[BugFix] Add support for MTP num_speculative_tokens > 1 with sparse MLA (#34552)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>
Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>
Co-authored-by: Matthew Bonanni <mbonanni@redhat.com>

tests/v1/spec_decode/test_eagle.py

@@ -476,12 +476,12 @@ def test_set_inputs_first_pass_draft_model():
         proposer.max_num_tokens, dtype=torch.bool, device=device
     )
 
-    # Mock the attn_metadata_builder to avoid needing the full model setup
+    # Mock draft_attn_groups to avoid needing the full model setup
     mock_kv_cache_spec = mock.MagicMock()
     mock_kv_cache_spec.block_size = block_size
-    mock_builder = mock.MagicMock()
-    mock_builder.kv_cache_spec = mock_kv_cache_spec
-    proposer.attn_metadata_builder = mock_builder
+    mock_attn_group = mock.MagicMock()
+    mock_attn_group.kv_cache_spec = mock_kv_cache_spec
+    proposer.draft_attn_groups = [mock_attn_group]
 
     # Request 0: query_len=3 (but 1 rejected), Request 1: query_len=2
     batch_spec = BatchSpec(
@@ -616,12 +616,12 @@ def test_set_inputs_first_pass_parallel_drafting():
         proposer.max_num_tokens, dtype=torch.bool, device=device
     )
 
-    # Mock the attn_metadata_builder
+    # Mock draft_attn_groups
     mock_kv_cache_spec = mock.MagicMock()
     mock_kv_cache_spec.block_size = block_size
-    mock_builder = mock.MagicMock()
-    mock_builder.kv_cache_spec = mock_kv_cache_spec
-    proposer.attn_metadata_builder = mock_builder
+    mock_attn_group = mock.MagicMock()
+    mock_attn_group.kv_cache_spec = mock_kv_cache_spec
+    proposer.draft_attn_groups = [mock_attn_group]
 
     # Request 0: query_len=4 (1 rejected), Request 1: query_len=4 (all valid)
     batch_spec = BatchSpec(
@@ -916,7 +916,7 @@ def test_propose(method, attn_backend, num_speculative_tokens, monkeypatch):
     proposer.model = model_mock
 
     # Assign draft attn_layer_names since load_model is not invoked
-    proposer.attn_layer_names = ["layer.0"]
+    proposer._draft_attn_layer_names = {"layer.0"}
 
     # Create input tensors
     batch_spec = BatchSpec(
@@ -961,20 +961,18 @@ def test_propose(method, attn_backend, num_speculative_tokens, monkeypatch):
 
     attn_metadata_builder = attn_metadata_builder_cls(
         kv_cache_spec=create_standard_kv_cache_spec(proposer.vllm_config),
-        layer_names=proposer.attn_layer_names,
+        layer_names=proposer._draft_attn_layer_names,
         vllm_config=proposer.vllm_config,
         device=device,
     )
 
-    # Mock runner for attention metadata building
+    # Mock runner and draft_attn_groups for attention metadata building
     proposer.runner = mock.MagicMock()
-    proposer.runner.attn_groups.append([mock.MagicMock()])
-    proposer.runner.attn_groups[0][
-        0
-    ].get_metadata_builder.return_value = attn_metadata_builder
-    proposer._get_attention_metadata_builder = mock.MagicMock(
-        return_value=attn_metadata_builder
-    )
+    mock_attn_group = mock.MagicMock()
+    mock_attn_group.get_metadata_builder.return_value = attn_metadata_builder
+    mock_attn_group.layer_names = list(proposer._draft_attn_layer_names)
+    mock_attn_group.kv_cache_spec = attn_metadata_builder.kv_cache_spec
+    proposer.draft_attn_groups = [mock_attn_group]
 
     result = proposer.propose(
         target_token_ids=target_token_ids,
@@ -1089,7 +1087,7 @@ def test_propose_tree(spec_token_tree):
     proposer.model = model_mock
 
     # Assign draft attn_layer_names since load_model is not invoked
-    proposer.attn_layer_names = ["layer.0"]
+    proposer._draft_attn_layer_names = {"layer.0"}
 
