Key Takeaways About 12,226 ZEC remained in Sprout Thursday morning. Version-4 transactions become invalid at block 3,543,405. Exits slowed to about 40 ZEC on Wednesday. Stranded Sprout balanc
About 12,226 ZEC was still in Sprout on Thursday morning, down from roughly 22,263 a week earlier, according to ZecStats figures cited in CoinDesk’s report. Most of that exit happened on Sunday, when about 9,094 ZEC left the pool. At the $1,300 price CoinDesk used, what remains is worth about $16 million.
The cutoff is block 3,543,405, the Mainnet activation height set in ZIP 259, the NU7 deployment specification, and repeated in the Zcash Foundation’s Zebra 7.0.0 release notice. CoinDesk reported that Blockchair showed the chain at block 3,542,391 at 05:24 UTC on Thursday, which at the current 75-second block target put activation near 02:30 UTC on Friday. Miners set the real pace, so that estimate can drift in either direction.
The rule behind the deadline is wider than Sprout. ZIP 2003 disallows every version-4 transaction from NU7 onward, and ZIP 259 states that this includes transparent-only and Sapling payments. Sprout gets caught because neither surviving format supports it: version 5, introduced with NU5, and version 6, introduced with NU6.3.
A wallet that refuses to send can therefore be facing either of two problems, and only one of them survives activation.
Coins still in Sprout
What breaks: the only format able to spend them stops being valid.
After the block: no supported replacement format exists.
Window: closes at block 3,543,405.
Transparent or Sapling ZEC in an outdated wallet
What breaks: the software builds version-4 payments the network rejects.
After the block: the balance stays spendable, and software that builds version 5 or 6 can recreate the payment. Re-signing the old one does not work.
Window: stays open with updated software.
Payments prepared in advance need their own check. ZIP 259 cautions that version-5 and version-6 transactions are not automatically valid across activation either, because their signatures commit to a consensus branch ID. A payment saved or signed offline before the upgrade may fail if it reaches a block after the cutoff, whatever its format.
Custodial balances sit with the platform holding them. The Zcash Foundation’s notice addresses the version-4 rule directly to wallets, exchanges and other services, which build transactions on their users’ behalf.
For Sprout holders, the harder problem is finding software that can still build the transfer. ZIP 2003 notes that the deprecation of zcashd, shortly before NU6.3, removed the only maintained software that still offered Sprout wallet functionality. Consensus rules accept Sprout spends until activation, but holders are left relying on third-party recovery tools to create them.
Identifying the coins comes first. Zcash’s address documentation lists Sprout shielded addresses with a zc prefix and Sapling addresses with zs. The prefix shows which pool an address belongs to, not whether any balance remains there.
The backup decides more than the address does. Zcash’s backup instructions treat legacy shielded keys separately from the seed behind Sapling addresses. The developer of the Argos recovery app adds two details in its documentation. A ZecWallet Lite seed cannot reconstruct the Sprout keys that wallet generated independently. A zcashd wallet.dat has no master seed at all, because its keys are stored one by one, so only the keys inside the file can be recovered from it.
What the backup contains also determines how long a rescue takes. According to the Argos 1.2.0 release notes, a wallet file that already stores spendable Sprout note data can be swept without a scan, in seconds. Keys without that note data, such as standalone SK keys or keys imported into zcashd without a rescan, need a scan that attempts to decrypt every shielded transaction in the chain’s history, because no index of Sprout addresses exists. The developer’s site puts a bare-key Sprout scan at roughly 26 GB of transfer on Mainnet and warns that scans run for hours. With the cutoff measured in hours, that gap can decide whether a transfer is possible at all.
The destination is fixed as well. Sprout value can only move into the Sapling pool, so the release notes require a zs1 address or a Unified Address with a Sapling receiver. Moving the funds on to Orchard is a separate transaction the holder makes afterwards.
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Argos is a third-party desktop app from Sovright, built after ZecWallet Lite stopped being maintained, according to the developer’s site. The site says the app was independently audited by Least Authority, handles seeds and keys locally, and connects to the internet only to read public block data and send the final sweep. For large balances, the developer recommends a brand-new computer, because a factory reset does not remove firmware-level malware from a long-used machine.
Before pressing sweep
Once a sweep confirms, the developer advises retiring the old recovery material, since malware on the recovery machine may have captured it. The original wallet file should be kept until every pool it covers has been emptied.
A signed, broadcast Sprout transfer counts only once it is included in a block before the activation height. Where that confirmation is read matters too. ZIP 259 waives the usual requirement that old node software expire before an upgrade, and warns that unsupported nodes may remain online and follow a pre-NU7 branch. The Zcash Foundation’s notice says nodes that have not upgraded will follow a different chain. A holder checking an outdated node could see a transfer the upgraded network does not recognize, so confirmation should come from NU7-ready software such as Zebra 7.0.0, the first stable Zebra release with Mainnet NU7 support.
The exit is also public. ZIP 2003’s privacy section notes that value leaving Sprout changes the pool’s visible balance, and that a short exit period makes timing and distinctive amounts easier to link to particular holders. With only about 40 ZEC moving on Wednesday, a late withdrawal has little surrounding traffic to blend into.
ZIP 2003, authored by Daira-Emma Hopwood, does not unissue or burn Sprout funds. The pool’s balance at activation stays inside Zcash’s issued supply, including for the Network Sustainability Mechanism calculations in ZIP 237. Zcash’s supply accounting therefore keeps counting coins that cannot currently move.
The proposal leaves a theoretical route back, either re-enabling version-4 transactions or adding another retrieval facility if the community considers it worthwhile. It also states that Sprout spending may never return.
The case for accepting that cost rests on node security. ZIP 2003 notes that Sprout has shared Sapling’s Groth16 proving system and BLS12-381 curves since the Sapling upgrade. Its JoinSplit circuit and its handling of nullifiers and note commitments still burden the code that accepts new transactions, including as a denial-of-service surface. Because Sprout sees almost no use, node implementations have little reason to optimize that path. How the upgrade’s scope was settled is covered in Coindoo’s report on the NU7 coinholder vote.
The cost lands on a narrow group: holders whose Sprout keys sit in backups they have not checked, or have lost. The public pool balance cannot separate the two. The figure that settles the story is the Sprout pool value in ZecStats data once block 3,543,405 is mined, which shows how much ZEC the upgrade left behind. A later ZIP proposing a retrieval facility would be the only sign that access might return.
The post Zcash Upgrade Locks Sprout Coins Even With the Right Keys appeared first on Coindoo.
