diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 47c9de7..6a24aee 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -8,12 +8,12 @@ repos:
       - id: trailing-whitespace
       - id: end-of-file-fixer
   - repo: https://github.com/psf/black
-    rev: "23.9.1"
+    rev: "23.10.0"
     hooks:
       - id: black
         language_version: python3
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.0.292
+    rev: v0.1.1
     hooks:
       - id: ruff
         args: ["--fix"]
@@ -29,3 +29,9 @@ repos:
     hooks:
       - id: taplo
         language_version: stable
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v1.6.1
+    hooks:
+      - id: mypy
+        additional_dependencies:
+          - pytest
diff --git a/.vscode/settings.json b/.vscode/settings.json
index 946aed9..b538f66 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,13 +1,13 @@
 {
     "python.testing.unittestEnabled": false,
     "python.testing.pytestEnabled": true,
-    "python.testing.pytestArgs": [],
+    "python.testing.pytestArgs": ["--color=yes"],
     "[python]": {
         "editor.formatOnSave": true,
         "editor.defaultFormatter": "ms-python.black-formatter",
         "editor.codeActionsOnSave": {
-            "source.fixAll.ruff": true,
-            "source.organizeImports.ruff": true,
+            "source.fixAll": true,
+            "source.organizeImports": true,
         },
     },
     "[toml]": {
diff --git a/pyproject.toml b/pyproject.toml
index 3176476..55b78f6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,8 +33,8 @@ ignore = [
 ]
 allowed-confusables = ["’"]
 [tool.ruff.isort]
-
 known-first-party = ["legacy_api_wrap"]
+required-imports = ["from __future__ import annotations"]
 [tool.ruff.extend-per-file-ignores]
 "src/testing/*.py" = ["INP001"]
 "tests/**/test_*.py" = [
@@ -45,6 +45,11 @@ known-first-party = ["legacy_api_wrap"]
   "S101",    # tests use `assert`
 ]
 
+[tool.mypy]
+strict = true
+explicit_package_bases = true
+mypy_path = "src"
+
 [tool.pytest.ini_options]
 addopts = [
   "--import-mode=importlib",
diff --git a/src/legacy_api_wrap/__init__.py b/src/legacy_api_wrap/__init__.py
index e569116..3b75edd 100644
--- a/src/legacy_api_wrap/__init__.py
+++ b/src/legacy_api_wrap/__init__.py
@@ -10,13 +10,15 @@
 
 from __future__ import annotations
 
+import sys
 from functools import wraps
 from inspect import Parameter, signature
-from typing import TYPE_CHECKING, Callable, TypeVar
+from typing import TYPE_CHECKING
 from warnings import warn
 
 if TYPE_CHECKING:
-    from typing import ParamSpec
+    from collections.abc import Callable
+    from typing import ParamSpec, TypeVar
 
     P = ParamSpec("P")
     R = TypeVar("R")
@@ -25,6 +27,8 @@
 POS_TYPES = {Parameter.POSITIONAL_ONLY, Parameter.POSITIONAL_OR_KEYWORD}
 
 
+# The actual returned Callable of course accepts more positional parameters,
+# but we want the type to lie so end users don’t rely on the deprecated API.
 def legacy_api(*old_positionals: str) -> Callable[[Callable[P, R]], Callable[P, R]]:
     """Legacy API wrapper.
 
@@ -57,30 +61,33 @@ def wrapper(fn: Callable[P, R]) -> Callable[P, R]:
         par_types = [p.kind for p in sig.parameters.values()]
         has_var = Parameter.VAR_POSITIONAL in par_types
         n_required = sum(1 for p in sig.parameters.values() if p.default is Parameter.empty)
-        n_positional = INF if has_var else sum(1 for p in par_types if p in POS_TYPES)
+        n_positional = sys.maxsize if has_var else sum(1 for p in par_types if p in POS_TYPES)
 
         @wraps(fn)
-        def fn_compatible(*args: P.args, **kw: P.kwargs) -> R:
-            if len(args) > n_positional:
-                args, args_rest = args[:n_positional], args[n_positional:]
-                if args_rest:
-                    if len(args_rest) > len(old_positionals):
-                        n_max = n_positional + len(old_positionals)
-                        msg = (
-                            f"{fn.__name__}() takes from {n_required} to {n_max} parameters, "
-                            f"but {len(args) + len(args_rest)} were given."
-                        )
-                        raise TypeError(msg)
-                    warn(
-                        f"The specified parameters {old_positionals[:len(args_rest)]!r} are "
-                        "no longer positional. "
-                        f"Please specify them like `{old_positionals[0]}={args_rest[0]!r}`",
-                        DeprecationWarning,
-                        stacklevel=2,
-                    )
-                    kw = {**kw, **dict(zip(old_positionals, args_rest))}
-
-            return fn(*args, **kw)
+        def fn_compatible(*args_all: P.args, **kw: P.kwargs) -> R:
+            if len(args_all) <= n_positional:
+                return fn(*args_all, **kw)
+
+            args_pos: P.args
+            args_pos, args_rest = args_all[:n_positional], args_all[n_positional:]
+
+            if len(args_rest) > len(old_positionals):
+                n_max = n_positional + len(old_positionals)
+                msg = (
+                    f"{fn.__name__}() takes from {n_required} to {n_max} parameters, "
+                    f"but {len(args_pos) + len(args_rest)} were given."
+                )
+                raise TypeError(msg)
+            warn(
+                f"The specified parameters {old_positionals[:len(args_rest)]!r} are "
+                "no longer positional. "
+                f"Please specify them like `{old_positionals[0]}={args_rest[0]!r}`",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            kw_new: P.kwargs = {**kw, **dict(zip(old_positionals, args_rest))}
+
+            return fn(*args_pos, **kw_new)
 
         return fn_compatible
 
diff --git a/src/testing/legacy_api_wrap/py.typed b/src/testing/legacy_api_wrap/py.typed
new file mode 100644
index 0000000..e69de29
diff --git a/src/testing/legacy_api_wrap/pytest.py b/src/testing/legacy_api_wrap/pytest.py
index 6c45acd..d71d41d 100644
--- a/src/testing/legacy_api_wrap/pytest.py
+++ b/src/testing/legacy_api_wrap/pytest.py
@@ -1,15 +1,21 @@
 """Pytest plugin for legacy_api_wrap."""
 
+from __future__ import annotations
+
 import sys
 import warnings
+from typing import TYPE_CHECKING
 
 import pytest
 
+if TYPE_CHECKING:
+    from collections.abc import Generator
+
 __all__ = ["_doctest_env", "pytest_itemcollected"]
 
 
 @pytest.fixture()
-def _doctest_env() -> None:
+def _doctest_env() -> Generator[None, None, None]:
     """Pytest fixture to make doctests not error on expected warnings."""
     sys.stderr, stderr_orig = sys.stdout, sys.stderr
     with warnings.catch_warnings():
diff --git a/tests/test_basic.py b/tests/test_basic.py
index 6058f77..0d18e40 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -10,7 +10,7 @@
 # def old(a, b=None, d=1, c=2):
 # 	pass
 @legacy_api("d", "c")
-def new(a, b=None, *, c=2, d=1, e=3):  # noqa: ANN001, ANN201
+def new(a, b=None, *, c=2, d=1, e=3):  # type: ignore[no-untyped-def] # noqa: ANN001, ANN201
     return {"a": a, "b": b, "c": c, "d": d, "e": e}
 
 
@@ -24,13 +24,13 @@ def test_new_param_available() -> None:
 
 def test_old_positional_order() -> None:
     with pytest.deprecated_call():
-        res = new(12, 13, 14)
+        res = new(12, 13, 14)  # type: ignore[misc]
     assert res["d"] == 14
 
 
 def test_warning_stack() -> None:
     with pytest.deprecated_call() as record:
-        new(12, 13, 14)
+        new(12, 13, 14)  # type: ignore[misc]
     w = record.pop()
     assert w.filename == __file__
 
@@ -40,4 +40,4 @@ def test_too_many_args() -> None:
         TypeError,
         match=r"new\(\) takes from 1 to 4 parameters, but 5 were given\.",
     ):
-        new(1, 2, 3, 4, 5)
+        new(1, 2, 3, 4, 5)  # type: ignore[misc]