[UX] Fail if an invalid attention backend is specified (#22217)

Signed-off-by: mgoin <michael@neuralmagic.com>

tests/kernels/attention/test_attention_selector.py

@@ -278,23 +278,13 @@ def test_flash_attn(monkeypatch: pytest.MonkeyPatch):
 
 @pytest.mark.parametrize("use_v1", [True, False])
 def test_invalid_env(use_v1: bool, monkeypatch: pytest.MonkeyPatch):
-
+    """Test that invalid attention backend names raise ValueError."""
     with monkeypatch.context() as m, patch(
             "vllm.attention.selector.current_platform", CudaPlatform()):
         m.setenv("VLLM_USE_V1", "1" if use_v1 else "0")
         m.setenv(STR_BACKEND_ENV_VAR, STR_INVALID_VAL)
 
-        # Test with head size 32
-        backend = get_attn_backend(32, torch.float16, None, 16, False)
-        EXPECTED = "FLASH_ATTN_VLLM_V1" if use_v1 else "FLASH_ATTN"
-        assert backend.get_name() == EXPECTED
-
-        # when block size == 16, backend will fall back to XFORMERS
-        # this behavior is not yet supported on V1.
-        if use_v1:
-            # TODO: support fallback on V1!
-            # https://github.com/vllm-project/vllm/issues/14524
-            pass
-        else:
-            backend = get_attn_backend(16, torch.float16, None, 16, False)
-            assert backend.get_name() == "XFORMERS"
+        # Should raise ValueError for invalid backend
+        with pytest.raises(ValueError) as exc_info:
+            get_attn_backend(32, torch.float16, None, 16, False)
+        assert "Invalid attention backend: 'INVALID'" in str(exc_info.value)

vllm/attention/selector.py

@@ -193,6 +193,10 @@ def _cached_get_attn_backend(
         backend_by_env_var: Optional[str] = envs.VLLM_ATTENTION_BACKEND
         if backend_by_env_var is not None:
             selected_backend = backend_name_to_enum(backend_by_env_var)
+            if selected_backend is None:
+                raise ValueError(
+                    f"Invalid attention backend: '{backend_by_env_var}'. "
+                    f"Valid backends are: {list(_Backend.__members__.keys())}")
 
     # get device-specific attn_backend
     attention_cls = current_platform.get_attn_backend_cls(