7 Bugs That Taught Us How to Build Better Diagnostic Tools

We shipped node-loop-detective v1.1.0 with a feature we were proud of: slow async I/O tracking. It could monkey-patch a running Node.js process, track slow HTTP requests, DNS lookups, and TCP connections, then report them with caller stack traces. All without restarting the target process. Then we opened 16 GitHub issues against our own code. Seven of them were bugs. Not edge cases. Not "nice to haves." Real bugs that could affect users in production. This is the story of those 7 bugs, what caused them, and what we learned fixing them. If you build developer tools — especially ones that attach to production processes — these patterns will look familiar. Bug #1: We Left Our Fingerprints Behind The problem: When node-loop-detective disconnects from a target process, it should leave no trace. But our I/O tracking feature monkey-patched http.request, https.request, dns.lookup, and net.Socket.connect — and never restored the originals. // What cleanup looked like before cleanup: () => {