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BIP 433 Pay to Anchor: Keyless Fee-Bump Guide
BIP 433 guide to Draft keyless Pay to Anchor outputs, CPFP fee bumping, standardness, ephemeral dust, TRUC anti-pinning and wallet safety.
This guide explains BIP 433 Pay to Anchor (P2A) in plain English. It covers the problem behind the BIP, why it matters and whether the proposal is part of Bitcoin today.
TL;DR
- What it is: BIP 433 is a Draft informational description of Pay to Anchor, a compact keyless output intended to support child-pays-for-parent fee bumping of presigned transactions. The output uses a fixed witness-v1 program and spends with no signature or witness data.
- Why it matters: P2A is a deliberately keyless fee-bump hook, not a protected output. It is useful when combined with correct package economics, topology controls and monitoring that assumes any network participant may construct the child spend.
- Current position: BIP 433 is a Draft informational description of Pay to Anchor, a compact keyless output intended to support child-pays-for-parent fee bumping of presigned transactions.
What this means in simple English
A BIP is a document that proposes or explains a Bitcoin idea, standard or rule. A BIP number does not mean the idea is active. Some BIPs are widely used, while others are drafts or historical proposals.
You do not need to read code to understand the main point. Start with the status, the problem being addressed and the practical effect on ordinary users, wallets, miners or node operators.
Simple example
Think of a mempool as one node’s waiting room for valid transactions that have not yet entered a block. Different nodes can have different waiting rooms, and the contents can change from one second to the next.
Key terms in plain English
- BIP:
- Bitcoin Improvement Proposal: a document describing a proposed rule, standard or process. Its status must be checked separately.
- Consensus:
- The shared rules full nodes use to decide whether blocks and transactions are valid.
- Node:
- A computer running Bitcoin software that checks data and communicates with other peers.
Status and scope
BIP 433 is Draft and Informational at the applications layer. It documents a standard output and spending policy rather than changing consensus validity.
Script form
The P2A script is OP_1 followed by the two-byte 0x4e73 witness program. Its published mainnet, testnet and regtest addresses are fixed representations, not user-derived custody addresses.
Keyless spend
A standard P2A input carries no witness. There is no privileged key, so any party can attach a child that contributes fees to the package.
Fee-bump purpose
The pattern gives a presigned parent transaction a small hook for later CPFP when the required feerate is known. Watchtowers can bump without receiving special anchor keys.
Griefing risk
Public spendability also lets an adversary attach an undesirable child. The BIP recommends considering TRUC topology limits and other anti-pinning measures rather than treating P2A alone as protection.
Related policies
P2A and ephemeral dust are distinct. Its default dust threshold is 240 satoshis; ephemeral-dust rules may allow a single dust output in a zero-fee parent that is spent within a package.
How specialists test it
Developers test the proposal with made-up data on an isolated test network. They check normal cases and deliberately invalid cases. Different implementations should reach the same result before anyone relies on the proposal.
Frequently asked questions
What is BIP 433 Pay to Anchor?
BIP 433 is a Draft informational description of Pay to Anchor, a compact keyless output intended to support child-pays-for-parent fee bumping of presigned transactions. The output uses a fixed witness-v1 program and spends with no signature or witness data.
Is BIP 433 Pay to Anchor active or supported today?
BIP 433 is a Draft informational description of Pay to Anchor, a compact keyless output intended to support child-pays-for-parent fee bumping of presigned transactions.
Why does BIP 433 Pay to Anchor matter?
P2A is a deliberately keyless fee-bump hook, not a protected output. It is useful when combined with correct package economics, topology controls and monitoring that assumes any network participant may construct the child spend.
Do beginners need to use the technical details?
No. Most readers only need to understand the idea and its current status. Developers and node operators can use the primary sources when they need the exact technical rules.
Conclusion
P2A is a deliberately keyless fee-bump hook, not a protected output. It is useful when combined with correct package economics, topology controls and monitoring that assumes any network participant may construct the child spend.
Primary sources
Check the current specification status and the documentation for the exact implementation you operate before moving production funds or changing a mining node.
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