     # Get the tree attention metadata builder.
     attn_metadata_builder_cls, _ = try_get_attention_backend(
@@ -1097,21 +1095,18 @@ def test_propose_tree(spec_token_tree):
     )
     attn_metadata_builder = attn_metadata_builder_cls(
         kv_cache_spec=create_standard_kv_cache_spec(proposer.vllm_config),
-        layer_names=proposer.attn_layer_names,
+        layer_names=proposer._draft_attn_layer_names,
         vllm_config=proposer.vllm_config,
         device=device,
     )
 
-    # Mock runner for attention metadata building.
+    # Mock runner and draft_attn_groups for attention metadata building.
     proposer.runner = mock.MagicMock()
-    proposer.runner.attn_groups.append([mock.MagicMock()])
-    proposer.runner.attn_groups[0][0].metadata_builders = [attn_metadata_builder]
-    proposer.runner.attn_groups[0][
-        0
-    ].get_metadata_builder.return_value = attn_metadata_builder
-    proposer._get_attention_metadata_builder = mock.MagicMock(
-        return_value=attn_metadata_builder
-    )
+    mock_attn_group = mock.MagicMock()
+    mock_attn_group.get_metadata_builder.return_value = attn_metadata_builder
+    mock_attn_group.layer_names = list(proposer._draft_attn_layer_names)
+    mock_attn_group.kv_cache_spec = attn_metadata_builder.kv_cache_spec
+    proposer.draft_attn_groups = [mock_attn_group]
 
     # Setup inputs for the proposer.
     target_token_ids = torch.randint(0, vocab_size, (total_tokens,), device=device)

tests/v1/spec_decode/test_mtp.py

@@ -162,7 +162,7 @@ def test_mtp_propose(num_speculative_tokens, monkeypatch):
         model_mock.compute_logits.side_effect = logits_returns
 
     proposer.model = model_mock
-    proposer.attn_layer_names = ["layer.0"]
+    proposer._draft_attn_layer_names = {"layer.0"}
 
     # Prepare inputs
     batch_spec = BatchSpec(seq_lens=seq_lens, query_lens=seq_lens)
@@ -190,13 +190,17 @@ def test_mtp_propose(num_speculative_tokens, monkeypatch):
 
     attn_metadata_builder = attn_metadata_builder_cls(
         kv_cache_spec=create_standard_kv_cache_spec(proposer.vllm_config),
-        layer_names=proposer.attn_layer_names,
+        layer_names=list(proposer._draft_attn_layer_names),
         vllm_config=proposer.vllm_config,
         device=device,
     )
 
     proposer.runner = mock.MagicMock()
-    proposer.attn_metadata_builder = attn_metadata_builder
+    mock_attn_group = mock.MagicMock()
+    mock_attn_group.get_metadata_builder.return_value = attn_metadata_builder
+    mock_attn_group.layer_names = list(proposer._draft_attn_layer_names)
+    mock_attn_group.kv_cache_spec = attn_metadata_builder.kv_cache_spec
+    proposer.draft_attn_groups = [mock_attn_group]
 
     # Run propose
     result = proposer.propose(

vllm/model_executor/layers/sparse_attn_indexer.py

@@ -79,6 +79,12 @@ def sparse_attn_indexer(
     has_prefill = attn_metadata.num_prefills > 0
     num_decode_tokens = attn_metadata.num_decode_tokens
 
+    # During speculative decoding, k may be padded to the CUDA graph batch
+    # size while slot_mapping only covers actual tokens. Truncate k to avoid
+    # out-of-bounds reads in the kernel.
+    num_tokens = slot_mapping.shape[0]
+    k = k[:num_tokens]
+
     ops.indexer_k_quant_and_cache(
         k,
         kv_cache,

vllm/v1/attention/backends/mla/indexer.py

@@ -12,6 +12,7 @@ from vllm.utils.deep_gemm import (
     get_paged_mqa_logits_metadata,
     is_deep_gemm_supported,
 )
+from vllm.utils.math_utils import cdiv
 from vllm.utils.platform_utils import num_compute_units
 from vllm.v1.attention.backend import (
     AttentionBackend,
@@ -24,6 +25,7 @@ from vllm.v1.attention.backends.utils import (
     split_decodes_and_prefills,
     split_prefill_chunks,
 )
+from vllm.v1.worker.cp_utils import get_total_cp_world_size
 
 logger = init_logger(__name__)
 
