feat(diffusion): select Wan VAE decode mode automatically

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/design/feature/vae_parallel.md

@@ -451,16 +451,16 @@ Complete examples in the codebase:
 
 ## Spatially-Sharded Decode (Wan)
 
-The tile-parallel executor above assigns independent spatial **tiles** to ranks. The Wan VAE additionally supports a **spatially-sharded decode** backend, selected through `DiffusionParallelConfig.vae_parallel_mode` (`"spatial_shard_height"` or `"spatial_shard_width"`; default is `"tile"`).
+The tile-parallel executor above assigns independent spatial **tiles** to ranks. The Wan VAE additionally supports a **spatially-sharded decode** backend. `DiffusionParallelConfig.vae_parallel_mode="auto"` is the default; explicit `"tile"`, `"spatial_shard_height"`, and `"spatial_shard_width"` modes remain available.
 
 ### How it differs from tile parallel
 
-| Aspect | Tile parallel (`"tile"`) | Spatially-sharded (`"spatial_shard_height"`/`"spatial_shard_width"`) |
-|--------|--------------------------|-----------------------------------------------|
-| Unit of work | Independent overlapping tiles | A single global feature map sharded along H or W |
-| Cross-rank communication | Gather tiles to rank 0, stitch + blend | Per-conv **halo exchange** of boundary rows/cols (P2P) |
-| Output assembly | Blend overlapping tiles | All-gather shards on rank 0, trim padding (matches `broadcast_result=False`) |
-| Scope | Decode + encode | Decode only |
+| Aspect | Tile parallel (`"tile"`) | Automatic (`"auto"`) | Spatially-sharded (`"spatial_shard_height"`/`"spatial_shard_width"`) |
+|--------|--------------------------|----------------------|-----------------------------------------------|
+| Unit of work | Independent overlapping tiles | Chooses tile or a spatial shard per request | A single global feature map sharded along H or W |
+| Cross-rank communication | Gather tiles to rank 0, stitch + blend | Depends on the selected path | Per-conv **halo exchange** of boundary rows/cols (P2P) |
+| Output assembly | Blend overlapping tiles | Depends on the selected path | All-gather shards on rank 0, trim padding (matches `broadcast_result=False`) |
+| Scope | Decode + encode | Wan decode selection; encode remains tiled | Decode only |
 
 Spatial-shard decode swaps the decoder's spatial convolutions/padding for halo-exchanging variants (`WanDistConv2d`, `WanDistCausalConv3d`, `WanDistZeroPad2d`) so each rank only holds a shard of the activations but still sees the correct receptive field at shard boundaries. Implementation lives in `vllm_omni/diffusion/distributed/autoencoders/wan_spatial_shard.py`.
 
@@ -476,11 +476,11 @@ serve.py (--vae-parallel-mode) / OmniEngineArgs
   -> DistributedAutoencoderKLWan.tiled_decode dispatch
 ```
 
-`DistributedAutoencoderKLWan._spatial_shard_decode_enabled()` gates the path: it requires distributed decode to be enabled, a 5D latent, and `vae_patch_parallel_size == DiT group size`. Otherwise it logs a warning and falls back to tile-parallel decode.
+`DistributedAutoencoderKLWan._spatial_shard_decode_enabled()` gates the path: it requires distributed decode to be enabled, a 5D latent, and `vae_patch_parallel_size == DiT group size`. The method is reached only after Diffusers has selected tiled decode for the request. In `auto` mode the longer axis that can cover every rank is selected, with width winning ties. This policy is cached only through normal kernel shape caches; it is reevaluated for every request.
 
 ### Notes
 
-- The decoder is patched **in place** the first time spatial-shard decode runs and is bound to a single split dimension for the lifetime of the VAE instance.
+- The decoder is patched **in place** the first time spatial-shard decode runs. Its wrappers consult a context-local split dimension and retain an exact direct fallback, allowing later requests to select tile, height, or width safely.
 - Numerical correctness vs. single-GPU decode is covered by `tests/diffusion/distributed/test_wan_spatial_shard.py::test_spatial_shard_decode_matches_reference` (multi-GPU, nightly `full_model` + `distributed_cuda`).
 
 ---

docs/user_guide/diffusion/parallelism/vae_parallelism.md

@@ -113,7 +113,7 @@ In `DiffusionParallelConfig`:
 | Parameter | Type | Default | Description |
 |-----------|------|---------|-------------|
 | `vae_patch_parallel_size` | int | 1 | Number of GPUs for VAE patch/tile parallelism. Set to 2 or higher to enable. Should typically match `tensor_parallel_size` as they share the same process group. |
-| `vae_parallel_mode` | str | `"tile"` | VAE parallel decode strategy: `"tile"` (default tile/patch parallel decode), `"spatial_shard_height"`, or `"spatial_shard_width"` (spatially-sharded decode, Wan only). See [Spatially-Sharded Decode](#spatially-sharded-decode-wan). |
+| `vae_parallel_mode` | str | `"auto"` | VAE parallel decode strategy: `"auto"` (shape/model/topology-aware selection), `"tile"`, `"spatial_shard_height"`, or `"spatial_shard_width"`. Spatial sharding is currently Wan-only. See [Spatially-Sharded Decode](#spatially-sharded-decode-wan). |
 
 Additional requirements:
 
@@ -128,7 +128,7 @@ Additional requirements:
 
 ## Spatially-Sharded Decode (Wan)
 
-The default `vae_parallel_mode="tile"` distributes whole tiles across ranks. For the **Wan** VAE there is an alternative decode strategy, **spatially-sharded decode**, selected via `vae_parallel_mode="spatial_shard_height"` or `vae_parallel_mode="spatial_shard_width"`.
+The default `vae_parallel_mode="auto"` selects spatial-shard decode only after a Wan request is already large enough to enter Diffusers' tiled-decode path and the VAE uses the full DiT decode group. It splits the longer eligible spatial axis to reduce halo traffic. Direct-decode requests, partial groups, and other VAE families keep their existing path. Set `vae_parallel_mode="tile"` to force the previous behavior, or `"spatial_shard_height"` / `"spatial_shard_width"` to force an axis.
 
 Instead of assigning independent tiles to ranks, spatial-shard decode shards the decoder feature maps along the height (`spatial_shard_height`) or width (`spatial_shard_width`) dimension and exchanges halo rows/columns between neighboring ranks around the spatial convolutions. This keeps the receptive field correct across shard boundaries, so the result matches the single-GPU decode within numerical tolerance.
 
@@ -141,7 +141,7 @@ omni = Omni(
     parallel_config=DiffusionParallelConfig(
         tensor_parallel_size=2,
         vae_patch_parallel_size=2,               # must match the DiT group size
-        vae_parallel_mode="spatial_shard_width", # or "spatial_shard_height"
+        vae_parallel_mode="auto",                # default
     ),
 )
 ```
@@ -152,14 +152,16 @@ Or from the CLI / serving entrypoint:
 vllm serve Wan-AI/Wan2.1-T2V-1.3B-Diffusers --omni \
     --tensor-parallel-size 2 \
     --vae-patch-parallel-size 2 \
-    --vae-parallel-mode spatial_shard_width
+    --vae-parallel-mode auto
 ```
 
 **Constraints and behavior:**
 
 - Spatial-shard decode is **decode-only** and currently implemented for the **Wan** VAE. Other models ignore `spatial_shard_*` modes.
-- It requires `vae_patch_parallel_size` to **match the DiT process group size**. If it does not, the VAE logs a warning and **falls back to tile-parallel decode** at runtime.
-- `spatial_shard_height` and `spatial_shard_width` are mutually exclusive for a given VAE instance (the decoder is patched in place for a single split dimension).
+- It requires `vae_patch_parallel_size` to **match the DiT process group size**. `auto` quietly falls back to tile decode for a partial group; an explicitly forced spatial mode logs a warning.
+- The decoder's context-aware wrappers retain the direct path, so one process can alternate small/large and portrait/landscape requests safely.
+
+The automatic policy is adapted from [SGLang #28071](https://github.com/sgl-project/sglang/pull/28071) and narrowed to vLLM-Omni's parity-tested Wan backend.
