[Bugfix] Treat generation_config max_tokens as default not ceiling (#34063)

Signed-off-by: almogtavor <almogtavor@gmail.com>

tests/entrypoints/test_utils.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
-from vllm.entrypoints.utils import sanitize_message
+
+from vllm.entrypoints.utils import get_max_tokens, sanitize_message
 
 
 def test_sanitize_message():
@@ -8,3 +9,74 @@ def test_sanitize_message():
         sanitize_message("<_io.BytesIO object at 0x7a95e299e750>")
         == "<_io.BytesIO object>"
     )
+
+
+class TestGetMaxTokens:
+    """Tests for get_max_tokens() to ensure generation_config's max_tokens
+    acts as a default when from model author, and as a ceiling when
+    explicitly set by the user."""
+
+    def test_default_sampling_params_used_when_no_request_max_tokens(self):
+        """When user doesn't specify max_tokens, generation_config default
+        should apply."""
+        result = get_max_tokens(
+            max_model_len=24000,
+            max_tokens=None,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+        )
+        assert result == 2048
+
+    def test_request_max_tokens_not_capped_by_default_sampling_params(self):
+        """When user specifies max_tokens in request, model author's
+        generation_config max_tokens must NOT cap it (fixes #34005)."""
+        result = get_max_tokens(
+            max_model_len=24000,
+            max_tokens=5000,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+        )
+        assert result == 5000
+
+    def test_override_max_tokens_caps_request(self):
+        """When user explicitly sets max_tokens, it acts as a ceiling."""
+        result = get_max_tokens(
+            max_model_len=24000,
+            max_tokens=5000,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+            override_max_tokens=2048,
+        )
+        assert result == 2048
+
+    def test_override_max_tokens_used_as_default(self):
+        """When no request max_tokens, override still applies as default."""
+        result = get_max_tokens(
+            max_model_len=24000,
+            max_tokens=None,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+            override_max_tokens=2048,
+        )
+        assert result == 2048
+
+    def test_max_model_len_still_caps_output(self):
+        """max_model_len - input_length is always the hard ceiling."""
+        result = get_max_tokens(
+            max_model_len=3000,
+            max_tokens=5000,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+        )
+        assert result == 2900  # 3000 - 100
+
+    def test_request_max_tokens_smaller_than_default(self):
+        """When user explicitly requests fewer tokens than gen_config default,
+        that should be respected."""
+        result = get_max_tokens(
+            max_model_len=24000,
+            max_tokens=512,
+            input_length=100,
+            default_sampling_params={"max_tokens": 2048},
+        )
+        assert result == 512