[Bugfix] Fix negative max_tokens when input prompt is too long (#36789)

Signed-off-by: Isotr0py <mozf@mail2.sysu.edu.cn>

tests/entrypoints/test_utils.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import pytest
+
 from vllm.entrypoints.utils import get_max_tokens, sanitize_message
 
 
@@ -80,3 +82,15 @@ class TestGetMaxTokens:
             default_sampling_params={"max_tokens": 2048},
         )
         assert result == 512
+
+    def test_input_length_exceeds_max_model_len(self):
+        with pytest.raises(
+            ValueError,
+            match="Input length .* exceeds model's maximum context length .*",
+        ):
+            get_max_tokens(
+                max_model_len=100,
+                max_tokens=50,
+                input_length=150,
+                default_sampling_params={"max_tokens": 2048},
+            )

vllm/entrypoints/utils.py

@@ -178,6 +178,11 @@ def get_max_tokens(
     default_sampling_params: dict,
     override_max_tokens: int | None = None,
 ) -> int:
+    if max_model_len < input_length:
+        raise ValueError(
+            f"Input length ({input_length}) exceeds model's maximum "
+            f"context length ({max_model_len})."
+        )
     model_max_tokens = max_model_len - input_length
     platform_max_tokens = current_platform.get_max_output_tokens(input_length)
     fallback_max_tokens = (