diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 21d449cf..6a174a04 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -74,10 +74,7 @@ jobs:
           path: ./coverage.lcov
       
       - name: Analyse with MyPy
-        run: mypy src
-      
-      - name: Type tests with MyPy
-        run: mypy --warn-unused-ignores typing_tests
+        run: mypy
 
   test-with-unpinned-deps:
     runs-on: ubuntu-latest
@@ -105,7 +102,7 @@ jobs:
       
       - name: Analyse with MyPy
         if: success() || failure()
-        run: mypy src
+        run: mypy
 
       - name: Test with pytest
         if: success() || failure()
diff --git a/pyproject.toml b/pyproject.toml
index d1f03472..7b002e3f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -108,7 +108,12 @@ property-decorators = [
 ]
 
 [tool.mypy]
+files = ["src/", "typing_tests/"]
 plugins = ["pydantic.mypy"]
+disallow_untyped_defs = true
+check_untyped_defs = true
+disallow_untyped_decorators = true
+warn_unused_ignores = true
 
 
 [tool.flake8]
diff --git a/src/labthings_fastapi/actions/__init__.py b/src/labthings_fastapi/actions/__init__.py
index 81e5c738..b526ea23 100644
--- a/src/labthings_fastapi/actions/__init__.py
+++ b/src/labthings_fastapi/actions/__init__.py
@@ -168,7 +168,7 @@ def status(self) -> InvocationStatus:
             return self._status
 
     @property
-    def action(self):
+    def action(self) -> ActionDescriptor:
         """The `.ActionDescriptor` object running in this thread."""
         action = self.action_ref()
         assert action is not None, "The action for an `Invocation` has been deleted!"
@@ -210,10 +210,12 @@ def response(self, request: Optional[Request] = None) -> InvocationModel:
             LinkElement(rel="self", href=href),
             LinkElement(rel="output", href=href + "/output"),
         ]
-        return self.action.invocation_model(
+        # The line below confuses MyPy because self.action **evaluates to** a Descriptor
+        # object (i.e. we don't call __get__ on the descriptor).
+        return self.action.invocation_model(  # type: ignore[call-overload]
             status=self.status,
             id=self.id,
-            action=self.thing.path + self.action.name,
+            action=self.thing.path + self.action.name,  # type: ignore[call-overload]
             href=href,
             timeStarted=self._start_time,
             timeCompleted=self._end_time,
@@ -249,24 +251,25 @@ def run(self) -> None:
 
         See `.Invocation.status` for status values.
         """
+        # self.action evaluates to an ActionDescriptor. This confuses mypy,
+        # which thinks we are calling ActionDescriptor.__get__.
+        action: ActionDescriptor = self.action  # type: ignore[call-overload]
         try:
-            self.action.emit_changed_event(self.thing, self._status)
+            action.emit_changed_event(self.thing, self._status.value)
 
             # Capture just this thread's log messages
             handler = DequeLogHandler(dest=self._log)
             logger = invocation_logger(self.id)
             logger.addHandler(handler)
 
-            action = self.action
             thing = self.thing
             kwargs = model_to_dict(self.input)
-            assert action is not None
             assert thing is not None
 
             with self._status_lock:
                 self._status = InvocationStatus.RUNNING
                 self._start_time = datetime.datetime.now()
-                self.action.emit_changed_event(self.thing, self._status)
+                action.emit_changed_event(self.thing, self._status.value)
 
             # The next line actually runs the action.
             ret = action.__get__(thing)(**kwargs, **self.dependencies)
@@ -274,12 +277,12 @@ def run(self) -> None:
             with self._status_lock:
                 self._return_value = ret
                 self._status = InvocationStatus.COMPLETED
-                self.action.emit_changed_event(self.thing, self._status)
+                action.emit_changed_event(self.thing, self._status.value)
         except InvocationCancelledError:
             logger.info(f"Invocation {self.id} was cancelled.")
             with self._status_lock:
                 self._status = InvocationStatus.CANCELLED
-                self.action.emit_changed_event(self.thing, self._status)
+                action.emit_changed_event(self.thing, self._status.value)
         except Exception as e:  # skipcq: PYL-W0703
             # First log
             if isinstance(e, InvocationError):
@@ -291,7 +294,7 @@ def run(self) -> None:
             with self._status_lock:
                 self._status = InvocationStatus.ERROR
                 self._exception = e
-                self.action.emit_changed_event(self.thing, self._status)
+                action.emit_changed_event(self.thing, self._status.value)
         finally:
             with self._status_lock:
                 self._end_time = datetime.datetime.now()
@@ -341,9 +344,9 @@ def emit(self, record: logging.LogRecord) -> None:
 class ActionManager:
     """A class to manage a collection of actions."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         """Set up an `.ActionManager`."""
-        self._invocations = {}
+        self._invocations: dict[uuid.UUID, Invocation] = {}
         self._invocations_lock = Lock()
 