@@ -214,20 +216,39 @@ class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
             if self.vllm_config.speculative_config
             else 0
         )
-        if self.num_speculative_tokens > 1:
-            raise ValueError(
-                "Sparse MLA only supports "
-                "num_speculative_tokens <= 1 because the DeepGEMM "
-                "fp8_paged_mqa_logits kernel does not support next_n > 2. "
-                f"Got num_speculative_tokens={self.num_speculative_tokens}."
-            )
         self.reorder_batch_threshold += self.num_speculative_tokens
 
         sm_count = num_compute_units(self.device.index)
         self.num_sms = sm_count
 
         self.decode_lens_buffer = torch.empty(
-            (scheduler_config.max_num_seqs,), dtype=torch.int32, device=self.device
+            (scheduler_config.max_num_batched_tokens,),
+            dtype=torch.int32,
+            device=self.device,
+        )
+
+        # Pre-allocated buffers for flattening (spec decode).
+        self.arange_buffer = torch.arange(
+            scheduler_config.max_num_seqs * (1 + self.num_speculative_tokens),
+            dtype=torch.int32,
+            device=self.device,
+        )
+        self.expanded_seq_lens_buffer = torch.zeros(
+            (scheduler_config.max_num_batched_tokens,),
+            dtype=torch.int32,
+            device=self.device,
+        )
+        max_num_blocks_per_req = cdiv(
+            self.vllm_config.model_config.max_model_len,
+            self.kv_cache_spec.block_size * get_total_cp_world_size(),
+        )
+        self.expanded_block_table_buffer = torch.zeros(
+            (
+                scheduler_config.max_num_batched_tokens,
+                max_num_blocks_per_req,
+            ),
+            dtype=torch.int32,
+            device=self.device,
         )
 
         # See: DeepGMM/csrc/apis/attention.hpp
@@ -326,42 +347,97 @@ class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
                 common_attn_metadata.query_start_loc_cpu[: num_decodes + 1]
             )
 
-            # Use CPU to avoid GPU sync; breaking async scheduling
-            requires_padding = (decode_lens_cpu.max() > decode_lens_cpu.min()).item()
+            seq_lens = common_attn_metadata.seq_lens[:num_decodes]
+            block_table = common_attn_metadata.block_table_tensor[:num_decodes, ...]
 
-            # Decide which top-k kernel to use based on batch size and sequence length
-            batch_size = num_decodes
-            _is_large_context = common_attn_metadata.max_seq_len > 8192
+            # Padded CUDA graph requests have block_table entries of -1.
+            # Clamp to 0 to prevent OOB access in the DeepGEMM kernel.
+            # This is safe because padded requests have seq_lens=0, so the
+            # kernel produces no meaningful output for those rows.
+            block_table.clamp_(min=0)
 
-            # Decision logic based on micro-benchmark results:
-            # - large_context_topk wins for batch <= 128 and seq_len > 8K
-            # - top_k_per_row_decode wins for batch > 128 or seq_len <= 8K
-            use_large_context_topk = batch_size <= 128 and _is_large_context
+            max_decode_len = int(decode_lens_cpu.max().item())
+            if max_decode_len > 1:
+                # Flatten multi-token decode requests into single-token
+                # batch entries, expanding seq_lens and block tables so
+                # the kernel always sees next_n=1.
+
+                # Assume 4 requests with seq_lens [10, 7, 12, 0] (the final req is
+                # padding) and decode_lens [3, 1, 4, 0] in the below example comments.
+                # The context lengths are therefore
+                # [10-3, 7-1, 12-4, 0-0] = [7, 6, 8, 0].
+
+                # 3 + 1 + 4 + 0 = 8
+                actual_expanded = int(decode_lens_cpu.sum().item())
+
+                # [7, 6, 8, 0] -> [7, 7, 7, 6, 8, 8, 8, 8]
+                expanded_base = torch.repeat_interleave(
+                    seq_lens - decode_lens, decode_lens
+                )
+
+                # [0, 3, 4, 8] -> [0, 0, 0, 3, 4, 4, 4, 4]
+                expanded_starts = torch.repeat_interleave(
+                    common_attn_metadata.query_start_loc[:num_decodes], decode_lens
+                )
 
