[Frontend] Added support for HF's new `continue_final_message` parameter (#8942)

tests/entrypoints/openai/test_chat_template.py

@@ -12,7 +12,7 @@ assert chatml_jinja_path.exists()
 
 # Define models, templates, and their corresponding expected outputs
 MODEL_TEMPLATE_GENERATON_OUTPUT = [
-    ("facebook/opt-125m", chatml_jinja_path, True, """<|im_start|>user
+    ("facebook/opt-125m", chatml_jinja_path, True, False, """<|im_start|>user
 Hello<|im_end|>
 <|im_start|>assistant
 Hi there!<|im_end|>
@@ -20,12 +20,20 @@ Hi there!<|im_end|>
 What is the capital of<|im_end|>
 <|im_start|>assistant
 """),
-    ("facebook/opt-125m", chatml_jinja_path, False, """<|im_start|>user
+    ("facebook/opt-125m", chatml_jinja_path, False, False, """<|im_start|>user
 Hello<|im_end|>
 <|im_start|>assistant
 Hi there!<|im_end|>
 <|im_start|>user
-What is the capital of""")
+What is the capital of"""),
+    ("facebook/opt-125m", chatml_jinja_path, False, True, """<|im_start|>user
+Hello<|im_end|>
+<|im_start|>assistant
+Hi there!<|im_end|>
+<|im_start|>user
+What is the capital of<|im_end|>
+<|im_start|>assistant
+The capital of"""),
 ]
 
 TEST_MESSAGES = [
@@ -42,6 +50,10 @@ TEST_MESSAGES = [
         'content': 'What is the capital of'
     },
 ]
+ASSISTANT_MESSAGE_TO_CONTINUE = {
+    'role': 'assistant',
+    'content': 'The capital of'
+}
 
 
 def test_load_chat_template():
@@ -73,10 +85,10 @@ def test_no_load_chat_template_literallike():
 
 
 @pytest.mark.parametrize(
-    "model,template,add_generation_prompt,expected_output",
+    "model,template,add_generation_prompt,continue_final_message,expected_output",
     MODEL_TEMPLATE_GENERATON_OUTPUT)
 def test_get_gen_prompt(model, template, add_generation_prompt,
-                        expected_output):
+                        continue_final_message, expected_output):
     # Initialize the tokenizer
     tokenizer = get_tokenizer(tokenizer_name=model)
     template_content = load_chat_template(chat_template=template)
@@ -84,8 +96,11 @@ def test_get_gen_prompt(model, template, add_generation_prompt,
     # Create a mock request object using keyword arguments
     mock_request = ChatCompletionRequest(
         model=model,
-        messages=TEST_MESSAGES,
-        add_generation_prompt=add_generation_prompt)
+        messages=TEST_MESSAGES + [ASSISTANT_MESSAGE_TO_CONTINUE]
+        if continue_final_message else TEST_MESSAGES,
+        add_generation_prompt=add_generation_prompt,
+        continue_final_message=continue_final_message,
+    )
 
     # Call the function and get the result
     result = apply_hf_chat_template(
@@ -93,6 +108,7 @@ def test_get_gen_prompt(model, template, add_generation_prompt,
         conversation=mock_request.messages,
         chat_template=mock_request.chat_template or template_content,
         add_generation_prompt=mock_request.add_generation_prompt,
+        continue_final_message=mock_request.continue_final_message,
     )
 
     # Test assertion

tests/entrypoints/openai/test_tokenization.py

@@ -104,17 +104,29 @@ async def test_tokenize_chat(client: openai.AsyncOpenAI, model_name: str,
                 "role": "user",
                 "content": "Can I ask a question? vllm1"
             }]
+            for continue_final in [False, True]:
+                if add_generation and continue_final:
+                    continue
+                if continue_final:
+                    conversation.append({
+                        "role": "assistant",
+                        "content": "Sure,"
+                    })
 
                 prompt = tokenizer.apply_chat_template(
                     add_generation_prompt=add_generation,
+                    continue_final_message=continue_final,
                     conversation=conversation,
                     tokenize=False)
-            tokens = tokenizer.encode(prompt, add_special_tokens=add_special)
+                tokens = tokenizer.encode(prompt,
+                                          add_special_tokens=add_special)
 
                 response = requests.post(base_url + "/tokenize",
                                          json={
                                              "add_generation_prompt":
                                              add_generation,
+                                             "continue_final_message":
+                                             continue_final,
                                              "add_special_tokens": add_special,
                                              "messages": conversation,
                                              "model": model_name

vllm/entrypoints/chat_utils.py

@@ -542,6 +542,14 @@ def apply_mistral_chat_template(
     if chat_template is not None:
         logger.warning(
             "'chat_template' cannot be overridden for mistral tokenizer.")
+    if "add_generation_prompt" in kwargs:
+        logger.warning(
+            "'add_generation_prompt' is not supported for mistral tokenizer, "
+            "so it will be ignored.")
+    if "continue_final_message" in kwargs:
+        logger.warning(
+            "'continue_final_message' is not supported for mistral tokenizer, "
+            "so it will be ignored.")
 
     return tokenizer.apply_chat_template(
         messages=messages,

vllm/entrypoints/llm.py

@@ -501,6 +501,7 @@ class LLM:
         lora_request: Optional[LoRARequest] = None,
         chat_template: Optional[str] = None,
         add_generation_prompt: bool = True,
+        continue_final_message: bool = False,
         tools: Optional[List[Dict[str, Any]]] = None,
     ) -> List[RequestOutput]:
         """
@@ -528,6 +529,9 @@ class LLM:
               If not provided, the model's default chat template will be used.
             add_generation_prompt: If True, adds a generation template
                 to each message.
+            continue_final_message: If True, continues the final message in
+                the conversation instead of starting a new one. Cannot be `True`
+                if `add_generation_prompt` is also `True`.
 