     @property
@@ -443,10 +446,13 @@ def list_invocations(
             i.response(request=request)
             for i in self.invocations
             if thing is None or i.thing == thing
-            if action is None or i.action == action
+            if action is None or i.action == action  # type: ignore[call-overload]
+            # i.action evaluates to an ActionDescriptor, which confuses mypy - it
+            # thinks we are calling ActionDescriptor.__get__ but this isn't ever
+            # called.
         ]
 
-    def expire_invocations(self):
+    def expire_invocations(self) -> None:
         """Delete invocations that have passed their expiry time."""
         to_delete = []
         with self._invocations_lock:
@@ -465,7 +471,9 @@ def attach_to_app(self, app: FastAPI) -> None:
         """
 
         @app.get(ACTION_INVOCATIONS_PATH, response_model=list[InvocationModel])
-        def list_all_invocations(request: Request, _blob_manager: BlobIOContextDep):
+        def list_all_invocations(
+            request: Request, _blob_manager: BlobIOContextDep
+        ) -> list[InvocationModel]:
             return self.list_invocations(request=request)
 
         @app.get(
@@ -512,7 +520,9 @@ def action_invocation(
                 503: {"description": "No result is available for this invocation"},
             },
         )
-        def action_invocation_output(id: uuid.UUID, _blob_manager: BlobIOContextDep):
+        def action_invocation_output(
+            id: uuid.UUID, _blob_manager: BlobIOContextDep
+        ) -> Any:
             """Get the output of an action invocation.
 
             This returns just the "output" component of the action invocation. If the
diff --git a/src/labthings_fastapi/client/__init__.py b/src/labthings_fastapi/client/__init__.py
index 320c9fd8..9c5ce620 100644
--- a/src/labthings_fastapi/client/__init__.py
+++ b/src/labthings_fastapi/client/__init__.py
@@ -148,7 +148,7 @@ def get_property(self, path: str) -> Any:
         r.raise_for_status()
         return r.json()
 
-    def set_property(self, path: str, value: Any):
+    def set_property(self, path: str, value: Any) -> None:
         """Make a PUT request to set the value of a property.
 
         :param path: the URI of the ``getproperty`` endpoint, relative
@@ -159,7 +159,7 @@ def set_property(self, path: str, value: Any):
         r = self.client.put(urljoin(self.path, path), json=value)
         r.raise_for_status()
 
-    def invoke_action(self, path: str, **kwargs):
+    def invoke_action(self, path: str, **kwargs: Any) -> Any:
         r"""Invoke an action on the Thing.
 
         This method will make the initial POST request to invoke an action,
@@ -180,8 +180,17 @@ def invoke_action(self, path: str, **kwargs):
         :raise RuntimeError: is raised if the action does not complete successfully.
         """
         for k in kwargs.keys():
-            if isinstance(kwargs[k], ClientBlobOutput):
-                kwargs[k] = {"href": kwargs[k].href, "media_type": kwargs[k].media_type}
+            value = kwargs[k]
+            if isinstance(value, ClientBlobOutput):
+                # ClientBlobOutput objects may be used as input to a subsequent
+                # action. When this is done, they should be serialised to a dict
+                # with `href` and `media_type` keys, as done below.
+                # Ideally this should be replaced with `Blob` and the use of
+                # `pydantic` models to serialise action inputs.
+                #
+                # Note that the blob will not be uploaded: we rely on the blob
+                # still existing on the server.
+                kwargs[k] = {"href": value.href, "media_type": value.media_type}
         r = self.client.post(urljoin(self.path, path), json=kwargs)
         r.raise_for_status()
         invocation = poll_invocation(self.client, r.json())
@@ -270,7 +279,9 @@ class Client(cls):  # type: ignore[valid-type, misc]
 class PropertyClientDescriptor:
     """A base class for properties on `.ThingClient` objects."""
 