 
 For end-to-end latency/throughput, launch serving with the desired `vae_parallel_mode` and use the existing diffusion serving benchmark:
 

tests/config/test_omni_config.py

@@ -579,6 +579,7 @@ def test_diffusion_parallel_config_keeps_current_diffusion_parallel_surface():
     assert cfg.data_parallel_size == 3
     assert cfg.cfg_parallel_size == 3
     assert cfg.mask_sp_padding is True
+    assert cfg.vae_parallel_mode == "auto"
     assert cfg.world_size == 72
 
 

tests/diffusion/distributed/test_wan_spatial_shard.py

@@ -1,4 +1,5 @@
 import os
+from contextlib import contextmanager
 from types import SimpleNamespace
 
 import pytest
@@ -10,6 +11,8 @@ from tests.helpers.mark import hardware_test
 from vllm_omni.diffusion.distributed.autoencoders import wan_spatial_shard
 from vllm_omni.diffusion.distributed.autoencoders.autoencoder_kl_wan import (
     DistributedAutoencoderKLWan,
+    prepare_pipeline_wan_spatial_shard_decode,
+    select_auto_spatial_shard_split_dim,
 )
 from vllm_omni.platforms import current_omni_platform
 
@@ -17,6 +20,23 @@ from vllm_omni.platforms import current_omni_platform
 # the end uses hardware_test (H100 x2) plus full_model / diffusion / parallel.
 
 
+@contextmanager
+def _active_spatial_shard(split_dim: str):
+    token = wan_spatial_shard._SPATIAL_SHARD_CONTEXT.set(
+        wan_spatial_shard.SpatialShardContext(
+            input_extent=2,
+            local_input_extent=2,
+            split_dim=split_dim,
+            rank=0,
+            world_size=3,
+        )
+    )
+    try:
+        yield
+    finally:
+        wan_spatial_shard._SPATIAL_SHARD_CONTEXT.reset(token)
+
+
 @pytest.mark.core_model
 @pytest.mark.cpu
 def test_split_for_parallel_decode_pads_uneven_height():
@@ -255,12 +275,14 @@ def test_dist_zero_pad_only_applies_global_height_edges(monkeypatch: pytest.Monk
 
     monkeypatch.setattr(wan_spatial_shard, "_rank_world", lambda group: (1, 3))
     mid_rank_pad = wan_spatial_shard.WanDistZeroPad2d((0, 1, 1, 1), group=object())
-    mid = mid_rank_pad(x)
+    with _active_spatial_shard("height"):
+        mid = mid_rank_pad(x)
     assert mid.shape == (1, 1, 2, 3)
 
     monkeypatch.setattr(wan_spatial_shard, "_rank_world", lambda group: (2, 3))
     last_rank_pad = wan_spatial_shard.WanDistZeroPad2d((0, 1, 1, 1), group=object())
-    last = last_rank_pad(x)
+    with _active_spatial_shard("height"):
+        last = last_rank_pad(x)
     assert last.shape == (1, 1, 3, 3)
 
 
@@ -270,13 +292,15 @@ def test_dist_zero_pad_only_applies_global_width_edges(monkeypatch: pytest.Monke
     x = torch.ones((1, 1, 2, 2))
 
     monkeypatch.setattr(wan_spatial_shard, "_rank_world", lambda group: (1, 3))
-    mid_rank_pad = wan_spatial_shard.WanDistZeroPad2d((1, 1, 0, 0), group=object(), split_dim="width")
-    mid = mid_rank_pad(x)
+    mid_rank_pad = wan_spatial_shard.WanDistZeroPad2d((1, 1, 0, 0), group=object())
+    with _active_spatial_shard("width"):
+        mid = mid_rank_pad(x)
     assert mid.shape == (1, 1, 2, 2)
 
     monkeypatch.setattr(wan_spatial_shard, "_rank_world", lambda group: (2, 3))
-    last_rank_pad = wan_spatial_shard.WanDistZeroPad2d((1, 1, 0, 0), group=object(), split_dim="width")
-    last = last_rank_pad(x)
+    last_rank_pad = wan_spatial_shard.WanDistZeroPad2d((1, 1, 0, 0), group=object())
+    with _active_spatial_shard("width"):
+        last = last_rank_pad(x)
     assert last.shape == (1, 1, 2, 3)
 
 
@@ -299,6 +323,113 @@ def test_spatial_shard_height_gate_falls_back_for_partial_group(monkeypatch: pyt
     assert vae._spatial_shard_decode_enabled(z) is False
 
 
+@pytest.mark.core_model
+@pytest.mark.cpu
+@pytest.mark.parametrize(
+    "z_shape,world_size,expected",
+    [
+        ((1, 16, 1, 60, 104), 4, "width"),
+        ((1, 16, 5, 60, 104), 4, "width"),
+        ((1, 16, 5, 104, 60), 4, "height"),
+        ((1, 16, 5, 2, 2), 4, None),
+        ((1, 16, 5, 64, 64), 4, "width"),
+    ],
+)
+def test_auto_spatial_shard_selector_is_shape_aware(z_shape, world_size, expected):
+    assert select_auto_spatial_shard_split_dim(z_shape, world_size) == expected
+
+
+@pytest.mark.core_model
+@pytest.mark.cpu
+def test_auto_spatial_shard_gate_requires_full_group(monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setattr(
+        "vllm_omni.diffusion.distributed.autoencoders.autoencoder_kl_wan.dist.get_world_size",
+        lambda group=None: 4,
+    )
+
+    vae = DistributedAutoencoderKLWan.__new__(DistributedAutoencoderKLWan)
+    vae.use_tiling = True
+    vae.distributed_executor = SimpleNamespace(group=object(), parallel_size=4, parallel_mode="auto")
+    vae.is_distributed_enabled = lambda: True
+    large = torch.zeros((1, 16, 5, 60, 104))
+    unshardable = torch.zeros((1, 16, 1, 2, 2))
+
+    assert vae._spatial_shard_decode_split_dim(large, 4) == "width"
+    assert vae._spatial_shard_decode_enabled(large) is True
+    assert vae._spatial_shard_decode_enabled(unshardable) is False
+
+    vae.distributed_executor.parallel_size = 2
+    assert vae._spatial_shard_decode_enabled(large) is False
+
+
+@pytest.mark.core_model
+@pytest.mark.cpu
+def test_spatial_wrappers_are_prepared_only_for_a_full_group(monkeypatch: pytest.MonkeyPatch):
+    vae = DistributedAutoencoderKLWan.__new__(DistributedAutoencoderKLWan)
+    object.__setattr__(
+        vae,
+        "distributed_executor",
+        SimpleNamespace(group=object(), parallel_size=2, parallel_mode="auto"),
+    )
+    installed = []
+
+    monkeypatch.setattr(
+        "vllm_omni.diffusion.offloader.module_collector.ModuleDiscovery.discover",
+        lambda pipeline: SimpleNamespace(vaes=[vae]),
+    )
+    monkeypatch.setattr(
+        "vllm_omni.diffusion.distributed.autoencoders.autoencoder_kl_wan.dist.is_initialized",
+        lambda: True,
+    )
+    monkeypatch.setattr(
+        "vllm_omni.diffusion.distributed.autoencoders.autoencoder_kl_wan.dist.get_world_size",
+        lambda group=None: 4,
+    )
+    monkeypatch.setattr(
+        wan_spatial_shard,
+        "install_wan_spatial_shard_decode",
+        lambda candidate, group, split_dim: installed.append((candidate, group, split_dim)),
+    )
+
+    prepare_pipeline_wan_spatial_shard_decode(torch.nn.Identity())
+    assert installed == []
+
+    vae.distributed_executor.parallel_size = 4
+    prepare_pipeline_wan_spatial_shard_decode(torch.nn.Identity())
+    assert installed == [(vae, vae.distributed_executor.group, "height")]
+
+
+@pytest.mark.core_model
+@pytest.mark.cpu
+@torch.no_grad()
+def test_installed_spatial_decoder_keeps_direct_fallback_and_can_change_axis():
+    vae = DistributedAutoencoderKLWan(
+        base_dim=8,
+        decoder_base_dim=8,
+        z_dim=4,
+        dim_mult=[1, 2],
+        num_res_blocks=1,
+        attn_scales=[],
+        temperal_downsample=[False],
+        latents_mean=[0.0] * 4,
+        latents_std=[1.0] * 4,
+        scale_factor_temporal=1,
+        scale_factor_spatial=2,
+    ).eval()