+                # [0, 1, 2, 0, 0, 1, 2, 3]
+                positions_within = (
+                    self.arange_buffer[:actual_expanded] - expanded_starts
+                )
+
+                # [8, 9, 10, 7, 9, 10, 11, 12, ...] where ... is unused buffer space
+                self.expanded_seq_lens_buffer[:actual_expanded] = (
+                    expanded_base + positions_within + 1
+                )
+                self.expanded_seq_lens_buffer[actual_expanded:] = 0
+                seq_lens = self.expanded_seq_lens_buffer[:num_decode_tokens]
+
+                # Give each of the flattened entries the same block table row as the
+                # original request.
+                self.expanded_block_table_buffer[:actual_expanded] = (
+                    torch.repeat_interleave(block_table, decode_lens, dim=0)
+                )
+                if actual_expanded < num_decode_tokens:
+                    self.expanded_block_table_buffer[
+                        actual_expanded:num_decode_tokens, 0
+                    ] = 0
+                block_table = self.expanded_block_table_buffer[:num_decode_tokens]
+
+                # All reqs now have decode_len=1
+                self.decode_lens_buffer[:num_decode_tokens] = 1
+                decode_lens = self.decode_lens_buffer[:num_decode_tokens]
+                offsets = None
+                batch_size = num_decode_tokens
+            else:
                 next_n = 1 + self.num_speculative_tokens
                 if next_n > 1:
-                offsets = torch.arange(next_n, device=self.device, dtype=torch.int32)
+                    offsets = torch.arange(
+                        next_n, device=self.device, dtype=torch.int32
+                    )
                 else:
                     offsets = None
-
-            seq_lens = common_attn_metadata.seq_lens[:num_decodes]
+                batch_size = num_decodes
 
             # DeepGEMM is required for the paged MQA logits on CUDA devices
             if current_platform.is_cuda() and is_deep_gemm_supported():
                 self.scheduler_metadata_buffer[:] = get_paged_mqa_logits_metadata(
-                    seq_lens, self.kv_cache_spec.block_size, self.num_sms
+                    seq_lens,
+                    self.kv_cache_spec.block_size,
+                    self.num_sms,
                 )
-            block_table = common_attn_metadata.block_table_tensor[:num_decodes, ...]
-            # Padded CUDA graph requests have block_table entries of -1.
-            # Clamp to 0 to prevent OOB access in the DeepGEMM kernel.
-            # This is safe because padded requests have seq_lens=0, so the
-            # kernel produces no meaningful output for those rows.
-            block_table.clamp_(min=0)
+
+            # Decide which top-k kernel to use based on batch size and sequence length
+            # Decision logic based on micro-benchmark results:
+            # - large_context_topk wins for batch <= 128 and seq_len > 8K
+            # - top_k_per_row_decode wins for batch > 128 or seq_len <= 8K
+            _is_large_context = common_attn_metadata.max_seq_len > 8192
+            use_large_context_topk = batch_size <= 128 and _is_large_context
+
             decode_metadata = DeepSeekV32IndexerDecodeMetadata(
                 block_table=block_table,
-                seq_lens=common_attn_metadata.seq_lens[:num_decodes],
+                seq_lens=seq_lens,
                 decode_lens=decode_lens,
-                requires_padding=requires_padding,
+                requires_padding=False,
                 schedule_metadata=self.scheduler_metadata_buffer,
                 use_large_context_topk=use_large_context_topk,
                 offsets=offsets,

