diff --git a/tests/config/test_pipeline_registry.py b/tests/config/test_pipeline_registry.py
index 3483d530c..6cc7c9258 100644
--- a/tests/config/test_pipeline_registry.py
+++ b/tests/config/test_pipeline_registry.py
@@ -6,11 +6,7 @@ from __future__ import annotations
 
 import pytest
 
-from vllm_omni.config.pipeline_registry import (
-    _DIFFUSION_PIPELINES,
-    _OMNI_PIPELINES,
-    _VLLM_OMNI_PIPELINES,
-)
+from vllm_omni.config.pipeline_registry import _OMNI_PIPELINES
 from vllm_omni.config.stage_config import (
     _PIPELINE_REGISTRY,
     PipelineConfig,
@@ -23,17 +19,9 @@ from vllm_omni.config.stage_config import (
 class TestCentralRegistryDeclarations:
     """Every in-tree pipeline must be declared exactly once in the central registry."""
 
-    def test_union_contains_all_omni(self):
+    def test_omni_entries_visible_in_registry(self):
         for key in _OMNI_PIPELINES:
-            assert key in _VLLM_OMNI_PIPELINES
-
-    def test_union_contains_all_diffusion(self):
-        for key in _DIFFUSION_PIPELINES:
-            assert key in _VLLM_OMNI_PIPELINES
-
-    def test_no_duplicate_model_type_between_omni_and_diffusion(self):
-        overlap = set(_OMNI_PIPELINES) & set(_DIFFUSION_PIPELINES)
-        assert not overlap, f"Duplicate model_types across omni/diffusion: {overlap}"
+            assert key in _PIPELINE_REGISTRY
 
     def test_expected_omni_pipelines_present(self):
         # Guard against accidental removal during future refactors.
diff --git a/tests/test_config_factory.py b/tests/test_config_factory.py
index 3148d8dc6..16d49034f 100644
--- a/tests/test_config_factory.py
+++ b/tests/test_config_factory.py
@@ -701,7 +701,7 @@ stages:
 
 
 class TestPipelineDiscovery:
-    """Tests for the central pipeline registry (``pipeline_registry._VLLM_OMNI_PIPELINES``)."""
+    """Tests for the central pipeline registry (``pipeline_registry._OMNI_PIPELINES``)."""
 
     def test_registry_has_known_models(self):
         """Built-in pipelines are lazy-loaded from the central declaration
diff --git a/vllm_omni/config/pipeline_registry.py b/vllm_omni/config/pipeline_registry.py
index d9351842d..937220914 100644
--- a/vllm_omni/config/pipeline_registry.py
+++ b/vllm_omni/config/pipeline_registry.py
@@ -15,7 +15,13 @@ they just no longer need to self-register via ``register_pipeline(...)``.
 Adding a new pipeline:
     1. Define the ``PipelineConfig`` instance as a module-level variable in
        ``vllm_omni/.../pipeline.py``.
-    2. Add one line to ``_OMNI_PIPELINES`` or ``_DIFFUSION_PIPELINES`` below.
+    2. Add one line to ``_OMNI_PIPELINES`` below.
+
+Single-stage diffusion models continue to use the
+``_create_default_diffusion_stage_cfg`` fallback in
+``async_omni_engine.py`` — they don't need a registry entry. The empty
+``_DIFFUSION_PIPELINES`` placeholder previously here (#2915) was removed
+once #2987 (which would have populated it) was deferred.
 
 ``register_pipeline(config)`` in ``stage_config`` is still supported for
 out-of-tree plugins and tests that create pipelines at runtime; those override
@@ -80,12 +86,3 @@ _OMNI_PIPELINES: dict[str, tuple[str, str]] = {
         "FISH_SPEECH_PIPELINE",
     ),
 }
-
-# --- Single-stage diffusion pipelines (populated in PR 3/N) ---
-_DIFFUSION_PIPELINES: dict[str, tuple[str, str]] = {}
-
-# Union view used by ``_LazyPipelineRegistry``; don't mutate at runtime.
-_VLLM_OMNI_PIPELINES: dict[str, tuple[str, str]] = {
-    **_OMNI_PIPELINES,
-    **_DIFFUSION_PIPELINES,
-}
diff --git a/vllm_omni/config/stage_config.py b/vllm_omni/config/stage_config.py
index f69e5c139..950685d62 100644
--- a/vllm_omni/config/stage_config.py
+++ b/vllm_omni/config/stage_config.py
@@ -274,9 +274,9 @@ class _LazyPipelineRegistry:
 
     def _get_lazy_map(self) -> dict[str, tuple[str, str]]:
         if self._lazy_map is None:
-            from vllm_omni.config.pipeline_registry import _VLLM_OMNI_PIPELINES
+            from vllm_omni.config.pipeline_registry import _OMNI_PIPELINES
 
-            self._lazy_map = _VLLM_OMNI_PIPELINES
+            self._lazy_map = _OMNI_PIPELINES
         return self._lazy_map
 
     def _load_lazy(self, model_type: str) -> PipelineConfig | None:
@@ -374,7 +374,7 @@ _PIPELINE_REGISTRY = _LazyPipelineRegistry()
 def register_pipeline(pipeline: PipelineConfig) -> None:
     """Register a pipeline config dynamically.
 
-    In-tree pipelines are declared in ``pipeline_registry._VLLM_OMNI_PIPELINES``
+    In-tree pipelines are declared in ``pipeline_registry._OMNI_PIPELINES``
     and loaded lazily; calling ``register_pipeline`` is only needed for
     out-of-tree plugins or tests that build a ``PipelineConfig`` at runtime.
     A dynamic registration overrides the central-registry entry with the same