+    latents = torch.randn(1, 4, 1, 4, 5)
+    parameter_names = {name for name, _ in vae.named_parameters()}
+    state_dict = {name: value.clone() for name, value in vae.state_dict().items()}
+    reference = vae.decode(latents, return_dict=False)[0]
+
+    wan_spatial_shard.install_wan_spatial_shard_decode(vae, group=object(), split_dim="height")
+    assert any(getattr(module, "_vllm_omni_dynamic_spatial_shard_conv", False) for module in vae.decoder.modules())
+    wan_spatial_shard.install_wan_spatial_shard_decode(vae, group=object(), split_dim="width")
+
+    assert {name for name, _ in vae.named_parameters()} == parameter_names
+    vae.load_state_dict(state_dict, strict=True)
+    assert torch.equal(vae.decode(latents, return_dict=False)[0], reference)
+
+
 @pytest.mark.core_model
 @pytest.mark.cpu
 def test_spatial_shard_width_gate_selects_width(monkeypatch: pytest.MonkeyPatch):
@@ -444,3 +575,99 @@ def test_spatial_shard_decode_matches_reference(split_dim: str):
     assert mean_abs_diff <= _SPATIAL_SHARD_TOLERANCE, (
         f"spatial_shard_{split_dim} mean_abs_diff {mean_abs_diff} exceeds {_SPATIAL_SHARD_TOLERANCE}"
     )
+
+
+def _auto_spatial_shard_decode_worker(rank: int, return_dict, master_port: str) -> None:
+    from vllm_omni.diffusion.distributed.parallel_state import (
+        destroy_model_parallel,
+        init_distributed_environment,
+        initialize_model_parallel,
+    )
+
+    os.environ["MASTER_ADDR"] = "127.0.0.1"
+    os.environ["MASTER_PORT"] = master_port
+    device = current_omni_platform.get_torch_device(rank)
+    current_omni_platform.set_device(device)
+    backend = current_omni_platform.dist_backend
+    init_distributed_environment(world_size=2, rank=rank, local_rank=rank, backend=backend)
+    initialize_model_parallel(sequence_parallel_size=2, ulysses_degree=2, backend=backend)
+
+    try:
+        vae = DistributedAutoencoderKLWan.from_pretrained(
+            _SPATIAL_SHARD_MODEL,
+            subfolder=_SPATIAL_SHARD_SUBFOLDER,
+            torch_dtype=torch.float32,
+        ).to(device=device, dtype=torch.float32)
+        vae.eval()
+        generator = torch.Generator(device=device).manual_seed(1)
+        small = torch.randn((1, vae.config.z_dim, 1, 60, 104), generator=generator, device=device)
+        landscape = torch.randn((1, vae.config.z_dim, 2, 60, 104), generator=generator, device=device)
+        portrait = torch.randn((1, vae.config.z_dim, 2, 104, 60), generator=generator, device=device)
+
+        with torch.inference_mode():
+            vae.use_tiling = False
+            vae.set_parallel_size(1, mode="tile")
+            small_reference = vae.decode(small, return_dict=False)[0]
+
+            vae.use_tiling = True
+            vae.set_parallel_size(2, mode="tile")
+            tile_before_install = vae.decode(small, return_dict=False)[0]
+
+            vae.set_parallel_size(2, mode="auto")
+            assert vae._spatial_shard_decode_split_dim(small, 2) == "width"
+            small_sharded = vae.decode(small, return_dict=False)[0]
+            assert vae._spatial_shard_decode_split_dim(landscape, 2) == "width"
+            vae.decode(landscape, return_dict=False)
+
+            vae.set_parallel_size(2, mode="tile")
+            tile_after_install = vae.decode(small, return_dict=False)[0]
+
+            vae.use_tiling = False
+            vae.set_parallel_size(1, mode="tile")
+            portrait_reference = vae.decode(portrait, return_dict=False)[0]
+
+            vae.use_tiling = True
+            vae.set_parallel_size(2, mode="auto")
+            assert vae._spatial_shard_decode_split_dim(portrait, 2) == "height"
+            portrait_sharded = vae.decode(portrait, return_dict=False)[0]
+
+        assert all(
+            buffer is None
+            for module in vae.decoder.modules()
+            for name, buffer in module._buffers.items()
+            if name in {"_halo_recv_top_buf", "_halo_recv_bottom_buf"}
+        )
+
+        if rank == 0:
+            small_diff = (small_sharded.float() - small_reference.float()).abs()
+            portrait_diff = (portrait_sharded.float() - portrait_reference.float()).abs()
+            return_dict["tile_fallback_exact"] = torch.equal(tile_after_install, tile_before_install)
+            return_dict["small_max_abs_diff"] = small_diff.max().item()
+            return_dict["small_mean_abs_diff"] = small_diff.mean().item()
+            return_dict["portrait_max_abs_diff"] = portrait_diff.max().item()
+            return_dict["portrait_mean_abs_diff"] = portrait_diff.mean().item()
+    finally:
+        destroy_model_parallel()
+        if dist.is_initialized():
+            dist.destroy_process_group()
+
+
+@pytest.mark.full_model
+@pytest.mark.diffusion
+@pytest.mark.parallel
+@hardware_test(res={"cuda": "H100"}, num_cards=2)
+def test_auto_spatial_shard_switches_shape_paths_safely():
+    manager = mp.get_context("spawn").Manager()
+    return_dict = manager.dict()
+    mp.spawn(
+        _auto_spatial_shard_decode_worker,
+        args=(return_dict, "29502"),
+        nprocs=2,
+        join=True,
+    )
+
+    assert return_dict.get("tile_fallback_exact") is True
+    assert return_dict["small_max_abs_diff"] <= _SPATIAL_SHARD_TOLERANCE
+    assert return_dict["small_mean_abs_diff"] <= _SPATIAL_SHARD_TOLERANCE
+    assert return_dict["portrait_max_abs_diff"] <= _SPATIAL_SHARD_TOLERANCE
+    assert return_dict["portrait_mean_abs_diff"] <= _SPATIAL_SHARD_TOLERANCE

tests/diffusion/models/minimax_h3/test_minimax_h3_parallel.py

@@ -120,3 +120,34 @@ def test_tp_accepts_checkpoint_supported_sizes():
     arch = MiniMaxH3DiTArchConfig()
     for tp_size in (1, 2, 4, 7):
         model._validate_tp_config(arch=arch, tp_size=tp_size)
+
+
+@pytest.mark.parametrize(("parallel_size", "enabled"), [(1, False), (2, True)])
+def test_video_vae_auto_mode_preserves_native_tile_parallelism(monkeypatch, parallel_size, enabled):
+    from vllm_omni.diffusion.models.minimax_h3 import vae as vae_module
+
+    parallel_state = {}
+    video_vae = object.__new__(vae_module.MiniMaxH3VideoVAE)
+    nn.Module.__init__(video_vae)
+    video_vae.remote = nn.Identity()
+    video_vae.model = type("FakeModel", (), {"parallel_tiling": False})()
+
+    monkeypatch.setattr(vae_module, "get_dit_group", lambda: object())
+    monkeypatch.setattr(vae_module.dist, "get_world_size", lambda group: 2)
+    monkeypatch.setattr(vae_module.dist, "get_rank", lambda group: 0)
+    monkeypatch.setattr(
+        vae_module.importlib,
+        "import_module",
+        lambda package: type(
+            "FakeParallelModule",
+            (),
+            {"get_parallel_state": staticmethod(lambda: parallel_state)},
+        )(),
+    )
+
+    video_vae.set_parallel_size(parallel_size, mode="auto")
+
+    assert video_vae.parallel_size == parallel_size
+    assert video_vae.model.parallel_tiling is enabled
+    assert parallel_state["group_size"] == parallel_size
