Hierarchical reasoning architectures split a problem into a coarse planning stage and a finer-grained execution stage, rather than solving the whole problem at one resolution. A coarse stage reasons over the full scope of the problem cheaply, producing a plan; a fine stage fills in detail conditioned on that plan, operating over a smaller effective context because the hard long-range consistency problem has already been resolved by the coarse stage.
Where it helps
This decomposition pays off when a problem has two distinct difficulties at different scales — long-range structural consistency, and short-range fine detail — that are expensive to solve jointly at full resolution. Spatiotemporal generation is a clear example: reasoning about motion and scene structure across an entire video clip and rendering per-pixel detail in a single frame are different kinds of hard.
The characteristic failure mode
The fine stage is only as good as the coarse plan it's conditioned on. If the coarse stage commits to a wrong or under-specified plan, the fine stage generally has no mechanism to override it — it can only add detail within what the plan allows. Output quality ends up bounded by the coarse stage's decisions.