diff --git a/aider/models.py b/aider/models.py
index 304232259..df28ae0e9 100644
--- a/aider/models.py
+++ b/aider/models.py
@@ -1,19 +1,31 @@
 import re
 
+known_tokens = {
+    "gpt-3.5-turbo": 4,
+    "gpt-4": 8,
+}
+
 
 class Model:
     always_available = False
     use_repo_map = False
     send_undo_reply = False
 
-    def __init__(self, name, tokens=None):
+    def __init__(self, name):
         self.name = name
+
+        tokens = None
+
+        match = re.search(r"-([0-9]+)k", name)
+        if match:
+            tokens = int(match.group(1))
+        else:
+            for m, t in known_tokens.items():
+                if name.startswith(m):
+                    tokens = t
+
         if tokens is None:
-            match = re.search(r"-([0-9]+)k", name)
-
-            default_tokens = 8
-
-            tokens = int(match.group(1)) if match else default_tokens
+            raise ValueError(f"Unknown context window size for model: {name}")
 
         self.max_context_tokens = tokens * 1024
 
@@ -40,6 +52,6 @@ class Model:
         return self.name
 
 
-GPT4 = Model("gpt-4", 8)
+GPT4 = Model("gpt-4")
 GPT35 = Model("gpt-3.5-turbo")
 GPT35_16k = Model("gpt-3.5-turbo-16k")
diff --git a/tests/test_models.py b/tests/test_models.py
new file mode 100644
index 000000000..af2a6f8d7
--- /dev/null
+++ b/tests/test_models.py
@@ -0,0 +1,28 @@
+import unittest
+
+from aider.models import Model
+
+
+class TestModels(unittest.TestCase):
+    def test_max_context_tokens(self):
+        model = Model("gpt-3.5-turbo")
+        self.assertEqual(model.max_context_tokens, 4 * 1024)
+
+        model = Model("gpt-3.5-turbo-16k")
+        self.assertEqual(model.max_context_tokens, 16 * 1024)
+
+        model = Model("gpt-4")
+        self.assertEqual(model.max_context_tokens, 8 * 1024)
+
+        model = Model("gpt-4-32k")
+        self.assertEqual(model.max_context_tokens, 32 * 1024)
+
+        model = Model("gpt-4-0101")
+        self.assertEqual(model.max_context_tokens, 8 * 1024)
+
+        model = Model("gpt-4-32k-2123")
+        self.assertEqual(model.max_context_tokens, 32 * 1024)
+
+
+if __name__ == "__main__":
+    unittest.main()