+    assert parallel_state["sp_enabled"] is enabled

tests/diffusion/test_diffusion_model_runner.py

@@ -728,6 +728,19 @@ def test_execute_model_runs_forward_after_kv_receive(monkeypatch):
 @pytest.mark.core_model
 @pytest.mark.cpu
 def test_load_model_clears_cache_backend_for_unsupported_pipeline(monkeypatch):
+    memory_pool_active = False
+    preparation_pool_states = []
+
+    @contextmanager
+    def _memory_pool_context(tag):
+        nonlocal memory_pool_active
+        assert tag == "weights"
+        memory_pool_active = True
+        try:
+            yield
+        finally:
+            memory_pool_active = False
+
     class _DummyLoader:
         def __init__(self, load_config, od_config=None):
             del load_config, od_config
@@ -775,16 +788,22 @@ def test_load_model_clears_cache_backend_for_unsupported_pipeline(monkeypatch):
     monkeypatch.setattr(model_runner_module, "LoadConfig", lambda: object())
     monkeypatch.setattr(model_runner_module, "DiffusersPipelineLoader", _DummyLoader)
     monkeypatch.setattr(model_runner_module, "DeviceMemoryProfiler", _DummyMemoryProfiler)
+    monkeypatch.setattr(
+        model_runner_module,
+        "prepare_pipeline_wan_spatial_shard_decode",
+        lambda pipeline: preparation_pool_states.append(memory_pool_active),
+    )
     monkeypatch.setattr(model_runner_module, "get_offload_backend", lambda od_config, device: None)
     monkeypatch.setattr(
         model_runner_module, "get_cache_backend", lambda cache_backend, cache_config: dummy_cache_backend
     )
 
-    DiffusionModelRunner.load_model(runner)
+    DiffusionModelRunner.load_model(runner, memory_pool_context_fn=_memory_pool_context)
 
     assert runner.cache_backend is None
     assert runner.od_config.cache_backend is None
     assert dummy_cache_backend.enabled is False
+    assert preparation_pool_states == [True]
 
 
 @pytest.mark.core_model

tests/entrypoints/test_serve.py

@@ -76,6 +76,18 @@ def test_serve_parser_accepts_four_way_cfg_parallelism() -> None:
     assert args.cfg_parallel_size == 4
 
 
+def test_serve_parser_defaults_vae_parallel_mode_to_auto_and_accepts_tile() -> None:
+    parser = TrackingArgumentParser()
+    subparsers = parser.add_subparsers(dest="subcommand")
+    OmniServeCommand().subparser_init(subparsers)
+
+    default_args = parser.parse_args(["serve", "fake-model", "--omni"])
+    tile_args = parser.parse_args(["serve", "fake-model", "--omni", "--vae-parallel-mode", "tile"])
+
+    assert default_args.vae_parallel_mode == "auto"
+    assert tile_args.vae_parallel_mode == "tile"
+
+
 def _make_headless_args(**kwargs) -> TrackingNamespace:
     defaults = {
         "model": "fake-model",

vllm_omni/config/omni_config.py

@@ -452,7 +452,7 @@ class OmniStageDiffusionParallelConfig(OmniStageParallelConfig):
     cfg_parallel_size: int = Field(default=1, ge=1)
     vae_patch_parallel_size: int = Field(default=1, ge=1)
     text_encoder_tp_size: int = Field(default=1, ge=1)
-    vae_parallel_mode: str = "tile"
+    vae_parallel_mode: str = "auto"
     use_hsdp: bool = False
     mask_sp_padding: bool = False
     hsdp_shard_size: int = -1
@@ -466,9 +466,10 @@ class OmniStageDiffusionParallelConfig(OmniStageParallelConfig):
             raise ValueError("allgather_degree > 1 is mutually exclusive with ulysses_degree/ring_degree > 1")
         if self.ulysses_mode not in {"strict", "advanced_uaa"}:
             raise ValueError("ulysses_mode must be 'strict' or 'advanced_uaa'")
-        if self.vae_parallel_mode not in {"tile", "spatial_shard_height", "spatial_shard_width"}:
+        if self.vae_parallel_mode not in {"auto", "tile", "spatial_shard_height", "spatial_shard_width"}:
             raise ValueError(
-                "vae_parallel_mode must be one of {'tile', 'spatial_shard_height', 'spatial_shard_width'}, "
+                "vae_parallel_mode must be one of "
+                "{'auto', 'tile', 'spatial_shard_height', 'spatial_shard_width'}, "
                 f"but got {self.vae_parallel_mode!r}."
             )
 

vllm_omni/diffusion/data.py

@@ -206,10 +206,13 @@ class DiffusionParallelConfig:
     process group.
     """
 
-    vae_parallel_mode: str = "tile"
+    vae_parallel_mode: str = "auto"
     """VAE parallel decode strategy.
 
-    - "tile": Patch/tile parallel decode (default). Each rank decodes a subset
+    - "auto": Select the longer spatial-shard axis for validated Wan tiled
+      decodes on a full DiT-group topology; otherwise use patch/tile parallel
+      decode.
+    - "tile": Patch/tile parallel decode. Each rank decodes a subset
       of spatial tiles and the results are stitched on rank 0.
     - "spatial_shard_height": Spatially-sharded decode that splits decoder
       feature maps along height and exchanges halo rows around spatial
@@ -250,8 +253,9 @@ class DiffusionParallelConfig:
         assert self.allgather_degree > 0, "AllGather degree must be > 0"
         assert self.cfg_parallel_size > 0, "CFG parallel size must be > 0"
         assert self.vae_patch_parallel_size > 0, "VAE patch parallel size must be > 0"
-        assert self.vae_parallel_mode in {"tile", "spatial_shard_height", "spatial_shard_width"}, (
-            "vae_parallel_mode must be one of {'tile', 'spatial_shard_height', 'spatial_shard_width'}, "
+        assert self.vae_parallel_mode in {"auto", "tile", "spatial_shard_height", "spatial_shard_width"}, (
+            "vae_parallel_mode must be one of "
+            "{'auto', 'tile', 'spatial_shard_height', 'spatial_shard_width'}, "
             f"but got {self.vae_parallel_mode!r}."
         )
         if self.allgather_degree > 1:

vllm_omni/diffusion/distributed/autoencoders/autoencoder_kl_wan.py

@@ -23,6 +23,51 @@ from vllm_omni.platforms import current_omni_platform
 logger = init_logger(__name__)
 
 
+def select_auto_spatial_shard_split_dim(
+    z_shape: tuple[int, ...],
+    world_size: int,
+) -> str | None:
+    """Select a profitable Wan spatial-shard axis for one latent shape.
+
+    This selector runs only after Diffusers has already chosen ``tiled_decode``
+    for a spatially large request. Splitting the longer eligible axis reduces
+    halo traffic for rectangular requests and improves per-rank balance.
+    """
+    if len(z_shape) != 5 or world_size <= 1:
+        return None
+
+    _, _, _, height, width = z_shape
+    candidates = [
+        (extent, split_dim) for extent, split_dim in ((height, "height"), (width, "width")) if extent >= world_size
+    ]
+    if not candidates:
+        return None
+    # Prefer width for square latents: it was modestly faster in the existing
+    # four-GPU Wan benchmark and keeps the choice deterministic.
+    return max(candidates, key=lambda candidate: (candidate[0], candidate[1] == "width"))[1]
+
+
+def prepare_pipeline_wan_spatial_shard_decode(pipeline: torch.nn.Module) -> None:
+    """Install eligible Wan wrappers while weight allocations are still tagged."""