vllm/v1/spec_decode/eagle.py

@@ -20,17 +20,13 @@ from vllm.logger import init_logger
 from vllm.model_executor.layers.attention_layer_base import AttentionLayerBase
 from vllm.model_executor.model_loader import get_model
 from vllm.model_executor.models import supports_multimodal
-from vllm.model_executor.models.deepseek_v2 import DeepseekV32IndexerCache
 from vllm.model_executor.models.interfaces import SupportsMultiModal
 from vllm.model_executor.models.llama_eagle3 import Eagle3LlamaForCausalLM
 from vllm.multimodal import MULTIMODAL_REGISTRY
 from vllm.platforms import current_platform
 from vllm.triton_utils import triton
 from vllm.utils.platform_utils import is_pin_memory_available
-from vllm.v1.attention.backend import (
-    AttentionMetadataBuilder,
-    CommonAttentionMetadata,
-)
+from vllm.v1.attention.backend import CommonAttentionMetadata
 from vllm.v1.attention.backends.registry import AttentionBackendEnum
 from vllm.v1.attention.backends.tree_attn import (
     TreeAttentionMetadata,
@@ -38,7 +34,7 @@ from vllm.v1.attention.backends.tree_attn import (
 )
 from vllm.v1.attention.backends.triton_attn import TritonAttentionMetadata
 from vllm.v1.cudagraph_dispatcher import CudagraphDispatcher
-from vllm.v1.kv_cache_interface import KVCacheConfig
+from vllm.v1.kv_cache_interface import KVCacheConfig, UniformTypeKVCacheSpecs
 from vllm.v1.sample.metadata import SamplingMetadata
 from vllm.v1.sample.sampler import _SAMPLING_EPS
 from vllm.v1.spec_decode.metadata import SpecDecodeMetadata
@@ -53,6 +49,7 @@ from vllm.v1.spec_decode.utils import (
 from vllm.v1.utils import CpuGpuBuffer
 from vllm.v1.worker.dp_utils import coordinate_batch_across_dp
 from vllm.v1.worker.gpu_input_batch import CachedRequestState, InputBatch
+from vllm.v1.worker.utils import AttentionGroup
 
 logger = init_logger(__name__)
 
@@ -113,10 +110,8 @@ class SpecDecodeBaseProposer:
             vllm_config.model_config
         )
 
-        self.attn_metadata_builder: AttentionMetadataBuilder | None = None
-        self.draft_indexer_metadata_builder: AttentionMetadataBuilder | None = None
-        self.attn_layer_names: list[str] = []
-        self.indexer_layer_names: list[str] = []
+        self.draft_attn_groups: list[AttentionGroup] = []
+        self.kv_cache_gid: int = -1
         self.eagle3_use_aux_hidden_state: bool = (
             self._get_eagle3_use_aux_hidden_state_from_config()
         )
@@ -353,7 +348,7 @@ class SpecDecodeBaseProposer:
                 self._slot_mapping_buffer[num_actual:num_tokens].fill_(PADDING_SLOT_ID)
 
         view = self._slot_mapping_buffer[:num_tokens]
-        return {name: view for name in self.attn_layer_names + self.indexer_layer_names}
+        return {name: view for name in self._draft_attn_layer_names}
 
     def initialize_cudagraph_keys(self, cudagraph_mode: CUDAGraphMode) -> None:
         """Initialize cudagraph dispatcher keys for eagle.
@@ -420,34 +415,14 @@ class SpecDecodeBaseProposer:
 
         assert self.runner is not None
 
-        if self.attn_metadata_builder is None:
-            attn_metadata_builder = self._get_attention_metadata_builder()
-        else:
-            attn_metadata_builder = self.attn_metadata_builder
-
-        attn_metadata = attn_metadata_builder.build_for_drafting(
+        per_layer_attn_metadata: dict[str, object] = {}
+        for attn_group in self.draft_attn_groups:
+            attn_metadata = attn_group.get_metadata_builder().build_for_drafting(
                 common_attn_metadata=common_attn_metadata, draft_index=0
             )
-        # FIXME: support hybrid kv for draft model (remove separate indexer)
-        if self.draft_indexer_metadata_builder:
-            draft_indexer_metadata = (
-                self.draft_indexer_metadata_builder.build_for_drafting(
-                    common_attn_metadata=common_attn_metadata,
-                    draft_index=0,
-                )
-            )
-        else:
-            draft_indexer_metadata = None
-        # At this moment, we assume all eagle layers belong to the same KV
-        # cache group, thus using the same attention metadata.
-        per_layer_attn_metadata = {}
-        for layer_name in self.attn_layer_names:
+            for layer_name in attn_group.layer_names:
                 per_layer_attn_metadata[layer_name] = attn_metadata
 
