Seems pretty cool. It looks like it has similar trust assumptions as a standard light client: the light client trusts the merkle root once it is buried under several blocks of proof of work, believing that an attacker is unlikely to do all that work just to fool a light client (especially when they could have been actually mining bitcoin with all that hashrate). A nice property is that, to get started, a fly client does not have to download a variable number of block headers (namely, all of them, however many there are), only a constant number of block headers, and it's a pretty small total number. That property seems to make fly clients more efficient than standard light clients.

On Wednesday, April 29, 2026 at 5:03:36 PM UTC-4 Zac Mitton wrote:

Hi, Ive been looking into FlyClient first described here. I don't see any BIPs, or previous discussion in this forum about it either.

On bitcoin It could allow a light-client to verify the entire work of the heaviest chain with a single ~100KB proof.

It can theoretically be done as a soft-fork by injecting a single hash into the coinbase tx (similar to how segwit is committed to).

What do you guy's think?

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