+    from vllm_omni.diffusion.offloader.module_collector import ModuleDiscovery
+
+    if not dist.is_initialized():
+        return
+    for vae in ModuleDiscovery.discover(pipeline).vaes:
+        if not isinstance(vae, DistributedAutoencoderKLWan):
+            continue
+        executor = vae.distributed_executor
+        mode = executor.parallel_mode
+        if mode not in {"auto", "spatial_shard_height", "spatial_shard_width"}:
+            continue
+        requested_size = int(executor.parallel_size)
+        world_size = dist.get_world_size(group=executor.group)
+        if requested_size <= 1 or requested_size != world_size:
+            continue
+        split_dim = "width" if mode == "spatial_shard_width" else "height"
+        wan_spatial_shard.install_wan_spatial_shard_decode(vae, executor.group, split_dim=split_dim)
+
+
 class OmniAutoencoderKLWan(AutoencoderKLWan):
     def _execution_context(self):
         try:
@@ -270,20 +315,26 @@ class DistributedAutoencoderKLWan(OmniAutoencoderKLWan, DistributedVaeMixin):
         dec = torch.clamp(dec, min=-1.0, max=1.0)
         return dec
 
-    def _spatial_shard_decode_split_dim(self) -> str | None:
+    def _spatial_shard_decode_split_dim(
+        self,
+        z: torch.Tensor | None = None,
+        world_size: int | None = None,
+    ) -> str | None:
         mode = self.distributed_executor.parallel_mode
         if mode == "spatial_shard_width":
             return "width"
         if mode == "spatial_shard_height":
             return "height"
+        if mode == "auto" and z is not None and world_size is not None:
+            return select_auto_spatial_shard_split_dim(tuple(z.shape), world_size)
         return None
 
     def _spatial_shard_decode_enabled(self, z: torch.Tensor) -> bool:
-        split_dim = self._spatial_shard_decode_split_dim()
-        if split_dim is None:
+        if self.distributed_executor.parallel_mode == "tile":
             return False
         if z.ndim != 5:
-            logger.warning("Wan VAE spatial sharded decode expects 5D latent input; falling back to tiled decode.")
+            if self.distributed_executor.parallel_mode != "tile":
+                logger.warning("Wan VAE spatial sharded decode expects 5D latent input; falling back to tiled decode.")
             return False
         if not self.is_distributed_enabled():
             return False
@@ -292,23 +343,32 @@ class DistributedAutoencoderKLWan(OmniAutoencoderKLWan, DistributedVaeMixin):
         world_size = dist.get_world_size(group=group)
         requested_size = int(self.distributed_executor.parallel_size)
         if requested_size != world_size:
-            logger.warning(
-                "Wan VAE spatial sharded decode currently requires vae_patch_parallel_size "
-                "to match the DIT group size; falling back to tiled decode. "
-                "requested=%s dit_group=%s split_dim=%s",
-                requested_size,
-                world_size,
-                split_dim,
-            )
+            if self.distributed_executor.parallel_mode == "auto":
+                logger.debug(
+                    "Wan VAE auto decode selected tile mode for a partial DiT group (requested=%s dit_group=%s)",
+                    requested_size,
+                    world_size,
+                )
+            elif self._spatial_shard_decode_split_dim() is not None:
+                logger.warning(
+                    "Wan VAE spatial sharded decode currently requires vae_patch_parallel_size "
+                    "to match the DIT group size; falling back to tiled decode. "
+                    "requested=%s dit_group=%s split_dim=%s",
+                    requested_size,
+                    world_size,
+                    self._spatial_shard_decode_split_dim(),
+                )
             return False
-        return True
+        return self._spatial_shard_decode_split_dim(z, world_size) is not None
 
     def tiled_decode(self, z: torch.Tensor, return_dict: bool = True):
         if not self.is_distributed_enabled():
             return super().tiled_decode(z, return_dict=return_dict)
 
         if self._spatial_shard_decode_enabled(z):
-            split_dim = self._spatial_shard_decode_split_dim()
+            world_size = dist.get_world_size(group=self.distributed_executor.group)
+            split_dim = self._spatial_shard_decode_split_dim(z, world_size)
+            assert split_dim is not None
             logger.debug("Decode running with Wan VAE spatial_shard_%s mode", split_dim)
             return wan_spatial_shard.spatial_shard_decode(
                 self,

vllm_omni/diffusion/distributed/autoencoders/wan_spatial_shard.py

@@ -52,6 +52,11 @@ _SPATIAL_SHARD_CONTEXT: ContextVar[SpatialShardContext | None] = ContextVar(
 )
 
 
+def _active_spatial_shard_split_dim() -> str | None:
+    context = _SPATIAL_SHARD_CONTEXT.get()
+    return context.split_dim if context is not None else None
+
+
 def _spatial_dim(split_dim: str) -> int:
     if split_dim == "height":
         return -2
@@ -323,21 +328,27 @@ class WanDistZeroPad2d(nn.Module):
         padding: tuple[int, int, int, int],
         group: dist.ProcessGroup,
         *,
-        split_dim: str = "height",
-        split_padding: tuple[int, int] | None = None,
+        defer_split_after_padding: bool = False,
     ) -> None:
         super().__init__()
         self.padding = padding
-        self.split_dim = split_dim
-        default_split_padding = (padding[2], padding[3]) if split_dim == "height" else (padding[0], padding[1])
-        self.split_padding = split_padding or default_split_padding
+        self.defer_split_after_padding = defer_split_after_padding
         self.group = group
 
     def forward(self, x: torch.Tensor) -> torch.Tensor:
+        split_dim = _active_spatial_shard_split_dim()
+        if split_dim is None:
+            return F.pad(x, self.padding)
+
         rank, world_size = _rank_world(self.group)
         left, right, top, bottom = self.padding
+        if self.defer_split_after_padding:
+            if split_dim == "height":
+                bottom = 0
+            else:
+                right = 0
         if world_size > 1:
-            if self.split_dim == "height":
+            if split_dim == "height":
                 top = top if rank == 0 else 0
                 bottom = bottom if rank == world_size - 1 else 0
             else:
@@ -351,8 +362,7 @@ class WanDistConv2d(nn.Conv2d):
         self,
         source: nn.Conv2d,
         group: dist.ProcessGroup,
-        split_dim: str = "height",
-        split_padding: tuple[int, int] | None = None,
+        deferred_padding: tuple[int, int, int, int] | None = None,
     ):
         super().__init__(
             source.in_channels,
@@ -369,75 +379,117 @@ class WanDistConv2d(nn.Conv2d):
         )
         self.load_state_dict(source.state_dict())
         self.group = group
-        self.split_dim = split_dim
-        self.split_tensor_dim = _spatial_dim(split_dim)
-        kernel_extent = self.kernel_size[-2] if split_dim == "height" else self.kernel_size[-1]
-        self.halo_size = (kernel_extent - 1) // 2
-        pad_h = source.padding[-2] if isinstance(source.padding, tuple) else source.padding
-        pad_w = source.padding[-1] if isinstance(source.padding, tuple) else source.padding
-        if split_dim == "height":
-            if split_padding is None:
-                split_padding = (pad_h, pad_h)
-            self._non_split_padding = (pad_w, pad_w, 0, 0)
-            self.kernel_extent = self.kernel_size[-2]
-            self.stride_extent = self.stride[-2]
-        else:
-            if split_padding is None:
-                split_padding = (pad_w, pad_w)
-            self._non_split_padding = (0, 0, pad_h, pad_h)
-            self.kernel_extent = self.kernel_size[-1]
-            self.stride_extent = self.stride[-1]
-        self.split_pad_left, self.split_pad_right = split_padding
-        self._halo_recv_top_buf: torch.Tensor | None = None
-        self._halo_recv_bottom_buf: torch.Tensor | None = None
-        self._trim_cache: dict[int, tuple[int, int, int]] = {}
+        self._direct_padding = source.padding
+        self._direct_padding_mode = source.padding_mode
+        self._direct_reversed_padding = source._reversed_padding_repeated_twice
+        self._deferred_padding = deferred_padding
+        source_padding = source.padding if isinstance(source.padding, tuple) else (source.padding, source.padding)
+        if len(source_padding) != 2 or not all(isinstance(value, int) for value in source_padding):
+            raise ValueError(f"Wan spatial-shard Conv2d requires integer padding, got {source.padding!r}.")
