diff --git a/pyproject.toml b/pyproject.toml
index 4b514db..60aefd2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -12,7 +12,7 @@ license = { file = "LICENSE" }
 dependencies = []
 
 [project.optional-dependencies]
-dev = ["pytest>=8"]
+dev = ["pytest>=9,<10", "ruff>=0.16,<0.17", "mypy>=2.4,<3"]
 
 [tool.setuptools.packages.find]
 where = ["src"]
@@ -20,3 +20,16 @@ where = ["src"]
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 pythonpath = ["src"]
+
+[tool.ruff]
+line-length = 88
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "UP", "FLY", "B"]
+
+[tool.ruff.lint.per-file-ignores]
+"src/higher_lower/game_data.py" = ["E501"] # assignment data kept verbatim
+
+[tool.mypy]
+strict = true
+mypy_path = "src"
diff --git a/src/higher_lower/art.py b/src/higher_lower/art.py
index bd09c03..ed221f8 100644
--- a/src/higher_lower/art.py
+++ b/src/higher_lower/art.py
@@ -9,23 +9,37 @@ string, so Doxygen does not read the backticks in the art as markup.
 ## The game logo.
 logo = (
     "\n"
-    r"    __  ___       __             " "\n"
-    r"   / / / (_)___ _/ /_  ___  _____" "\n"
-    r"  / /_/ / / __ `/ __ \/ _ \/ ___/" "\n"
-    r" / __  / / /_/ / / / /  __/ /    " "\n"
-    r"/_/ ///_/\__, /_/ /_/\___/_/     " "\n"
-    r"   / /  /____/_      _____  _____" "\n"
-    r"  / /   / __ \ | /| / / _ \/ ___/" "\n"
-    r" / /___/ /_/ / |/ |/ /  __/ /    " "\n"
-    r"/_____/\____/|__/|__/\___/_/     " "\n"
+    r"    __  ___       __             "
+    "\n"
+    r"   / / / (_)___ _/ /_  ___  _____"
+    "\n"
+    r"  / /_/ / / __ `/ __ \/ _ \/ ___/"
+    "\n"
+    r" / __  / / /_/ / / / /  __/ /    "
+    "\n"
+    r"/_/ ///_/\__, /_/ /_/\___/_/     "
+    "\n"
+    r"   / /  /____/_      _____  _____"
+    "\n"
+    r"  / /   / __ \ | /| / / _ \/ ___/"
+    "\n"
+    r" / /___/ /_/ / |/ |/ /  __/ /    "
+    "\n"
+    r"/_____/\____/|__/|__/\___/_/     "
+    "\n"
 )
 
 ## The "vs" shown between the two accounts.
 vs = (
     "\n"
-    r" _    __    " "\n"
-    r"| |  / /____" "\n"
-    r"| | / / ___/" "\n"
-    r"| |/ (__  ) " "\n"
-    r"|___/____(_)" "\n"
+    r" _    __    "
+    "\n"
+    r"| |  / /____"
+    "\n"
+    r"| | / / ___/"
+    "\n"
+    r"| |/ (__  ) "
+    "\n"
+    r"|___/____(_)"
+    "\n"
 )
diff --git a/src/higher_lower/constants.py b/src/higher_lower/constants.py
index 90574fc..cfd1993 100644
--- a/src/higher_lower/constants.py
+++ b/src/higher_lower/constants.py
@@ -2,11 +2,13 @@
 @brief Constants shared by the game modules.
 """
 
+from typing import Final
+
 ## Keys of an account dictionary.
-KEY_NAME = "name"
-KEY_FOLLOWER_COUNT = "follower_count"
-KEY_DESCRIPTION = "description"
-KEY_COUNTRY = "country"
+KEY_NAME: Final = "name"
+KEY_FOLLOWER_COUNT: Final = "follower_count"
+KEY_DESCRIPTION: Final = "description"
+KEY_COUNTRY: Final = "country"
 
 ## The two choices the player can type, in lower case.
 CHOICE_A = "a"
diff --git a/src/higher_lower/game_logic.py b/src/higher_lower/game_logic.py
index c28223d..a8c5260 100644
--- a/src/higher_lower/game_logic.py
+++ b/src/higher_lower/game_logic.py
@@ -1,5 +1,5 @@
 """@file game_logic.py
-@brief Pure functions of the game: drawing accounts, describing them and checking guesses.
+@brief Pure game functions: draw accounts, describe them, check guesses.
 """
 
 import random
@@ -9,9 +9,9 @@ from higher_lower.constants import (
     ACCOUNT_TEMPLATE,
     CHOICE_A,
     CHOICE_B,
+    KEY_COUNTRY,
     KEY_DESCRIPTION,
     KEY_NAME,
-    KEY_COUNTRY,
 )
 from higher_lower.game_data import Account, data
 
diff --git a/tests/test_game.py b/tests/test_game.py
index 427f9d1..285a3aa 100644
--- a/tests/test_game.py
+++ b/tests/test_game.py
@@ -14,20 +14,37 @@ from higher_lower.game import ask_choice, play_game, show_round
 from higher_lower.game_data import Account
 
 ACCOUNTS: list[Account] = [
-    {"name": "First", "follower_count": 30, "description": "Test one", "country": "Denmark"},
-    {"name": "Second", "follower_count": 20, "description": "Test two", "country": "Norway"},
-    {"name": "Third", "follower_count": 10, "description": "Test three", "country": "Sweden"},
+    {
+        "name": "First",
+        "follower_count": 30,
+        "description": "Test one",
+        "country": "Denmark",
+    },
+    {
+        "name": "Second",
+        "follower_count": 20,
+        "description": "Test two",
+        "country": "Norway",
+    },
+    {
+        "name": "Third",
+        "follower_count": 10,
+        "description": "Test three",
+        "country": "Sweden",
+    },
 ]
 
 
 class FirstRng(random.Random):
-    """@brief Random generator that always takes the first entries, so a game is fixed."""
+    """@brief Random generator that takes the first entries, so a game is fixed."""
 
-    def sample(self, population: Sequence, k: int, **kwargs: object) -> list:  # type: ignore[override]
+    def sample(  # type: ignore[override]
+        self, population: Sequence[Account], k: int, **kwargs: object
+    ) -> list[Account]:
         """@brief Return the first k entries."""
         return list(population)[:k]
 
-    def choice(self, seq: Sequence) -> Account:  # type: ignore[override]
+    def choice(self, seq: Sequence[Account]) -> Account:  # type: ignore[override]
         """@brief Return the first entry."""
         return seq[0]
 
diff --git a/tests/test_game_logic.py b/tests/test_game_logic.py
index b153b65..5c6b0fb 100644
--- a/tests/test_game_logic.py
+++ b/tests/test_game_logic.py
@@ -58,8 +58,7 @@ def test_format_data_describes_the_account() -> None:
         "country": "United States",
     }
     assert (
-        format_data(account)
-        == "Nike, a Sportswear multinational, from United States"
+        format_data(account) == "Nike, a Sportswear multinational, from United States"
     )