Splice-In

Splice-in is the act of adding fresh on-chain bitcoin to an already-open Lightning channel using one cooperative transaction, while the channel stays alive. Before splicing, the only way to grow a channel was to close it and open a larger one, which cost two on-chain transactions, paused the channel, and reset its history. Splice-in folds new UTXOs into the existing funding output so capacity increases without the channel ever going offline.

One transaction, no downtime

Both peers cooperatively build a transaction that spends the current funding output plus the new inputs, producing a new, larger funding output that the channel immediately continues from. Off-chain balances and the channel's operating history carry over, so payments can resume as soon as the splice is broadcast, often before it even confirms in implementations that support unconfirmed-spend chaining.

Why operators use it

Splice-in is how a routing node tops up outbound liquidity on a profitable channel, or how a merchant adds capacity to a channel that is performing well, all without the fee and downtime penalty of close-and-reopen. It also avoids fragmenting funds across many small channels, keeping capital concentrated where it earns routing fees.

Splice-in is the capacity-increasing half of channel splicing; its counterpart for withdrawing funds is covered in our splice-out entry, and both operate on the underlying payment channel funding output.