Git's “503 After CONNECT” Is a Tunnel Failure
A gateway status returned to CONNECT occurs before the normal HTTPS repository conversation. Check the requested host and proxy path before changing Git credentials.
“Received HTTP code 503 from proxy after CONNECT” is more specific than “GitHub is down.” Git asked the proxy to open a tunnel, and the proxy returned a failure status.
The normal HTTPS conversation with the repository has not necessarily started. A 502, 503, or 504 at this stage points toward the intermediary's attempt to establish the onward connection, its upstream path, or its policy.
Check the exact hostname and port in the CONNECT request. A typo in the remote URL can reach the proxy successfully and still fail when the proxy tries to resolve or contact the destination. Adding a random www. prefix or replacing the name with an IP is not a general correction.
Compare the intended route for an internal repository with that for an external one. If a host should be direct, express that policy explicitly. If the proxy is required, removing it is not a diagnosis.
Correlate the redacted Git trace with the proxy log. Repository passwords and Git buffer sizes do not repair a tunnel that was never established.