diff --git a/nanochat/engine.py b/nanochat/engine.py
index aa2e6a9..d0657c4 100644
--- a/nanochat/engine.py
+++ b/nanochat/engine.py
@@ -118,7 +118,11 @@ class KVCache:
 
     def advance(self, num_tokens):
         """Advance the cache position by num_tokens."""
-        self.cache_seqlens += num_tokens
+        # Validate that we don't exceed max sequence length
+        new_seqlens = self.cache_seqlens + num_tokens
+        if torch.any(new_seqlens > self.max_seq_len):
+            raise ValueError(f"Cache overflow: attempted to advance beyond max_seq_len={self.max_seq_len}")
+        self.cache_seqlens.copy_(new_seqlens)
 
     def prefill(self, other):
         """
diff --git a/nanochat/tokenizer.py b/nanochat/tokenizer.py
index a2146c2..b06d756 100644
--- a/nanochat/tokenizer.py
+++ b/nanochat/tokenizer.py
@@ -232,15 +232,16 @@ class RustBPETokenizer:
 
         if isinstance(text, str):
             ids = self.enc.encode_ordinary(text)
+            # Use list concatenation instead of insert(0, ...) for O(1) prepend
             if prepend is not None:
-                ids.insert(0, prepend_id) # TODO: slightly inefficient here? :( hmm
+                ids = [prepend_id] + ids
             if append is not None:
                 ids.append(append_id)
         elif isinstance(text, list):
             ids = self.enc.encode_ordinary_batch(text, num_threads=num_threads)
+            # Use list concatenation instead of insert(0, ...) for O(1) prepend per row
             if prepend is not None:
-                for ids_row in ids:
-                    ids_row.insert(0, prepend_id) # TODO: same
+                ids = [[prepend_id] + row for row in ids]
             if append is not None:
                 for ids_row in ids:
                     ids_row.append(append_id)