Ephemeral Anchors

P2A is a keyless standard output with scriptPubKey OP_1 <0x4e73>. Spending it requires neither a signature nor witness data, so anyone can use it as a compact attachment point for a child that adds fees. [1]

Bitcoin Core 28.0 made P2A spends standard and allowed limited relay of a 1-parent-1-child package. A low-fee or zero-fee parent can therefore be evaluated together with a child that pays enough for the complete package. [2]

Ephemeral dust policy in Bitcoin Core 29.0 permits one dust output when a zero-fee transaction creates it and the first unconfirmed descendant spends it together with the other intended outputs. Dust creation and removal therefore belong in the same package. [3]

Anyone can spend P2A and create the sponsoring child, improving interoperability with services such as watchtowers but also allowing an outsider to attach an unattractive child. BIP 433 therefore recommends combining it with TRUC when in doubt. [1]

P2A spending has been standard in Bitcoin Core since version 28.0, but its relay depends on node support. A typical use is a presigned contract transaction such as a Lightning commitment, whose required fee is added by a child only when the parent is published. [2]

Neither P2A nor ephemeral dust is a consensus change. They are standardness, mempool, and relay policy, so a valid block may include a transaction that an older or differently configured node did not accept into its mempool. [3]

P2A and ephemeral dust are not synonyms and do not inherently depend on each other. The historical term Ephemeral Anchors usually meant their combination; a keyed anchor can use ephemeral dust, and P2A itself can also be used without TRUC. [1]

Before use, verify the node version and policy, the P2A script, the parent's zero fee under ephemeral dust, the single dust output, its joint spend in the package, the TRUC topology, and replacement rules. Acceptance by one node does not guarantee relay or confirmation. [4]