diff --git a/.yamllint.yml b/.yamllint.yml
index 54fcc0d..e35c665 100644
--- a/.yamllint.yml
+++ b/.yamllint.yml
@@ -6,3 +6,6 @@ rules:
     max: 100
   truthy:
     check-keys: false
+  # Git on Windows checks files out with CRLF; the repository blob has LF.
+  new-lines:
+    type: platform
diff --git a/README.md b/README.md
index 8f8f260..87fac0c 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ There are no runtime dependencies: the program uses only the Python standard lib
 ## Set up
 
 Create a local virtual environment named `.venv` in the project folder, upgrade `pip`
-and install the project with its development tools (pytest, ruff, mypy).
+and install the project with its development tools (pytest, ruff, mypy, yamllint, actionlint).
 
 ### Windows powershell
 
diff --git a/pyproject.toml b/pyproject.toml
index f8768e9..4ddbdcf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,48 +1,49 @@
-# Project configuration for the squirrel census assignment (day 25).
-# Read by pip, setuptools and pytest. The decision to use the standard library
-# only is recorded in docs/adr/adr-0001-standard-library-instead-of-pandas.md.
-
-[build-system]
-requires = ["setuptools>=68"]
-build-backend = "setuptools.build_meta"
-
-[project]
-name = "squirrel"
-version = "0.1.0"
-description = "Count the squirrels of the 2018 Central Park Squirrel Census by primary fur colour."
-readme = "README.md"
-requires-python = ">=3.13"
-license = "AGPL-3.0-or-later"
-license-files = ["LICENSE"]
-dependencies = []
-classifiers = [
-  "Programming Language :: Python :: 3.13",
-  "Intended Audience :: Education",
-]
-
-[project.optional-dependencies]
-dev = ["pytest==9.1.1", "ruff==0.17.0", "mypy==2.4.0",
-  "pyyaml==6.0.3", "types-PyYAML==6.0.12.20260906"]
-
-[tool.setuptools.packages.find]
-where = ["src"]
-
-[tool.pytest.ini_options]
-pythonpath = ["src"]
-testpaths = ["tests"]
-
-[tool.ruff]
-line-length = 88
-src = ["src", "tests"]
-extend-exclude = ["docs"]
-
-[tool.ruff.lint]
-select = ["E", "F", "I", "UP", "B", "D"]
-ignore = ["D203", "D213"]
-
-[tool.mypy]
-strict = true
-files = ["src", "tests"]
-
-[tool.ruff.lint.per-file-ignores]
-"tests/**" = ["D"]
+# Project configuration for the squirrel census assignment (day 25).
+# Read by pip, setuptools and pytest. The decision to use the standard library
+# only is recorded in docs/adr/adr-0001-standard-library-instead-of-pandas.md.
+
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "squirrel"
+version = "0.1.0"
+description = "Count the squirrels of the 2018 Central Park Squirrel Census by primary fur colour."
+readme = "README.md"
+requires-python = ">=3.13"
+license = "AGPL-3.0-or-later"
+license-files = ["LICENSE"]
+dependencies = []
+classifiers = [
+  "Programming Language :: Python :: 3.13",
+  "Intended Audience :: Education",
+]
+
+[project.optional-dependencies]
+dev = ["pytest==9.1.1", "ruff==0.17.0", "mypy==2.4.0",
+  "pyyaml==6.0.3", "types-PyYAML==6.0.12.20260906",
+  "yamllint==1.38.0", "actionlint-py==1.7.12.25"]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+pythonpath = ["src"]
+testpaths = ["tests"]
+
+[tool.ruff]
+line-length = 88
+src = ["src", "tests"]
+extend-exclude = ["docs"]
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "UP", "B", "D"]
+ignore = ["D203", "D213"]
+
+[tool.mypy]
+strict = true
+files = ["src", "tests"]
+
+[tool.ruff.lint.per-file-ignores]
+"tests/**" = ["D"]
diff --git a/wiki/Install.da.md b/wiki/Install.da.md
index cefc86a..bbe1124 100644
--- a/wiki/Install.da.md
+++ b/wiki/Install.da.md
@@ -2,7 +2,7 @@
 
 # Installation
 
-Projektet kræver **Python 3.13 eller nyere**. Det har ingen kørselsafhængigheder; `pytest`, `ruff` og `mypy` er udviklingsværktøjer.
+Projektet kræver **Python 3.13 eller nyere**. Det har ingen kørselsafhængigheder; `pytest`, `ruff`, `mypy`, `yamllint` og `actionlint` er udviklingsværktøjer.
 
 ## Windows PowerShell
 
diff --git a/wiki/Install.md b/wiki/Install.md
index 7d85995..d8c418b 100644
--- a/wiki/Install.md
+++ b/wiki/Install.md
@@ -2,7 +2,7 @@
 
 # Install
 
-The project needs **Python 3.13 or later**. It has no runtime dependencies; `pytest`, `ruff` and `mypy` are development tools.
+The project needs **Python 3.13 or later**. It has no runtime dependencies; `pytest`, `ruff`, `mypy`, `yamllint` and `actionlint` are development tools.
 
 ## Windows PowerShell
 
diff --git a/wiki/Install.ms.md b/wiki/Install.ms.md
index 3fa7812..ee54e37 100644
--- a/wiki/Install.ms.md
+++ b/wiki/Install.ms.md
@@ -2,7 +2,7 @@
 
 # Pemasangan
 
-Projek ini memerlukan **Python 3.13 atau lebih baharu**. Ia tiada kebergantungan masa jalan; `pytest`, `ruff` dan `mypy` ialah alat pembangunan.
+Projek ini memerlukan **Python 3.13 atau lebih baharu**. Ia tiada kebergantungan masa jalan; `pytest`, `ruff`, `mypy`, `yamllint` dan `actionlint` ialah alat pembangunan.
 
 ## Windows PowerShell