Subject: [bitcoindev] Unbreaking testnet4

From: "'Antoine Poinsot' via Bitcoin Development Mailing List" <[email-masked]>
Date: Tue, 18 Mar 2025 14:29:35 +0000

Hi,

Testnet4 was rolled out a year ago to address the shortcomings of testnet3. One of those shortcomings was the difficulty reset creating havoc. [0] In spite of this a similar rule was adopted for testnet4. [1] As a result, testnet4 is similarly creating havoc. [2]

The goal of testnet is to mimic the Bitcoin mainnet. This is why it is useful to have in addition to a more control testing environment such as Signet.

The given rationale for a difficulty reset was to let developers occasionally mine blocks on their laptop. But you cannot have your cake and eat it too: either the network is permissionless (PoW) or you assign identities and privileges to some (Signet). By trying to do both at the same time testnet4 created a loophole for abuse. As a result it failed on both count: it neither mimics mainnet nor allows developers to mine active blocks on their laptop.

I propose to fix this by removing the difficulty reset rule from testnet4 through a flag day hard fork on 2026-01-01. I picked a date well in the future to minimize disruption. This leaves enough time for a patch to be reviewed, merged, included in the next major Bitcoin Core release, backported to previous releases and adopted by the infrastructure running on testnet4. That should be enough for a test network.

Let me know what you think,
Antoine

[0] https://gnusha.org/pi/bitcoindev/[email-masked]
[1] https://github.com/bitcoin/bips/blob/master/bip-0094.mediawiki#rule-specification
[2] [https://fork.observer](https://fork.observer/) - pick the network on the top right corner

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