SIGHASH_ANYPREVOUT (APO)

SIGHASH_ANYPREVOUT (APO), specified in BIP 118 and formerly known as SIGHASH_NOINPUT, is a proposed Taproot signature mode. A standard signature commits to the specific previous output (UTXO) being spent. An ANYPREVOUT signature omits that commitment, so the same signature can later be rebound to any other output that shares the matching script. APO is a proposed soft fork and is not active on Bitcoin mainnet, though it has been deployed on test networks such as Signet for experimentation.

Two modes

BIP 118 defines two related sighash flags. SIGHASH_ANYPREVOUT still commits to the spent output's script (its scriptPubKey and tapleaf), so it binds to outputs using that script but not to a particular UTXO. SIGHASH_ANYPREVOUTANYSCRIPT is broader, committing to even less, which makes a signature reusable across a wider set of outputs.

Primary use case: eltoo

APO's flagship motivation is eltoo, a simplified Lightning Network channel-update mechanism. In today's penalty-based channels, publishing an outdated state risks losing funds. With rebindable signatures, each new channel state can simply spend any earlier state, so honest parties only ever need to publish the latest one, removing the punishment model. APO can also support statechains and certain covenant-like constructions, though less efficiently than purpose-built opcodes.

APO is often grouped with the broader covenant discussion even though its core purpose is signature flexibility. This entry describes an experimental proposal under review, not a settled feature.