-        for layer_name in self.indexer_layer_names:
-            assert draft_indexer_metadata is not None
-            per_layer_attn_metadata[layer_name] = draft_indexer_metadata
-
         cudagraph_runtime_mode, num_input_tokens, num_tokens_across_dp = (
             self._determine_batch_execution_and_padding(num_tokens)
         )
@@ -503,12 +478,7 @@ class SpecDecodeBaseProposer:
             positions = self.mrope_positions[:, token_indices_to_sample]
         else:
             positions = self.positions[token_indices_to_sample]
-        if self.method in (
-            "deepseek_mtp",
-            "ernie_mtp",
-            "longcat_flash_mtp",
-            "pangu_ultra_moe_mtp",
-        ):
+        if self.method == "mtp":
             hidden_states = self.hidden_states[token_indices_to_sample]
         else:
             hidden_states = hidden_states[token_indices_to_sample]
@@ -613,7 +583,8 @@ class SpecDecodeBaseProposer:
                 common_attn_metadata._num_computed_tokens_cpu += 1
 
             # Compute the slot mapping.
-            block_size = attn_metadata_builder.kv_cache_spec.block_size
+            # Use the first draft attention group's kv_cache_spec for block_size
+            block_size = self.draft_attn_groups[0].kv_cache_spec.block_size
             if self.uses_mrope:
                 # all dimensions of positions are the same
                 block_numbers = clamped_positions[0] // block_size
@@ -639,10 +610,12 @@ class SpecDecodeBaseProposer:
             )
 
             # Rebuild attention metadata
-            attn_metadata = attn_metadata_builder.build_for_drafting(  # type: ignore
-                common_attn_metadata=common_attn_metadata, draft_index=token_index + 1
+            for attn_group in self.draft_attn_groups:
+                attn_metadata = attn_group.get_metadata_builder().build_for_drafting(
+                    common_attn_metadata=common_attn_metadata,
+                    draft_index=token_index + 1,
                 )
-            for layer_name in self.attn_layer_names:
+                for layer_name in attn_group.layer_names:
                     per_layer_attn_metadata[layer_name] = attn_metadata
 
             # copy inputs to buffer for cudagraph
@@ -805,18 +778,17 @@ class SpecDecodeBaseProposer:
             # 2.
             # Recompute the slot mapping based on the new positions and
             # rejection mask.
-            builder = (
-                self._get_attention_metadata_builder()
-                if self.attn_metadata_builder is None
-                else self.attn_metadata_builder
-            )
+            # Use the first draft attention group's kv_cache_spec for block_size
+            # (all draft layers share the same kv-cache group)
+            assert len(self.draft_attn_groups) > 0
+            block_size = self.draft_attn_groups[0].kv_cache_spec.block_size
             new_slot_mapping = compute_new_slot_mapping(
                 cad=cad,
                 new_positions=self.positions[:total_num_output_tokens],
                 is_rejected_token_mask=self.is_rejected_token_mask[
                     :total_num_output_tokens
                 ],
-                block_size=builder.kv_cache_spec.block_size,
+                block_size=block_size,
                 num_new_tokens=self.net_num_new_slots_per_request,
                 max_model_len=self.max_model_len,
             )
@@ -1000,9 +972,7 @@ class SpecDecodeBaseProposer:
         | list[dict[str, torch.Tensor]]
         | None = None,
     ) -> list[torch.Tensor]:
-        tree_attn_metadata_builder = self.runner.attn_groups[0][
-            0
-        ].get_metadata_builder()
+        tree_attn_metadata_builder = self.draft_attn_groups[0].get_metadata_builder()
         assert isinstance(tree_attn_metadata_builder, TreeAttentionMetadataBuilder)
 
         total_num_drafts = self.cu_drafts_per_level[0]
@@ -1078,9 +1048,10 @@ class SpecDecodeBaseProposer:
                 common_attn_metadata=common_attn_metadata, draft_index=level + 1
             )
 
-            # Apply new attention metadata to all layers.
+            # Apply new attention metadata to all draft layers.
             per_layer_attn_metadata = {}
-            for layer_name in self.attn_layer_names:
+            for attn_group in self.draft_attn_groups:
+                for layer_name in attn_group.layer_names:
                     per_layer_attn_metadata[layer_name] = attn_metadata
 
