fix(glm): classify long DCP profile rows as prefill

.lil/changes/vllm-822.json

@@ -0,0 +1,16 @@
+{
+  "schema": "local-inference-release-change/v1",
+  "id": "vllm-822",
+  "category": "fix",
+  "summary": "Keep long GLM DCP profile prefills out of bounded KDA decode",
+  "models": ["GLM-5.3-Flash"],
+  "compatibility": "No configuration change is required. Valid MTP decode rows retain the B12X KDA decode path.",
+  "details": [
+    "A row enters speculative KDA decode only when its length is one target token plus its declared draft count.",
+    "This prevents a zero-draft marker on a long memory-profile prefill from exceeding the decode kernel capacity during TP4/DCP4 startup."
+  ],
+  "pull_requests": [822],
+  "authors": ["Local Inference Lab"],
+  "evidence": ["https://github.com/local-inference-lab/vllm/pull/822"],
+  "requires": []
+}

tests/v1/attention/test_gdn_metadata_builder.py

@@ -16,6 +16,7 @@
 )
 from vllm.config import SpeculativeConfig
 from vllm.config.compilation import CUDAGraphMode
+from vllm.v1.attention.backends.b12x_gdn_metadata import B12xGdnMixedMetadata
 from vllm.v1.attention.backends.gdn_attn import (
     GDNAttentionMetadata,
     GDNAttentionMetadataBuilder,
@@ -174,6 +175,26 @@ class GDNBuildTestCase:
         expected_num_prefill_tokens=0,
         expected_num_spec_decodes=0,
     ),
+    "long_prefill_with_zero_draft_profile_marker": GDNBuildTestCase(
+        seq_lens=[4112],
+        query_lens=[4096],
+        num_decode_draft_tokens=[0],
+        num_speculative_tokens=3,
+        expected_num_decodes=0,
+        expected_num_prefills=1,
+        expected_num_prefill_tokens=4096,
+        expected_num_spec_decodes=0,
+    ),
+    "single_token_zero_draft_decode": GDNBuildTestCase(
+        seq_lens=[33],
+        query_lens=[1],
+        num_decode_draft_tokens=[0],
+        num_speculative_tokens=3,
+        expected_num_decodes=1,
+        expected_num_prefills=0,
+        expected_num_prefill_tokens=0,
+        expected_num_spec_decodes=0,
+    ),
     # Multi-token prefill alongside spec decode — no decode to reclassify
     "spec_decode_with_real_prefill": GDNBuildTestCase(
         seq_lens=[100, 20],
@@ -442,6 +463,24 @@ def test_gdn_build_classification(test_case: GDNBuildTestCase):
     assert meta.num_spec_decodes == test_case.expected_num_spec_decodes
 
 
+def test_b12x_mixed_metadata_keeps_zero_draft_profile_in_prefill() -> None:
+    builder = _create_gdn_builder(num_speculative_tokens=3)
+    builder._b12x_mixed = B12xGdnMixedMetadata(
+        max_tokens=4096, max_seqs=1, state_columns=4, device=DEVICE
+    )
+    metadata = _build(
+        builder,
+        BatchSpec(seq_lens=[4112], query_lens=[4096]),
+        num_decode_draft_tokens=[0],
+    )
+
+    assert metadata.num_prefills == 1
+    assert metadata.num_spec_decodes == 0
+    assert metadata.b12x_mixed is not None
+    assert metadata.b12x_mixed._num_non_spec == 1
+    assert metadata.b12x_mixed._num_spec == 0
+
+
 def test_fresh_single_token_prompt_uses_prefill_state_initialization() -> None:
     builder = _create_gdn_builder()
     batch = BatchSpec(seq_lens=[1], query_lens=[1])

vllm/v1/attention/backends/b12x_gdn_metadata.py

@@ -100,7 +100,11 @@ def stage(
         for row, (start, end) in enumerate(zip(starts, starts[1:])):
             if end == start:
                 continue
-            if drafts is not None and drafts[row] >= 0 and end - start > 1:
+            if (
+                drafts is not None
+                and drafts[row] >= 0
+                and end - start == drafts[row] + 1
+            ):
                 if end - start > self.state_columns:
                     raise ValueError("GDN verification exceeds planned state columns")
                 spec_rows.append(row)

vllm/v1/attention/backends/gdn_attn.py

@@ -475,7 +475,15 @@ def build(  # type: ignore[override]
             spec_sequence_masks = None
             num_spec_decodes = 0
         else:
-            spec_sequence_masks_cpu = num_decode_draft_tokens_cpu >= 0
+            # A speculative row contains exactly one target token followed by
+            # its draft tokens. Profiling may provide a zero-draft marker for
+            # a long prefill, which must not enter the bounded decode kernel.
+            query_lens_cpu = query_start_loc_cpu.diff()
+            spec_sequence_masks_cpu = (
+                (num_decode_draft_tokens_cpu >= 0)
+                & (num_decode_draft_tokens_cpu <= self.num_spec)
+                & (query_lens_cpu == num_decode_draft_tokens_cpu + 1)
+            )
             num_spec_decodes = spec_sequence_masks_cpu.sum().item()
             # A zero-draft varlen batch still recovers the previous step's
             # accepted recurrent state before consuming its bonus tokens.