-    pass
+    name: str
+    type: type | BaseModel
+    path: str
 
 
 def property_descriptor(
@@ -312,10 +323,10 @@ class P(PropertyClientDescriptor):
     if readable:
 
         def __get__(
-            self,
+            self: PropertyClientDescriptor,
             obj: Optional[ThingClient] = None,
             _objtype: Optional[type[ThingClient]] = None,
-        ):
+        ) -> Any:
             if obj is None:
                 return self
             return obj.get_property(self.name)
@@ -324,7 +335,9 @@ def __get__(
         P.__get__ = __get__  # type: ignore[attr-defined]
     if writeable:
 
-        def __set__(self, obj: ThingClient, value: Any):
+        def __set__(
+            self: PropertyClientDescriptor, obj: ThingClient, value: Any
+        ) -> None:
             obj.set_property(self.name, value)
 
         __set__.__annotations__["value"] = model
@@ -349,7 +362,7 @@ def add_action(cls: type[ThingClient], action_name: str, action: dict) -> None:
         format.
     """
 
-    def action_method(self, **kwargs):
+    def action_method(self: ThingClient, **kwargs: Any) -> Any:
         return self.invoke_action(action_name, **kwargs)
 
     if "output" in action and "type" in action["output"]:
@@ -359,7 +372,7 @@ def action_method(self, **kwargs):
     setattr(cls, action_name, action_method)
 
 
-def add_property(cls: type[ThingClient], property_name: str, property: dict):
+def add_property(cls: type[ThingClient], property_name: str, property: dict) -> None:
     """Add a property to a ThingClient subclass.
 
     A descriptor will be added to the provided class that makes the
diff --git a/src/labthings_fastapi/client/in_server.py b/src/labthings_fastapi/client/in_server.py
index 7bcdfbe7..9c3d6ff6 100644
--- a/src/labthings_fastapi/client/in_server.py
+++ b/src/labthings_fastapi/client/in_server.py
@@ -113,18 +113,22 @@ class P(PropertyClientDescriptor):
         path = property_path or property_name
 
     def __get__(
-        self,
+        self: PropertyClientDescriptor,
         obj: Optional[DirectThingClient] = None,
         _objtype: Optional[type[DirectThingClient]] = None,
-    ):
+    ) -> Any:
         if obj is None:
             return self
         return getattr(obj._wrapped_thing, self.name)
 
-    def __set__(self, obj: DirectThingClient, value: Any):
+    def __set__(
+        self: PropertyClientDescriptor, obj: DirectThingClient, value: Any
+    ) -> None:
         setattr(obj._wrapped_thing, self.name, value)
 
-    def set_readonly(self, obj: DirectThingClient, value: Any):
+    def set_readonly(
+        self: PropertyClientDescriptor, obj: DirectThingClient, value: Any
+    ) -> None:
         raise AttributeError("This property is read-only.")
 
     if readable:
@@ -198,7 +202,7 @@ def add_action(
     """
 
     @wraps(action.func)
-    def action_method(self, **kwargs):
+    def action_method(self: DirectThingClient, **kwargs: Any) -> Any:
         dependency_kwargs = {
             param.name: self._dependencies[param.name]
             for param in action.dependency_params
@@ -273,7 +277,7 @@ def direct_thing_client_class(
 
     def init_proxy(
         self: DirectThingClient, request: Request, **dependencies: Mapping[str, Any]
-    ):
+    ) -> None:
         r"""Initialise a DirectThingClient (this docstring will be replaced).
 
         :param self: The DirectThingClient instance we're initialising.
diff --git a/src/labthings_fastapi/decorators/__init__.py b/src/labthings_fastapi/decorators/__init__.py
index b1950b76..f1c61460 100644
--- a/src/labthings_fastapi/decorators/__init__.py
+++ b/src/labthings_fastapi/decorators/__init__.py
@@ -37,7 +37,7 @@
 """
 
 from functools import wraps, partial
-from typing import Optional, Callable, overload
+from typing import Any, Optional, Callable, overload
 from ..descriptors import (
     ActionDescriptor,
     EndpointDescriptor,
@@ -45,7 +45,7 @@
 )
 
 
-def mark_thing_action(func: Callable, **kwargs) -> ActionDescriptor:
+def mark_thing_action(func: Callable, **kwargs: Any) -> ActionDescriptor:
     r"""Mark a method of a Thing as an Action.
 
     We replace the function with a descriptor that's a
@@ -65,12 +65,12 @@ class ActionDescriptorSubclass(ActionDescriptor):
 
 
 @overload
-def thing_action(func: Callable, **kwargs) -> ActionDescriptor: ...
+def thing_action(func: Callable, **kwargs: Any) -> ActionDescriptor: ...
 
 
 @overload
 def thing_action(
-    **kwargs,
+    **kwargs: Any,
 ) -> Callable[
     [
         Callable,
@@ -81,7 +81,7 @@ def thing_action(
 
 @wraps(mark_thing_action)
 def thing_action(
-    func: Optional[Callable] = None, **kwargs
+    func: Optional[Callable] = None, **kwargs: Any
 ) -> (
     ActionDescriptor
     | Callable[
@@ -121,7 +121,7 @@ def thing_action(
 
 
 def fastapi_endpoint(
-    method: HTTPMethod, path: Optional[str] = None, **kwargs
+    method: HTTPMethod, path: Optional[str] = None, **kwargs: Any
 ) -> Callable[[Callable], EndpointDescriptor]:
     r"""Mark a function as a FastAPI endpoint without making it an action.
 
diff --git a/src/labthings_fastapi/dependencies/invocation.py b/src/labthings_fastapi/dependencies/invocation.py
index 9777df40..85b0bf6e 100644
--- a/src/labthings_fastapi/dependencies/invocation.py
+++ b/src/labthings_fastapi/dependencies/invocation.py
@@ -146,7 +146,7 @@ def __init__(self, id: InvocationID):
         threading.Event.__init__(self)
         self.invocation_id = id
 
-    def raise_if_set(self):
+    def raise_if_set(self) -> None:
         """Raise an exception if the event is set.
 
