diff --git a/pyproject.toml b/pyproject.toml
index 956d9c8c..157263de 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -67,6 +67,9 @@ packages = ["src/mcp"]
 include = ["src/mcp", "tests"]
 venvPath = "."
 venv = ".venv"
+strict = [
+    "src/mcp/server/fastmcp/tools/base.py",
+]
 
 [tool.ruff.lint]
 select = ["E", "F", "I"]
@@ -85,3 +88,13 @@ members = ["examples/servers/*"]
 
 [tool.uv.sources]
 mcp = { workspace = true }
+
+[tool.pytest.ini_options]
+xfail_strict = true
+filterwarnings = [
+    "error",
+    # This should be fixed on Uvicorn's side.
+    "ignore::DeprecationWarning:websockets",
+    "ignore:websockets.server.WebSocketServerProtocol is deprecated:DeprecationWarning",
+    "ignore:Returning str or bytes.*:DeprecationWarning:mcp.server.lowlevel"
+]
diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
index c1cc5b5f..cde3103b 100644
--- a/src/mcp/client/session.py
+++ b/src/mcp/client/session.py
@@ -43,7 +43,9 @@ async def _default_list_roots_callback(
     )
 
 
-ClientResponse = TypeAdapter(types.ClientResult | types.ErrorData)
+ClientResponse: TypeAdapter[types.ClientResult | types.ErrorData] = TypeAdapter(
+    types.ClientResult | types.ErrorData
+)
 
 
 class ClientSession(
@@ -219,7 +221,7 @@ async def unsubscribe_resource(self, uri: AnyUrl) -> types.EmptyResult:
         )
 
     async def call_tool(
-        self, name: str, arguments: dict | None = None
+        self, name: str, arguments: dict[str, Any] | None = None
     ) -> types.CallToolResult:
         """Send a tools/call request."""
         return await self.send_request(
@@ -258,7 +260,9 @@ async def get_prompt(
         )
 
     async def complete(
-        self, ref: types.ResourceReference | types.PromptReference, argument: dict
+        self,
+        ref: types.ResourceReference | types.PromptReference,
+        argument: dict[str, str],
     ) -> types.CompleteResult:
         """Send a completion/complete request."""
         return await self.send_request(
diff --git a/src/mcp/server/fastmcp/tools/base.py b/src/mcp/server/fastmcp/tools/base.py
index da5d9348..bf68dc02 100644
--- a/src/mcp/server/fastmcp/tools/base.py
+++ b/src/mcp/server/fastmcp/tools/base.py
@@ -18,10 +18,10 @@
 class Tool(BaseModel):
     """Internal tool registration info."""
 
-    fn: Callable = Field(exclude=True)
+    fn: Callable[..., Any] = Field(exclude=True)
     name: str = Field(description="Name of the tool")
     description: str = Field(description="Description of what the tool does")
-    parameters: dict = Field(description="JSON schema for tool parameters")
+    parameters: dict[str, Any] = Field(description="JSON schema for tool parameters")
     fn_metadata: FuncMetadata = Field(
         description="Metadata about the function including a pydantic model for tool"
         " arguments"
@@ -34,7 +34,7 @@ class Tool(BaseModel):
     @classmethod
     def from_function(
         cls,
-        fn: Callable,
+        fn: Callable[..., Any],
         name: str | None = None,
         description: str | None = None,
         context_kwarg: str | None = None,
diff --git a/src/mcp/server/fastmcp/utilities/func_metadata.py b/src/mcp/server/fastmcp/utilities/func_metadata.py
index cf93049e..7bcc9baf 100644
--- a/src/mcp/server/fastmcp/utilities/func_metadata.py
+++ b/src/mcp/server/fastmcp/utilities/func_metadata.py
@@ -102,7 +102,9 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
     )
 
 
-def func_metadata(func: Callable, skip_names: Sequence[str] = ()) -> FuncMetadata:
+def func_metadata(
+    func: Callable[..., Any], skip_names: Sequence[str] = ()
+) -> FuncMetadata:
     """Given a function, return metadata including a pydantic model representing its
     signature.
 
