diff --git a/pygls/client.py b/pygls/client.py
index b2ea96bd..577f05e0 100644
--- a/pygls/client.py
+++ b/pygls/client.py
@@ -79,7 +79,10 @@ def __init__(
         protocol_cls: Type[JsonRPCProtocol] = JsonRPCProtocol,
         converter_factory: Callable[[], Converter] = default_converter,
     ):
-        self.protocol = protocol_cls(self, converter_factory())
+        # Strictly speaking `JsonRPCProtocol` wants a `LanguageServer`, not a
+        # `JsonRPCClient`. However there similar enough for our purposes, which is
+        # that this client will mostly be used in testing contexts.
+        self.protocol = protocol_cls(self, converter_factory())  # type: ignore
 
         self._server: Optional[asyncio.subprocess.Process] = None
         self._stop_event = Event()
diff --git a/pygls/protocol.py b/pygls/protocol.py
index 8cafa4ca..1733c2fc 100644
--- a/pygls/protocol.py
+++ b/pygls/protocol.py
@@ -28,10 +28,20 @@
 from concurrent.futures import Future
 from functools import lru_cache, partial
 from itertools import zip_longest
-from typing import Any, Callable, List, Optional, Type, TypeVar, Union, TYPE_CHECKING
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Type,
+    TypeVar,
+    Union,
+    TYPE_CHECKING,
+)
 
 if TYPE_CHECKING:
-    from pygls.server import Server
+    from pygls.server import LanguageServer
 
 
 import attrs
@@ -243,19 +253,19 @@ class JsonRPCProtocol(asyncio.Protocol):
 
     VERSION = "2.0"
 
-    def __init__(self, server: Server, converter):
+    def __init__(self, server: LanguageServer, converter):
         self._server = server
         self._converter = converter
 
         self._shutdown = False
 
         # Book keeping for in-flight requests
-        self._request_futures = {}
-        self._result_types = {}
+        self._request_futures: Dict[str, Future[Any]] = {}
+        self._result_types: Dict[str, Any] = {}
 
         self.fm = FeatureManager(server)
-        self.transport = None
-        self._message_buf = []
+        self.transport: Optional[asyncio.BaseTransport] = None
+        self._message_buf: List[bytes] = []
 
         self._send_only_body = False
 
diff --git a/pygls/server.py b/pygls/server.py
index be661fde..fc06498a 100644
--- a/pygls/server.py
+++ b/pygls/server.py
@@ -25,7 +25,12 @@
 
 from pygls import IS_PYODIDE
 from pygls.lsp import ConfigCallbackType, ShowDocumentCallbackType
-from pygls.exceptions import PyglsError, JsonRpcException, FeatureRequestError
+from pygls.exceptions import (
+    JsonRpcInternalError,
+    PyglsError,
+    JsonRpcException,
+    FeatureRequestError,
+)
 from lsprotocol.types import (
     ClientCapabilities,
     Diagnostic,
@@ -51,6 +56,8 @@
 
 F = TypeVar("F", bound=Callable)
 
+ServerErrors = Union[PyglsError, JsonRpcException, type[JsonRpcInternalError]]
+
 
 async def aio_readline(loop, executor, stop_event, rfile, proxy):
     """Reads data from stdin in separate thread (asynchronously)."""
@@ -370,6 +377,7 @@ def __init__(
 
         self.name = name
         self.version = version
+        self.process_id: Optional[Union[int, None]] = None
         super().__init__(protocol_cls, converter_factory, loop, max_workers)
 
     def apply_edit(
@@ -501,7 +509,9 @@ def show_message_log(self, message, msg_type=MessageType.Log) -> None:
         self.lsp.show_message_log(message, msg_type)
 
     def _report_server_error(
-        self, error: Exception, source: Union[PyglsError, JsonRpcException]
+        self,
+        error: Exception,
+        source: ServerErrors,
     ):
         # Prevent recursive error reporting
         try:
@@ -509,9 +519,7 @@ def _report_server_error(
         except Exception:
             logger.warning("Failed to report error to client")
 
-    def report_server_error(
-        self, error: Exception, source: Union[PyglsError, JsonRpcException]
-    ):
+    def report_server_error(self, error: Exception, source: ServerErrors):
         """
         Sends error to the client for displaying.
 
diff --git a/pygls/uris.py b/pygls/uris.py
index 69a6ce3e..8c40f70b 100644
--- a/pygls/uris.py
+++ b/pygls/uris.py
@@ -21,7 +21,7 @@
 
 https://github.com/Microsoft/vscode-uri/blob/e59cab84f5df6265aed18ae5f43552d3eef13bb9/lib/index.ts
 """
-from typing import Tuple
+from typing import Optional, Tuple
 
 import re
 from urllib import parse
@@ -118,12 +118,12 @@ def uri_scheme(uri: str):
 # TODO: Use `URLParts` type
 def uri_with(
     uri: str,
-    scheme: str | None = None,
-    netloc: str | None = None,
-    path: str | None = None,
-    params: str | None = None,
-    query: str | None = None,
-    fragment: str | None = None,
+    scheme: Optional[str] = None,
+    netloc: Optional[str] = None,
+    path: Optional[str] = None,
+    params: Optional[str] = None,
+    query: Optional[str] = None,
+    fragment: Optional[str] = None,
 ):
     """
     Return a URI with the given part(s) replaced.
diff --git a/pygls/workspace/document.py b/pygls/workspace/document.py
index 96093f61..c0a16c93 100644
--- a/pygls/workspace/document.py
+++ b/pygls/workspace/document.py
@@ -54,7 +54,7 @@ def __init__(
             raise Exception("`path` cannot be None")
         self.path = path
         self.language_id = language_id
-        self.filename: str | None = os.path.basename(self.path)
+        self.filename: Optional[str] = os.path.basename(self.path)
 
         self._local = local
         self._source = source
diff --git a/tests/conftest.py b/tests/conftest.py
index 7ea5d434..7d0b50ae 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -24,7 +24,6 @@
 
 from pygls import uris, IS_PYODIDE, IS_WIN
 from pygls.feature_manager import FeatureManager
-from pygls.workspace.workspace import Document
 from pygls.workspace.workspace import Workspace
 
 from .ls_setup import (