diff --git a/tests/spec_decode/e2e/conftest.py b/tests/spec_decode/e2e/conftest.py
index 59fb8311fc5b..5d3469c4210e 100644
--- a/tests/spec_decode/e2e/conftest.py
+++ b/tests/spec_decode/e2e/conftest.py
@@ -1,10 +1,127 @@
-from typing import List, Tuple
+import asyncio
+from typing import List, Optional, Tuple, Union
 
 import pytest
+import ray
 
 from tests.conftest import cleanup
 from vllm import LLM
+from vllm.engine.arg_utils import AsyncEngineArgs
+from vllm.engine.async_llm_engine import AsyncLLMEngine
+from vllm.lora.request import LoRARequest
 from vllm.model_executor.utils import set_random_seed
+from vllm.outputs import RequestOutput
+from vllm.sampling_params import SamplingParams
+from vllm.sequence import MultiModalData
+from vllm.usage.usage_lib import UsageContext
+from vllm.utils import Counter, random_uuid
+
+
+class AsyncLLM:
+    """AsyncLLM
+
+    Note: Current LLM class in vllm don't support async mode, for test purpose,
+    we implement async one in here. Maybe we could move to
+    vllm/entrypoints/llm.py in future.
+
+    Below AsyncLLM is directly borrow from vllm/entrypoints/llm.py with changes
+    to make to work in async mode.
+    """
+
+    def __init__(
+        self,
+        model: str,
+        tokenizer: Optional[str] = None,
+        tokenizer_mode: str = "auto",
+        skip_tokenizer_init: bool = False,
+        trust_remote_code: bool = False,
+        tensor_parallel_size: int = 1,
+        dtype: str = "auto",
+        quantization: Optional[str] = None,
+        revision: Optional[str] = None,
+        tokenizer_revision: Optional[str] = None,
+        seed: int = 0,
+        gpu_memory_utilization: float = 0.9,
+        swap_space: int = 4,
+        enforce_eager: bool = False,
+        max_context_len_to_capture: int = 8192,
+        disable_custom_all_reduce: bool = False,
+        **kwargs,
+    ) -> None:
+        if "disable_log_stats" not in kwargs:
+            kwargs["disable_log_stats"] = True
+        self.engine_args = AsyncEngineArgs(
+            model=model,
+            tokenizer=tokenizer,
+            tokenizer_mode=tokenizer_mode,
+            skip_tokenizer_init=skip_tokenizer_init,
+            trust_remote_code=trust_remote_code,
+            tensor_parallel_size=tensor_parallel_size,
+            dtype=dtype,
+            quantization=quantization,
+            revision=revision,
+            tokenizer_revision=tokenizer_revision,
+            seed=seed,
+            gpu_memory_utilization=gpu_memory_utilization,
+            swap_space=swap_space,
+            enforce_eager=enforce_eager,
+            max_context_len_to_capture=max_context_len_to_capture,
+            engine_use_ray=True,
+            disable_custom_all_reduce=disable_custom_all_reduce,
+            **kwargs,
+        )
+        self.request_counter = Counter()
+
+    def generate(
+        self,
+        prompts: Optional[Union[str, List[str]]] = None,
+        sampling_params: Optional[Union[SamplingParams,
+                                        List[SamplingParams]]] = None,
+        prompt_token_ids: Optional[List[List[int]]] = None,
+        use_tqdm: bool = True,
+        lora_request: Optional[LoRARequest] = None,
+        multi_modal_data: Optional[MultiModalData] = None,
+    ) -> List[RequestOutput]:
+
+        llm_engine = AsyncLLMEngine.from_engine_args(
+            self.engine_args, usage_context=UsageContext.LLM_CLASS)
+
+        if prompts is None:
+            raise ValueError("prompts must be provided.")
+        if isinstance(prompts, str):
+            # Convert a single prompt to a list.
+            prompts = [prompts]
+
+        if prompts is not None:
+            num_requests = len(prompts)
+
+        if sampling_params is None:
+            # Use default sampling params.
+            sampling_params = SamplingParams()
+
+        elif isinstance(sampling_params,
+                        list) and len(sampling_params) != num_requests:
+            raise ValueError("The lengths of prompts and "
+                             "sampling_params must be the same.")
+
+        async def get_output(prompt, sampling_param) -> str:
+            request_id = random_uuid()
+            results_generator = llm_engine.generate(prompt, sampling_param,
+                                                    request_id)
+            final_output = None
+            async for request_output in results_generator:
+                final_output = request_output
+            return final_output
+
+        outputs = []
+        try:
+            for i in range(num_requests):
+                prompt = prompts[i] if prompts is not None else None
+                res = asyncio.run(get_output(prompt, sampling_params))
+                outputs.append(res)
+        finally:
+            ray.shutdown()
+        return outputs
 
