The Obsidian forum has threads reporting Sync data loss for over 2 years. To help reproduce bugs, I built a fuzzer that runs Obsidian instances in containers, makes them edit notes while cutting off their network, and documents the failure rates.

The fuzzer finds amusingly short sequences that cause data loss and that you can reproduce on your own in a minute (if you have an Obsidian Sync subscription). Steps in the repro README.

I guided Claude Code to build the fuzzer, but I didn't read the code. This didn't go well. The project ballooned to a painful 60h, so I wrote a separate experience report on the many failure modes and what I'll do differently next time: https://hmijail.substack.com/p/building-a-semantic-fuzzer-fo...