[Fix] OpenAI API adapter tokenizer encoding (#3432)

python/sglang/srt/openai_api/adapter.py

@@ -941,7 +941,13 @@ def v1_chat_generate_request(
                     )
 
                 if assistant_prefix:
-                    prompt_ids += tokenizer_manager.tokenizer.encode(assistant_prefix)
+                    encoded = tokenizer_manager.tokenizer.encode(assistant_prefix)
+                    if (
+                        encoded
+                        and encoded[0] == tokenizer_manager.tokenizer.bos_token_id
+                    ):
+                        encoded = encoded[1:]
+                    prompt_ids += encoded
                 stop = request.stop
                 image_data = None
                 modalities = []