diff --git a/pyproject.toml b/pyproject.toml
index 74a4a37..0548285 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -106,6 +106,7 @@ warn_redundant_casts = true
 warn_return_any = true
 check_untyped_defs = true
 no_implicit_optional = true
+disallow_untyped_defs = true
 
 [[tool.mypy.overrides]]
 # Third-party deps without inline type stubs.
@@ -113,8 +114,9 @@ module = ["kokoro_onnx", "sounddevice", "soundfile"]
 ignore_missing_imports = true
 
 [[tool.mypy.overrides]]
-# Tests use pytest-mock's mocker.patch which returns Any-heavy values; don't
-# require full strictness there.
+# Tests rely on pytest-mock's `mocker` (Any-heavy) and pytest fixtures with
+# implicit Any-typed kwargs. Strict untyped-def bans buy little here, so
+# tests stay on the looser config.
 module = "tests.*"
 disallow_untyped_defs = false
 
diff --git a/stackvox/daemon.py b/stackvox/daemon.py
index e25c5ac..299f126 100644
--- a/stackvox/daemon.py
+++ b/stackvox/daemon.py
@@ -130,7 +130,10 @@ def _read_default_device() -> int:
 
     last_device = [_read_default_device()]
 
-    def _on_device_change(obj_id: int, n: int, addrs, data) -> int:
+    def _on_device_change(obj_id: int, n: int, addrs: Any, data: Any) -> int:
+        # `addrs` is a ctypes.POINTER(_PropAddr); `data` is a c_void_p — neither
+        # is needed because we already know which property fired (only one is
+        # registered) and we re-read the device ID directly.
         try:
             current = _read_default_device()
             if current and current != last_device[0]: