To make a counter argument to mine, and assuming Bitcoin soft forks are not impossible, FlyClient has an advantage that is impossible to replicate with ZK proofs; they are simple enough that one can reason about and implement from scratch similar to implementing an SPV, whereas formally proving ZK Starks end to end is very unlikely to happen any time soon, and reviewing all of its complexity is impossible for any single mortal.<br /><br />The same argument applies for validating Sidechains by the way, you can use a ZK proof, but I am increasingly thinking that a FlyClient verifier native in Bitcoin as an opcode, is orders of magnitude safer and easier to implement.<br /><br />So I encourage you and everyone to not give up on FlyClient in Bitcoin yet.<br /><br /><div class="gmail_quote"><div dir="auto" class="gmail_attr">On Tuesday, 19 May 2026 at 00:07:54 UTC+3 Zac Mitton wrote:<br/></div><blockquote class="gmail_quote" style="margin: 0 0 0 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div dir="ltr">Ok, Let&#39;s keep this thread about Flyclient on Bitcoin. The challenges related to actually getting a soft-fork into bitcoin are best discussed elsewhere.<div><br></div><div>If anyone has information related to progress on this, or implementation ideas please help by posting here or DMing me. I will try to aggregate.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, May 4, 2026 at 12:26 PM Tom Harding &lt;<a href data-email-masked rel="nofollow">to...@thinlink.com</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>

  
    
  
  <div>
    <p>Zac, </p>
    <p>That&#39;s probably because the question, and my post, are
      off-topic.  They are not about FlyClient, which is not interesting
      to me because bitcoin&#39;s header chain is tiny.</p>
    <p>Instead they touch on two techniques which would improve on most
      historical implementations of SPV clients by adding protocol
      support for them.</p>
    <p>Sorry for the distraction.</p>
    <p>Tom</p>
    <p><br>
    </p>
    <div>On 5/3/26 12:26, Zac Mitton wrote:<br>
    </div>
    <blockquote type="cite">
      
      <div dir="auto">Tom I can’t seem to grok the question or your
        explanation of it. Could you spell it out for us in detail?</div>
      <div><br clear="all">
        <br clear="all">
        <div>
          <div dir="ltr" class="gmail_signature">Thanks, Zac</div>
        </div>
      </div>
      <div><br>
        <div class="gmail_quote">
          <div dir="ltr" class="gmail_attr">On Sun, May 3, 2026 at
            12:58 PM Tom Harding &lt;<a href data-email-masked rel="nofollow">to...@thinlink.com</a>&gt;
            wrote:<br>
          </div>
          <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
            <div>In the linked presentation by Benedict Bünz, it&#39;s worth
              listening to the first audience question/answer. 
              Questioner is spot on that an SPV client polling the
              network to gain probabilistic confidence of unspentness
              could easily subsume the task of gaining input inclusion
              proofs from the network, with no forking change necessary.</div>
            <br>
            <div class="gmail_quote">
              <div dir="auto" class="gmail_attr">On Wednesday, April 29,
                2026 at 2:03:36 PM UTC-7 Zac Mitton wrote:<br>
              </div>
              <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi,
                Ive been looking into FlyClient first described <a href="https://www.youtube.com/watch?v=BPNs9EVxWrA&amp;t=8386s" rel="nofollow" target="_blank" data-saferedirecturl="https://www.google.com/url?hl=en-GB&amp;q=https://www.youtube.com/watch?v%3DBPNs9EVxWrA%26t%3D8386s&amp;source=gmail&amp;ust=1779443545937000&amp;usg=AOvVaw2Ps5gyOqwxcGUGEY0Ne50v">here</a>.
                I don&#39;t see any BIPs, or previous discussion in this
                forum about it either.
                <div><br>
                </div>
                <div>On bitcoin It could allow a light-client to verify
                  the entire work of the heaviest chain with a single
                  ~100KB proof.<br>
                  <div><br>
                  </div>
                  <div>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). </div>
                  <div><br>
                  </div>
                  <div>What do you guy&#39;s think?</div>
                </div>
              </blockquote>
            </div>
            <br>
          </blockquote>
        </div>
      </div>
    </blockquote>
  </div>

</blockquote></div>
</blockquote></div>

<p></p>

-- <br />
You received this message because you are subscribed to the Google Groups &quot;Bitcoin Development Mailing List&quot; group.<br />
To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:bitcoindev+unsubscribe@googlegroups.com">bitcoindev+unsubscribe@googlegroups.com</a>.<br />
To view this discussion visit <a href="https://groups.google.com/d/msgid/bitcoindev/8e069e17-c819-4c30-b4d7-917b9561813bn%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/bitcoindev/8e069e17-c819-4c30-b4d7-917b9561813bn%40googlegroups.com</a>.<br />