diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ea87b39..f492739 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -22,3 +22,16 @@ jobs:
           cache: pip
       - run: python -m pip install -e ".[dev]"
       - run: pytest
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
+        with:
+          python-version: "3.12"
+          cache: pip
+      # The dev extra pins ruff, so CI and contributors run the same version.
+      - run: python -m pip install -e ".[dev]"
+      - run: ruff check .
+      - run: ruff format --check .
diff --git a/millionsend/_client.py b/millionsend/_client.py
index a11c132..61f7e26 100644
--- a/millionsend/_client.py
+++ b/millionsend/_client.py
@@ -169,7 +169,12 @@ def request(
 
     try:
         resp = requests.request(
-            method, url, headers=headers, data=data, params=query, timeout=_config("timeout") or DEFAULT_TIMEOUT
+            method,
+            url,
+            headers=headers,
+            data=data,
+            params=query,
+            timeout=_config("timeout") or DEFAULT_TIMEOUT,
         )
     except requests.exceptions.RequestException as exc:
         raise MillionSendError(
diff --git a/millionsend/contacts.py b/millionsend/contacts.py
index 9685d94..a9e755a 100644
--- a/millionsend/contacts.py
+++ b/millionsend/contacts.py
@@ -33,9 +33,7 @@ def list(
 
         ``params`` is resend-python's pagination dict, forwarded as the query.
         """
-        return request(
-            "GET", f"/contacts/{_key(contact_id or id, email)}/topics", query=list_query(params)
-        )
+        return request("GET", f"/contacts/{_key(contact_id or id, email)}/topics", query=list_query(params))
 
     @classmethod
     def update(cls, params: Dict[str, Any]) -> Any:
@@ -84,9 +82,7 @@ def create(
         )
 
     @classmethod
-    def get(
-        cls, params: List[Union[str, Dict[str, Any]]], include: Optional[List[str]] = None
-    ) -> Any:
+    def get(cls, params: List[Union[str, Dict[str, Any]]], include: Optional[List[str]] = None) -> Any:
         """POST /contacts/batch/get — 1..1000 contacts by ``{"id": ...}`` or ``{"email": ...}``
         (a bare string is an id), returned in request order.
 
diff --git a/pyproject.toml b/pyproject.toml
index 905a3a1..251dc2c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -18,10 +18,26 @@ Homepage = "https://github.com/MillionSend/millionsend-python"
 Repository = "https://github.com/MillionSend/millionsend-python"
 
 [project.optional-dependencies]
-dev = ["pytest>=7"]
+dev = ["pytest>=7", "ruff==0.16.9"]
 
 [tool.hatch.build.targets.wheel]
 packages = ["millionsend"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
+
+[tool.ruff]
+target-version = "py39"
+line-length = 110
+# The README's Python samples keep their hand-aligned comments.
+extend-exclude = ["*.md"]
+
+[tool.ruff.lint]
+ignore = [
+    # Public signatures keep typing.Optional/Dict/List; rewriting the annotations changes them.
+    "FA100",
+    "UP006",
+    "UP035",
+    # __all__ is grouped by purpose (config, resources, errors), not alphabetized.
+    "RUF022",
+]
diff --git a/tests/conftest.py b/tests/conftest.py
index 21d6fd3..a3dff3b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -5,7 +5,7 @@
 import pytest
 
 import millionsend
-import millionsend._client as _client
+from millionsend import _client
 
 
 class FakeResponse:
diff --git a/tests/test_client.py b/tests/test_client.py
index 6f16e6f..f0f9cc1 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -4,12 +4,12 @@
 import requests
 
 import millionsend
-import millionsend._client as _client
 from millionsend import (
     MillionSendError,
     MissingApiKeyError,
     NotFoundError,
     ValidationError,
+    _client,
 )
 
 
diff --git a/tests/test_resources.py b/tests/test_resources.py
index 17730b5..9d5e5b4 100644
--- a/tests/test_resources.py
+++ b/tests/test_resources.py
@@ -292,9 +292,7 @@ def test_contacts_topics_list(http):
 
 def test_contacts_topics_update_bare_array(http):
     http.body = {"id": "c1"}
-    millionsend.Contacts.Topics.update(
-        {"id": "c1", "topics": [{"id": "t1", "subscription": "opt_out"}]}
-    )
+    millionsend.Contacts.Topics.update({"id": "c1", "topics": [{"id": "t1", "subscription": "opt_out"}]})
     assert http.calls[0]["method"] == "PATCH"
     assert http.calls[0]["path"] == "/contacts/c1/topics"
     assert http.calls[0]["body"] == [{"id": "t1", "subscription": "opt_out"}]