             # Consider max model length.
@@ -1288,43 +1259,17 @@ class SpecDecodeBaseProposer:
                 AttentionLayerBase,  # type: ignore[type-abstract]
             ).keys()
         )
-        # FIXME: support hybrid kv for draft model
-        target_indexer_layer_names = set(
-            get_layers_from_vllm_config(
-                self.vllm_config, DeepseekV32IndexerCache
-            ).keys()
-        )
 
         self.model = self._get_model()
 
-        draft_attn_layer_names = (
-            get_layers_from_vllm_config(
+        # Find draft layers (attention layers added by draft model)
+        all_attn_layers = get_layers_from_vllm_config(
             self.vllm_config,
             AttentionLayerBase,  # type: ignore[type-abstract]
-            ).keys()
-            - target_attn_layer_names
-        )
-        indexer_layers = get_layers_from_vllm_config(
-            self.vllm_config, DeepseekV32IndexerCache
-        )
-        draft_indexer_layer_names = indexer_layers.keys() - target_indexer_layer_names
-        self.attn_layer_names = list(draft_attn_layer_names - draft_indexer_layer_names)
-        self.indexer_layer_names = list(draft_indexer_layer_names)
-
-        if self.indexer_layer_names:
-            first_layer = self.indexer_layer_names[0]
-            self.draft_indexer_metadata_builder = (
-                indexer_layers[first_layer]
-                .get_attn_backend()
-                .get_builder_cls()(
-                    indexer_layers[first_layer].get_kv_cache_spec(self.vllm_config),
-                    self.indexer_layer_names,
-                    self.vllm_config,
-                    self.device,
         )
+        self._draft_attn_layer_names = (
+            set(all_attn_layers.keys()) - target_attn_layer_names
         )
-        else:
-            self.draft_indexer_metadata_builder = None
 
         if self.supports_mm_inputs:
             # Even if the target model is multimodal, we can also use
@@ -1562,9 +1507,9 @@ class SpecDecodeBaseProposer:
 
             # Make sure to use EAGLE's own buffer during cudagraph capture.
             if (
-                self.attn_layer_names
+                self._draft_attn_layer_names
                 and slot_mappings is not None
-                and self.attn_layer_names[0] in slot_mappings
+                and next(iter(self._draft_attn_layer_names)) in slot_mappings
             ):
                 slot_mapping_dict = self._get_slot_mapping(num_input_tokens)
             else:
@@ -1594,31 +1539,6 @@ class SpecDecodeBaseProposer:
                     kwargs["hidden_states"] = self.hidden_states[:num_input_tokens]
                 self.model(**kwargs)
 
-    def _get_attention_metadata_builder(self) -> AttentionMetadataBuilder:
-        """Find and return the attention metadata builders for EAGLE layers.
-
-        Returns:
-            The metadata builders for EAGLE layers.
-
-        Raises:
-            AssertionError: If no metadata builders are found for EAGLE layers.
-        """
-        builder = None
-        chosen_layer = self.attn_layer_names[0]
-
-        for kv_cache_group in self.runner.attn_groups:
-            for attn_group in kv_cache_group:
-                if chosen_layer in attn_group.layer_names:
-                    builder = attn_group.get_metadata_builder()
-                    break
-            if builder is not None:
-                break
-
-        assert builder is not None, (
-            "Failed to find attention metadata builder for EAGLE layers."
-        )
-        return builder
-
     def _get_eagle3_use_aux_hidden_state_from_config(self) -> bool:
         """
         Some eagle3 heads (e.g., nvidia/gpt-oss-120b-Eagle3-v2) do not use auxiliary
@@ -1651,13 +1571,71 @@ class SpecDecodeBaseProposer:
                 set(
                     [
                         kv_cache_groups[layer_name]
-                        for layer_name in self.attn_layer_names
+                        for layer_name in self._draft_attn_layer_names
                     ]
                 )
             )
             == 1
         ), "All drafting layers should belong to the same kv cache group"
 