 
 @pytest.fixture
@@ -36,8 +153,12 @@ def create_llm_generator(baseline_or_test, request, common_llm_kwargs,
 
     def generator_inner():
         print(f'Creating {baseline_or_test=} LLM for {test_name=}. {kwargs=}')
-        llm = LLM(**kwargs)
 
+        use_async = False
+        if "use_async" in kwargs:
+            use_async = kwargs.pop("use_async")
+
+        llm = AsyncLLM(**kwargs) if use_async else LLM(**kwargs)
         set_random_seed(seed)
 
         yield llm
diff --git a/tests/spec_decode/e2e/test_compatibility.py b/tests/spec_decode/e2e/test_compatibility.py
index fde950c14382..60c20ed7db7a 100644
--- a/tests/spec_decode/e2e/test_compatibility.py
+++ b/tests/spec_decode/e2e/test_compatibility.py
@@ -42,10 +42,17 @@ def test_spec_decode_xfail_ray(test_llm_generator):
         temperature=temperature,
     )
 
-    with pytest.raises(AssertionError,
-                       match="Speculative decoding not yet supported for "):
-        get_output_from_llm_generator(test_llm_generator, prompts,
-                                      sampling_params)
+    try:
+        with pytest.raises(
+                AssertionError,
+                match="Speculative decoding not yet supported for "):
+            get_output_from_llm_generator(test_llm_generator, prompts,
+                                          sampling_params)
+    finally:
+        # we need to free up ray resource,
+        # so that latter test could use the gpu we allocated here
+        import ray
+        ray.shutdown()
 
 
 @pytest.mark.parametrize(
diff --git a/tests/spec_decode/e2e/test_correctness.py b/tests/spec_decode/e2e/test_correctness.py
index 0536cc4ecde7..ab8d913fb894 100644
--- a/tests/spec_decode/e2e/test_correctness.py
+++ b/tests/spec_decode/e2e/test_correctness.py
@@ -40,17 +40,24 @@
 
 @pytest.mark.parametrize(
     "common_llm_kwargs",
-    [{
-        # Use a small model for a fast test.
-        # Note this is repeated in the test body; to initialize a tokenizer.
-        "model": "JackFram/llama-68m",
+    [
+        {
+            # Use a small model for a fast test.
+            # Note this is repeated in the test body; to initialize a tokenizer.
+            "model": "JackFram/llama-68m",
 
-        # Skip cuda graph recording for fast test.
-        "enforce_eager": True,
+            # Skip cuda graph recording for fast test.
+            "enforce_eager": True,
 
-        # Required for spec decode.
-        "use_v2_block_manager": True
-    }])
+            # Required for spec decode.
+            "use_v2_block_manager": True,
+
+            # whether use AsyncLLM engine
+            "use_async": async_mode,
+        }
+        # Try both async and sync engine execution
+        for async_mode in [True, False]
+    ])
 @pytest.mark.parametrize(
     "per_test_common_llm_kwargs",
     [
diff --git a/vllm/engine/async_llm_engine.py b/vllm/engine/async_llm_engine.py
index 7c1eb2ecbe55..4aceb19b5077 100644
--- a/vllm/engine/async_llm_engine.py
+++ b/vllm/engine/async_llm_engine.py
@@ -211,9 +211,11 @@ async def step_async(self) -> List[RequestOutput]:
         if not scheduler_outputs.is_empty():
             # Execute the model.
             output = await self.model_executor.execute_model_async(
-                seq_group_metadata_list, scheduler_outputs.blocks_to_swap_in,
+                seq_group_metadata_list,
+                scheduler_outputs.blocks_to_swap_in,
                 scheduler_outputs.blocks_to_swap_out,
-                scheduler_outputs.blocks_to_copy)
+                scheduler_outputs.blocks_to_copy,
+                num_lookahead_slots=scheduler_outputs.num_lookahead_slots)
         else:
             output = []
 
