Zcash's Planned NU7 Upgrade Would Freeze Sprout Funds Even With Private Keys
Zcash proposes closing the Sprout spending route
Zcash's proposed NU7 upgrade would remove the spending path out of Sprout, the network's oldest privacy pool. A pool is a separate part of the network where shielded transactions are handled.
The Sprout pool still held about 22,309 ZEC on Oct. 17, according to the zkp.baby dashboard. Under the proposal, holders could keep their private keys and still not be able to move those funds on the upgraded network.
The change appears in ZIP 2003, a Zcash improvement proposal. Its listed owner is Daira-Emma Hopwood. The proposal would reject version 4 transactions from NU7 onward.
Key numbers and dates
- About 22,309.44195128 ZEC sat in the Sprout pool at block 3,521,260, per the zkp.baby dashboard, updated at 19:10:09 UTC on Oct. 17.
- NU7 mainnet activation is still undecided. A final decision is planned for Oct. 20, with Nov. 5 a conditional target.
- Testnet activation was scheduled for testnet height 4,465,026 around Oct. 6, and an Oct. 10 engineering reply confirmed testnet had passed it.
- ZIP 2003 was created on Oct. 31, 2024.
- The Zcash Foundation announced Zebra 7.0.0-rc.0 on Oct. 2.
Why Sprout has no replacement format
Sprout transfers require version 4 transactions. Version 5 and version 6 do not support them. A private key gives a holder the right to spend, but the network must also accept the transaction that carries that spend.
The restriction covers every version 4 transaction, including transparent-only and Sapling transactions. The results differ by pool. Users can rebuild non-Sprout transactions in a supported format. Sprout has no equal replacement. Re-signing a version 4 transaction does not fix its invalid format.
ZIP 2003 keeps Sprout's remaining balance in issued supply. It allows the theoretical option of re-enabling version 4 transactions or creating another way to retrieve funds. It also warns that spending may never be restored.
What ZIP 2003 says about recovery
Moving to current wallet software does not automatically move the coins. Zallet's migration documentation excludes Sprout spending keys and funds from its import process and tells users to keep their original wallet.dat file.
The Argos recovery tool documents a route for holders who have a legacy wallet file or standalone Sprout spending keys. A wallet with usable note data and a cached witness may avoid a historical scan. With a key alone, recovery may require rebuilding the extra data needed to build a valid spend.
For a full-block scan, Argos estimates about 280 GB of transfer and potentially days of processing, while keeping less than 500 MB on disk. Argos says its Sprout sweeps land in Sapling, a newer shielded pool, including when a Unified Address is supplied. Moving the recovered funds afterward is a separate step.
The privacy trade-off of leaving Sprout
ZIP 308's migration design explains that value moving from Sprout to Sapling passes through public pool accounting. This exposes the amount without requiring a transparent address.
In the scheme specified by ZIP 308, transaction timing is coordinated and amounts are randomized to reduce the chance that individual transactions can be linked. ZIP 2003 warns that a short or poorly coordinated exit period can make distinctive amounts and timing easier to tie to holders.
According to the proposal, the security benefit is shared, while the migration burden falls on individual holders.
What is confirmed
The proposal text, ZIP 2003, sets out the rejection of version 4 transactions from NU7 onward and leaves Sprout's remaining balance in issued supply. The Sprout balance figure comes from the zkp.baby dashboard as of Oct. 17. Testnet implementation is running, and the Zcash Foundation announced Zebra 7.0.0-rc.0 with a testnet activation that an Oct. 10 engineering reply said had passed.
What is still unclear
Mainnet activation is not final. The NU7 deployment specification leaves the activation height unassigned. The Oct. 20 decision follows testnet experience, and Nov. 5 is only a target that depends on that decision. ZIP 2003 also leaves open the theoretical possibility of restoring spending later, while warning it may never return.
Why it matters
Sprout is a legacy privacy pool with little use, but it still holds funds. If NU7 activates as proposed, those holders would need to recover and exit before the change, using tools such as Argos, with the costs and privacy exposure described in the ZIPs.
Sources
Newisty Editorial Team covers technology, cryptocurrency, digital products, online platforms, developer tools and the wider digital economy. Our content is researched from official sources, company announcements, public documentation, market data and other primary or reputable sources. Articles are reviewed and edited before publication for clarity, accuracy and useful context.
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