What happened
On 13 August 2026,
BIP-89 "Chain Code Delegation",
by Jesse Posner and Jurvis Tan, was advanced to Deployed status in Bitcoin's proposal
repository. The pull request that moved it
carries the evidence that status requires: "Chain code delegation was rolled out and
deployed to all Bitkey customers on November 4th, 2025." It is an applications-layer
specification, so nothing about the network changed on that date.
What it changes
The problem it addresses is the ordinary way a multisig wallet is set up. To co-sign, every participant is handed an extended public key, an xpub - or a descriptor containing them. The BIP's motivation states the consequence plainly: "In multisig deployments, sharing extended public keys (xpubs) or descriptors enables all participants to scan the chain and infer counterparties' activity."
That is worth unpacking, because the leak is bigger than it sounds. An xpub is an ordinary public key from an asymmetric keypair with one extra 32-byte value attached: the chain code. The chain code is the ingredient that lets anyone holding it derive the next child public key, and the one after that, without limit. Give a collaborative-custody provider an xpub so they can hold one key of a 2-of-3, and you have also given them the ability to enumerate every address that branch will ever produce, and to look up all of them on the chain.
Chain code delegation withholds the chain code and replaces it with a per-input scalar tweak, handed over at the moment of signing. The co-signer holds what the specification calls a non-extended keypair, and receives only "the minimum per-spend data needed to sign". They can verify and sign the input in front of them. They cannot derive the next address, because they were never given the thing that derives addresses.
What it does not change
The co-signer still sees what it signs. This buys privacy against enumeration of the whole wallet, not invisibility, the inputs, amounts and destination of a transaction that is actually being co-signed are exactly as visible as before.
It is also not a security improvement. The quorum is unchanged, and a co-signer's key is worth precisely what it was worth yesterday to anyone who steals it. The asymmetry is deliberate and runs one way: the privileged participant still sees everything, so this protects a wallet owner from their provider and never the reverse.
And Deployed is a claim about the specification, not about your wallet.
BIP-3 defines the status as
reachable "upon request by any community member with evidence that the BIP is in active
use" - here, one product, since November 2025. A wallet that does not implement it did not
gain anything on 13 August.
Context
The tradeoff is the interesting part, and it is the same one that runs through multisig generally: a participant who cannot derive your addresses also cannot independently reconstruct the wallet from their key alone. Deciding whether that matters means knowing what each key holder in your setup is expected to do in a recovery, which is a question worth answering before the setup rather than during it.