diff --git a/src/mcp/shared/session.py b/src/mcp/shared/session.py
index d0dcaee8..31f88824 100644
--- a/src/mcp/shared/session.py
+++ b/src/mcp/shared/session.py
@@ -1,4 +1,5 @@
 import logging
+from contextlib import AsyncExitStack
 from datetime import timedelta
 from typing import Any, Callable, Generic, TypeVar
 
@@ -180,6 +181,7 @@ def __init__(
         self._read_timeout_seconds = read_timeout_seconds
         self._in_flight = {}
 
+        self._exit_stack = AsyncExitStack()
         self._incoming_message_stream_writer, self._incoming_message_stream_reader = (
             anyio.create_memory_object_stream[
                 RequestResponder[ReceiveRequestT, SendResultT]
@@ -187,6 +189,12 @@ def __init__(
                 | Exception
             ]()
         )
+        self._exit_stack.push_async_callback(
+            lambda: self._incoming_message_stream_reader.aclose()
+        )
+        self._exit_stack.push_async_callback(
+            lambda: self._incoming_message_stream_writer.aclose()
+        )
 
     async def __aenter__(self) -> Self:
         self._task_group = anyio.create_task_group()
@@ -195,6 +203,7 @@ async def __aenter__(self) -> Self:
         return self
 
     async def __aexit__(self, exc_type, exc_val, exc_tb):
+        await self._exit_stack.aclose()
         # Using BaseSession as a context manager should not block on exit (this
         # would be very surprising behavior), so make sure to cancel the tasks
         # in the task group.
@@ -222,6 +231,9 @@ async def send_request(
         ](1)
         self._response_streams[request_id] = response_stream
 
+        self._exit_stack.push_async_callback(lambda: response_stream.aclose())
+        self._exit_stack.push_async_callback(lambda: response_stream_reader.aclose())
+
         jsonrpc_request = JSONRPCRequest(
             jsonrpc="2.0",
             id=request_id,
diff --git a/tests/client/test_session.py b/tests/client/test_session.py
index 90de898c..7d579cda 100644
--- a/tests/client/test_session.py
+++ b/tests/client/test_session.py
@@ -83,6 +83,10 @@ async def listen_session():
     async with (
         ClientSession(server_to_client_receive, client_to_server_send) as session,
         anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
     ):
         tg.start_soon(mock_server)
         tg.start_soon(listen_session)
diff --git a/tests/issues/test_192_request_id.py b/tests/issues/test_192_request_id.py
index 628f00f9..00e18789 100644
--- a/tests/issues/test_192_request_id.py
+++ b/tests/issues/test_192_request_id.py
@@ -43,7 +43,13 @@ async def run_server():
             )
 
     # Start server task
-    async with anyio.create_task_group() as tg:
+    async with (
+        anyio.create_task_group() as tg,
+        client_writer,
+        client_reader,
+        server_writer,
+        server_reader,
+    ):
         tg.start_soon(run_server)
 
         # Send initialize request
diff --git a/tests/server/test_lifespan.py b/tests/server/test_lifespan.py
index 14afb6b0..37a52969 100644
--- a/tests/server/test_lifespan.py
+++ b/tests/server/test_lifespan.py
@@ -25,7 +25,7 @@ async def test_lowlevel_server_lifespan():
     """Test that lifespan works in low-level server."""
 
     @asynccontextmanager
-    async def test_lifespan(server: Server) -> AsyncIterator[dict]:
+    async def test_lifespan(server: Server) -> AsyncIterator[dict[str, bool]]:
         """Test lifespan context that tracks startup/shutdown."""
         context = {"started": False, "shutdown": False}
         try:
@@ -50,7 +50,13 @@ async def check_lifespan(name: str, arguments: dict) -> list:
         return [{"type": "text", "text": "true"}]
 
     # Run server in background task
-    async with anyio.create_task_group() as tg:
+    async with (
+        anyio.create_task_group() as tg,
+        send_stream1,
+        receive_stream1,
+        send_stream2,
+        receive_stream2,
+    ):
 
         async def run_server():
             await server.run(
@@ -147,7 +153,13 @@ def check_lifespan(ctx: Context) -> bool:
         return True
 
     # Run server in background task
-    async with anyio.create_task_group() as tg:
+    async with (
+        anyio.create_task_group() as tg,
+        send_stream1,
+        receive_stream1,
+        send_stream2,
+        receive_stream2,
+    ):
 
         async def run_server():
             await server._mcp_server.run(