+    def initialize_attn_backend(
+        self,
+        kv_cache_config: KVCacheConfig,
+        kernel_block_sizes: list[int] | None = None,
+    ) -> None:
+        """
+        Initialize AttentionGroups for draft layers using kv_cache_config.
+        Called from the model runner's initialize_metadata_builders.
+        """
+        all_attn_layers = get_layers_from_vllm_config(
+            self.vllm_config,
+            AttentionLayerBase,  # type: ignore[type-abstract]
+        )
+
+        # Find which kv_cache_group the draft layers belong to
+        self.validate_same_kv_cache_group(kv_cache_config)
+        kv_cache_spec = None
+        for gid, group in enumerate(kv_cache_config.kv_cache_groups):
+            if self._draft_attn_layer_names & set(group.layer_names):
+                self.kv_cache_gid = gid
+                kv_cache_spec = group.kv_cache_spec
+                break
+
+        attention_groups: dict[tuple[str, str], AttentionGroup] = {}
+        if kv_cache_spec is not None:
+            for layer_name in self._draft_attn_layer_names:
+                attn_backend = all_attn_layers[layer_name].get_attn_backend()
+                backend_key = attn_backend.full_cls_name()
+                if backend_key not in attention_groups:
+                    layer_kv_cache_spec = kv_cache_spec
+                    if isinstance(layer_kv_cache_spec, UniformTypeKVCacheSpecs):
+                        layer_kv_cache_spec = layer_kv_cache_spec.kv_cache_specs[
+                            layer_name
+                        ]
+
+                    kernel_block_size = (
+                        kernel_block_sizes[self.kv_cache_gid]
+                        if kernel_block_sizes is not None
+                        and self.kv_cache_gid < len(kernel_block_sizes)
+                        else None
+                    )
+                    attn_group = AttentionGroup(
+                        backend=attn_backend,
+                        layer_names=[layer_name],
+                        kv_cache_spec=layer_kv_cache_spec,
+                        kv_cache_group_id=self.kv_cache_gid,
+                    )
+                    attn_group.create_metadata_builders(
+                        self.vllm_config,
+                        self.device,
+                        kernel_block_size=kernel_block_size,
+                    )
+                    attention_groups[backend_key] = attn_group
+                else:
+                    attention_groups[backend_key].layer_names.append(layer_name)
+
+        self.draft_attn_groups = list(attention_groups.values())
+
     def _determine_batch_execution_and_padding(
         self,
         num_tokens: int,

vllm/v1/worker/gpu_model_runner.py

@@ -1936,7 +1936,7 @@ class GPUModelRunner(
 
             if self.speculative_config and spec_decode_common_attn_metadata is None:
                 if isinstance(self.drafter, EagleProposer):
-                    if self.drafter.attn_layer_names[0] in kv_cache_group.layer_names:
+                    if self.drafter.kv_cache_gid == kv_cache_gid:
                         spec_decode_common_attn_metadata = cm
                 else:
                     spec_decode_common_attn_metadata = cm
@@ -5559,6 +5559,14 @@ class GPUModelRunner(
         # because some of them change the threshold at init time.
         self.calculate_reorder_batch_threshold()
 
+        # Initialize drafter attention backend
+        if self.speculative_config and (
+            self.speculative_config.use_eagle()
+            or self.speculative_config.uses_draft_model()
+        ):
+            assert isinstance(self.drafter, EagleProposer | DraftModelProposer)
+            self.drafter.initialize_attn_backend(kv_cache_config, kernel_block_sizes)
+
     def _check_and_update_cudagraph_mode(
         self,
         attention_backends: list[set[type[AttentionBackend]]],
@@ -6079,15 +6087,11 @@ class GPUModelRunner(
             kv_cache_config, kernel_block_sizes
         )
 
-        if self.speculative_config and (
-            self.speculative_config.use_eagle()
-            or self.speculative_config.uses_draft_model()
-            or self.speculative_config.uses_extract_hidden_states()
+        if (
+            self.speculative_config
+            and self.speculative_config.uses_extract_hidden_states()
         ):
-            assert isinstance(
-                self.drafter,
-                EagleProposer | DraftModelProposer | ExtractHiddenStatesProposer,
-            )
+            assert isinstance(self.drafter, ExtractHiddenStatesProposer)
             # validate all draft model layers belong to the same kv cache
             # group
             self.drafter.validate_same_kv_cache_group(kv_cache_config)

vllm/v1/worker/utils.py

@@ -48,7 +48,7 @@ class AttentionGroup:
         self,
         vllm_config,
         device,
-        kernel_block_size: int | None,
+        kernel_block_size: int | None = None,
         num_metadata_builders: int = 1,
     ):
         kv_cache_spec_builder = (