diff --git a/vllm/entrypoints/openai/api_server.py b/vllm/entrypoints/openai/api_server.py
index e91e7c1a07b2..d260396e47c4 100644
--- a/vllm/entrypoints/openai/api_server.py
+++ b/vllm/entrypoints/openai/api_server.py
@@ -130,6 +130,8 @@ async def check_length(
         input_ids = tokenizer(prompt).input_ids
     token_num = len(input_ids)
 
+    if request.max_tokens is None:
+        request.max_tokens = max_model_len - token_num
     if token_num + request.max_tokens > max_model_len:
         return input_ids, create_error_response(
             HTTPStatus.BAD_REQUEST,
diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index b45e4146e29f..473400a7faf9 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -58,7 +58,7 @@ class ChatCompletionRequest(BaseModel):
     temperature: Optional[float] = 0.7
     top_p: Optional[float] = 1.0
     n: Optional[int] = 1
-    max_tokens: Optional[int] = 16
+    max_tokens: Optional[int] = None
     stop: Optional[Union[str, List[str]]] = Field(default_factory=list)
     stream: Optional[bool] = False
     presence_penalty: Optional[float] = 0.0