diff --git a/Makefile b/Makefile
index 94b117d..ae5bd44 100644
--- a/Makefile
+++ b/Makefile
@@ -4,7 +4,7 @@
 
 .PHONY: mypy
 mypy: venv/bin/mypy
-	venv/bin/mypy --strict -p chap
+	venv/bin/mypy --strict --no-warn-unused-ignores -p chap
 
 venv/bin/mypy:
 	python -mvenv venv
diff --git a/pyproject.toml b/pyproject.toml
index 2bb0e55..6311380 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -42,3 +42,5 @@ write_to = "src/chap/__version__.py"
 [tool.setuptools.dynamic]
 readme = {file = ["README.md"], content-type="text/markdown"}
 dependencies = {file = "requirements.txt"}
+[tool.setuptools.package-data]
+"pkgname" = ["py.typed"]
diff --git a/src/chap/backends/openai_chatgpt.py b/src/chap/backends/openai_chatgpt.py
index a6d8fc7..6dd6e5a 100644
--- a/src/chap/backends/openai_chatgpt.py
+++ b/src/chap/backends/openai_chatgpt.py
@@ -4,6 +4,7 @@
 
 import functools
 import json
+import warnings
 from dataclasses import dataclass
 from typing import AsyncGenerator, cast
 
@@ -20,6 +21,7 @@ class EncodingMeta:
     encoding: tiktoken.Encoding
     tokens_per_message: int
     tokens_per_name: int
+    tokens_overhead: int
 
     @functools.lru_cache()
     def encode(self, s: str) -> list[int]:
@@ -27,47 +29,38 @@ def encode(self, s: str) -> list[int]:
 
     def num_tokens_for_message(self, message: Message) -> int:
         # n.b. chap doesn't use message.name yet
-        return len(self.encode(message.role)) + len(self.encode(message.content))
+        return (
+            len(self.encode(message.role))
+            + len(self.encode(message.content))
+            + self.tokens_per_message
+        )
 
     def num_tokens_for_messages(self, messages: Session) -> int:
-        return sum(self.num_tokens_for_message(message) for message in messages) + 3
+        return (
+            sum(self.num_tokens_for_message(message) for message in messages)
+            + self.tokens_overhead
+        )
 
     @classmethod
     @functools.cache
     def from_model(cls, model: str) -> "EncodingMeta":
-        if model == "gpt-3.5-turbo":
-            # print("Warning: gpt-3.5-turbo may update over time. Returning num tokens assuming gpt-3.5-turbo-0613.")
-            model = "gpt-3.5-turbo-0613"
-        if model == "gpt-4":
-            # print("Warning: gpt-4 may update over time. Returning num tokens assuming gpt-4-0613.")
-            model = "gpt-4-0613"
-
         try:
             encoding = tiktoken.encoding_for_model(model)
         except KeyError:
-            print("Warning: model not found. Using cl100k_base encoding.")
+            warnings.warn("Warning: model not found. Using cl100k_base encoding.")
             encoding = tiktoken.get_encoding("cl100k_base")
 
-        if model in {
-            "gpt-3.5-turbo-0613",
-            "gpt-3.5-turbo-16k-0613",
-            "gpt-4-0314",
-            "gpt-4-32k-0314",
-            "gpt-4-0613",
-            "gpt-4-32k-0613",
-        }:
-            tokens_per_message = 3
-            tokens_per_name = 1
-        elif model == "gpt-3.5-turbo-0301":
+        tokens_per_message = 3
+        tokens_per_name = 1
+        tokens_overhead = 3
+
+        if model == "gpt-3.5-turbo-0301":
             tokens_per_message = (
                 4  # every message follows <|start|>{role/name}\n{content}<|end|>\n
             )
             tokens_per_name = -1  # if there's a name, the role is omitted
-        else:
-            raise NotImplementedError(
-                f"""EncodingMeta is not implemented for model {model}. See https://github.com/openai/openai-python/blob/main/chatml.md for information on how messages are converted to tokens."""
-            )
-        return cls(encoding, tokens_per_message, tokens_per_name)
+
+        return cls(encoding, tokens_per_message, tokens_per_name, tokens_overhead)
 
 
 class ChatGPT:
diff --git a/src/chap/core.py b/src/chap/core.py
index 5f4e6a3..df05cf4 100644
--- a/src/chap/core.py
+++ b/src/chap/core.py
@@ -11,6 +11,7 @@
 import pkgutil
 import subprocess
 from dataclasses import MISSING, dataclass, fields
+from types import UnionType
 from typing import Any, AsyncGenerator, Callable, cast
 
 import click
@@ -171,7 +172,10 @@ def format_backend_help(api: Backend, formatter: click.HelpFormatter) -> None:
             if doc:
                 doc += " "
             doc += f"(Default: {default!r})"
-            typename = f.type.__name__
+            f_type = f.type
+            if isinstance(f_type, UnionType):
+                f_type = f_type.__args__[0]
+            typename = f_type.__name__
             rows.append((f"-B {name}:{typename.upper()}", doc))
         formatter.write_dl(rows)
 
@@ -191,8 +195,11 @@ def set_one_backend_option(kv: tuple[str, str]) -> None:
         field = all_fields.get(name)
         if field is None:
             raise click.BadParameter(f"Invalid parameter {name}")
+        f_type = field.type
+        if isinstance(f_type, UnionType):
+            f_type = f_type.__args__[0]
         try:
-            tv = field.type(value)
+            tv = f_type(value)
         except ValueError as e:
             raise click.BadParameter(
                 f"Invalid value for {name} with value {value}: {e}"
diff --git a/src/chap/py.typed b/src/chap/py.typed
new file mode 100644
index 0000000..e69de29