[Frontend] Fix request length check and add option to disallow auto truncation in scheduler (#2876)

python/sglang/srt/managers/scheduler.py

@@ -78,6 +78,7 @@ from sglang.srt.managers.schedule_policy import (
 from sglang.srt.managers.session_controller import Session
 from sglang.srt.managers.tp_worker import TpModelWorker
 from sglang.srt.managers.tp_worker_overlap_thread import TpModelWorkerClient
+from sglang.srt.managers.utils import validate_input_length
 from sglang.srt.mem_cache.chunk_cache import ChunkCache
 from sglang.srt.mem_cache.radix_cache import RadixCache
 from sglang.srt.metrics.collector import SchedulerMetricsCollector, SchedulerStats
@@ -690,14 +691,16 @@ class Scheduler:
             # By default, only return the logprobs for output tokens
             req.logprob_start_len = len(req.origin_input_ids) - 1
 
-        # Truncate prompts that are too long
-        if len(req.origin_input_ids) > self.max_req_input_len:
-            logger.warning(
-                "Request length is longer than the KV cache pool size or "
-                "the max context length. Truncated. "
-                f"{len(req.origin_input_ids)=}, {self.max_req_input_len=}."
-            )
-            req.origin_input_ids = req.origin_input_ids[: self.max_req_input_len]
+        # Validate prompts length
+        error_msg = validate_input_length(
+            req,
+            self.max_req_input_len,
+            self.server_args.allow_auto_truncate,
+        )
+
+        if error_msg:
+            self.waiting_queue.append(req)
+            return
 
         req.sampling_params.max_new_tokens = min(
             (
@@ -745,13 +748,12 @@ class Scheduler:
         )
         req.tokenizer = self.tokenizer
 
-        # Truncate prompts that are too long
-        if len(req.origin_input_ids) >= self.max_req_input_len:
-            logger.warning(
-                "Request length is longer than the KV cache pool size or "
-                "the max context length. Truncated!!!"
-            )
-            req.origin_input_ids = req.origin_input_ids[: self.max_req_input_len]
+        # Validate prompts length
+        validate_input_length(
+            req,
+            self.max_req_input_len,
+            self.server_args.allow_auto_truncate,
+        )
 
         self.waiting_queue.append(req)
 

python/sglang/srt/managers/tokenizer_manager.py

@@ -292,12 +292,28 @@ class TokenizerManager:
                 SessionParams(**obj.session_params) if obj.session_params else None
             )
 
-        if obj.input_ids is not None and len(input_ids) >= self.context_len:
+        input_token_num = len(input_ids) if input_ids is not None else 0
+        if input_token_num >= self.context_len:
             raise ValueError(
-                f"The input ({len(input_ids)} tokens) is longer than the "
+                f"The input ({input_token_num} tokens) is longer than the "
                 f"model's context length ({self.context_len} tokens)."
             )
 
+        if (
+            obj.sampling_params.get("max_new_tokens") is not None
+            and obj.sampling_params.get("max_new_tokens") + input_token_num
+            >= self.context_len
+        ):
+            raise ValueError(
+                f"Requested token count exceeds the model's maximum context length "
+                f"of {self.context_len} tokens. You requested a total of "
+                f"{obj.sampling_params.get('max_new_tokens') + input_token_num} "
+                f"tokens: {input_token_num} tokens from the input messages and "
+                f"{obj.sampling_params.get('max_new_tokens')} tokens for the "
+                f"completion. Please reduce the number of tokens in the input "
+                f"messages or the completion to fit within the limit."
+            )
+
         # Parse sampling parameters
         sampling_params = SamplingParams(**obj.sampling_params)
         sampling_params.normalize(self.tokenizer)

