LayerTree Models 1,149-vB Bitcoin Exits for 1 Million Members

Lightning Labs CTO Olaoluwa Osuntokun published LayerTree on Thursday, Oct. 15, as a design for placing many users’ Bitcoin (BTC) balances behind a shared on-chain output while allowing each user to leave without the operator’s permission. The proposal would consolidate balances into one output, but it depends on Bitcoin script capabilities that are not active.

Bitcoin tracks funds in discrete outputs. LayerTree would keep individual balances in an off-chain Merkle-sum tree and commit the tree’s root to the shared output. Bitcoin script would check that changes match the commitment.

A participant could exit in a single transaction whether the tree held dozens of members or more than a million. The proof included in that transaction expands as the tree gains levels, increasing its size even though the transaction count stays the same.

A FULL-model exit from a 1,048,576-member tree is estimated at 1,149 virtual bytes (vB). That figure measures transaction size, not the fee. Earlier coverage of the opcode-dependent estimate examined a separate, lower 581-vB scenario that relies on OP_MERKLESUMUPDATE, an opcode that has not been proposed for Bitcoin. The broader design also requires script capabilities Bitcoin has not activated, so these are modeled exits rather than a feature users can currently use.

The operator can reject new members but holds no key over existing members’ balances. Before funding a grafted branch, participants must check their own proof path; an unverifiable commitment could leave value frozen.

LayerTree also outlines a way to place Lightning channels in tree leaves and receive payments through a “credit leaf” using vouchers. In that flow, the operator settles a payment before signing a voucher for its recipient, who relies on the operator in the interval between those steps.

Each update would spend and recreate the shared output, which can make exits from one tree wait in sequence. Large deployments could use multiple trees, while a single tree may struggle if all its members try to leave at once without cooperation.