diff --git a/.github/actions/build-site/action.yml b/.github/actions/build-site/action.yml
new file mode 100644
index 0000000..51c3b27
--- /dev/null
+++ b/.github/actions/build-site/action.yml
@@ -0,0 +1,61 @@
+name: Build and check the site
+description: >-
+  Installs the pinned Hugo, builds the production site into public/, and tests
+  invariants. Shared by the deploy and the pull request workflows.
+
+runs:
+  using: composite
+  steps:
+    # Standard edition is sufficient: the site writes plain CSS, uses no Sass
+    # and no image processing.
+    - name: Install Hugo
+      shell: bash
+      env:
+        # The entire dependency manifest for this site is these two values.
+        # To bump: change the version, download the tarball, and replace the
+        # checksum with `sha256sum hugo_<version>_linux-amd64.tar.gz`.
+        HUGO_VERSION: 0.162.1
+        HUGO_SHA256: 4bfcdb092d0306586f1b72e5687787ead053faab2d71f09951d3c5fecde66873
+      run: |
+        set -euo pipefail
+        url="https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_${HUGO_VERSION}_linux-amd64.tar.gz"
+        curl -sSLf -o hugo.tar.gz "$url"
+        echo "${HUGO_SHA256}  hugo.tar.gz" | sha256sum --check --strict -
+        tar -xzf hugo.tar.gz hugo
+        rm hugo.tar.gz
+        ./hugo version
+
+    # No -D, so drafts stay off the rendered site.
+    - name: Build
+      shell: bash
+      run: ./hugo --minify --printPathWarnings
+      env:
+        HUGO_ENVIRONMENT: production
+
+    # baseURL is the only place the domain may appear. Templates must build
+    # URLs with .Permalink/.RelPermalink/relURL/absURL so that changing the
+    # domain stays a one-line edit.
+    - name: Assert no hardcoded host
+      shell: bash
+      run: |
+        if grep -rnE 'https?://(www\.)?lnfuzz\.(org|github\.io)' layouts/ content/ data/; then
+          echo "::error::A template or page hardcodes the site's own host. Use relURL/absURL instead."
+          exit 1
+        fi
+
+    # No JavaScript and no third-party requests.
+    - name: Assert no scripts or third-party requests
+      shell: bash
+      run: |
+        if grep -rniE '<script|googleapis|platform\.twitter|buttons\.github|jsdelivr|cdnjs' public/; then
+          echo "::error::Found a script tag or third-party reference in the built site."
+          exit 1
+        fi
+
+    - name: Assert Atom feed at /feed.xml
+      shell: bash
+      run: |
+        test -s public/feed.xml || {
+          echo "::error::public/feed.xml is missing."
+          exit 1
+        }
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
new file mode 100644
index 0000000..ffd6e4e
--- /dev/null
+++ b/.github/workflows/build.yml
@@ -0,0 +1,23 @@
+name: Build site
+
+on:
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+# A check on a superseded commit tells nobody anything, so cancel it.
+concurrency:
+  group: "build-${{ github.ref }}"
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build and check
+        uses: ./.github/actions/build-site
diff --git a/.github/workflows/deploy.yml b/.github/workflows/deploy.yml
index 45d82b3..5b8088e 100644
--- a/.github/workflows/deploy.yml
+++ b/.github/workflows/deploy.yml
@@ -16,13 +16,6 @@ concurrency:
   group: "pages"
   cancel-in-progress: false
 
-env:
-  # The entire dependency manifest for this site is these two values.
-  # To bump: change the version, download the tarball, and replace the checksum
-  # with `sha256sum hugo_<version>_linux-amd64.tar.gz`.
-  HUGO_VERSION: 0.162.1
-  HUGO_SHA256: 4bfcdb092d0306586f1b72e5687787ead053faab2d71f09951d3c5fecde66873
-
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -30,50 +23,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      # Standard edition is sufficient: the site writes plain CSS, uses no Sass
-      # and no image processing.
-      - name: Install Hugo
-        run: |
-          set -euo pipefail
-          url="https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_${HUGO_VERSION}_linux-amd64.tar.gz"
-          curl -sSLf -o hugo.tar.gz "$url"
-          echo "${HUGO_SHA256}  hugo.tar.gz" | sha256sum --check --strict -
-          tar -xzf hugo.tar.gz hugo
-          ./hugo version
-
       - name: Setup Pages
         uses: actions/configure-pages@v5
 
-      # No -D, so drafts stay off the rendered site.
-      - name: Build
-        run: ./hugo --minify --printPathWarnings
-        env:
-          HUGO_ENVIRONMENT: production
-
-      # baseURL is the only place the domain may appear. Templates must build
-      # URLs with .Permalink/.RelPermalink/relURL/absURL so that changing the
-      # domain stays a one-line edit.
-      - name: Assert no hardcoded host
-        run: |
-          if grep -rnE 'https?://(www\.)?lnfuzz\.(org|github\.io)' layouts/ content/ data/; then
-            echo "::error::A template or page hardcodes the site's own host. Use relURL/absURL instead."
-            exit 1
-          fi
-
-      # No JavaScript and no third-party requests.
-      - name: Assert no scripts or third-party requests
-        run: |
-          if grep -rniE '<script|googleapis|platform\.twitter|buttons\.github|jsdelivr|cdnjs' public/; then
-            echo "::error::Found a script tag or third-party reference in the built site."
-            exit 1
-          fi
-
-      - name: Assert Atom feed at /feed.xml
-        run: |
-          test -s public/feed.xml || {
-            echo "::error::public/feed.xml is missing."
-            exit 1
-          }
+      - name: Build and check
+        uses: ./.github/actions/build-site
 
       # This action defaults to _site/, but Hugo publishes to public/.
       - name: Upload artifact