[BugFix][Spec Decoding] Fix negative accepted tokens metric crash (#33729)

Signed-off-by: Nick Hill <nickhill123@gmail.com>

tests/v1/core/test_scheduler.py

@@ -885,6 +885,66 @@ def test_schedule_spec_decoding_stats(spec_tokens, output_tokens, expected):
         assert stats.num_accepted_tokens_per_pos == expected[3]
 
 
+def test_spec_decoding_stats_empty_output():
+    """Test that spec decoding stats handle empty output tokens gracefully.
+
+    This is a regression test for a bug where empty sampled_token_ids
+    would cause num_accepted = len([]) - 1 = -1, leading to a
+    ValueError when incrementing a Prometheus counter with a negative value.
+    """
+    num_spec_tokens = 3
+    scheduler = create_scheduler(num_speculative_tokens=num_spec_tokens)
+    requests = create_requests(num_requests=1, num_tokens=1)
+    request = requests[0]
+    req_id = request.request_id
+
+    scheduler.add_request(request)
+
+    # Initial schedule (prefill)
+    output = scheduler.schedule()
+    assert len(output.scheduled_new_reqs) == 1
+
+    # Complete the prefill with a sampled token
+    model_runner_output = ModelRunnerOutput(
+        req_ids=[req_id],
+        req_id_to_index={req_id: 0},
+        sampled_token_ids=[[0]],
+        logprobs=None,
+        prompt_logprobs_dict={},
+        pooler_output=[],
+    )
+    scheduler.update_from_output(output, model_runner_output)
+
+    # Add draft tokens for speculation
+    draft_token_ids = DraftTokenIds([req_id], [[1, 2, 3]])
+    scheduler.update_draft_token_ids(draft_token_ids)
+
+    # Schedule the speculated tokens for validation
+    output = scheduler.schedule()
+    assert req_id in output.scheduled_spec_decode_tokens
+    assert len(output.scheduled_spec_decode_tokens[req_id]) == 3
+
+    # Simulate empty output tokens (e.g., due to request abortion or error)
+    # This would previously cause num_accepted = -1 and crash
+    model_runner_output = ModelRunnerOutput(
+        req_ids=[req_id],
+        req_id_to_index={req_id: 0},
+        sampled_token_ids=[[]],  # Empty output tokens
+        logprobs=None,
+        prompt_logprobs_dict={},
+        pooler_output=[],
+    )
+
+    # This should not raise an error
+    engine_core_outputs = scheduler.update_from_output(output, model_runner_output)
+
+    # Spec decoding stats should be None since no tokens were generated
+    scheduler_stats = (
+        engine_core_outputs[0].scheduler_stats if engine_core_outputs else None
+    )
+    assert scheduler_stats is None or scheduler_stats.spec_decoding_stats is None
+
+
 def _assert_right_scheduler_output(
     output: SchedulerOutput,
     num_requests: int,

vllm/v1/core/sched/scheduler.py

@@ -1276,7 +1276,7 @@ class Scheduler(SchedulerInterface):
             scheduled_spec_token_ids = (
                 scheduler_output.scheduled_spec_decode_tokens.get(req_id)
             )
-            if scheduled_spec_token_ids:
+            if scheduled_spec_token_ids and generated_token_ids:
                 num_draft_tokens = len(scheduled_spec_token_ids)
                 num_accepted = len(generated_token_ids) - 1
                 num_rejected = num_draft_tokens - num_accepted