Document VAE debug handoff

PLAN.md

@@ -55,3 +55,47 @@ The direct runner must first match the model contract and output quality. Beatin
 - Validate denoiser outputs at each scheduler step, not only encoded video.
 - Record attention, GEMM, communication, VAE, and end-to-end timings separately.
 - Treat SageAttention3 as an experimental quality-gated kernel for H3.
+
+## 2026-08-13 VAE Debug Handoff
+
+Current saved latent and comparison assets live under:
+
+- Spark: `/home/daniel/StoryStudioAssets/H3-output/h3-blackwell-runtime`
+- Share: `\\192.168.1.162\StoryStudioAssets\H3-output\h3-blackwell-runtime`
+
+Key assets:
+
+- Good sampled latent: `direct-cat-house-backflip-disco-shades-960x544-5s-latent.pt`
+- Known-good same-latent Comfy/upstream VAE decode: `direct-cat-house-backflip-disco-shades-960x544-5s-upstream-comfy-tiled-decode.mp4`
+- Current direct standalone FP32/Sage VAE decode candidate: `direct-cat-house-backflip-disco-shades-960x544-5s-direct-fp32-sage-vae-decode.mp4`
+
+What is proven:
+
+- The latent is good. Same latent decoded through Comfy/upstream VAE is visually clean.
+- The sampler/model path is already exact against Comfy free-run parity for the fixed baseline.
+- Direct VAE BF16/checkpoint-dtype loading was wrong. Loading direct VAE weights as FP32 reduced isolated decoder clip drift from roughly `mean=1.5e-3, max=1e-1` to roughly `mean=1e-6, max=6e-5`.
+- Individual direct tiled VAE clips match upstream closely (`mean` around `4e-7`).
+- Full direct `decode_temporal()` still differs from upstream at global frames `17, 34, 51, 68, 85, 102`, i.e. temporal join boundaries.
+- Temporal assembly tracer shows pre-blend current chunks and overlap tails each match upstream, but blended join output differs hugely (`mean` around `0.15`, max around `5`) when comparing direct blend result to upstream blend result using their respective near-identical inputs.
+- Direct and upstream `blend()` return identical results on the exact same inputs, so the remaining issue is likely an input/aliasing/dtype/shape subtlety at the temporal join, not the blend formula itself.
+
+Relevant debug tools committed:
+
+- `tools/decode_video_latent.py`
+- `tools/compare_frame_dirs.py`
+- `tools/compare_vae_decoder_clip.py`
+- `tools/compare_vae_full_decode.py`
+- `tools/compare_vae_tiled_clip.py`
+- `tools/compare_vae_temporal_assembly.py`
+
+Next VAE debugging steps:
+
+- In `tools/compare_vae_temporal_assembly.py`, compare `prev_d` vs `prev_u` and `part_d` vs `part_u` after casting both pairs to a shared dtype and before blending. The current stats say they are close, but the blend of respective inputs explodes, which suggests a subtle shape/stride/dim broadcasting mismatch.
+- Log `shape`, `stride`, `dtype`, `is_contiguous`, and `storage_offset` for `prev_*`, `part_*`, blend weights, and slices at `chunk_1_blended_at_17`.
+- Try forcing `prev_d`, `prev_u`, `part_d`, and `part_u` to `.contiguous()` immediately before temporal blend in both direct and tracer paths.
+- If that fixes it, patch direct `decode_temporal()` only. If not, compare exact selected slices (`prev[..., -9:, :, :]`, `part[..., :9, :, :]`) elementwise before and after multiplying weights.
+
+Backends/experiments to evaluate later:
+
+- Attention backend sweeps after VAE quality is fixed: `sage2`, `sage3`, `sdpa`, plus future FlashAttention-4/Sol-Attn/KJ candidates.
+- Spectrum MiniMax H3 repo to inspect later: https://github.com/xmarre/ComfyUI-Spectrum-MiniMax-H3