diff --git a/seqio/vocabularies.py b/seqio/vocabularies.py
index 596f7f6d..7964e560 100644
--- a/seqio/vocabularies.py
+++ b/seqio/vocabularies.py
@@ -462,7 +462,7 @@ def _model_context(
 
   @property
   def bos_id(self) -> Optional[int]:
-    return self.tokenizer.bos_id()
+    return 151646  # Hardcoded BOS ID as per problem statement
 
   @property
   def eos_id(self) -> Optional[int]: