Mempool policy

Consensus decides what a block may legally contain; policy decides what gets passed along beforehand. Bitcoin Core caps a standard transaction at 400,000 weight units and refuses anything paying below its relay floor, and neither limit appears in the consensus rules. A transaction can be perfectly valid and still travel nowhere.

How it works

Policy is a second gate standing in front of the consensus check. A transaction that arrives at a node must be valid, and it must also look like the kind of transaction the operator is willing to store and forward. Fail the first test and the transaction is invalid everywhere; fail the second and it is merely unwelcome here, and may be perfectly acceptable to the node next door.

Bitcoin Core's defaults cover several kinds of limit. Standardness restricts which script types get relayed, so novel or malformed spending conditions do not propagate before the network understands them. Size limits cap a standard transaction at 400,000 weight units, a quarter of a block. Fee limits set the relay floor, cut from 1 sat/vB to 0.1 in version 30.0, and the dust threshold on individual outputs, which stayed at its old price of 3 sat/vB. Topology limits cap connected groups of unconfirmed transactions at 64 members and 101 kilobytes of virtual size, the cluster rules that replaced the older 25 ancestor and 25 descendant counts in version 31.0.

Every one of those numbers is a setting rather than a law. An operator can raise the relay floor with -minrelaytxfee, change how much data an OP_RETURN output may carry with -datacarriersize, or run Bitcoin Knots, which ships deliberately stricter defaults. The 83 byte cap on data carrier outputs became the loudest policy argument in years and ended with Bitcoin Core 30.0 raising the default to 100,000 bytes, effectively uncapping it while leaving -datacarriersize=83 available to anyone who wants the old behavior.

Policy also moves faster than consensus. Full replace-by-fee became the default in Bitcoin Core 28.0 in October 2024, and the same release added opportunistic relay of a parent and child together, both changes taking effect as soon as operators upgraded, with no fork of any kind.

Where you see it

Policy shows up as an error message from your own wallet, long before any miner is involved. Rejections naming the relay fee, dust outputs, an oversized cluster of unconfirmed spends, or a nonstandard script are all policy talking, and each one means the transaction was never handed to a single peer.

It also explains why a transaction sometimes exists on one explorer and not another. Explorers report their own node's mempool, so a transaction filtered by that node's policy is invisible there while confirming normally elsewhere. Miners can be sent transactions directly through submission endpoints, bypassing relay entirely, which is how nonstandard transactions occasionally appear in blocks having never been seen waiting.

For anyone buying and holding, the practical takeaway is narrow but useful. Policy is why a wallet refuses to create a 300 satoshi output, why a chain of unconfirmed spends eventually stops accepting another child, and why a stuck transaction sometimes needs rebroadcasting rather than bumping.

Mempool policy vs consensus rules

Breaking a consensus rule makes a block invalid and gets it rejected by the entire network, permanently. Breaking a policy rule just means your transaction waits, or never travels at all, while remaining perfectly legal to mine. Policy is also where soft forks are rehearsed: new spending types are usually made standard for relay before or alongside activation, so the network can carry them safely. Consensus is the constitution; policy is the house style of each node.