Steelhead: Interleaving Partially Synchronous and Asynchronous Commit Rules on a Shared DAG
Steelhead interleaves partially synchronous and asynchronous DAG commit rules and adapts without extra messages.
Steelhead is a dual-mode consensus mechanism that applies an asynchronous commit rule every k-th DAG round and a partially synchronous rule otherwise. Validators periodically replay the committed DAG, adopt the period with the fewest expected message delays, and fall back to k=1 if output stalls, while the asynchronous rule on coin rounds preserves liveness. It sends no messages beyond DAG blocks. Instantiations with Mysticeti and Mahi-Mahi at n at least 3f+1, and BlueBottle variants at n at least 5f+1, have safety and liveness proofs including Lean 4 mechanization; simulations match the partially synchronous protocol on a healthy network and stay near the asynchronous one when that protocol stalls.