         This is intended as a compact alternative to:
@@ -161,7 +161,7 @@ def raise_if_set(self):
         if self.is_set():
             raise InvocationCancelledError("The action was cancelled.")
 
-    def sleep(self, timeout: float):
+    def sleep(self, timeout: float) -> None:
         r"""Sleep for a given time in seconds, but raise an exception if cancelled.
 
         This function can be used in place of `time.sleep`. It will usually behave
diff --git a/src/labthings_fastapi/descriptors/action.py b/src/labthings_fastapi/descriptors/action.py
index 68a409a3..7c06d899 100644
--- a/src/labthings_fastapi/descriptors/action.py
+++ b/src/labthings_fastapi/descriptors/action.py
@@ -122,11 +122,11 @@ def __init__(
         self.invocation_model.__name__ = f"{self.name}_invocation"
 
     @overload
-    def __get__(self, obj: Literal[None], type=None) -> ActionDescriptor:  # noqa: D105
+    def __get__(self, obj: Literal[None], type: type[Thing]) -> ActionDescriptor:  # noqa: D105
         ...
 
     @overload
-    def __get__(self, obj: Thing, type=None) -> Callable:  # noqa: D105
+    def __get__(self, obj: Thing, type: type[Thing] | None = None) -> Callable:  # noqa: D105
         ...
 
     def __get__(
@@ -158,17 +158,17 @@ def __get__(
         return partial(self.func, obj)
 
     @property
-    def name(self):
+    def name(self) -> str:
         """The name of the wrapped function."""
         return self.func.__name__
 
     @property
-    def title(self):
+    def title(self) -> str:
         """A human-readable title."""
         return get_summary(self.func) or self.name
 
     @property
-    def description(self):
+    def description(self) -> str | None:
         """A description of the action."""
         return get_docstring(self.func, remove_summary=True)
 
@@ -189,7 +189,7 @@ def _observers_set(self, obj: Thing) -> WeakSet:
             ld.action_observers[self.name] = WeakSet()
         return ld.action_observers[self.name]
 
-    def emit_changed_event(self, obj: Thing, status: str):
+    def emit_changed_event(self, obj: Thing, status: str) -> None:
         """Notify subscribers that the action status has changed.
 
         This function is run from within the `.Invocation` thread that
@@ -266,12 +266,12 @@ def start_action(
             action_manager: ActionManagerContextDep,
             _blob_manager: BlobIOContextDep,
             request: Request,
-            body,
+            body: Any,  # This annotation will be overwritten below.
             id: InvocationID,
             cancel_hook: CancelHook,
             background_tasks: BackgroundTasks,
-            **dependencies,
-        ):
+            **dependencies: Any,
+        ) -> InvocationModel:
             action = action_manager.invoke_action(
                 action=self,
                 thing=thing,
@@ -318,6 +318,7 @@ def start_action(
         if hasattr(self.output_model, "media_type"):
             responses[200]["content"][self.output_model.media_type] = {}
         # Now we can add the endpoint to the app.
+        assert thing.path is not None, "Can't add the endpoint without thing.path!"
         app.post(
             thing.path + self.name,
             response_model=self.invocation_model,
@@ -341,7 +342,7 @@ def start_action(
         )
         def list_invocations(
             action_manager: ActionManagerContextDep, _blob_manager: BlobIOContextDep
-        ):
+        ) -> list[InvocationModel]:
             return action_manager.list_invocations(self, thing)
 
     def action_affordance(
@@ -359,6 +360,7 @@ def action_affordance(
         :return: An `.ActionAffordance` describing this action.
         """
         path = path or thing.path
+        assert path is not None, "Can't generate forms without a path!"
         forms = [
             Form[ActionOp](href=path + self.name, op=[ActionOp.invokeaction]),
         ]
diff --git a/src/labthings_fastapi/descriptors/endpoint.py b/src/labthings_fastapi/descriptors/endpoint.py
index 6beec6a1..47c542c7 100644
--- a/src/labthings_fastapi/descriptors/endpoint.py
+++ b/src/labthings_fastapi/descriptors/endpoint.py
@@ -19,6 +19,7 @@
 from ..utilities.introspection import get_docstring, get_summary
 
 from typing import (
+    Any,
     Callable,
     Literal,
     Mapping,
@@ -45,7 +46,7 @@ def __init__(
         func: Callable,
         http_method: HTTPMethod = "get",
         path: Optional[str] = None,
-        **kwargs: Mapping,
+        **kwargs: Mapping[str, Any],
     ):
         r"""Initialise an EndpointDescriptor.
 