diff --git a/vllm/executor/cpu_executor.py b/vllm/executor/cpu_executor.py
index e4436b2144bd..da1b500cddaf 100644
--- a/vllm/executor/cpu_executor.py
+++ b/vllm/executor/cpu_executor.py
@@ -109,12 +109,14 @@ async def execute_model_async(
         blocks_to_swap_in: Dict[int, int],
         blocks_to_swap_out: Dict[int, int],
         blocks_to_copy: Dict[int, List[int]],
+        num_lookahead_slots: int,
     ) -> List[SamplerOutput]:
         output = await make_async(self.driver_worker.execute_model)(
             seq_group_metadata_list=seq_group_metadata_list,
             blocks_to_swap_in=blocks_to_swap_in,
             blocks_to_swap_out=blocks_to_swap_out,
-            blocks_to_copy=blocks_to_copy)
+            blocks_to_copy=blocks_to_copy,
+            num_lookahead_slots=num_lookahead_slots)
         return output
 
     async def check_health_async(self) -> None:
diff --git a/vllm/executor/executor_base.py b/vllm/executor/executor_base.py
index c36aa18fb25b..96cd18250bb3 100644
--- a/vllm/executor/executor_base.py
+++ b/vllm/executor/executor_base.py
@@ -112,6 +112,7 @@ async def execute_model_async(
         blocks_to_swap_in: Dict[int, int],
         blocks_to_swap_out: Dict[int, int],
         blocks_to_copy: Dict[int, List[int]],
+        num_lookahead_slots: int,
     ) -> List[SamplerOutput]:
         """Executes one model step on the given sequences."""
         raise NotImplementedError
diff --git a/vllm/executor/gpu_executor.py b/vllm/executor/gpu_executor.py
index 5ac62f02b99c..489e66d58602 100644
--- a/vllm/executor/gpu_executor.py
+++ b/vllm/executor/gpu_executor.py
@@ -163,10 +163,12 @@ async def execute_model_async(
         blocks_to_swap_in: Dict[int, int],
         blocks_to_swap_out: Dict[int, int],
         blocks_to_copy: Dict[int, List[int]],
+        num_lookahead_slots: int,
     ) -> List[SamplerOutput]:
         output = await make_async(self.driver_worker.execute_model)(
             seq_group_metadata_list=seq_group_metadata_list,
             blocks_to_swap_in=blocks_to_swap_in,
             blocks_to_swap_out=blocks_to_swap_out,
-            blocks_to_copy=blocks_to_copy)
+            blocks_to_copy=blocks_to_copy,
+            num_lookahead_slots=num_lookahead_slots)
         return output
diff --git a/vllm/executor/neuron_executor.py b/vllm/executor/neuron_executor.py
index f406287f3c1d..8a3b9cde8431 100644
--- a/vllm/executor/neuron_executor.py
+++ b/vllm/executor/neuron_executor.py
@@ -84,6 +84,7 @@ async def execute_model_async(
         blocks_to_swap_in: Dict[int, int],
         blocks_to_swap_out: Dict[int, int],
         blocks_to_copy: Dict[int, List[int]],
+        num_lookahead_slots: int,
     ) -> List[SamplerOutput]:
         output = await make_async(self.driver_worker.execute_model)(
             seq_group_metadata_list=seq_group_metadata_list, )
diff --git a/vllm/executor/ray_gpu_executor.py b/vllm/executor/ray_gpu_executor.py
index b6bcda4e6b18..3eb3726bd5a6 100644
--- a/vllm/executor/ray_gpu_executor.py
+++ b/vllm/executor/ray_gpu_executor.py
@@ -196,6 +196,7 @@ def execute_model(self,
                 "blocks_to_swap_in": blocks_to_swap_in,
                 "blocks_to_swap_out": blocks_to_swap_out,
                 "blocks_to_copy": blocks_to_copy,
+                "num_lookahead_slots": num_lookahead_slots,
             },
             use_ray_compiled_dag=USE_RAY_COMPILED_DAG)