python/sglang/srt/managers/utils.py

+import logging
+from typing import Optional
+
+from sglang.srt.managers.schedule_batch import FINISH_ABORT, Req
+
+logger = logging.getLogger(__name__)
+
+
+def validate_input_length(
+    req: Req, max_req_input_len: int, allow_auto_truncate: bool
+) -> Optional[str]:
+    """Validate and potentially truncate input length.
+
+    Args:
+        req: The request containing input_ids to validate
+        max_req_input_len: Maximum allowed input length
+        allow_auto_truncate: Whether to truncate long inputs
+
+    Returns:
+        Error message if validation fails, None if successful
+    """
+    if len(req.origin_input_ids) >= max_req_input_len:
+        if allow_auto_truncate:
+            logger.warning(
+                "Request length is longer than the KV cache pool size or "
+                "the max context length. Truncated. "
+                f"{len(req.origin_input_ids)=}, {max_req_input_len=}."
+            )
+            req.origin_input_ids = req.origin_input_ids[:max_req_input_len]
+            return None
+        else:
+            error_msg = (
+                f"Input length ({len(req.origin_input_ids)} tokens) exceeds "
+                f"the maximum allowed length ({max_req_input_len} tokens). "
+                f"Use a shorter input or enable --allow-auto-truncate."
+            )
+            logger.error(error_msg)
+            req.finished_reason = FINISH_ABORT(error_msg)
+            return error_msg
+
+    return None

python/sglang/srt/server_args.py

@@ -157,6 +157,7 @@ class ServerArgs:
     num_continuous_decode_steps: int = 1
     delete_ckpt_after_loading: bool = False
     enable_memory_saver: bool = False
+    allow_auto_truncate: bool = False
 
     def __post_init__(self):
         # Set missing default values
@@ -859,6 +860,11 @@ class ServerArgs:
             action="store_true",
             help="Allow saving memory using release_memory_occupation and resume_memory_occupation",
         )
+        parser.add_argument(
+            "--allow-auto-truncate",
+            action="store_true",
+            help="Allow automatically truncating requests that exceed the maximum input length instead of returning an error.",
+        )
 
     @classmethod
     def from_cli_args(cls, args: argparse.Namespace):

test/srt/run_suite.py

@@ -31,6 +31,7 @@ suites = {
         "test_pytorch_sampling_backend.py",
         "test_radix_attention.py",
         "test_release_memory_occupation.py",
+        "test_request_length_validation.py",
         "test_retract_decode.py",
         "test_server_args.py",
         "test_session_control.py",

test/srt/test_request_length_validation.py

+import unittest
+
+import openai
+
+from sglang.srt.utils import kill_process_tree
+from sglang.test.test_utils import (
+    DEFAULT_SMALL_MODEL_NAME_FOR_TEST,
+    DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+    DEFAULT_URL_FOR_TEST,
+    popen_launch_server,
+)
+
+
+class TestRequestLengthValidation(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.base_url = DEFAULT_URL_FOR_TEST
+        cls.api_key = "sk-123456"
+
+        # Start server with auto truncate disabled
+        cls.process = popen_launch_server(
+            DEFAULT_SMALL_MODEL_NAME_FOR_TEST,
+            cls.base_url,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            api_key=cls.api_key,
+            other_args=("--max-total-tokens", "1000", "--context-length", "100"),
+        )
+
+    @classmethod
+    def tearDownClass(cls):
+        kill_process_tree(cls.process.pid)
+
+    def test_input_length_validation(self):
+        client = openai.Client(api_key=self.api_key, base_url=f"{self.base_url}/v1")
+
+        long_text = "hello " * 100  # Will tokenize to more than context length
+
+        with self.assertRaises(openai.BadRequestError) as cm:
+            client.chat.completions.create(
+                model=DEFAULT_SMALL_MODEL_NAME_FOR_TEST,
+                messages=[
+                    {"role": "user", "content": long_text},
+                ],
+                temperature=0,
+            )
+
+        self.assertIn("is longer than the model's context length", str(cm.exception))
+
+    def test_max_tokens_validation(self):
+        client = openai.Client(api_key=self.api_key, base_url=f"{self.base_url}/v1")
+
+        long_text = "hello "
+
+        with self.assertRaises(openai.BadRequestError) as cm:
+            client.chat.completions.create(
+                model=DEFAULT_SMALL_MODEL_NAME_FOR_TEST,
+                messages=[
+                    {"role": "user", "content": long_text},
+                ],
+                temperature=0,
+                max_tokens=500,
+            )
+
+        self.assertIn(
+            "Requested token count exceeds the model's maximum context",
+            str(cm.exception),
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()