npm@11.x incompatible with FastGithub proxy (127.0.0.1:38457) on Ubuntu 24.04, returning 404 for all requests

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Environment:

• OS: Ubuntu 24.04.4 LTS (x86_64)
• Node.js: v24.14.1 (via nvm)
• npm: v11.11.0
• Proxy tool: FastGithub (listening on 127.0.0.1:38457)
• Shell: zsh 5.9

Description:

When http_proxy and https_proxy are set to http://127.0.0.1:38457 (FastGithub proxy), npm install commands consistently fail with 404 Not Found, even though:

• The proxy itself is working correctly (verified via curl -x http://127.0.0.1:38457 https://registry.npmjs.org/ -I returns HTTP/2 200).
• npm config get registry returns https://registry.npmjs.org/
• no_proxy includes registry.npmjs.org (confirmed in npm config list).
• The same request works flawlessly when proxy environment variables are unset before running npm, or when using bun (which uses the same proxy without issues).

Steps to Reproduce:

1. Configure FastGithub proxy to listen on 127.0.0.1:38457.
2. Set environment variables:

export http_proxy="http://127.0.0.1:38457"
export https_proxy="http://127.0.0.1:38457"
export no_proxy="localhost,127.0.0.1,registry.npmjs.org"

3. Ensure npm registry is set to official:

npm config set registry https://registry.npmjs.org/

4. Run any install command:

npm install -g npm-check

Expected Behavior:

Package installs successfully (as it does with curl or bun).

Actual Behavior:

Additional Context

• The problem persists even after npm cache clean --force and deleting ~/.npmrc.
• Diagnostic outputs confirming proxy works and npm config is clean:

- Attempts to clear npm's own proxy configuration did not resolve the issue: