Zcash activates NU7, cutting target blocks to 25 seconds

Zcash activated its NU7 network upgrade at block 3,543,405 on Nov. 5, cutting target block spacing from 75 seconds to 25. The change is designed to shorten confirmation waits and expand shielded payment capacity, while also changing how transaction fees support the network.

The Zcash Foundation’s Zebra 7.0.0 release notes describe the upgrade’s consensus changes. The block interval is the target time between blocks, not a guarantee that every payment will confirm in 25 seconds. CoinDesk reported that the network was averaging about 39 seconds between blocks shortly after activation as mining difficulty adjusted; services also set their own confirmation requirements.

How does NU7 increase shielded payment capacity?

NU7 combines faster target blocks with per-block limits on shielded transaction activity. The Foundation says the limits apply to Orchard, Ironwood and Sapling activity, and that they more than double Orchard throughput, from 2.9 to 6.6 transactions per second. The same limits reduce the worst-case sync bandwidth a spammer could impose on light wallets by about 37%.

That bandwidth matters because wallets need to scan blockchain data to find payments addressed to them. Without limits, a surge of shielded activity could force light wallets to download and check more data. By capping activity in each block, NU7 constrains that load while more frequent blocks make room for more payment activity over time. The Foundation says Zebra applies the limits both when building block templates and when admitting transactions to its mempool, so oversized transactions can be rejected before inclusion in a block.

Faster blocks can also leave miners less time to receive and validate each other’s work, increasing the chance that competing blocks are discarded. CoinDesk reported that a test of 25-second blocks across 80 computers in 11 regions discarded 3.43% of blocks under demanding conditions, compared with 1.57% at the former pace. That test result describes the tested setup, not a confirmed mainnet rate.

What changes for ZEC issuance and transaction fees?

NU7 reduces the subsidy in each block by the same factor that it increases block frequency, while tripling the number of blocks between halvings. The Foundation says this keeps the rate of new ZEC issuance essentially unchanged and preserves both the four-year halving schedule and the 21 million ZEC supply cap.

The upgrade also activates the Network Sustainability Mechanism. Under ZIP 235, 60% of each block’s transaction fees are removed from circulation and placed in a reserve; miners receive the other 40%. ZIP 237 schedules gradual reissuance of the reserve through future block rewards, beginning at mainnet height 8,939,664. The Foundation says the unchanged supply cap and halving schedule remain in place under this fee arrangement.

Which old Zcash funds are affected by NU7?

NU7 makes version 4 transactions invalid, while version 5 and version 6 remain valid. Because Sprout, Zcash’s original shielded pool, can only use version 4 transactions, any ZEC left there became unspendable at activation, according to the Foundation. CoinDesk reported that about 12,224 ZEC remained in the pool at that point.

The change gives users and infrastructure operators different concerns: the shorter target interval and shielded limits affect payment timing and wallet scanning, while the transaction-version cutoff affects funds that were not moved out of Sprout. NU7’s activation therefore changes both the network’s operating pace and the rules for spending coins held in its oldest privacy pool.

Sources and documents

• Zebra 7.0.0 release notes — forum.zcashcommunity.com