During a seemingly quiet upgrade window, the Zcash network is turning its attention to an early privacy architecture that has been running since 2016—the Sprout privacy pool. According to a single source, Zcash plans to activate the NU7 network upgrade soon, and it is expected to stop accepting version 4 transactions around block height 3543405, which is currently the only format that allows funds to be transferred out of the Sprout pool. In other words, once NU7 takes effect, Sprout will be "retired" at the protocol level, and assets that have not yet been migrated from the pool will not be spendable on-chain, even if the holders have valid private keys. According to a single source, on a Sunday before the upgrade expectations fermented, about 9094 ZEC had already been transferred out of the Sprout pool, but by Thursday morning, approximately 12226 ZEC still remained in the pool. At an estimated value of about 1300 dollars each, this amounts to approximately 16 million dollars' worth of assets being locked in a privacy pool that is about to lose its exit. Several public documents have therefore urged holders to migrate their assets as soon as possible, while NU7 represents a technological iteration focused on security and compliance, which may also push a batch of on-chain assets toward "permanent lock-up," clearly exposing the tension between technological evolution and user asset protection.
Countdown to Evacuation: Nearly 10,000 ZEC Depart Before Upgrade
On the Sunday before NU7 is expected to activate, a wave of concentrated withdrawals that fit the “deadline” narrative finally emerged from the Sprout pool. According to a single source, about 9094 ZEC were transferred out of this early privacy pool that day, with some reports simplifying it to "about 10,000" for easier dissemination, marking this departure as a key point before the upgrade. This timing echoes the claim that NU7 is expected to activate around block height 3543405, and it provides a concrete response on-chain to the migration reminders that had been issued continuously before.
However, this seemingly spectacular evacuation did not empty the Sprout pool. According to the same source, as of the following Thursday morning, approximately 12226 ZEC still remained in the pool, presenting a stark contrast to the over 9000 that had departed just a few days earlier. The "unevenness" of the migration response is evident: some holders acted before the clear upgrade window, while a portion of assets continued to quietly remain in the old structure, which was about to lose its exit. Around the official statements about NU7 and several public reports, a continual reminder was given during this phase that Sprout holders must complete their asset migration before the upgrade takes effect, or they may face the risk of being unable to transfer out; although the specific numbers and timing come from a single source and may have statistical discrepancies, they collectively outline a reality of a privacy pool that, as the timeframe for a clear technological "cut-off" approaches, still has not achieved a complete withdrawal.
Keys Won't Save You: Version 4 Channel Permanently Shut Down
Under the Sprout architecture, the so-called "privacy pool" is not a wallet that can be entered and exited at will, but more like a secret door that opens only under specific protocol conditions. The only corresponding key for this door is the version 4 transaction format: according to a single source, all funds transferred out of the Sprout privacy pool must be packaged into a version 4 transaction in order to be validated on-chain and written into new blocks. The protocol layer did not design a second or third optional exit path for Sprout. Because of this, when Zcash writes one of the core changes of the NU7 upgrade as "stop accepting version 4 transactions around block height 3543405," it essentially marks a clear "permanent shut down" point for this door on-chain.
The technical logic of NU7 is not to directly remove the Sprout pool but to cut off its only exit channel by no longer accepting version 4 transactions. According to a single source, once the upgrade is activated as planned around the evening of November 5 (Eastern Time, corresponding to the morning of November 6 in Beijing), the transaction outputs in the Sprout pool will no longer be regarded as valid data by network nodes, even if holders possess completely correct private keys, they will not be able to construct spend transactions accepted on-chain. The NU7 setting to cease version 4 transactions is equivalent to establishing a hard deadline on the protocol level for the Sprout pool that has both time coordinates and technical conditions; once this line is crossed, the state of the remaining approximately 12226 ZEC in the pool will shift from "migratable assets" to "unspendable historical records" on the chain level, and for holders, this code-fixed rule represents the most genuine boundary of ownership.
