diff --git a/src/codeas/core/agent.py b/src/codeas/core/agent.py
index 192da77..a810c42 100644
--- a/src/codeas/core/agent.py
+++ b/src/codeas/core/agent.py
@@ -1,4 +1,4 @@
-from typing import Union
+from typing import Optional, Union
 
 from pydantic import BaseModel
 from tokencost import (
@@ -46,7 +46,7 @@ class Agent(BaseModel):
     instructions: str
     model: str
     response_format: object = None
-    system_prompt: str = None
+    system_prompt: Optional[str] = None
 
     def run(
         self,
@@ -218,4 +218,4 @@ def _calculate_tokens_and_cost(self, messages: list, response=None):
     repo = Repo(repo_path=".")
     incl_files = repo.filter_files()
     files_paths = [path for path, incl in zip(repo.files_paths, incl_files) if incl]
-    ...
+    ...
\ No newline at end of file