+        self._spatial_pad_h = int(source_padding[-2])
+        self._spatial_pad_w = int(source_padding[-1])
+        self.register_buffer("_halo_recv_top_buf", None, persistent=False)
+        self.register_buffer("_halo_recv_bottom_buf", None, persistent=False)
+        self._trim_cache: dict[tuple[str, int], tuple[int, int, int]] = {}
 
     def forward(self, x: torch.Tensor) -> torch.Tensor:
-        x = F.pad(x, self._non_split_padding)
+        split_dim = _active_spatial_shard_split_dim()
+        if split_dim is None:
+            if self._direct_padding_mode != "zeros":
+                x = F.pad(x, self._direct_reversed_padding, mode=self._direct_padding_mode)
+                padding: str | tuple[int, int] = (0, 0)
+            else:
+                padding = self._direct_padding
+            return F.conv2d(x, self.weight, self.bias, self.stride, padding, self.dilation, self.groups)
+
+        split_tensor_dim = _spatial_dim(split_dim)
+        if split_dim == "height":
+            kernel_extent = self.kernel_size[-2]
+            stride_extent = self.stride[-2]
+            non_split_padding = (self._spatial_pad_w, self._spatial_pad_w, 0, 0)
+            default_split_padding = (self._spatial_pad_h, self._spatial_pad_h)
+            deferred_split_padding = (
+                (self._deferred_padding[2], self._deferred_padding[3]) if self._deferred_padding is not None else None
+            )
+        else:
+            kernel_extent = self.kernel_size[-1]
+            stride_extent = self.stride[-1]
+            non_split_padding = (0, 0, self._spatial_pad_h, self._spatial_pad_h)
+            default_split_padding = (self._spatial_pad_w, self._spatial_pad_w)
+            deferred_split_padding = (
+                (self._deferred_padding[0], self._deferred_padding[1]) if self._deferred_padding is not None else None
+            )
+        split_pad_left, split_pad_right = deferred_split_padding or default_split_padding
+        halo_size = (kernel_extent - 1) // 2
+
+        x = F.pad(x, non_split_padding)
         x_padded, self._halo_recv_top_buf, self._halo_recv_bottom_buf = halo_exchange(
             x,
             group=self.group,
-            halo_size=self.halo_size,
-            split_dim=self.split_dim,
+            halo_size=halo_size,
+            split_dim=split_dim,
             recv_top_buf=self._halo_recv_top_buf,
             recv_bottom_buf=self._halo_recv_bottom_buf,
         )
-        shift, start, upper_bound = self._get_trim_params(x.shape[self.split_tensor_dim])
+        shift, start, upper_bound = self._get_trim_params(
+            x.shape[split_tensor_dim],
+            split_dim=split_dim,
+            halo_size=halo_size,
+            split_pad_left=split_pad_left,
+            split_pad_right=split_pad_right,
+            kernel_extent=kernel_extent,
+            stride_extent=stride_extent,
+        )
         if shift:
             x_padded = _narrow_along_dim(
                 x_padded,
-                self.split_tensor_dim,
+                split_tensor_dim,
                 shift,
-                x_padded.shape[self.split_tensor_dim] - shift,
+                x_padded.shape[split_tensor_dim] - shift,
             )
         out = super().forward(x_padded)
-        out = _trim_local_conv_output(out, self.halo_size, start, upper_bound, split_dim=self.split_dim)
-        return _zero_invalid_extent(out, split_dim=self.split_dim)
+        out = _trim_local_conv_output(out, halo_size, start, upper_bound, split_dim=split_dim)
+        return _zero_invalid_extent(out, split_dim=split_dim)
 
-    def _get_trim_params(self, local_extent: int) -> tuple[int, int, int]:
-        trim_params = self._trim_cache.get(local_extent)
+    def _get_trim_params(
+        self,
+        local_extent: int,
+        *,
+        split_dim: str,
+        halo_size: int,
+        split_pad_left: int,
+        split_pad_right: int,
+        kernel_extent: int,
+        stride_extent: int,
+    ) -> tuple[int, int, int]:
+        cache_key = (split_dim, local_extent)
+        trim_params = self._trim_cache.get(cache_key)
         if trim_params is None:
             rank, world_size = _rank_world(self.group)
             trim_params = _compute_conv_trim_params(
                 local_extent=local_extent,
                 rank=rank,
                 world_size=world_size,
-                halo_size=self.halo_size,
-                pad_before=self.split_pad_left,
-                pad_after=self.split_pad_right,
-                kernel_extent=self.kernel_extent,
-                stride_extent=self.stride_extent,
+                halo_size=halo_size,
+                pad_before=split_pad_left,
+                pad_after=split_pad_right,
+                kernel_extent=kernel_extent,
+                stride_extent=stride_extent,
             )
-            self._trim_cache[local_extent] = trim_params
+            self._trim_cache[cache_key] = trim_params
         return trim_params
 
 
 class WanDistCausalConv3d(nn.Conv3d):
+    # Cross-PR compatibility contract for the lossless Wan data-movement
+    # installer: it may fuse the no-context direct path, but must leave this
+    # wrapper in control whenever a spatial-shard context is active.
+    _vllm_omni_dynamic_spatial_shard_conv = True
+
     def __init__(
         self,
         source: nn.Conv3d,
         group: dist.ProcessGroup,
-        split_dim: str = "height",
     ):
         super().__init__(
             source.in_channels,
@@ -454,88 +506,98 @@ class WanDistCausalConv3d(nn.Conv3d):
         )
         self.load_state_dict(source.state_dict())
         self.group = group
-        self.split_dim = split_dim
-        self.split_tensor_dim = _spatial_dim(split_dim)
         source_padding = getattr(source, "_padding", None)
         if source_padding is None:
             p_t, p_h, p_w = source.padding
             source_padding = (p_w, p_w, p_h, p_h, 2 * p_t, 0)
         self._source_padding = tuple(source_padding)
-        if split_dim == "height":
-            self.split_pad_left = int(self._source_padding[2])
-            self.split_pad_right = int(self._source_padding[3])
-            self.kernel_extent = self.kernel_size[-2]
-            self.stride_extent = self.stride[-2]
-            padding = (
-                self._source_padding[0],
-                self._source_padding[1],
-                0,
-                0,
-                self._source_padding[4],
-                self._source_padding[5],
-            )
-        else:
-            self.split_pad_left = int(self._source_padding[0])
-            self.split_pad_right = int(self._source_padding[1])
-            self.kernel_extent = self.kernel_size[-1]
-            self.stride_extent = self.stride[-1]
-            padding = (
-                0,
-                0,
-                self._source_padding[2],
-                self._source_padding[3],
-                self._source_padding[4],
-                self._source_padding[5],
-            )
-        self.halo_size = (self.kernel_extent - 1) // 2
-        self._padding = padding if self.halo_size > 0 else self._source_padding
-        self._halo_recv_top_buf: torch.Tensor | None = None
-        self._halo_recv_bottom_buf: torch.Tensor | None = None
-        self._trim_cache: dict[int, tuple[int, int, int]] = {}
+        self.register_buffer("_halo_recv_top_buf", None, persistent=False)
+        self.register_buffer("_halo_recv_bottom_buf", None, persistent=False)
+        self._trim_cache: dict[tuple[str, int], tuple[int, int, int]] = {}
 
     def forward(self, x: torch.Tensor, cache_x: torch.Tensor | None = None) -> torch.Tensor:
-        padding = list(self._padding)
+        split_dim = _active_spatial_shard_split_dim()
+        padding = list(self._source_padding)
         if cache_x is not None and padding[4] > 0:
             cache_x = cache_x.to(x.device)
             x = torch.cat([cache_x, x], dim=2)
             padding[4] -= cache_x.shape[2]
 
+        if split_dim is None:
+            return super().forward(F.pad(x, padding))
+
+        split_tensor_dim = _spatial_dim(split_dim)
+        if split_dim == "height":
+            split_pad_left = int(padding[2])
+            split_pad_right = int(padding[3])
+            kernel_extent = self.kernel_size[-2]
+            stride_extent = self.stride[-2]
+            padding[2] = 0
+            padding[3] = 0