@@ -67,10 +68,10 @@ def __init__(
         self.kwargs = kwargs
 
     @overload
-    def __get__(self, obj: Literal[None], type=None) -> Self: ...  # noqa: D105
+    def __get__(self, obj: Literal[None], type: type[Thing] | None = None) -> Self: ...  # noqa: D105
 
     @overload
-    def __get__(self, obj: Thing, type=None) -> Callable: ...  # noqa: D105
+    def __get__(self, obj: Thing, type: type[Thing] | None = None) -> Callable: ...  # noqa: D105
 
     def __get__(
         self, obj: Optional[Thing], type: type[Thing] | None = None
@@ -96,26 +97,26 @@ def __get__(
         return wraps(self.func)(partial(self.func, obj))
 
     @property
-    def name(self):
+    def name(self) -> str:
         """The name of the wrapped function."""
         return self.func.__name__
 
     @property
-    def path(self):
+    def path(self) -> str:
         """The path of the endpoint (relative to the Thing)."""
         return self._path or self.name
 
     @property
-    def title(self):
+    def title(self) -> str:
         """A human-readable title."""
         return get_summary(self.func) or self.name
 
     @property
-    def description(self):
+    def description(self) -> str | None:
         """A description of the endpoint."""
         return get_docstring(self.func, remove_summary=True)
 
-    def add_to_fastapi(self, app: FastAPI, thing: Thing):
+    def add_to_fastapi(self, app: FastAPI, thing: Thing) -> None:
         """Add an endpoint for this function to a FastAPI app.
 
         We will add an endpoint to the app, bound to a particular `.Thing`.
@@ -126,11 +127,12 @@ def add_to_fastapi(self, app: FastAPI, thing: Thing):
         :param app: the `fastapi.FastAPI` application we are adding to.
         :param thing: the `.Thing` we're bound to.
         """
+        assert thing.path is not None
         # fastapi_endpoint is equivalent to app.get/app.post/whatever
         fastapi_endpoint = getattr(app, self.http_method)
         bound_function = partial(self.func, thing)
         # NB the line above can't use self.__get__ as wraps() confuses FastAPI
-        kwargs = {  # Auto-populate description and summary
+        kwargs: dict[str, Any] = {  # Auto-populate description and summary
             "description": f"## {self.title}\n\n {self.description}",
             "summary": self.title,
         }
diff --git a/src/labthings_fastapi/example_things/__init__.py b/src/labthings_fastapi/example_things/__init__.py
index def9bb57..9d6daba3 100644
--- a/src/labthings_fastapi/example_things/__init__.py
+++ b/src/labthings_fastapi/example_things/__init__.py
@@ -74,7 +74,7 @@ def make_a_dict(
         return out
 
     @thing_action
-    def increment_counter(self):
+    def increment_counter(self) -> None:
         """Increment the counter property.
 
         This action doesn't do very much - all it does, in fact,
@@ -84,7 +84,7 @@ def increment_counter(self):
         self.counter += 1
 
     @thing_action
-    def slowly_increase_counter(self, increments: int = 60, delay: float = 1):
+    def slowly_increase_counter(self, increments: int = 60, delay: float = 1) -> None:
         """Increment the counter slowly over a minute.
 
         :param increments: how many times to increment.
@@ -118,7 +118,7 @@ class ThingWithBrokenAffordances(Thing):
     """A Thing that raises exceptions in actions/properties."""
 
     @thing_action
-    def broken_action(self):
+    def broken_action(self) -> None:
         """Do something that raises an exception.
 
         :raise RuntimeError: every time.
@@ -126,7 +126,7 @@ def broken_action(self):
         raise RuntimeError("This is a broken action")
 
     @lt_property
-    def broken_property(self):
+    def broken_property(self) -> None:
         """Raise an exception when the property is accessed.
 
         :raise RuntimeError: every time.
@@ -137,7 +137,7 @@ def broken_property(self):
 class ThingThatCantInstantiate(Thing):
     """A Thing that raises an exception in __init__."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         """Fail to initialise.
 
         :raise RuntimeError: every time.
@@ -148,14 +148,14 @@ def __init__(self):
 class ThingThatCantStart(Thing):
     """A Thing that raises an exception in __enter__."""
 
