[Spec decode] automatically disable mm for text-only draft models (#25667)

Signed-off-by: Jonas Kuebler <kuebj@amazon.com>

tests/v1/e2e/test_spec_decode.py

@@ -8,7 +8,7 @@ from typing import Any, Union
 import pytest
 import torch
 
-from tests.utils import get_attn_backend_list_based_on_platform
+from tests.utils import get_attn_backend_list_based_on_platform, large_gpu_mark
 from vllm import LLM, SamplingParams
 from vllm.assets.base import VLLM_S3_BUCKET_URL
 from vllm.assets.image import VLM_IMAGES_DIR
@@ -88,69 +88,66 @@ def test_ngram_correctness(
     Compare the outputs of an original LLM and a speculative LLM
     should be the same when using ngram speculative decoding.
     '''
-    with monkeypatch.context() as m:
-        m.setenv("VLLM_USE_V1", "1")
-        test_prompts = get_test_prompts(mm_enabled=False)
-
-        ref_llm = LLM(model=model_name, max_model_len=1024)
-        ref_outputs = ref_llm.chat(test_prompts, sampling_config)
-        del ref_llm
-        torch.cuda.empty_cache()
-        cleanup_dist_env_and_memory()
-
-        spec_llm = LLM(
-            model=model_name,
-            speculative_config={
-                "method": "ngram",
-                "prompt_lookup_max": 5,
-                "prompt_lookup_min": 3,
-                "num_speculative_tokens": 3,
-            },
-            max_model_len=1024,
-        )
-        spec_outputs = spec_llm.chat(test_prompts, sampling_config)
-        matches = 0
-        misses = 0
-        for ref_output, spec_output in zip(ref_outputs, spec_outputs):
-            if ref_output.outputs[0].text == spec_output.outputs[0].text:
-                matches += 1
-            else:
-                misses += 1
-                print(f"ref_output: {ref_output.outputs[0].text}")
-                print(f"spec_output: {spec_output.outputs[0].text}")
-
-        # Heuristic: expect at least 66% of the prompts to match exactly
-        # Upon failure, inspect the outputs to check for inaccuracy.
-        assert matches >= int(0.66 * len(ref_outputs))
-        del spec_llm
-        torch.cuda.empty_cache()
-        cleanup_dist_env_and_memory()
-
-
-@pytest.mark.parametrize(["model_setup", "mm_enabled"], [
-    (("eagle3", "Qwen/Qwen3-8B", "AngelSlim/Qwen3-8B_eagle3", 1), False),
-    (("eagle", "meta-llama/Llama-3.1-8B-Instruct",
-      "yuhuili/EAGLE-LLaMA3.1-Instruct-8B", 1), False),
-    (("eagle3", "meta-llama/Llama-3.1-8B-Instruct",
-      "yuhuili/EAGLE3-LLaMA3.1-Instruct-8B", 1), False),
-    pytest.param(
-        ("eagle", "meta-llama/Llama-4-Scout-17B-16E-Instruct",
-         "morgendave/EAGLE-Llama-4-Scout-17B-16E-Instruct", 4),
-        False,
-        marks=pytest.mark.skip(reason="Skipping due to CI OOM issues")),
-    pytest.param(
-        ("eagle", "meta-llama/Llama-4-Scout-17B-16E-Instruct",
-         "morgendave/EAGLE-Llama-4-Scout-17B-16E-Instruct", 4),
-        True,
-        marks=pytest.mark.skip(reason="Skipping due to CI OOM issues")),
-    (("eagle", "eagle618/deepseek-v3-random",
-      "eagle618/eagle-deepseek-v3-random", 1), False),
-],
-                         ids=[
-                             "qwen3_eagle3", "llama3_eagle", "llama3_eagle3",
-                             "llama4_eagle", "llama4_eagle_mm",
-                             "deepseek_eagle"
-                         ])
+    test_prompts = get_test_prompts(mm_enabled=False)
+
+    ref_llm = LLM(model=model_name, max_model_len=1024)
+    ref_outputs = ref_llm.chat(test_prompts, sampling_config)
+    del ref_llm
+    torch.cuda.empty_cache()
+    cleanup_dist_env_and_memory()
+
+    spec_llm = LLM(
+        model=model_name,
+        speculative_config={
+            "method": "ngram",
+            "prompt_lookup_max": 5,
+            "prompt_lookup_min": 3,
+            "num_speculative_tokens": 3,
+        },
+        max_model_len=1024,
+    )