+        else:
+            split_pad_left = int(padding[0])
+            split_pad_right = int(padding[1])
+            kernel_extent = self.kernel_size[-1]
+            stride_extent = self.stride[-1]
+            padding[0] = 0
+            padding[1] = 0
+        halo_size = (kernel_extent - 1) // 2
+
         x = F.pad(x, padding)
         x_padded, self._halo_recv_top_buf, self._halo_recv_bottom_buf = halo_exchange(
             x,
             group=self.group,
-            halo_size=self.halo_size,
-            split_dim=self.split_dim,
+            halo_size=halo_size,
+            split_dim=split_dim,
             recv_top_buf=self._halo_recv_top_buf,
             recv_bottom_buf=self._halo_recv_bottom_buf,
         )
-        shift, start, upper_bound = self._get_trim_params(x.shape[self.split_tensor_dim])
+        shift, start, upper_bound = self._get_trim_params(
+            x.shape[split_tensor_dim],
+            split_dim=split_dim,
+            halo_size=halo_size,
+            split_pad_left=split_pad_left,
+            split_pad_right=split_pad_right,
+            kernel_extent=kernel_extent,
+            stride_extent=stride_extent,
+        )
         if shift:
             x_padded = _narrow_along_dim(
                 x_padded,
-                self.split_tensor_dim,
+                split_tensor_dim,
                 shift,
-                x_padded.shape[self.split_tensor_dim] - shift,
+                x_padded.shape[split_tensor_dim] - shift,
             )
         out = super().forward(x_padded)
-        out = _trim_local_conv_output(out, self.halo_size, start, upper_bound, split_dim=self.split_dim)
-        return _zero_invalid_extent(out, split_dim=self.split_dim)
+        out = _trim_local_conv_output(out, halo_size, start, upper_bound, split_dim=split_dim)
+        return _zero_invalid_extent(out, split_dim=split_dim)
 
-    def _get_trim_params(self, local_extent: int) -> tuple[int, int, int]:
-        trim_params = self._trim_cache.get(local_extent)
+    def _get_trim_params(
+        self,
+        local_extent: int,
+        *,
+        split_dim: str,
+        halo_size: int,
+        split_pad_left: int,
+        split_pad_right: int,
+        kernel_extent: int,
+        stride_extent: int,
+    ) -> tuple[int, int, int]:
+        cache_key = (split_dim, local_extent)
+        trim_params = self._trim_cache.get(cache_key)
         if trim_params is None:
             rank, world_size = _rank_world(self.group)
             trim_params = _compute_conv_trim_params(
                 local_extent=local_extent,
                 rank=rank,
                 world_size=world_size,
-                halo_size=self.halo_size,
-                pad_before=self.split_pad_left,
-                pad_after=self.split_pad_right,
-                kernel_extent=self.kernel_extent,
-                stride_extent=self.stride_extent,
+                halo_size=halo_size,
+                pad_before=split_pad_left,
+                pad_after=split_pad_right,
+                kernel_extent=kernel_extent,
+                stride_extent=stride_extent,
             )
-            self._trim_cache[local_extent] = trim_params
+            self._trim_cache[cache_key] = trim_params
         return trim_params
 
 
@@ -582,12 +644,15 @@ def _trim_local_conv_output(
     return out
 
 
-def _patch_attention_block(module: nn.Module, group: dist.ProcessGroup, split_dim: str) -> None:
+def _patch_attention_block(module: nn.Module, group: dist.ProcessGroup) -> None:
     if getattr(module, "_vllm_omni_spatial_shard_attention", False):
         return
     orig_forward = module.forward
 
     def _forward(self: nn.Module, x: torch.Tensor, *args: Any, **kwargs: Any) -> torch.Tensor:
+        split_dim = _active_spatial_shard_split_dim()
+        if split_dim is None:
+            return orig_forward(x, *args, **kwargs)
         _, world_size = _rank_world(group)
         if world_size <= 1:
             return orig_forward(x, *args, **kwargs)
@@ -609,7 +674,6 @@ def _replace_child(
     name: str,
     child: nn.Module,
     group: dist.ProcessGroup,
-    split_dim: str,
 ) -> None:
     if child.__class__.__name__ == "WanCausalConv3d":
         setattr(
@@ -618,51 +682,39 @@ def _replace_child(
             WanDistCausalConv3d(
                 child,
                 group,
-                split_dim=split_dim,
             ),
         )
         return
     if isinstance(child, nn.ZeroPad2d):
         padding = tuple(int(p) for p in child.padding)
-        module_padding = padding
-        if parent.__class__.__name__ == "Sequential":
-            # Let the following WanDistConv2d account for global after-edge
-            # padding; this module handles the non-split dimension and before edge.
-            if split_dim == "height":
-                module_padding = (padding[0], padding[1], padding[2], 0)
-            else:
-                module_padding = (padding[0], 0, padding[2], padding[3])
         setattr(
             parent,
             name,
             WanDistZeroPad2d(
-                module_padding,
+                padding,
                 group,
-                split_dim=split_dim,
-                split_padding=(padding[2], padding[3]) if split_dim == "height" else (padding[0], padding[1]),
+                defer_split_after_padding=parent.__class__.__name__ == "Sequential",
             ),
         )
         return
     if isinstance(child, nn.Conv2d):
-        split_padding = None
+        deferred_padding = None
         if name == "1" and parent.__class__.__name__ == "Sequential":
             # WanResample downsample uses ZeroPad2d((0, 1, 0, 1)) before a
             # stride-2 conv with padding=0.  Only the last rank should see the
             # bottom/right global padding, which is approximated by split_padding.
             prev = getattr(parent, "0", None)
             if isinstance(prev, WanDistZeroPad2d):
-                split_padding = prev.split_padding
+                deferred_padding = prev.padding
             elif isinstance(prev, nn.ZeroPad2d):
-                pad = prev.padding
-                split_padding = (int(pad[2]), int(pad[3])) if split_dim == "height" else (int(pad[0]), int(pad[1]))
+                deferred_padding = tuple(int(value) for value in prev.padding)
         setattr(
             parent,
             name,
             WanDistConv2d(
                 child,
                 group,
-                split_dim=split_dim,
-                split_padding=split_padding,
+                deferred_padding=deferred_padding,
             ),
         )
 
@@ -670,21 +722,20 @@ def _replace_child(
 def _patch_decoder_modules(
     module: nn.Module,
     group: dist.ProcessGroup,
-    split_dim: str,
     inside_attention: bool = False,
 ) -> None:
     if module.__class__.__name__ == "WanAttentionBlock":
-        _patch_attention_block(module, group, split_dim)
+        _patch_attention_block(module, group)
         inside_attention = True
 
     for name, child in list(module.named_children()):
         if child.__class__.__name__ == "WanCausalConv3d":
-            _replace_child(module, name, child, group, split_dim)
+            _replace_child(module, name, child, group)
             continue
         if isinstance(child, nn.Conv2d) and not inside_attention:
-            _replace_child(module, name, child, group, split_dim)
+            _replace_child(module, name, child, group)
             continue
-        _patch_decoder_modules(child, group, split_dim, inside_attention=inside_attention)
+        _patch_decoder_modules(child, group, inside_attention=inside_attention)
 
 
 def _decoder_upsample_count(decoder: nn.Module) -> int:
@@ -695,16 +746,25 @@ def _decoder_upsample_count(decoder: nn.Module) -> int:
     return count
 
 
+def _clear_wan_spatial_shard_runtime_buffers(vae: Any) -> None:
+    decoder = getattr(vae, "decoder", None)
+    if decoder is None:
+        return
+    for module in decoder.modules():
+        if "_halo_recv_top_buf" in module._buffers:
+            module._halo_recv_top_buf = None
+        if "_halo_recv_bottom_buf" in module._buffers:
+            module._halo_recv_bottom_buf = None
+
+
 def install_wan_spatial_shard_decode(vae: Any, group: dist.ProcessGroup, split_dim: str = "height") -> None:
     """Patch ``vae.decoder`` once for spatially-sharded decode.
