On compute capability > (9, 0) GPUs (Blackwell sm_120, e.g. RTX PRO 6000),
xformers' memory_efficient_attention has no kernel that accepts a tensor
attn_bias — fa3F/cutlassF require cap <= (9,0), fa2F/fa3F reject a tensor
bias. So unmasked attention works (i2v ANCHOR scenes generate) but MASKED
attention crashes: scenes generated from GUIDES hit the LTX guide path
(comfy/ldm/lightricks/model.py _attention_with_guide_mask) which passes a
tensor mask → "No operator found for memory_efficient_attention_forward".
This is the ComfyUI-wide issue normally worked around with
--disable-xformers --use-pytorch-cross-attention; when that flag isn't in
effect on an instance, the guide path still reaches xformers.
Fix: _funpack_install_mask_safe_attention(model), called at the top of the
chain sampler's sample(). When the active backend is xformers AND the device
capability is > (9,0), install ComfyUI's per-call optimized_attention_override
(the wrap_attn hook) so MASKED attention falls back to attention_pytorch
(SDPA handles masks on every GPU) while UNMASKED attention stays on the fast
xformers path. Threaded via transformer_options so it reaches every scene's
forward. No global monkeypatch; no recursion (wrap_attn stamps
_inside_attn_wrapper before calling the override). No-op on supported GPUs,
non-xformers backends, or when another override is already installed;
idempotent.
Tests: tests/test_mask_safe_attention.py — 9 cases covering override routing
(masked→pytorch via kwarg or positional, unmasked→original) and install
gating (xformers+Blackwell installs; supported cap / non-xformers / no-CUDA
skip; existing override respected; idempotent). Run with the ComfyUI venv.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
