[Bugfix] Respect min_tokens in scheduler stop check (#26317)

Signed-off-by: Elaine Zhao <elaineyz@amazon.com>

tests/v1/core/test_scheduler.py

@@ -497,6 +497,96 @@ def test_stop_via_update_from_output():
     assert list(requests[0].output_token_ids) == [EOS_TOKEN_ID, 10, 11]
 
 
+def test_check_stop_min_tokens():
+    """Test that requests don't stop when min_tokens requirement isn't met."""
+    from vllm.v1.core.sched.utils import check_stop
+
+    # Test case 1: num_output_tokens < min_tokens
+    # Should return False (don't stop)
+    sampling_params = SamplingParams(
+        ignore_eos=False,
+        max_tokens=20,
+        min_tokens=5,
+    )
+    request = Request(
+        request_id="0",
+        prompt_token_ids=[0, 1, 2],
+        sampling_params=sampling_params,
+        pooling_params=None,
+        eos_token_id=EOS_TOKEN_ID,
+    )
+    # Simulate having generated 3 output tokens (less than min_tokens=5)
+    request.append_output_token_ids([10, 11, EOS_TOKEN_ID])  # EOS token present
+
+    result = check_stop(request, max_model_len=100)
+    assert result is False, "Should not stop when num_output_tokens<min_tokens"
+
+    # Test case 2: num_output_tokens >= min_tokens
+    # Should follow normal stopping logic (stop on EOS)
+    request.append_output_token_ids(
+        [
+            10,
+            11,
+            12,
+            13,
+            14,
+            EOS_TOKEN_ID,
+        ]
+    )  # 6 tokens > min_tokens
+
+    result = check_stop(request, max_model_len=100)
+    assert result is True, "Should stop on EOS when min_tokens met"
+    assert request.status == RequestStatus.FINISHED_STOPPED
+
+    # Test case 3: min_tokens = 0, should follow normal stopping logic
+    sampling_params_no_min = SamplingParams(
+        ignore_eos=False,
+        max_tokens=20,
+        min_tokens=0,
+    )
+    request_no_min = Request(
+        request_id="1",
+        prompt_token_ids=[0, 1, 2],
+        sampling_params=sampling_params_no_min,
+        pooling_params=None,
+        eos_token_id=EOS_TOKEN_ID,
+    )
+    request_no_min.append_output_token_ids([10, EOS_TOKEN_ID])
+
+    result = check_stop(request_no_min, max_model_len=100)
+    assert result is True, "Should stop on EOS when min_tokens=0"
+    assert request_no_min.status == RequestStatus.FINISHED_STOPPED
+
+    # Test case 4: min_tokens > 0 with stop token (not EOS)
+    sampling_params_stop = SamplingParams(
+        ignore_eos=False,
+        max_tokens=20,
+        min_tokens=5,
+        stop_token_ids=[42],
+    )
+    request_stop = Request(
+        request_id="2",
+        prompt_token_ids=[0, 1, 2],
+        sampling_params=sampling_params_stop,
+        pooling_params=None,
+        eos_token_id=EOS_TOKEN_ID,
+    )
+    # Only 3 output tokens, less than min_tokens=5, but has stop token
+    request_stop.append_output_token_ids([10, 11, 42])
+    result = check_stop(request_stop, max_model_len=100)
+    assert result is False, "Should not stop when num_output_tokens<min_tokens"
+
+    # Test case 5: min_tokens met, should stop on stop token
+    request_stop.append_output_token_ids(
+        [10, 11, 12, 13, 14, 42]
+    )  # 6 tokens >= min_tokens=5
+
+    result = check_stop(request_stop, max_model_len=100)
+    assert result is True, "Should stop on stop token when min_tokens met"
+    assert request_stop.status == RequestStatus.FINISHED_STOPPED
+    assert request_stop.stop_reason == 42
+
+
 @pytest.mark.parametrize(
     "enable_prefix_caching, prompt_logprobs",
     [

vllm/v1/core/sched/utils.py

@@ -58,6 +58,11 @@ def check_stop(
 
     sampling_params = request.sampling_params
     assert sampling_params is not None
+
+    min_tokens = sampling_params.min_tokens
+    if request.num_output_tokens < min_tokens:
+        return False
+
     last_token_id = request.output_token_ids[-1]
     if not sampling_params.ignore_eos and last_token_id == request.eos_token_id:
         request.status = RequestStatus.FINISHED_STOPPED