+    spec_outputs = spec_llm.chat(test_prompts, sampling_config)
+    matches = 0
+    misses = 0
+    for ref_output, spec_output in zip(ref_outputs, spec_outputs):
+        if ref_output.outputs[0].text == spec_output.outputs[0].text:
+            matches += 1
+        else:
+            misses += 1
+            print(f"ref_output: {ref_output.outputs[0].text}")
+            print(f"spec_output: {spec_output.outputs[0].text}")
+
+    # Heuristic: expect at least 66% of the prompts to match exactly
+    # Upon failure, inspect the outputs to check for inaccuracy.
+    assert matches >= int(0.66 * len(ref_outputs))
+    del spec_llm
+    torch.cuda.empty_cache()
+    cleanup_dist_env_and_memory()
+
+
+@pytest.mark.parametrize(
+    ["model_setup", "mm_enabled"],
+    [
+        (("eagle3", "Qwen/Qwen3-8B", "AngelSlim/Qwen3-8B_eagle3", 1), False),
+        (("eagle", "meta-llama/Llama-3.1-8B-Instruct",
+          "yuhuili/EAGLE-LLaMA3.1-Instruct-8B", 1), False),
+        (("eagle3", "meta-llama/Llama-3.1-8B-Instruct",
+          "yuhuili/EAGLE3-LLaMA3.1-Instruct-8B", 1), False),
+        pytest.param(("eagle", "meta-llama/Llama-4-Scout-17B-16E-Instruct",
+                      "morgendave/EAGLE-Llama-4-Scout-17B-16E-Instruct", 4),
+                     False,
+                     marks=large_gpu_mark(min_gb=80)),  # works on 4x H100
+        pytest.param(("eagle", "meta-llama/Llama-4-Scout-17B-16E-Instruct",
+                      "morgendave/EAGLE-Llama-4-Scout-17B-16E-Instruct", 4),
+                     True,
+                     marks=large_gpu_mark(min_gb=80)),  # works on 4x H100
+        (("eagle", "eagle618/deepseek-v3-random",
+          "eagle618/eagle-deepseek-v3-random", 1), False),
+    ],
+    ids=[
+        "qwen3_eagle3", "llama3_eagle", "llama3_eagle3", "llama4_eagle",
+        "llama4_eagle_mm", "deepseek_eagle"
+    ])
 @pytest.mark.parametrize("attn_backend",
                          get_attn_backend_list_based_on_platform())
 def test_eagle_correctness(
@@ -174,9 +171,14 @@ def test_eagle_correctness(
     model_setup: (method, model_name, eagle_model_name, tp_size)
     '''
     with monkeypatch.context() as m:
-        m.setenv("VLLM_USE_V1", "1")
-        m.setenv("VLLM_MLA_DISABLE", "1")
-        m.setenv("VLLM_ATTENTION_BACKEND", attn_backend)
+        if "Llama-4-Scout" in model_setup[1] and attn_backend == "FLASH_ATTN":
+            # Scout requires default backend selection
+            # because vision encoder has head_dim 88 being incompatible
+            #  with FLASH_ATTN and needs to fall back to Flex Attn
+            pass
+        else:
+            m.setenv("VLLM_MLA_DISABLE", "1")
+            m.setenv("VLLM_ATTENTION_BACKEND", attn_backend)
 
         if (attn_backend == "TRITON_ATTN" and not current_platform.is_rocm()):
             pytest.skip("TRITON_ATTN does not support "

vllm/v1/spec_decode/eagle.py

@@ -804,6 +804,20 @@ class EagleProposer:
 
         self.attn_layer_names = list(draft_attn_layer_names)
 
+        if self.is_multimodal_model:
+            # Even if the target model is multimodal, we can also use
+            # text-only draft models
+            try:
+                dummy_input_ids = torch.tensor([[1]],
+                                               device=self.input_ids.device)
+                self.model.get_input_embeddings(dummy_input_ids,
+                                                multimodal_embeddings=None)
+            except (NotImplementedError, AttributeError, TypeError):
+                logger.warning(
+                    "Draft model does not support multimodal inputs, "
+                    "falling back to text-only mode")
+                self.is_multimodal_model = False
+
         if supports_multimodal(target_model):
             # handle multimodality
             self.model.config.image_token_index = (