Pay-to-anchor
Pay-to-anchor (P2A) is a keyless native Segregated Witness version 1 output type described by BIP 433. The scriptPubKey is OP_1 <0x4e73>, serialized as the bytes 0x51 0x02 0x4e 0x73. On mainnet the corresponding Bech32m address is bc1pfeessrawgf. Spending requires an empty witness. Anyone able to broadcast a child that spends the output can attach fees. P2A names the output template. It is not the same as a keyed Lightning option_anchors output, and it is not the same as an ephemeral anchor, which is a usage pattern that combines a P2A output with TRUC policy and ephemeral dust.
Template
Because the program is a native witness output, BIP 141 requires a blank scriptSig, so the spending transaction's txid is stable against scriptSig malleation. Bitcoin Core treats a non-empty witness as nonstandard for this template, which keeps the wtxid stable against witness padding under standard relay. Consensus still accepts any witness. BIP 433 recommends combining P2A with TRUC to bound pinning by an oversized low-feerate child. Bitcoin Core 28.0 made P2A spends standard. Core 29.0 added ephemeral-dust policy for a zero-value P2A spent in the same package.
The fixed mainnet address bc1pfeessrawgf is shared by all P2A outputs of that template. Block explorers therefore show many unrelated protocols paying to the same script. Wallet coin selection should treat P2A outputs as fee anchors rather than as ordinary user funds.
Use with TRUC
P2A is intended as a shared, anyone-can-spend fee anchor for packages that need a stable txid and a place to attach fees without a key ceremony. Contracting protocols can create a zero-fee or low-fee parent that pays to P2A and then broadcast a TRUC child that spends the anchor at a competitive feerate. Because the witness is empty under policy, counterparties do not need to coordinate a signature just to attach fees. Keyed Lightning anchor outputs remain a separate template used inside many channel commitment designs.
Because anyone can spend a P2A output under policy, fee-bumping races are open. Counterparties may broadcast competing children. TRUC sibling eviction keeps only the more incentive-compatible child beside the parent. Without TRUC, keyed anchors or carve-out-era policies were used to reserve a bump path for each party.
Ephemeral-anchor packages are the usual contracting pattern: create the P2A output and spend it in the same unconfirmed TRUC child so the UTXO set never retains the anchor after confirmation.