 
     This mutates the already-loaded decoder in place by swapping its spatial
-    convolutions/padding for halo-exchanging variants and wrapping
-    ``decoder.forward``. The patch is permanent for the lifetime of the VAE
-    instance and is applied only once (subsequent calls are no-ops). A given
-    instance is bound to a single ``split_dim``; switching between
-    ``"height"`` and ``"width"`` requires a fresh VAE instance and raises here
-    otherwise.
+    convolutions/padding for context-aware variants and attaching a dedicated
+    spatial forward. Outside that forward the replacements execute their exact
+    local PyTorch behavior, so a later auto-selected tile request remains safe.
+    The same installed decoder can alternate height and width sharding.
 
     Only group-relative rank 0 assembles the final decoded frame, mirroring the
     distributed tiled-decode ``broadcast_result=False`` contract; the other ranks
@@ -712,18 +772,12 @@ def install_wan_spatial_shard_decode(vae: Any, group: dist.ProcessGroup, split_d
     """
     _spatial_dim(split_dim)
     if getattr(vae, "_vllm_omni_wan_spatial_shard_installed", False):
-        installed_split_dim = getattr(vae, "_vllm_omni_wan_spatial_shard_split_dim", "height")
-        if installed_split_dim != split_dim:
-            raise ValueError(
-                "Wan spatial-shard VAE decoder was already patched for "
-                f"{installed_split_dim!r} split; create a fresh VAE instance to use {split_dim!r} split."
-            )
         return
     decoder = getattr(vae, "decoder", None)
     if decoder is None:
         raise ValueError("Wan spatial-shard VAE decode requires a decoder module.")
 
-    _patch_decoder_modules(decoder, group, split_dim)
+    _patch_decoder_modules(decoder, group)
     upsample_count = _decoder_upsample_count(decoder)
     orig_forward = decoder.forward
 
@@ -733,6 +787,8 @@ def install_wan_spatial_shard_decode(vae: Any, group: dist.ProcessGroup, split_d
         feat_cache: list[torch.Tensor] | None = None,
         feat_idx: list[int] | None = None,
         first_chunk: bool = False,
+        *,
+        split_dim: str,
     ) -> torch.Tensor:
         if feat_idx is None:
             feat_idx = [0]
@@ -760,10 +816,9 @@ def install_wan_spatial_shard_decode(vae: Any, group: dist.ProcessGroup, split_d
             _SPATIAL_SHARD_CONTEXT.reset(token)
         return gather_and_trim_extent(out, expected_extent=expected_extent, split_dim=split_dim, group=group, dst=0)
 
-    decoder.forward = MethodType(_forward, decoder)
+    decoder._vllm_omni_spatial_shard_forward = MethodType(_forward, decoder)
     vae._vllm_omni_wan_spatial_shard_installed = True
-    vae._vllm_omni_wan_spatial_shard_split_dim = split_dim
-    logger.info("Installed Wan VAE %s-sharded decode.", split_dim)
+    logger.info("Installed Wan VAE dynamic-axis spatial-shard decode.")
 
 
 def spatial_shard_decode(
@@ -792,11 +847,12 @@ def spatial_shard_decode(
             decoded_chunks = []
             for i in range(z.shape[2]):
                 vae._conv_idx = [0]
-                chunk = vae.decoder(
+                chunk = vae.decoder._vllm_omni_spatial_shard_forward(
                     x[:, :, i : i + 1, :, :],
                     feat_cache=vae._feat_map,
                     feat_idx=vae._conv_idx,
                     first_chunk=(i == 0),
+                    split_dim=split_dim,
                 )
                 if produce_output:
                     decoded_chunks.append(chunk)
@@ -810,6 +866,10 @@ def spatial_shard_decode(
                 out = z.new_zeros(0)
     finally:
         vae.clear_cache()
+        # Halo buffers are request-shape scratch space allocated outside the
+        # tagged weight pool. Drop live references so sleep/offload never has
+        # to preserve or discard them as model state.
+        _clear_wan_spatial_shard_runtime_buffers(vae)
 
     if not return_dict:
         return (out,)

vllm_omni/diffusion/models/minimax_h3/vae.py

@@ -155,6 +155,11 @@ class MiniMaxH3VideoVAE(nn.Module, DistributedVaeMixin):
         parallel_size: int,
         mode: str = "tile",
     ) -> None:
+        # ``auto`` is the cross-model default. Spatial selection is Wan-only,
+        # so preserve MiniMax H3's native tile path for that default while
+        # continuing to reject explicitly requested Wan spatial modes.
+        if mode == "auto":
+            mode = "tile"
         if mode != "tile":
             raise ValueError(f"MiniMax H3 VAE supports its native tile parallel mode only, got {mode!r}")
         group = get_dit_group()

vllm_omni/diffusion/worker/diffusion_model_runner.py

@@ -30,6 +30,9 @@ from vllm_omni.diffusion.cache.prompt_embed_cache import (
 from vllm_omni.diffusion.cache.selector import get_cache_backend
 from vllm_omni.diffusion.compile import regionally_compile
 from vllm_omni.diffusion.data import DiffusionOutput, OmniDiffusionConfig
+from vllm_omni.diffusion.distributed.autoencoders.autoencoder_kl_wan import (
+    prepare_pipeline_wan_spatial_shard_decode,
+)
 from vllm_omni.diffusion.forward_context import set_forward_context
 from vllm_omni.diffusion.model_loader.diffusers_loader import DiffusersPipelineLoader
 from vllm_omni.diffusion.models.interface import (
@@ -243,6 +246,7 @@ class DiffusionModelRunner(OmniConnectorModelRunnerMixin):
                     custom_pipeline_name=custom_pipeline_name,
                     device=self.device,
                 )
+                prepare_pipeline_wan_spatial_shard_decode(self.pipeline)
         time_after_load = time.perf_counter()
 
         logger.info(

vllm_omni/engine/arg_utils.py

@@ -505,7 +505,7 @@ class OrchestratorArgs:
     diffusion_kv_cache_skip_layers: str | None = None
     cfg_parallel_size: int = 1
     vae_patch_parallel_size: int = 1
-    vae_parallel_mode: str = "tile"
+    vae_parallel_mode: str = "auto"
     text_encoder_tp_size: int = 1
     default_sampling_params: str | None = None
     max_generated_image_size: int | None = None

vllm_omni/engine/async_omni_engine.py

@@ -972,7 +972,7 @@ class AsyncOmniEngine:
             cfg_parallel_size = normalized_kwargs.get("cfg_parallel_size") or 1
             pipeline_parallel_size = normalized_kwargs.get("pipeline_parallel_size") or 1
             vae_patch_parallel_size = normalized_kwargs.get("vae_patch_parallel_size") or 1
-            vae_parallel_mode = normalized_kwargs.get("vae_parallel_mode") or "tile"
+            vae_parallel_mode = normalized_kwargs.get("vae_parallel_mode") or "auto"
             text_encoder_tp_size = normalized_kwargs.get("text_encoder_tp_size") or 1
             enable_expert_parallel = normalized_kwargs.get("enable_expert_parallel") or False
             use_hsdp = normalized_kwargs.get("use_hsdp", False)

vllm_omni/entrypoints/cli/serve.py

@@ -750,10 +750,11 @@ class OmniServeCommand(CLISubcommand):
         omni_config_group.add_argument(
             "--vae-parallel-mode",
             type=str,
-            default="tile",
-            choices=["tile", "spatial_shard_height", "spatial_shard_width"],
+            default="auto",
+            choices=["auto", "tile", "spatial_shard_height", "spatial_shard_width"],
             help="VAE parallel decode strategy for diffusion models. "
-            "'tile' (default) uses patch/tile parallel decode; "
+            "'auto' (default) selects the longer spatial-shard axis for validated Wan tiled decodes "
+            "on a full DiT group, then falls back to patch/tile decode; 'tile' always uses patch/tile decode; "
             "'spatial_shard_height'/'spatial_shard_width' use spatially-sharded decode that splits "
             "decoder feature maps along height/width and exchanges halo regions. The "
             "'spatial_shard_*' modes require vae_patch_parallel_size to match the DiT group size. "