[Fix] Fix Qwen3.5 MTP startup with HiCache (#34560)

Co-authored-by: hjzhang <76768149+1e4ves@users.noreply.github.com>
Co-authored-by: YAMY <74099316+YAMY1234@users.noreply.github.com>

python/sglang/srt/configs/model_config.py

@@ -707,6 +707,7 @@ class ModelConfig:
                 )
             self.hf_config.architectures[0] = "Qwen3_5ForCausalLMMTP"
             self.hf_config.num_nextn_predict_layers = 1
+            self.hf_text_config.num_nextn_predict_layers = 1
 
         if is_draft_model and self.hf_config.architectures[0] == "ExaoneMoEForCausalLM":
             self.hf_config.architectures[0] = "ExaoneMoEForCausalLMMTP"

python/sglang/srt/mem_cache/hybrid_cache/hybrid_pool_assembler.py

@@ -930,9 +930,15 @@ def build_full_draft_pools(
     server_args: ServerArgs,
 ) -> tuple[list[SidecarPoolSpec], list[PoolEntry]]:
     """Build draft KV/DSA sidecars whose indices follow target full KV."""
-    from sglang.srt.mem_cache.memory_pool import DSATokenToKVPool
+    from sglang.srt.mem_cache.memory_pool import (
+        DSATokenToKVPool,
+        HybridLinearKVPool,
+    )
 
     pool = draft_kv_pool
+    if isinstance(pool, HybridLinearKVPool):
+        # Hybrid draft runners keep their sole attention layer in this sub-pool.
+        pool = pool.full_kv_pool
     if pool.layer_num == 0:
         return [], []
 

test/registered/hicache/test_qwen35_hicache.py

@@ -57,6 +57,14 @@ class TestQwen35WithHiCache(CustomTestCase):
                 "128",
                 "--mamba-ssm-dtype",
                 "bfloat16",
+                "--speculative-algorithm",
+                "NEXTN",
+                "--speculative-num-steps",
+                "3",
+                "--speculative-eagle-topk",
+                "1",
+                "--speculative-num-draft-tokens",
+                "4",
                 "--max-running-requests",
                 "128",
                 "--reasoning-parser",

test/registered/unit/configs/test_model_config.py

@@ -4,6 +4,7 @@ import unittest
 from types import SimpleNamespace
 
 from sglang.srt.configs.model_config import (
+    ModelConfig,
     get_hybrid_layer_ids,
     is_embedding_gemma,
 )
@@ -52,5 +53,22 @@ class TestEmbeddingGemmaConfig(CustomTestCase):
         self.assertFalse(is_embedding_gemma(config))
 
 
+class TestDraftModelConfig(CustomTestCase):
+    def test_qwen35_mtp_depth_is_synced_to_text_config(self):
+        config = object.__new__(ModelConfig)
+        config.is_draft_model = True
+        config.speculative_algorithm = "EAGLE"
+        config.hf_config = SimpleNamespace(
+            architectures=["Qwen3_5MoeForConditionalGeneration"]
+        )
+        config.hf_text_config = SimpleNamespace()
+
+        config._config_draft_model()
+
+        self.assertEqual(config.hf_config.architectures, ["Qwen3_5ForCausalLMMTP"])
+        self.assertEqual(config.hf_config.num_nextn_predict_layers, 1)
+        self.assertEqual(config.hf_text_config.num_nextn_predict_layers, 1)
+
+
 if __name__ == "__main__":
     unittest.main()

test/registered/unit/mem_cache/test_hybrid_pool_assembler.py

@@ -1,8 +1,13 @@
-"""Unit test for hybrid HiCache fixed-size budget splitting."""
+"""Unit tests for hybrid HiCache pool assembly."""
 
 import unittest
+from types import SimpleNamespace
 
-from sglang.srt.mem_cache.hybrid_cache.hybrid_pool_assembler import _split_hicache_size
+from sglang.srt.mem_cache.hybrid_cache.hybrid_pool_assembler import (
+    _split_hicache_size,
+    build_full_draft_pools,
+)
+from sglang.srt.mem_cache.memory_pool import HybridLinearKVPool
 from sglang.test.ci.ci_register import register_cpu_ci
 from sglang.test.test_utils import CustomTestCase
 
@@ -35,5 +40,20 @@ class TestSplitHicacheSize(CustomTestCase):
         self.assertEqual(sum(shares), 100)  # total budget preserved, not doubled
 
 
+class TestDraftSidecarPoolDispatch(CustomTestCase):
+    def test_full_builder_unwraps_empty_hybrid_linear_pool(self):
+        draft_kv_pool = object.__new__(HybridLinearKVPool)
+        draft_kv_pool.full_kv_pool = SimpleNamespace(layer_num=0)
+
+        specs, entries = build_full_draft_pools(
+            draft_kv_pool=draft_kv_pool,
+            tree_cache=None,
+            server_args=None,
+        )
+
+        self.assertEqual(specs, [])
+        self.assertEqual(entries, [])
+
+
 if __name__ == "__main__":
     unittest.main()