check user-specified model_max_len with hf derived max_model_len (#1778)

python/sglang/srt/configs/model_config.py

@@ -13,6 +13,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 """
 
+import logging
+import os
 from enum import IntEnum, auto
 from typing import Optional
 
@@ -20,6 +22,8 @@ from transformers import PretrainedConfig
 
 from sglang.srt.hf_transformers_utils import get_config, get_context_length
 
+logger = logging.getLogger(__name__)
+
 
 class AttentionArch(IntEnum):
     MLA = auto()
@@ -46,10 +50,29 @@ class ModelConfig:
             model_override_args=model_override_args,
         )
         self.hf_text_config = get_hf_text_config(self.hf_config)
+        derived_context_len = get_context_length(self.hf_text_config)
+        allow_long_context = os.environ.get(
+            "SGLANG_ALLOW_OVERWRITE_LONGER_CONTEXT_LEN", None
+        )
+
         if context_length is not None:
-            self.context_len = context_length
+            if context_length > derived_context_len:
+                if allow_long_context:
+                    logger.warning(
+                        f"Warning: User-specified context_length ({context_length}) is greater than the derived context_length ({derived_context_len}). "
+                        f"This may lead to incorrect model outputs or CUDA errors."
+                    )
+                    self.context_len = context_length
+                else:
+                    raise ValueError(
+                        f"User-specified context_length ({context_length}) is greater than the derived context_length ({derived_context_len}). "
+                        f"This may lead to incorrect model outputs or CUDA errors. Note that the derived context_length may differ from max_position_embeddings in the model's config. "
+                        f"To allow overriding this maximum, set the env var SGLANG_ALLOW_OVERWRITE_LONGER_CONTEXT_LEN=1"
+                    )
+            else:
+                self.context_len = context_length
         else:
-            self.context_len = get_context_length(self.hf_text_config)
+            self.context_len = derived_context_len
 
         # Unify the config keys for hf_text_config
         self.head_dim = getattr(