diff --git a/ollama/_client.py b/ollama/_client.py
index d3b2a4a..1ac5372 100644
--- a/ollama/_client.py
+++ b/ollama/_client.py
@@ -7,7 +7,7 @@ from pathlib import Path
 from hashlib import sha256
 from base64 import b64encode
 
-from typing import Any, AnyStr, Union, Optional, Sequence, Mapping
+from typing import Any, AnyStr, Union, Optional, Sequence, Mapping, Literal
 
 import sys
 
@@ -81,7 +81,7 @@ class Client(BaseClient):
     context: Optional[Sequence[int]] = None,
     stream: bool = False,
     raw: bool = False,
-    format: str = '',
+    format: Literal['', 'json'] = '',
     images: Optional[Sequence[AnyStr]] = None,
     options: Optional[Options] = None,
   ) -> Union[Mapping[str, Any], Iterator[Mapping[str, Any]]]:
@@ -121,7 +121,7 @@ class Client(BaseClient):
     model: str = '',
     messages: Optional[Sequence[Message]] = None,
     stream: bool = False,
-    format: str = '',
+    format: Literal['', 'json'] = '',
     options: Optional[Options] = None,
   ) -> Union[Mapping[str, Any], Iterator[Mapping[str, Any]]]:
     """
@@ -338,7 +338,7 @@ class AsyncClient(BaseClient):
     context: Optional[Sequence[int]] = None,
     stream: bool = False,
     raw: bool = False,
-    format: str = '',
+    format: Literal['', 'json'] = '',
     images: Optional[Sequence[AnyStr]] = None,
     options: Optional[Options] = None,
   ) -> Union[Mapping[str, Any], AsyncIterator[Mapping[str, Any]]]:
@@ -377,7 +377,7 @@ class AsyncClient(BaseClient):
     model: str = '',
     messages: Optional[Sequence[Message]] = None,
     stream: bool = False,
-    format: str = '',
+    format: Literal['', 'json'] = '',
     options: Optional[Options] = None,
   ) -> Union[Mapping[str, Any], AsyncIterator[Mapping[str, Any]]]:
     """
diff --git a/ollama/_types.py b/ollama/_types.py
index 226626e..8258825 100644
--- a/ollama/_types.py
+++ b/ollama/_types.py
@@ -1,4 +1,4 @@
-from typing import Any, TypedDict, Sequence
+from typing import Any, TypedDict, Sequence, Literal
 
 import sys
 
@@ -27,7 +27,8 @@ class GenerateResponse(BaseGenerateResponse):
 
 
 class Message(TypedDict):
-  role: str
+  role: Literal['user', 'assistant', 'system']
+
   content: str
   images: NotRequired[Sequence[Any]]