         Returns:
             A list of ``RequestOutput`` objects containing the generated
@@ -559,6 +563,7 @@ class LLM:
                     messages=msgs,
                     chat_template=chat_template,
                     add_generation_prompt=add_generation_prompt,
+                    continue_final_message=continue_final_message,
                     tools=tools,
                 )
             else:
@@ -567,6 +572,7 @@ class LLM:
                     conversation=conversation,
                     chat_template=chat_template,
                     add_generation_prompt=add_generation_prompt,
+                    continue_final_message=continue_final_message,
                     tools=tools,
                 )
 

vllm/entrypoints/openai/protocol.py

@@ -211,6 +211,15 @@ class ChatCompletionRequest(OpenAIBaseModel):
          "This is a parameter used by chat template in tokenizer config of the "
          "model."),
     )
+    continue_final_message: bool = Field(
+        default=False,
+        description=
+        ("If this is set, the chat will be formatted so that the final "
+         "message in the chat is open-ended, without any EOS tokens. The "
+         "model will continue this message rather than starting a new one. "
+         "This allows you to \"prefill\" part of the model's response for it. "
+         "Cannot be used at the same time as `add_generation_prompt`."),
+    )
     add_special_tokens: bool = Field(
         default=False,
         description=(
@@ -431,6 +440,15 @@ class ChatCompletionRequest(OpenAIBaseModel):
                         " of the specified `tools`")
         return data
 
+    @model_validator(mode="before")
+    @classmethod
+    def check_generation_prompt(cls, data):
+        if data.get("continue_final_message") and data.get(
+                "add_generation_prompt"):
+            raise ValueError("Cannot set both `continue_final_message` and "
+                             "`add_generation_prompt` to True.")
+        return data
+
 
 class CompletionRequest(OpenAIBaseModel):
     # Ordered by official OpenAI API documentation
@@ -862,8 +880,18 @@ class TokenizeChatRequest(OpenAIBaseModel):
     messages: List[ChatCompletionMessageParam]
 
     add_generation_prompt: bool = Field(default=True)
+    continue_final_message: bool = Field(default=False)
     add_special_tokens: bool = Field(default=False)
 
+    @model_validator(mode="before")
+    @classmethod
+    def check_generation_prompt(cls, data):
+        if data.get("continue_final_message") and data.get(
+                "add_generation_prompt"):
+            raise ValueError("Cannot set both `continue_final_message` and "
+                             "`add_generation_prompt` to True.")
+        return data
+
 
 TokenizeRequest = Union[TokenizeCompletionRequest, TokenizeChatRequest]
 

vllm/entrypoints/openai/serving_chat.py

@@ -140,6 +140,7 @@ class OpenAIServingChat(OpenAIServing):
                     messages=request.messages,
                     chat_template=request.chat_template or self.chat_template,
                     add_generation_prompt=request.add_generation_prompt,
+                    continue_final_message=request.continue_final_message,
                     tools=tool_dicts,
                     documents=request.documents,
                     **(request.chat_template_kwargs or {}),
@@ -150,6 +151,7 @@ class OpenAIServingChat(OpenAIServing):
                     conversation=conversation,
                     chat_template=request.chat_template or self.chat_template,
                     add_generation_prompt=request.add_generation_prompt,
+                    continue_final_message=request.continue_final_message,
                     tools=tool_dicts,
                     documents=request.documents,
                     **(request.chat_template_kwargs or {}),
@@ -361,7 +363,7 @@ class OpenAIServingChat(OpenAIServing):
 
                     # Send response to echo the input portion of the
                     # last message
-                    if request.echo:
+                    if request.echo or request.continue_final_message:
                         last_msg_content: str = ""
                         if conversation and "content" in conversation[
                                 -1] and conversation[-1].get("role") == role:
@@ -716,7 +718,7 @@ class OpenAIServingChat(OpenAIServing):
                 stop_reason=output.stop_reason)
             choices.append(choice_data)
 
-        if request.echo:
+        if request.echo or request.continue_final_message:
             last_msg_content = ""
             if conversation and "content" in conversation[-1] and conversation[
                     -1].get("role") == role:

vllm/entrypoints/openai/serving_tokenization.py

@@ -87,6 +87,7 @@ class OpenAIServingTokenization(OpenAIServing):
                     messages=request.messages,
                     chat_template=self.chat_template,
                     add_generation_prompt=request.add_generation_prompt,
+                    continue_final_message=request.continue_final_message,
                 )
             else:
                 prompt = apply_hf_chat_template(
@@ -94,6 +95,7 @@ class OpenAIServingTokenization(OpenAIServing):
                     conversation=conversation,
                     chat_template=self.chat_template,
                     add_generation_prompt=request.add_generation_prompt,
+                    continue_final_message=request.continue_final_message,
                 )
         else:
             prompt = request.prompt