Use of HTTP_PROXY & NODE_USE_ENV_PROXY fails with an infinite loop

[ArnaudBuchholz]

Bug Description

I have been experimenting with HTTP_PROXY & NODE_USE_ENV_PROXY and realized that the created ProxyAgent uses proxyTunnel: true by default which may fail if the proxy does not implement CONNECT. As a result, undici loops forever.

Reproducible By

See https://github.com/ArnaudBuchholz/training-fetch-proxy
run the proxy using node reserve-proxy
use npm run native-fetch to reproduce the issue.
use NODE_DEBU=undici npm run native-fetch to see the infinite loop

Expected Behavior

Have a way to disable proxyTunnel

Logs & Screenshots

UNDICI 13766: connecting to www.google.com using http:undefined
UNDICI 13766: connecting to localhost:8989:8989 using http:undefined
UNDICI 13766: connected to localhost:8989:8989 using http:h1
UNDICI 13766: sending request to CONNECT http://localhost:8989www.google.com:80
UNDICI 13766: request to CONNECT http://localhost:8989www.google.com:80 errored - other side closed
UNDICI 13766: connection to www.google.com using http:undefined errored - other side closed
UNDICI 13766: connecting to www.google.com using http:undefined
UNDICI 13766: connecting to localhost:8989:8989 using http:undefined
UNDICI 13766: connected to localhost:8989:8989 using http:h1
UNDICI 13766: sending request to CONNECT http://localhost:8989www.google.com:80
UNDICI 13766: request to CONNECT http://localhost:8989www.google.com:80 errored - other side closed
UNDICI 13766: connection to www.google.com using http:undefined errored - other side closed
UNDICI 13766: connecting to www.google.com using http:undefined
UNDICI 13766: connecting to localhost:8989:8989 using http:undefined
UNDICI 13766: connected to localhost:8989:8989 using http:h1
UNDICI 13766: sending request to CONNECT http://localhost:8989www.google.com:80
UNDICI 13766: request to CONNECT http://localhost:8989www.google.com:80 errored - other side closed
UNDICI 13766: connection to www.google.com using http:undefined errored - other side closed
UNDICI 13766: connecting to www.google.com using http:undefined
UNDICI 13766: connecting to localhost:8989:8989 using http:undefined
UNDICI 13766: connected to localhost:8989:8989 using http:h1

Environment

Mac OS / Tahoe 26.3.1
Node 24.14

Additional context

[metcoder95]

A new flag can suffice this, would you like to send a PR for it?

[ArnaudBuchholz]

I am new to this project but I can have a look. However, I am more concerned about the infinite loop that "breaks" Node.js. Do you think I should split this issue in two ?

[mcollina]

Added this to Node.js: nodejs/node#62926.