Zcash’s planned Sprout freeze would leave private keys unable to move funds

Zcash's proposed NU7 upgrade would close the spending route out of its oldest privacy pool, Sprout, which still held about 22,309 ZEC on Oct. 17. Holders could keep their private keys and still lose the ability to move those funds on the upgraded network.

Mainnet activation remains undecided. The NU7 deployment specification leaves the activation height unassigned, with a final decision planned for Oct. 20 after testnet experience. Nov. 5 remains the target, conditional on that decision.

The zkp.baby dashboard recorded 22,309.44195128 ZEC in Sprout at block 3,521,260, updated at 19:10:09 UTC on Oct. 17.

The change is set out in ZIP 2003, a Zcash improvement proposal whose listed owner is Daira-Emma Hopwood. It proposes rejecting version 4 transactions from NU7 onward. Sprout transfers require that format; neither version 5 nor version 6 supports them.

A private key authorizes a spend, but the network must also accept the transaction carrying it. Under the proposed rules, Sprout holders would have no supported format in which to exercise that authority.

The restriction reaches every version 4 transaction, including transparent-only and Sapling transactions. The consequences differ by pool. Users can recreate non-Sprout transactions in a supported format where possible. Sprout has no equivalent replacement. Merely re-signing a version 4 transaction cannot fix its invalid format, and even newer formats need signatures appropriate to the consensus branch.

ZIP 2003 explicitly leaves Sprout's remaining balance in issued supply. It allows the theoretical possibility of re-enabling version 4 transactions or creating another retrieval facility, while warning that spending may never be restored.

Moving to current wallet software does not necessarily move the coins. Zallet's migration documentation explicitly excludes Sprout spending keys and funds from its import process and tells users to preserve their original wallet.dat.

The Argos recovery tool documents a route for holders with a legacy wallet file or standalone Sprout spending keys. A wallet containing usable note data and a cached witness can avoid a historical scan. With a key alone, recovery may require reconstructing the additional data needed to build a valid spend.

For that full-block scan, Argos estimates roughly 280 GB of transfer and potentially days of processing, while retaining less than 500 MB on disk.

Argos also documents a specific destination: its Sprout sweeps land in Sapling, a newer shielded pool, including when a Unified Address is supplied. Moving the recovered funds onward is a separate operation.