diff --git a/pyproject.toml b/pyproject.toml
index 285367a..7cc7d61 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,3 +5,10 @@ requires = [
 ]
 
 build-backend = "setuptools.build_meta"
+
+[tool.mypy]
+files = [
+    "soupsieve/*.py"
+]
+strict = true
+show_error_codes = true
diff --git a/tox.ini b/tox.ini
index add98cb..81a4fd6 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@ passenv = *
 deps =
     -rrequirements/tests.txt
 commands =
-    mypy --strict --show-error-codes soupsieve
+    mypy
     py.test --cov soupsieve --cov-append {toxinidir}
     coverage html -d {envtmpdir}/coverage
     coverage xml