diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..2dc42a7
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,57 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+# Cancel superseded runs on the same ref.
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  ci:
+    name: make ci (PHP ${{ matrix.php }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ['8.4']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up PHP ${{ matrix.php }}
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          # PCOV drives Infection's coverage in `make test/mutation`.
+          coverage: pcov
+          extensions: mbstring
+          tools: composer:v2
+
+      - name: Validate composer.json
+        run: composer validate --strict
+
+      - name: Resolve Composer cache directory
+        id: composer-cache
+        run: echo "dir=$(composer config cache-files-dir)" >> "$GITHUB_OUTPUT"
+
+      - name: Cache Composer packages
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: composer-${{ matrix.php }}-${{ hashFiles('composer.json') }}
+          restore-keys: composer-${{ matrix.php }}-
+
+      - name: Install dependencies
+        run: composer install --no-interaction --no-progress --prefer-dist
+
+      - name: Run the full quality gate
+        # check -> build -> unit -> PHPStan (L9) -> mutation (100% MSI) -> demo
+        run: make ci
diff --git a/composer.json b/composer.json
index 57a7bf9..b586cf1 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
         "source": "https://github.com/xphp-lang/collections"
     },
     "require": {
-        "php": "^8.3"
+        "php": "^8.4.0"
     },
     "require-dev": {
         "xphp-lang/xphp": "0.3.0",