Originally published on hexisteme notes.
If nslookup resolves a host fine but curl, pip, and Python (requests/httpx) fail with "Could not resolve host," your mDNSResponder daemon has almost certainly entered a non-responsive "zombie" state — and the fix is to restart it (sudo killall -9 mDNSResponder), not to touch your API keys, SDK versions, or code.
This failure is maddening because every signal points the wrong way. nslookup example.com returns a clean IP, so DNS "works." Your network is up. Your code didn't change. Yet curl, pip install, and every Python HTTP call die with "Could not resolve host." People burn hours rotating API keys, downgrading SDKs, and editing config files. None of that is the problem.
The two-DNS-path asymmetry (your fastest diagnosis)
macOS resolves DNS over two independent paths, and the asymmetry is the diagnosis:
