diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..31ce555
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,48 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+# This repo is public and publishes `affitor` to npm; a workflow here never needs write access.
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      # Actions are pinned to a commit, not a moving tag: a tag can be repointed,
+      # and this repo publishes to npm. The comment records which tag was pinned.
+      - name: Checkout
+        uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4
+        with:
+          # This job runs code from the pull request. Do not leave the token in
+          # .git/config where that code can read it.
+          persist-credentials: false
+
+      - name: Setup Node
+        uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      # No `--if-present`: all three workspaces declare build, lint and test today,
+      # so a missing script means a package quietly stopped being checked. Fail loudly.
+      - name: Build
+        run: npm run build --workspaces
+
+      - name: Lint
+        run: npm run lint --workspaces
+
+      - name: Unit tests
+        run: npm run test --workspaces