Two-Block Reorg Hits Bitcoin as Foundry Secures Longest Chain Advantage

Foundry won a rare Bitcoin reorg, exposing how mining concentration can impact rewards and network competition.

Bitcoin experienced a rare two-block reorganization after competing mining pools briefly produced parallel versions of the blockchain. Foundry USA pulled ahead by quickly extending its chain, causing valid blocks from AntPool and ViaBTC to be dropped. While those stale blocks weren’t invalid, miners who produced them lost their rewards and their transactions returned to the mempool.

Foundry Leads Chain After Rare Bitcoin Reorganization Event

Bitcoin recorded a rare two-block chain reorganization after rival mining pools briefly built parallel versions of the blockchain. Foundry USA, the largest mining pool, emerged ahead after adding a run of new blocks faster than competitors.

AntPool and ViaBTC saw their valid blocks dropped from Bitcoin’s final record. Chain reorganizations happen when two versions of Bitcoin’s ledger compete for dominance. 

Nodes temporarily split between the rival chains, then settle on the one with the most cumulative proof of work. That is what happened around block heights 941,881 and 941,882, according to Bitcoin developer and observer b10c.

At height 941,881, AntPool and Foundry each mined a valid block just 12 seconds apart. AntPool found its block at 15:49:35 UTC, while Foundry followed at 15:49:47 UTC. Since both blocks were valid, some parts of the network accepted one version and others accepted the other.

Competition continued at a height of 941,882 as ViaBTC added a block to AntPool’s branch. Meanwhile, Foundry extended its own branch with another block. For a short period, Bitcoin had two competing chains, each consisting of two blocks.

Blocks 941,883 through 941,886 were all mined by Foundry, giving its branch a clear lead in cumulative work. Once that happened, Bitcoin nodes switched to Foundry’s chain as the canonical ledger.

Mining Race Leaves Valid Blocks Orphaned in Rare Bitcoin Reorganization

Blocks mined by AntPool and ViaBTC became stale, but they were not invalid. Often called “orphaned,” they were fully valid blocks that lost the race because another chain grew faster. Since stale blocks do not remain in the accepted chain, miners who produced them received no block reward.

Transactions included in the stale blocks were not destroyed. Instead, they returned to the mempool and waited to be mined again in later blocks. That process is a normal part of Bitcoin’s consensus rules when short-lived forks appear.

Two-block reorganizations are unusual, though not unheard of. Short one-block races happen from time to time across decentralized networks. A deeper reorg draws more attention because it gives a clearer picture of mining concentration and block production power.

Foundry mining multiple blocks consecutively has raised concerns about the influence of large pools. As mining conditions tighten, smaller operators may shut down or consolidate, allowing larger pools to control a greater share of hashrate.

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