diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..7e8566a
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,55 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+# A new push supersedes the previous run on the same ref.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+# Least privilege: this workflow only ever reads the repository.
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Python ${{ matrix.python-version }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    strategy:
+      # One failing interpreter must not mask the others.
+      fail-fast: false
+      matrix:
+        # pyproject declares requires-python >= 3.11, so every supported
+        # interpreter is exercised.
+        python-version: ["3.11", "3.12", "3.13"]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          # No credential is needed after checkout; leaving one in .git/config
+          # would expose it to anything later in the job.
+          persist-credentials: false
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: pip
+          cache-dependency-path: pyproject.toml
+
+      - name: Install (editable, with dev extras)
+        run: python -m pip install -e ".[dev]"
+
+      - name: Lint
+        run: ruff check headersvalidator/
+
+      - name: Test
+        # Network I/O is isolated in the *_utils modules and mocked there, so
+        # the suite needs neither a live network nor any external binary.
+        run: pytest --tb=short -q
diff --git a/CLAUDE.md b/CLAUDE.md
index 3f31e5e..20b4613 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -72,7 +72,8 @@ Score = `round(earned / (graded_count * 2) * 100)`.
 - Conventional commits: `fix:`, `feat:`, `fix(scope):`, `refactor:`, `test:`, `docs:`
 - Input validation lives in `cli.py` (URL normalisation delegated to `normalise_url()` in `http_utils.py`)
 - `fetch_headers()` from `http_utils` is the single I/O abstraction; patch it in tests via `monkeypatch`
-- No CI config currently present
+- CI: `.github/workflows/ci.yml` runs `ruff check` and the full suite on
+  push and PR to `main`, across Python 3.11-3.13
 
 ## Before Every Commit
 
diff --git a/pyproject.toml b/pyproject.toml
index bf31e85..d83b72c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -60,7 +60,17 @@ where = ["."]
 include = ["headersvalidator*"]
 
 [project.optional-dependencies]
-dev = ["pytest>=8", "pytest-cov>=5", "pytest-mock>=3.12"]
+dev = ["pytest>=8", "pytest-cov>=5", "ruff>=0.6", "pytest-mock>=3.12"]
+
+[tool.ruff]
+target-version = "py311"
+
+[tool.ruff.lint]
+# Ruff's default rule set as of 0.6, made explicit. Without this the linted
+# rule set is whatever the installed ruff defaults to, which widens on every
+# release — so a green local run and a red CI run can disagree purely by
+# version. Pinning it here makes the bar reviewable and stable.
+select = ["E4", "E7", "E9", "F"]
 
 [tool.pytest.ini_options]
 pythonpath = ["."]