diff --git a/.gitignore b/.gitignore
index 0d69b13..a2578ad 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ __pycache__
 *.egg-info/
 /build
 /dist
+/venv
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..1437bd3
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,15 @@
+# SPDX-FileCopyrightText: 2023 Jeff Epler <jepler@gmail.com>
+#
+# SPDX-License-Identifier: MIT
+
+.PHONY: mypy
+mypy: venv/bin/mypy
+	venv/bin/mypy --strict -p chap
+
+venv/bin/mypy:
+	python -mvenv venv
+	venv/bin/pip install chap mypy
+
+.PHONY: clean
+clean:
+	rm -rf venv
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 0000000..4dbbc58
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,6 @@
+# SPDX-FileCopyrightText: 2023 Jeff Epler <jepler@gmail.com>
+#
+# SPDX-License-Identifier: MIT
+
+[mypy]
+mypy_path = src
diff --git a/src/chap/backends/replay.py b/src/chap/backends/replay.py
index 19e242b..ea55f03 100644
--- a/src/chap/backends/replay.py
+++ b/src/chap/backends/replay.py
@@ -3,10 +3,10 @@
 # SPDX-License-Identifier: MIT
 
 import asyncio
-import functools
 import os
 import random
 from dataclasses import dataclass
+from typing import Iterable, AsyncGenerator, Callable, Any
 
 import click
 
@@ -14,10 +14,20 @@ from ..session import (  # pylint: disable=relative-beyond-top-level
     Assistant,
     Session,
     User,
+    session_from_file
 )
+from ..core import Backend
 
+from typing import TYPE_CHECKING, TypeVar
+if TYPE_CHECKING:
+    F = TypeVar('F', bound=Callable[..., Any])
+    def cached(f: F) -> F: return f
+else:
+    from functools import lru_cache
+    def cached(f):
+        return lru_cache()(f)
 
-def ipartition(s, sep=" "):
+def ipartition(s: str, sep:str=" ") -> Iterable[tuple[str, str]]:
     rest = s
     while rest:
         first, opt_sep, rest = rest.partition(sep)
@@ -27,43 +37,46 @@ def ipartition(s, sep=" "):
 class Replay:
     @dataclass
     class Parameters:
-        session: str = None
+        session: str | None = None
         """Complete path to an existing session file"""
         delay_mu: float = 0.035
         """Average delay between tokens"""
         delay_sigma: float = 0.02
         """Standard deviation of token delay"""
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.parameters = self.Parameters()
 
     @property
-    @functools.lru_cache()
-    def _session(self):
+    @cached
+    def _session(self) -> Session:
         if self.parameters.session is None:
             raise click.BadParameter(
                 "Must specify -B session:/full/path/to/existing_session.json"
             )
         session_file = os.path.expanduser(self.parameters.session)
-        with open(session_file, "r", encoding="utf-8") as f:
-            return Session.from_json(f.read())
+        return session_from_file(session_file)
 
     @property
-    @functools.lru_cache()
-    def _assistant_responses(self):
+    @cached
+    def _assistant_responses(self) -> Session:
         return [
-            message for message in self._session.session if message.role == "assistant"
+            message for message in self._session if message.role == "assistant"
         ]
 
     @property
-    def system_message(self):
+    def system_message(self) -> str:
         num_assistant_responses = len(self._assistant_responses)
         return (
             f"Replay of {self.parameters.session} with {num_assistant_responses} responses. "
-            f"The original session system message was:\n\n{self._session.session[0].content}"
+            f"The original session system message was:\n\n{self._session[0].content}"
         )
 
-    async def aask(self, session, query):
+    @system_message.setter
+    def system_message(self, value: str) -> None:
+        raise AttributeError("Read-only attribute 'system_message'")
+
+    async def aask(self, session: Session, query: str) -> AsyncGenerator[str, None]:
         data = self.ask(session, query)
         for word, opt_sep in ipartition(data):
             yield word + opt_sep
@@ -72,20 +85,20 @@ class Replay:
             )
 
     def ask(
-        self, session, query, *, max_query_size=5, timeout=60
-    ):  # pylint: disable=unused-argument
+            self, session: Session, query: str
+    ) -> str:  # pylint: disable=unused-argument
         if self._assistant_responses:
             idx = sum(
-                1 for message in session.session if message.role == "assistant"
+                1 for message in session if message.role == "assistant"
             ) % len(self._assistant_responses)
 
             new_content = self._assistant_responses[idx].content
         else:
             new_content = "(No assistant responses in session)"
-        session.session.extend([User(query), Assistant(new_content)])
+        session.extend([User(query), Assistant(new_content)])
         return new_content
 
 
-def factory():
+def factory() -> Backend:
     """Replay an existing session file. Useful for testing."""
     return Replay()