-    def __enter__(self):
+    def __enter__(self) -> None:
         """Fail to start the thing.
 
         :raise RuntimeError: every time.
         """
         raise RuntimeError("This thing can't start")
 
-    def __exit__(self, exc_t: Any, exc_v: Any, exc_tb: Any):
+    def __exit__(self, exc_t: Any, exc_v: Any, exc_tb: Any) -> None:
         """Don't leave the thing as we never entered.
 
         :param exc_t: Exception type.
diff --git a/src/labthings_fastapi/outputs/blob.py b/src/labthings_fastapi/outputs/blob.py
index 420ad7f8..201dfff5 100644
--- a/src/labthings_fastapi/outputs/blob.py
+++ b/src/labthings_fastapi/outputs/blob.py
@@ -46,6 +46,7 @@ def get_image(self) -> MyImageBlob:
 import shutil
 from typing import (
     Annotated,
+    Any,
     AsyncGenerator,
     Callable,
     Literal,
@@ -215,7 +216,7 @@ class BlobFile:
     id: Optional[uuid.UUID] = None
     """A unique ID to identify the data in a `.BlobManager`."""
 
-    def __init__(self, file_path: str, media_type: str, **kwargs):
+    def __init__(self, file_path: str, media_type: str, **kwargs: Any):
         r"""Create a `.BlobFile` to wrap data stored on disk.
 
         `.BlobFile` objects wrap data stored on disk as files. They
diff --git a/src/labthings_fastapi/outputs/mjpeg_stream.py b/src/labthings_fastapi/outputs/mjpeg_stream.py
index 3b0d3300..efdd882e 100644
--- a/src/labthings_fastapi/outputs/mjpeg_stream.py
+++ b/src/labthings_fastapi/outputs/mjpeg_stream.py
@@ -139,7 +139,7 @@ def __init__(self, ringbuffer_size: int = 10):
         self._ringbuffer: list[RingbufferEntry] = []
         self.reset(ringbuffer_size=ringbuffer_size)
 
-    def reset(self, ringbuffer_size: Optional[int] = None):
+    def reset(self, ringbuffer_size: Optional[int] = None) -> None:
         """Reset the stream and optionally change the ringbuffer size.
 
         Discard all frames from the ringbuffer and reset the frame index.
@@ -361,7 +361,7 @@ class MJPEGStreamDescriptor:
     This descriptor does not currently show up in the :ref:`wot_td`.
     """
 
-    def __init__(self, **kwargs: dict[str, Any]):
+    def __init__(self, **kwargs: Any):
         r"""Initialise an MJPEGStreamDescriptor.
 
         :param \**kwargs: keyword arguments are passed to the initialiser of
@@ -381,10 +381,10 @@ def __set_name__(self, _owner: Thing, name: str) -> None:
         self.name = name
 
     @overload
-    def __get__(self, obj: Literal[None], type=None) -> Self: ...  # noqa: D105
+    def __get__(self, obj: Literal[None], type: type | None = None) -> Self: ...  # noqa: D105
 
     @overload
-    def __get__(self, obj: Thing, type=None) -> MJPEGStream: ...  # noqa: D105
+    def __get__(self, obj: Thing, type: type | None = None) -> MJPEGStream: ...  # noqa: D105
 
     def __get__(
         self, obj: Optional[Thing], type: type[Thing] | None = None
@@ -452,5 +452,5 @@ class Camera(lt.Thing):
             f"{thing.path}{self.name}/viewer",
             response_class=HTMLResponse,
         )
-        async def viewer_page():
+        async def viewer_page() -> HTMLResponse:
             return await self.viewer_page(f"{thing.path}{self.name}")
diff --git a/src/labthings_fastapi/properties.py b/src/labthings_fastapi/properties.py
index 9af1bbf9..a88cbe47 100644
--- a/src/labthings_fastapi/properties.py
+++ b/src/labthings_fastapi/properties.py
@@ -381,7 +381,7 @@ def add_to_fastapi(self, app: FastAPI, thing: Thing) -> None:
         # the function to the decorator.
         if not self.readonly:
 
-            def set_property(body):  # We'll annotate body later
+            def set_property(body: Any) -> None:  # We'll annotate body later
                 if isinstance(body, RootModel):
                     body = body.root
                 return self.__set__(thing, body)
@@ -402,7 +402,7 @@ def set_property(body):  # We'll annotate body later
             summary=self.title,
             description=f"## {self.title}\n\n{self.description or ''}",
         )
-        def get_property():
+        def get_property() -> Any:
             return self.__get__(thing)
 
     def property_affordance(
@@ -445,6 +445,19 @@ def property_affordance(
             }
         )
 
