Thanks for the response, and sorry to take so long to update this. To answer your question, I was using the same browser to request the link and then open it (I actually tried with both Safari and Firefox, being consistent each time). But I think I’ve found why I was having trouble. I was opening the following URL directly:
This is from a script I use to add notes to transactions, and it has the URL hard-coded into the script. That URL was from the redirect I got when clicking on
Sign in with your Monzo account from developers.monzo.com. This used to work for me. However, I think this is now what’s causing the state mismatch. When I started directly from developers.monzo.com, it all worked fine. I’ve now changed the URL in the script to:
and all works as expected.