing a release
-        if: startsWith(github.ref, 'refs/tags/v')
-        env:
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          while true; do
-            blocking_run=$(gh api \
-              "repos/${GITHUB_REPOSITORY}/actions/workflows/cd.yml/runs?per_page=100" \
-              --jq ".workflow_runs[] | select(.run_number < ${GITHUB_RUN_NUMBER} and (.status == \"queued\" or .status == \"in_progress\")) | .id" \
-              | head -n 1)
-            if [[ -z "$blocking_run" ]]; then
-              break
-            fi
-            echo "Waiting for earlier CD run $blocking_run"
-            sleep 15
-          done
-
       - name: Checkout
         uses: actions/checkout@v6
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@v4
-
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
@@ -70,32 +71,26 @@ jobs:
         uses: docker/metadata-action@v6
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
-          tags: |
-            type=raw,value=dev,enable={{is_default_branch}}
-            type=sha,format=short
-            type=semver,pattern={{version}}
-            type=semver,pattern={{major}}.{{minor}}
-            type=raw,value=stable,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
 
-      - name: Build amd64 image for smoke test
+      - name: Build image for smoke test
         uses: docker/build-push-action@v7
         with:
           context: .
           file: ./Dockerfile
-          # Smoke test runs on the runner, so this stage stays native amd64:
-          # buildx cannot `load` a multi-platform result into the daemon.
-          platforms: linux/amd64
+          platforms: ${{ matrix.platform }}
           push: false
           load: true
-          tags: ${{ steps.meta.outputs.tags }}
+          tags: api-tools:smoke-${{ matrix.arch }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          # Per-arch cache scope: the two matrix jobs run concurrently and
+          # would otherwise race writing the same `type=gha` scope.
+          cache-from: type=gha,scope=cd-${{ matrix.arch }}
+          cache-to: type=gha,mode=max,scope=cd-${{ matrix.arch }}
 
       - name: Smoke-test production lifecycle offline
         run: |
-          image=$(printf '%s\n' '${{ steps.meta.outputs.tags }}' | head -n 1)
-          container=api-tools-release-smoke-${{ github.run_id }}
+          image=api-tools:smoke-${{ matrix.arch }}
+          container=api-tools-release-smoke-${{ github.run_id }}-${{ matrix.arch }}
           trap 'docker rm -f "$container" >/dev/null 2>&1 || true' EXIT
           docker run -d --name "$container" --network none --read-only --tmpfs /tmp \
             -e PYTHONDONTWRITEBYTECODE=1 \
@@ -114,19 +109,115 @@ jobs:
           docker exec "$container" python -c \
             "import json, urllib.request; request=urllib.request.Request('http://127.0.0.1:8000/api/MarkAccent/', data=json.dumps({'text':'東京'}).encode(), headers={'Content-Type':'application/json'}); response=json.load(urllib.request.urlopen(request, timeout=30)); assert response['status'] == 200 and response['result']"
 
-      - name: Push smoke-tested image (amd64 + arm64)
+      - name: Push smoke-tested image by digest
+        id: build
         uses: docker/build-push-action@v7
         with:
           context: .
           file: ./Dockerfile
-          # arm64 so images run natively on the arm64 (Oracle Ampere) deploy
-          # host. amd64 layers come from the cache the smoke build just wrote,
-          # so only the arm64 half is rebuilt (under QEMU).
-          platforms: linux/amd64,linux/arm64
-          push: true
+          platforms: ${{ matrix.platform }}
+          # Push untagged, addressed only by digest: `merge` stitches the
+          # per-arch digests into the tagged index once *both* smoke tests
+          # pass, so a broken arm64 image can never take `dev`/`stable`.
+          outputs: >-
+            type=image,name=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }},push-by-digest=true,name-canonical=true,push=true
           provenance: true
           sbom: true
-          tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          # Every layer is already in the scope the smoke build just wrote, so
+          # this re-exports rather than rebuilds. No cache-to: nothing new.
+          cache-from: type=gha,scope=cd-${{ matrix.arch }}
+
+      - name: Export digest
+        run: |
+          mkdir -p /tmp/digests
+          digest='${{ steps.build.outputs.digest }}'
+          touch "/tmp/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v7
+        with:
+          name: digest-${{ matrix.arch }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    name: Publish multi-arch manifest
+    needs: [build]
+    runs-on: ubuntu-latest
+    steps:
+      # Serialising happens here rather than before the builds: only tagging is
+      # order-sensitive (an older run must not drag `stable` backwards), and
+      # the per-arch builds are free to run alongside an earlier run.
+      - name: Wait for earlier CD runs before publishing a release
+        if: startsWith(github.ref, 'refs/tags/v')
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          while true; do
+            blocking_run=$(gh api \
+              "repos/${GITHUB_REPOSITORY}/actions/workflows/cd.yml/runs?per_page=100" \
+              --jq ".workflow_runs[] | select(.run_number < ${GITHUB_RUN_NUMBER} and (.status == \"queued\" or .status == \"in_progress\")) | .id" \
+              | head -n 1)
+            if [[ -z "$blocking_run" ]]; then
+              break
+            fi
+            echo "Waiting for earlier CD run $blocking_run"
+            sleep 15
+          done
+
+      - name: Download digests
+        uses: actions/download-artifact@v8
+        with:
+          path: /tmp/digests
+          pattern: digest-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v4
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v4
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v6
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=dev,enable={{is_default_branch}}
+            type=sha,format=short
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=stable,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
+
+      - name: Create and push multi-arch manifest
+        working-directory: /tmp/digests
+        run: |
+          shopt -s nullglob
+          digests=(*)
+          if [[ ${#digests[@]} -eq 0 ]]; then
+            echo "No per-arch digests were downloaded" >&2
+            exit 1
+          fi
+          args=()
+          while IFS= read -r tag; do
+            args+=(--tag "$tag")
+          done < <(jq -r '.tags[]' <<<"$DOCKER_METADATA_OUTPUT_JSON")
+          # Each source is the single-platform index buildx pushed above: it
+          # carries the platform manifest plus its provenance/SBOM attestation
+          # manifests, and imagetools copies all of them into the merged index.
+          for digest in "${digests[@]}"; do
+            args+=("${REGISTRY}/${IMAGE_NAME}@sha256:${digest}")
+          done
+          docker buildx imagetools create "${args[@]}"
+
+      - name: Inspect published index
+        run: |
+          docker buildx imagetools inspect \
+            "${REGISTRY}/${IMAGE_NAME}:${{ steps.meta.outputs.version }}"