Bitcoin TRUC Transaction

Overview

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. TRUC transactions are treated as replaceable under the relevant node policy, which supports package replacement and sibling-eviction designs intended to reduce pinning.

TRUC restrictions apply until confirmation. Once an output is confirmed, a later spender does not inherit the same unconfirmed topology simply because the confirmed parent used version 3. 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.

For wallet developers, using TRUC requires constructing transactions that remain within the policy limits and ensuring fee-bumping paths are available. A transaction can be consensus-valid yet fail to enter a node’s mempool if it violates local policy. Applications should monitor relay acceptance, parent-child state, replacement results, and eventual block confirmation. They should also avoid presenting TRUC as a guarantee against every form of pinning, because protection depends on policy adoption and the surrounding transaction design.