From Sprout in 2016 to Today: The Old Privacy Architecture Retires
If we pull the "cut-off line" set by NU7 backward, starting at 2016, Sprout once represented one of the Zcash network's most core privacy commitments. As the earliest privacy pool on the mainnet, it fulfilled the storage and transfer functions of privacy assets for quite a while and served as an early testing ground for zero-knowledge privacy technology transitioning from papers to production environments on public chains. At that time, there were almost no mature alternatives to that generation architecture, and privacy users naturally had to route their funding paths around Sprout, which consequently locked a large amount of assets in a closed space that was later deemed "outdated" both officially and by the community.
In the following years, Zcash continuously iterated on privacy technology, successively launching newer privacy solutions, and the role of Sprout shifted from "default entry" to "historical compatibility pool," accumulating discussions on how to safely phase out the old privacy pool. Now, NU7's choice to stop accepting version 4 transactions at the protocol level is one of the most critical technical steps in this retirement process: by cutting off the only on-chain transfer format, it explicitly categorizes Sprout as historical architecture instead of designing a new migration bridge for it. Public materials also emphasize that there are still considerable assets remaining in the Sprout pool, which makes this seemingly purely technical upgrade inevitably colored by the legacy of assets, and lays bare the tension between "upgrade speed" and "legacy compatibility" before the entire privacy network, becoming a long-term point of contention that future project planning cannot avoid when shaping their own iterative paths.
Approximately 16 Million Locked: The Cost and Controversy of Protocol Upgrades
According to a single source, as of Thursday morning, approximately 12226 ZEC remained in the Sprout pool, estimated to be worth around 16 million dollars at about 1300 dollars each. This number itself is enough to shift the focus of the debate: from "how to gracefully retire the old privacy architecture" to "who will be responsible for potentially permanently locked assets." More complicated is the fact that this valuation does not disclose specific price points or market sources, and the price itself carries uncertainty yet has already been embedded into a technical decision with significant real-world consequences.
From a protocol perspective, NU7's cessation of version 4 transactions means long-term gains in security auditing, maintenance costs, and overall structural simplification, definitively closing the book on a privacy pool that has been continually viewed as a "product of the old era" since 2016. But from an asset perspective, version 4 transactions are the only on-chain exit for Sprout funds; once this channel is closed, the approximately 12226 ZEC may still be recorded on-chain as balance but will no longer have spending meaning—the fact that holders possess private keys will be directly negated by technical rules. Before this, multiple public materials had repeatedly reminded users to migrate their assets before NU7 activation, but as the countdown window ended, there remained a notable volume of unmigrated assets, indicating that education and migration guidance for holders clearly exhibit blind spots—whether addresses had long been dormant, whether users had lost keys, or whether the technical understanding threshold was too high for some holders to keep pace with the complex evolution of privacy pools is currently unanswered by the materials, leaving this unresolved approximately 16 million dollar amount as a long-standing dispute between "promoting security upgrades" and "safeguarding historical assets" within the privacy network.
The Next Lesson for Privacy Networks: Caring for Lagging Users During Iteration
The batch of on-chain assets from the Sprout pool that might be permanently locked serves as a very specific alarm bell for governance and upgrade methods in privacy networks: under the premise of “code is law,” once an upgrade like NU7 that ceases to accept version 4 transactions takes effect at the predetermined block height, untransferred assets in the old privacy pool will instantaneously shift from "technical debt" to being unspendable historical remnants on-chain, and the current public materials have not confirmed whether NU7 has been fully activated as expected, leaving the fate of this approximately 12226 ZEC in a temporarily unresolved risk state. For all privacy public chains, iterations of zero-knowledge proofs and transaction formats will inevitably continue to occur, but the Sprout event has demonstrated: if the design of transitional periods, asset migration tools, and communication strategies for ordinary users do not keep pace, such iterations may create a layer of assets that are "legitimate but unusable" on the chain, directly testing the community's tolerance and willingness to protect lagging users. What remains to be observed in the future is whether other privacy projects outside of Zcash will handle the rhythm of retirement with greater caution when phasing out outdated architectures, such as extending reminder periods, reserving clearer migration channels at the protocol level, or even crafting "how to safely exit the old pool" into governance consensus from the very beginning, allowing cases like Sprout to no longer be merely collateral damage of technical evolution, but instead a starting point for the entire privacy ecosystem to reassess upgrade decisions, user education, and asset safeguarding responsibilities.
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