Definition
AnyPrevOut, abbreviated APO and specified in BIP-118, is a proposed new signature-hash (SIGHASH) mode for Bitcoin. A normal signature commits to the exact previous output (UTXO) being spent, so it is valid for that one input and no other. An ANYPREVOUT signature deliberately omits the input reference, so the same signature can later be "rebound" to any output protected by a matching script and key.
From SIGHASH_NOINPUT to APO
The idea was first floated as SIGHASH_NOINPUT and refined into BIP-118's two flags: SIGHASH_ANYPREVOUT (which still commits to the script) and SIGHASH_ANYPREVOUTANYSCRIPT (which does not). Pairing it with Taproot keeps the feature opt-in and contained to specific spending paths, so it does not weaken ordinary transactions.
Why it matters for Lightning
APO's headline use case is eltoo, a simplified Lightning channel-update mechanism. In today's penalty-based channels, each party must store every old channel state forever and risks losing funds if they ever broadcast a stale one. With rebindable signatures, any newer state can spend an older state's transaction, so the protocol simply enforces "latest state wins" — no toxic data to retain, no catastrophic penalty for a mistake. APO also underpins designs like blind merged mining for spacechains.
AnyPrevOut is a soft-fork proposal, not yet active consensus. For wider context on the second-layer mechanics it supports, D-Central's reference also covers ephemeral anchors and other fee-bumping tools used in modern channel constructions.
In Simple Terms
AnyPrevOut, abbreviated APO and specified in BIP-118, is a proposed new signature-hash (SIGHASH) mode for Bitcoin. A normal signature commits to the exact previous output…