[Misc] clear and separate error messages for input too long and input +...

[Misc] clear and separate error messages for input too long and input + max-tokens too long (#22803)
Signed-off-by: Roger Wang <hey@rogerw.me>

vllm/entrypoints/openai/serving_engine.py

@@ -585,6 +585,8 @@ class OpenAIServing:
                       (EmbeddingChatRequest, EmbeddingCompletionRequest,
                        ScoreRequest, RerankRequest, ClassificationRequest)):
 
+            # Note: input length can be up to the entire model context length
+            # since these requests don't generate tokens.
             if token_num > self.max_model_len:
                 operations: dict[type[AnyRequest], str] = {
                     ScoreRequest: "score",
@@ -613,21 +615,24 @@ class OpenAIServing:
             max_tokens = request.max_completion_tokens or request.max_tokens
         else:
             max_tokens = getattr(request, "max_tokens", None)
-        if max_tokens is None:
+
+        # Note: input length can be up to model context length - 1 for
+        # completion-like requests.
         if token_num >= self.max_model_len:
             raise ValueError(
                 f"This model's maximum context length is "
-                    f"{self.max_model_len} tokens. However, you requested "
-                    f"{token_num} tokens in the messages, "
-                    f"Please reduce the length of the messages.")
-        elif token_num + max_tokens > self.max_model_len:
+                f"{self.max_model_len} tokens. However, your request has "
+                f"{token_num} input tokens. Please reduce the length of "
+                "the input messages.")
+
+        if max_tokens is not None and \
+            token_num + max_tokens > self.max_model_len:
             raise ValueError(
-                f"This model's maximum context length is "
-                f"{self.max_model_len} tokens. However, you requested "
-                f"{max_tokens + token_num} tokens "
-                f"({token_num} in the messages, "
-                f"{max_tokens} in the completion). "
-                f"Please reduce the length of the messages or completion.")
+                "'max_tokens' or 'max_completion_tokens' is too large: "
+                f"{max_tokens}. This model's maximum context length is "
+                f"{self.max_model_len} tokens and your request has "
+                f"{token_num} input tokens ({max_tokens} > {self.max_model_len}"
+                f" - {token_num}).")
 
         return TextTokensPrompt(prompt=input_text, prompt_token_ids=input_ids)