Bitcoin Core #35724 adds logging to measure compact-block reconstruction

Victoria Shinder17 October 2026, 08:542 min read

Bitcoin Core merged pull request #35724 on 16 October 2026, adding more detailed logging around compact block (cmpctblock) reconstruction. The change is observability, not a consensus or relay change: it makes it easier to see how well a node rebuilds a compact block from the pieces it already holds. The author is davidgumberg, and the PR is split from #35558 and described as "part of [the] prefill series".

What compact-block reconstruction is. When a node receives a cmpctblock message, it does not get every transaction in full. It reconstructs the block from transactions it already has — in its mempool, in an "extrapool", or included directly in the message's prefill section — and requests only the ones it is missing. How often a node has to ask for missing transactions, and how much of the prefill it actually needed, determines how much bandwidth compact blocks save.

What the PR logs. At debuglevel=trace, the node now logs all missing transactions in a received CMPCTBLOCK, and all missing transactions requested from it by peers. It also logs the sizes and counts of transactions used from the prefill, the mempool and the extrapool, and the sizes and sources of redundant transactions in the prefill section — that is, transactions a sender prefilled that the receiver already had and did not need. Together these let an operator or developer see where reconstruction is spending bandwidth and whether senders are prefilling usefully.

Why it is only a step. The PR itself changes nothing about how blocks are relayed or validated; it adds measurements. It was opened on 14 July 2026 and merged on 16 October, and its own description places it within a wider "prefill series" of work on compact-block behaviour. So the news here is diagnostic plumbing that later optimisation work can build on, not a user-visible change to node behaviour. Anyone wanting the exact log format should read the PR and the files it touches rather than infer it from this summary.

Source: Bitcoin Core, "cmpctblock: Improve logging of cmpctblock message reconstruction statistics" (PR #35724) — https://github.com/bitcoin/bitcoin/pull/35724