diff --git a/.envrc b/.envrc
index 5669729a..d23567b5 100644
--- a/.envrc
+++ b/.envrc
@@ -1,2 +1 @@
-export VIRTUAL_ENV=.venv
-layout python3
+layout python python3.9
diff --git a/.gitignore b/.gitignore
index 62044c9b..26ea2c44 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,6 @@ dist/
 
 # Logs
 *.log
+
+# direnv
+.direnv/
diff --git a/.vscode/extensions.json b/.vscode/extensions.json
index b3dd12ff..e0a962d0 100644
--- a/.vscode/extensions.json
+++ b/.vscode/extensions.json
@@ -3,6 +3,8 @@
     "esbenp.prettier-vscode",
     "ms-python.python",
     "streetsidesoftware.code-spell-checker",
-    "tamasfe.even-better-toml"
+    "tamasfe.even-better-toml",
+    "ms-python.mypy-type-checker",
+    "ms-python.black-formatter"
   ]
 }
diff --git a/.vscode/settings.json b/.vscode/settings.json
index 841650c0..333df77f 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -9,7 +9,6 @@
     "**/.pytest_cache": true,
     "**/*.egg-info": true
   },
-  "python.formatting.provider": "black",
   "python.testing.pytestArgs": [
     "tests"
   ],
@@ -17,8 +16,9 @@
   "[jsonc][markdown]": {
     "editor.defaultFormatter": "esbenp.prettier-vscode"
   },
-  "python.linting.mypyEnabled": true,
-  "python.linting.enabled": true,
+  "[python]": {
+    "editor.defaultFormatter": "ms-python.black-formatter"
+  },
   "cSpell.words": [
     "aocd",
     "cj's",
@@ -26,5 +26,12 @@
     "parta",
     "partb",
     "popleft"
-  ]
+  ],
+  "evenBetterToml.formatter.allowedBlankLines": 1,
+  "evenBetterToml.formatter.alignComments": false,
+  "evenBetterToml.formatter.arrayAutoCollapse": false,
+  "evenBetterToml.formatter.arrayTrailingComma": true,
+  "evenBetterToml.formatter.columnWidth": 0,
+  "evenBetterToml.formatter.trailingNewline": true,
+  "evenBetterToml.formatter.arrayAutoExpand": false
 }
diff --git a/pyproject.toml b/pyproject.toml
index abb64241..5d9dce40 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -9,7 +9,7 @@ homepage = "https://github.com/cj81499/advent-of-code"
 python = ">=3.9"
 advent-of-code-data = "^2.0.1"
 more-itertools = "^10.0.0"
-numpy = "^1.25.2" # remove me
+numpy = "^1.25.2" # TODO: remove me
 z3-solver = "^4.12.2"
 lark = "^1.1.7"
 
@@ -33,9 +33,9 @@ omit = []
 
 [tool.coverage.report]
 exclude_lines = [
-	'# pragma: no cover',
-	'if __name__ == "__main__":',
-	'if TYPE_CHECKING:',
+  '# pragma: no cover',
+  'if __name__ == "__main__":',
+  'if TYPE_CHECKING:',
   'raise NotImplementedError',
   '@abc\.abstractmethod',
   'assert False',
@@ -54,9 +54,9 @@ all = true
 # skip slow tests by default
 addopts = "-m 'not slow'"
 markers = [
-    "slow: marks tests as slow"
+  "slow: marks tests as slow",
 ]
-xfail_strict=true
+xfail_strict = true
 
 [tool.mypy]
 strict = true
@@ -81,12 +81,12 @@ ignore_missing_imports = true
 # I may eventually go back and add types to these.
 [[tool.mypy.overrides]]
 module = [
-    'aoc_cj.aoc2015.*',
-    'aoc_cj.aoc2016.*',
-    'aoc_cj.aoc2017.*',
-    'aoc_cj.aoc2018.*',
-    'aoc_cj.aoc2019.*',
-    'aoc_cj.aoc2020.*',
+  'aoc_cj.aoc2015.*',
+  'aoc_cj.aoc2016.*',
+  'aoc_cj.aoc2017.*',
+  'aoc_cj.aoc2018.*',
+  'aoc_cj.aoc2019.*',
+  'aoc_cj.aoc2020.*',
 ]
 ignore_errors = true
 
diff --git a/src/aoc_cj/__init__.py b/src/aoc_cj/__init__.py
index 43df1381..a13b70bd 100644
--- a/src/aoc_cj/__init__.py
+++ b/src/aoc_cj/__init__.py
@@ -44,11 +44,11 @@ def solve(year: int, day: int, data: str) -> tuple[Answer, Answer]:  # pragma: n
 
 def _solve_part(module: ModuleType, data: str, part: Literal["a", "b"]) -> Answer:  # pragma: no cover
     if f := getattr(module, f"part{part}", None):
-        assert inspect.isfunction(f)
+        assert inspect.isfunction(f), f"f ({f}) is not a function"
         try:
             # TODO: consider checking that inspect.signature matches expected signature
             resp = f(data)
-            assert isinstance(resp, (int, str))
+            assert isinstance(resp, (int, str)) or resp is None, f"resp ({resp}) must be an int, str, or None"
             return resp
         except NotImplementedError:  # unsolved
             return None