Bitcoin Core saw a significant uptick in development activity during 2025, with mailing list traffic up 60% year-over-year, 135 unique code contributors, and approximately 285,000 lines of code changed, according to data compiled by Casa Chief Security Officer Jameson Lopp.
Bitcoin Core, the reference implementation that powers approximately 78% of Bitcoin full nodes, experienced a notable resurgence in development activity throughout 2025, according to annual metrics published by Casa CSO Jameson Lopp.
Email volume to the Bitcoin Development Mailing List — a key venue for proposing and debating protocol changes — increased 60% compared to 2024, Lopp reported on Saturday. The increase is particularly striking given that mailing list traffic had declined 25% the prior year, which Lopp had attributed partly to the list’s migration from Linux Foundation servers to Google Groups in early 2024, when subscriber counts dropped.
The contributor base also expanded. According to Lopp’s data, 135 different individuals contributed code to the Bitcoin Core repository during 2025, up from approximately 112 the year before. The number reverses a multi-year trend: contributor counts had peaked around 193 in 2018 before declining through most of the early 2020s.
The total volume of code changes, measured as lines added and deleted, reached approximately 285,000 for the year, roughly in line with 2024’s 276,000 and consistent with the project’s historical pace over the past decade, per Lopp’s data.
Lopp compiles his annual Bitcoin statistics through direct analysis of the Bitcoin Core GitHub repository and other public data sources. His methodology, which he has documented in previous annual reviews, relies on standard git commands such as “git shortlog” and “git log” to calculate contributor counts and code change volumes.
The statistics are self-reported and have not been independently verified by The Block, though Lopp’s analyses are widely cited in the Bitcoin community and consistent with his publicly available scripts.
The uptick in raw development metrics coincides with a year of significant technical activity for Bitcoin Core. In November, the project completed its first public third-party security audit, conducted by cybersecurity firm Quarkslab and funded by nonprofit Brink. The audit, which focused on Bitcoin Core’s peer-to-peer networking layer, found no critical or high-severity vulnerabilities.
The year also saw contentious debates over mempool policy changes, culminating in the merge of a controversial update to OP_RETURN limits in the v30 release. The change, which removed a long-standing 83-byte cap on data that can be embedded in transactions, sparked heated discussion among developers over data storage on the Bitcoin blockchain.
Organizations supporting Bitcoin Core development have continued funding efforts, with VanEck maintaining its pledge of 5% of spot Bitcoin ETF profits to developer nonprofit Brink.