+    def __set__(self, obj: Thing, value: Any) -> None:
+        """Set the property (stub method).
+
+        This is a stub ``__set__`` method to mark this as a data descriptor.
+
+        :param obj: The Thing on which we are setting the value.
+        :param value: The new value for the Thing.
+        :raises NotImplementedError: as this must be overridden by concrete classes.
+        """
+        raise NotImplementedError(
+            "__set__ must be overridden by property implementations."
+        )
+
 
 class DataProperty(BaseProperty[Value], Generic[Value]):
     """A Property descriptor that acts like a regular variable.
@@ -602,7 +615,7 @@ def __set__(
         if emit_changed_event:
             self.emit_changed_event(obj, value)
 
-    def _observers_set(self, obj: Thing):
+    def _observers_set(self, obj: Thing) -> WeakSet:
         """Return the observers of this property.
 
         Each observer in this set will be notified when the property is changed.
@@ -646,7 +659,7 @@ def emit_changed_event(self, obj: Thing, value: Value) -> None:
             value,
         )
 
-    async def emit_changed_event_async(self, obj: Thing, value: Value):
+    async def emit_changed_event_async(self, obj: Thing, value: Value) -> None:
         """Notify subscribers that the property has changed.
 
         This function may only be run in the `anyio` event loop. See
@@ -793,7 +806,7 @@ def instance_get(self, obj: Thing) -> Value:
         """
         return self.fget(obj)
 
-    def __set__(self, obj: Thing, value: Value):
+    def __set__(self, obj: Thing, value: Value) -> None:
         """Set the value of the property.
 
         :param obj: the `.Thing` on which the attribute is accessed.
diff --git a/src/labthings_fastapi/server/__init__.py b/src/labthings_fastapi/server/__init__.py
index 0ef9abd9..12763d7e 100644
--- a/src/labthings_fastapi/server/__init__.py
+++ b/src/labthings_fastapi/server/__init__.py
@@ -216,7 +216,7 @@ async def lifespan(self, app: FastAPI) -> AsyncGenerator[None]:
 
         self.blocking_portal = None
 
-    def add_things_view_to_app(self):
+    def add_things_view_to_app(self) -> None:
         """Add an endpoint that shows the list of attached things."""
         thing_server = self
 
diff --git a/src/labthings_fastapi/server/cli.py b/src/labthings_fastapi/server/cli.py
index 5a1aefbf..d8e32ba2 100644
--- a/src/labthings_fastapi/server/cli.py
+++ b/src/labthings_fastapi/server/cli.py
@@ -30,7 +30,7 @@
 from . import ThingServer, server_from_config
 
 
-def get_default_parser():
+def get_default_parser() -> ArgumentParser:
     """Return the default CLI parser for LabThings.
 
     This can be used to add more arguments, for custom CLIs that make use of
diff --git a/src/labthings_fastapi/thing.py b/src/labthings_fastapi/thing.py
index 3d3cb739..c2ca49d8 100644
--- a/src/labthings_fastapi/thing.py
+++ b/src/labthings_fastapi/thing.py
@@ -152,7 +152,7 @@ def thing_description(request: Request) -> ThingDescription:
             return self.thing_description(base=str(request.base_url))
 
         @server.app.websocket(self.path + "ws")
-        async def websocket(ws: WebSocket):
+        async def websocket(ws: WebSocket) -> None:
             await websocket_endpoint(self, ws)
 
     # A private variable to hold the list of settings so it doesn't need to be
@@ -230,13 +230,14 @@ def load_settings(self, setting_storage_path: str) -> None:
                 _LOGGER.warning("Error loading settings for %s", thing_name)
         self._setting_storage_path = setting_storage_path
 
-    def save_settings(self):
+    def save_settings(self) -> None:
         """Save settings to JSON.
 
         This is called whenever a setting is updated. All settings are written to
         the settings file every time.
         """
         if self._settings is not None:
+            assert self._setting_storage_path is not None
             setting_dict = {}
             for name in self._settings.keys():
                 value = getattr(self, name)
@@ -352,7 +353,7 @@ def observe_property(self, property_name: str, stream: ObjectSendStream) -> None
             raise KeyError(f"{property_name} is not a LabThings Property")
         prop._observers_set(self).add(stream)
 
-    def observe_action(self, action_name: str, stream: ObjectSendStream):
+    def observe_action(self, action_name: str, stream: ObjectSendStream) -> None:
         """Register a stream to receive action status change notifications.
 
