Key Pool

The key pool is a Bitcoin Core wallet implementation detail: a cache of pre-generated keys (and addresses) so that fresh addresses are instantly available without on-demand key generation, and so backups remain valid across some window of address creation.

In the original legacy (non-HD) wallets, the key pool was load-bearing. Each generated key was independent random data; if the user requested addresses faster than the wallet backed itself up, the backup could become stale and addresses created after the backup wouldn't be recoverable. The pool (default 100 keys) was a buffer: as long as you backed up after the pool was generated, you were covered for the next 100 addresses.

After HD wallet support landed in Bitcoin Core 0.13.0 (2016), the keypool became less load-bearing. HD derivation lets the wallet regenerate any future key from the seed, so a backup made at any time covers keys that did not exist yet. But the keypool stayed, for two reasons. Addresses are still pre-generated. And the pool doubles as a lookahead window: a restored backup only notices payments to keys it has already derived, so Bitcoin Core raised the default from 100 to 1,000 keys in 0.15.0 (2017) to make recovery more robust. Other HD wallets use a much smaller window, the BIP 44 gap limit: a recovering wallet stops scanning after 20 unused addresses in a row.

Bitcoin Core's descriptor wallets, the only kind since 30.0 (2025) dropped legacy wallets, keep the same idea in a different shape: each output descriptor covers a range of keys, 0 to 1,000 by default, and the wallet derives ahead within it. For users, it's invisible. Wallets just produce fresh addresses on demand, and backup is one seed phrase. The keypool plumbing is one of those things that makes "just use the wallet" work.