Fix the `--allow-auto-truncate` argument in tokenizer manager. (#9391)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

python/sglang/srt/managers/tokenizer_manager.py

@@ -565,10 +565,20 @@ class TokenizerManager:
         self, obj: Union[GenerateReqInput, EmbeddingReqInput], input_ids: List[int]
     ) -> None:
         """Validates that the input token count and the requested token count doesn't exceed the model's context length."""
+        # FIXME: unify the length validation logic with the one in the scheduler.
+        _max_req_len = self.context_len - 1
 
         input_token_num = len(input_ids) if input_ids is not None else 0
-        # Check if input alone exceeds context length
         if input_token_num >= self.context_len:
+            if self.server_args.allow_auto_truncate:
+                logger.warning(
+                    f"The input ({input_token_num} tokens) is longer than the "
+                    f"model's context length ({self.context_len} tokens). "
+                    "Truncating the input."
+                )
+                input_ids = input_ids[:_max_req_len]
+                input_token_num = len(input_ids)
+            else:
                 raise ValueError(
                     f"The input ({input_token_num} tokens) is longer than the "
                     f"model's context length ({self.context_len} tokens)."
@@ -584,8 +594,18 @@ class TokenizerManager:
         max_new_tokens = obj.sampling_params.get("max_new_tokens")
         if (
             max_new_tokens is not None
-            and (max_new_tokens + input_token_num) >= self.context_len
+            and (max_new_tokens + input_token_num) >= _max_req_len
         ):
+            if self.server_args.allow_auto_truncate:
+                logger.warning(
+                    f"Requested token count ({input_token_num} input + {max_new_tokens} new) "
+                    f"exceeds the model's context length ({self.context_len} tokens). "
+                    "Truncating max_new_tokens."
+                )
+                obj.sampling_params["max_new_tokens"] = max(
+                    0, _max_req_len - input_token_num
+                )
+            else:
                 total_tokens = max_new_tokens + input_token_num
                 error_msg = (
                     f"Requested token count exceeds the model's maximum context length "