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When building custom network dispatchers, fetch wrappers, or NO_PROXY merging logic, never let proxy environment variables (HTTP_PROXY/HTTPS_PROXY/ALL_PROXY, any casing) route traffic to loopback/local addresses through a proxy. Libraries like undici’s EnvHttpProxyAgent proxy everything, including localhost, whenever NO_PROXY is unset — this silently breaks local tooling (e.g. LM Studio/Ollama model servers) for any user who launches the app with proxy env vars set but no NO_PROXY.

Always special-case loopback explicitly rather than relying on the proxy library’s own matching:

• Enumerate every loopback spelling the user might hit: localhost, 127.0.0.1/127.x.x.x, ::1, [::1], 0.0.0.0, and IPv4-mapped IPv6 forms such as ::ffff:127.0.0.1 (which URL parsing can rewrite to [::ffff:7f00:1]). Match these with a real IP-aware matcher (e.g. Node’s BlockList) rather than string equality, since mapped/compressed forms won’t match literal lists.
• Route loopback hosts through a plain, non-proxying dispatcher/agent and send everything else through the proxy-aware one, mirroring the platform’s own proxy resolver (VS Code’s resolver always connects to loopback directly).
• When merging NO_PROXY values from multiple sources/casings, union the entries and always inject the loopback hosts, but preserve a * wildcard (opt-out of all proxying) on either side instead of overwriting it.
• Add regression tests that set a proxy env var (pointing at a rejecting/recording proxy) with NO_PROXY deliberately absent, and assert loopback requests connect directly while non-loopback requests hit the proxy. Don’t only test with both proxy vars unset — that misses the exact corporate-proxy configuration that breaks this.

Example pattern:

const LOOPBACK_HOSTS = new Set(["localhost", "0.0.0.0", "::1", "::ffff:127.0.0.1"]);
function isLoopbackHost(input: string | URL | Request): boolean {
  const host = new URL(toUrlString(input)).hostname.replace(/^\[|\]$/g, "").toLowerCase();
  return LOOPBACK_HOSTS.has(host) || host.startsWith("127.") || blockList.check(host, "ipv6");
}
// direct, no-timeout Agent for loopback; EnvHttpProxyAgent for everything else

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