Ancestor and Descendant Limits

Ancestor and descendant limits are mempool-policy caps on how many unconfirmed transactions may be linked together in a chain of spends. An "ancestor" set is every unconfirmed parent a transaction depends on; a "descendant" set is every unconfirmed child that builds on it. By default Bitcoin Core long capped each set at 25 transactions and 101 kvB of virtual size, rejecting anything that would exceed these with the error too-long-mempool-chain.

Why the Limits Exist

Every time a transaction enters or leaves the mempool, a node must recompute the aggregate fees and sizes of all connected transactions to keep mining priority and eviction decisions correct. Without a bound, an adversary could build enormous chains that force expensive recalculation on every node — a denial-of-service vector. The limits cap this worst-case work, and they also feed directly into CPFP fee-bumping: a descendant chain that is already at the limit leaves no room to attach a new high-fee child, which is precisely what a pinning attack exploits.

The Move to Cluster Mempool

Bitcoin Core 31.0 replaced separate ancestor/descendant size and count enforcement with a unified "cluster" model. Instead of bounding chains in each direction, it limits any connected component of related transactions to 64 transactions and 101 kvB, simplifying the accounting and improving mining-template construction.

These limits sit at the heart of fee-bumping economics. See Anchor Outputs and the CPFP carve-out for how protocols reserve room within them, and Mempool Policy for the configuration flags that adjust them.