Bitcoin TRUC Transaction

Overview

Meaning and scope

A Bitcoin TRUC transaction is an unconfirmed transaction that opts into Topologically Restricted Until Confirmation policy by using transaction version 3. Supporting Bitcoin Core nodes apply strict limits to its unconfirmed parent-child structure, size, and replacement behavior. These restrictions make fee bumping more predictable and reduce transaction pinning risks.

Mechanism and operation

A Bitcoin TRUC transaction is a transaction that signals Topologically Restricted Until Confirmation behavior by setting its version field to 3. Nodes implementing BIP 431 apply additional mempool and relay policy rules while the transaction remains unconfirmed. The goal is to constrain the dependency graph so wallets and contract protocols can reason more reliably about fee bumping and replacement. Under the policy, unconfirmed TRUC transactions form an all-TRUC topology. A TRUC transaction may have at most one unconfirmed ancestor and one unconfirmed descendant, producing a one-parent, one-child structure. Size limits also apply, including a tighter limit for a child that spends an unconfirmed TRUC parent.

Practical payment example

A wallet constructs a version-three unconfirmed Bitcoin transaction and checks relay-policy limits on its unconfirmed parent-child structure.

Operational checks

TRUC transactions are treated as replaceable under the relevant node policy, which supports package replacement and sibling-eviction designs intended to reduce pinning. The term therefore describes a temporary policy treatment, not a new class of coin or a permanent property of the output. It is also distinct from transaction type systems in which a type changes consensus serialization or execution semantics.

Distinctions and limits

Once an output is confirmed, a later spender does not inherit the same unconfirmed topology simply because the confirmed parent used version 3. For wallet developers, using TRUC requires constructing transactions that remain within the policy limits and ensuring fee-bumping paths are available.

Related reading

Related definitions: transaction, mempool.