Node.js 18+ global fetch() throws 'TypeError: fetch failed' with cause 'UND_ERR_CONNECT_TIMEOUT' (Connect Timeout Error) behind a proxy or on a network with broken IPv6. How do I fix it?

Cause: The built-in fetch is implemented by undici, and 'fetch failed' is a generic wrapper; the real reason is in error.cause. UND_ERR_CONNECT_TIMEOUT means the TCP connect did not finish in time. Two common reasons: (a) you are behind an HTTP(S) proxy, but fetch does not read HTTP_PROXY/HTTPS_PROXY by default, so it tries a direct connection that your network blocks; (b) the host resolves to IPv6 and IPv4 addresses and the IPv6 route is dead, so connections stall before falling back. Fix (best first): 1. Print the cause first: catch the error and log err.cause (code, address, port) so you know which case you have. 2. For a proxy on a recent Node (documented as added in v24.0.0 and v22.21.0), enable environment proxy support: NODE_USE_ENV_PROXY=1 node app.js, or the --use-env-proxy flag, with HTTP_PROXY, HTTPS_PROXY and NO_PROXY set to your authorized proxy and bypass list. On older Node, install undici and call setGlobalDispatcher(new EnvHttpProxyAgent()) once at startup, which makes fetch honor those variables. 3. For IPv6 trouble, Node 20+ already tries addresses in turn (autoSelectFamily). If the default per-attempt timeout is too short for your network, raise it with --network-family-autoselection-attempt-timeout (available from v20.13 and v22.1) or net.setDefaultAutoSelectFamilyAttemptTimeout(). As an alternative, node --dns-result-order=ipv4first prefers IPv4 results. Fixing the network's IPv6 routing is better if you can. 4. If a corporate proxy re-signs TLS and you then see certificate errors, add its root CA with NODE_EXTRA_CA_CERTS=/path/to/ca.pem. Do not disable certificate checking. Verify: rerun the request; it completes, and err.cause no longer shows a connect timeout. Confirm with curl -v to the same URL from the same machine that the network path works. Sources: https://nodejs.org/api/cli.html https://nodejs.org/api/net.html https://github.com/nodejs/undici/blob/main/docs/docs/api/EnvHttpProxyAgent.md