         :param action_name: the action to register for.
diff --git a/src/labthings_fastapi/thing_description/__init__.py b/src/labthings_fastapi/thing_description/__init__.py
index a06497f1..5691c838 100644
--- a/src/labthings_fastapi/thing_description/__init__.py
+++ b/src/labthings_fastapi/thing_description/__init__.py
@@ -278,7 +278,7 @@ def jsonschema_to_dataschema(
     return output
 
 
-def type_to_dataschema(t: type, **kwargs) -> DataSchema:
+def type_to_dataschema(t: type, **kwargs: Any) -> DataSchema:
     r"""Convert a Python type to a Thing Description DataSchema.
 
     This makes use of pydantic's `schema_of` function to create a
diff --git a/src/labthings_fastapi/thing_description/_model.py b/src/labthings_fastapi/thing_description/_model.py
index f5bdcf9a..8c3c1633 100644
--- a/src/labthings_fastapi/thing_description/_model.py
+++ b/src/labthings_fastapi/thing_description/_model.py
@@ -81,7 +81,7 @@ class Subprotocol(Enum):
 ]
 
 
-def uses_thing_context(v: ThingContextType):
+def uses_thing_context(v: ThingContextType) -> None:
     """Check the URLs in the ThingContextType are valid.
 
     This function makes ``assert`` statements, so will fail with an exception
diff --git a/src/labthings_fastapi/types/numpy.py b/src/labthings_fastapi/types/numpy.py
index e8d067fb..2fb27db0 100644
--- a/src/labthings_fastapi/types/numpy.py
+++ b/src/labthings_fastapi/types/numpy.py
@@ -71,7 +71,7 @@ def np_to_listoflists(arr: np.ndarray) -> NestedListOfNumbers:
     :param arr: a `numpy.ndarray`.
     :return: a nested list of numbers.
     """
-    return arr.tolist()  # type: ignore[return-value]
+    return arr.tolist()
 
 
 def listoflists_to_np(lol: Union[NestedListOfNumbers, np.ndarray]) -> np.ndarray:
diff --git a/src/labthings_fastapi/utilities/introspection.py b/src/labthings_fastapi/utilities/introspection.py
index 8cbeaf6a..f991f33e 100644
--- a/src/labthings_fastapi/utilities/introspection.py
+++ b/src/labthings_fastapi/utilities/introspection.py
@@ -114,9 +114,7 @@ def input_model_from_signature(
         p_type = Any if p.annotation is Parameter.empty else type_hints[name]
         # pydantic uses `...` to represent missing defaults (i.e. required params)
         default = Field(...) if p.default is Parameter.empty else p.default
-        # p_type below has a complicated type, but it is reasonable to
-        # call p_type a `type` and ignore the mypy error.
-        fields[name] = (p_type, default)  # type: ignore[assignment]
+        fields[name] = (p_type, default)
     model = create_model(  # type: ignore[call-overload]
         f"{func.__name__}_input",
         model_config=ConfigDict(extra="allow" if takes_v_kwargs else "forbid"),
@@ -176,7 +174,7 @@ def return_type(func: Callable) -> Type:
     """
     sig = inspect.signature(func)
     if sig.return_annotation == inspect.Signature.empty:
-        return Any  # type: ignore[return-value]
+        return Any
     else:
         # We use `get_type_hints` rather than just `sig.return_annotation`
         # because it resolves forward references, etc.
diff --git a/tests/test_property.py b/tests/test_property.py
index aad5db5b..2ffac7f9 100644
--- a/tests/test_property.py
+++ b/tests/test_property.py
@@ -220,6 +220,20 @@ class Example:
     assert set(entry.keys()) == set(["get", "put"])
 
 
+def test_baseproperty_set_error():
+    """Check `.Baseproperty.__set__` raises an error and is overridden."""
+    assert tp.DataProperty.__get__ is tp.BaseProperty.__get__
+    assert tp.DataProperty.__set__ is not tp.BaseProperty.__set__
+    assert tp.FunctionalProperty.__set__ is not tp.BaseProperty.__set__
+
+    class Example:
+        bp: int = tp.BaseProperty()
+
+    example = Example()
+    with pytest.raises(NotImplementedError):
+        example.bp = 0
+
+
 def test_decorator_exception():
     r"""Check decorators work as expected when the setter has a different name.
 
diff --git a/typing_tests/thing_definitions.py b/typing_tests/thing_definitions.py
index b0815e91..5650f2f1 100644
--- a/typing_tests/thing_definitions.py
+++ b/typing_tests/thing_definitions.py
@@ -59,7 +59,7 @@ def int_factory() -> int:
 
 
 @lt.property
-def strprop(self) -> str:
+def strprop(self: typing.Any) -> str:
     """A functional property that should not cause mypy errors."""
     return "foo"
 
@@ -213,7 +213,7 @@ def intprop2(self) -> int:
         return 0
 
     @intprop2.setter
-    def set_intprop2(self, value: int):
+    def set_intprop2(self, value: int) -> None:
         """Setter for intprop2."""
         pass