[BUG] OAuth error: The socket connection was closed unexpectedly. For more information, pass `verbose: true` in the second argument to fetch()

[atmclarke]

Preflight Checklist

• I have searched existing issues and this hasn't been reported yet
• This is a single bug report (please file separate reports for different bugs)
• I am using the latest version of Claude Code

What's Wrong?

Claude Code fails on networks with broken QUIC/UDP path — no TCP fallback.

Claude Code fails to connect to api.anthropic.com (and other backend services) on networks where UDP port 443 / QUIC traffic is dropped or broken, even though TCP HTTPS to the same endpoints works fine. The underlying Bun runtime appears to prefer QUIC/HTTP/3 and does not fall back to TCP HTTPS when QUIC fails, unlike curl and mainstream browsers which implement fallback behaviour.

What Should Happen?

It should login.

Error Messages/Logs

OAuth error: The socket connection was closed unexpectedly. For more information, pass `verbose: true` in the second argument to fetch()

Steps to Reproduce

1. Reproducible on specific networks where QUIC/UDP 443 is mangled in ways TCP HTTPS is not. I don't have a clean way to reproduce this on arbitrary hardware, and I suspect it'll be inconsistent across installs depending on firmware, MTU, and CGNAT state.
2. Install Claude Code (any recent version)
3. rm ~/.claude/.credentials.json to clear existing auth
4. tcpdump 'host api.anthropic.com'
5. run claude from terminal
6. /login — browser completes, terminal errors with OAuth error: The socket connection was closed unexpectedly
7. tcdump during Claude Code failure shows only QUIC (UDP 443) traffic — zero TCP SYN packets attempted
8. curl https://api.anthropic.com succeeds (TCP HTTPS)
9. Add rule: echo "block out proto udp from any to 160.79.104.0/24 port 443" | sudo tee /etc/pf.anchors/blockquic
10. sudo pfctl -a blockquic -f /etc/pf.anchors/blockquic
11. sudo pfctl -e
12. run claude from terminal
13. /login now works.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

No response

Claude Code Version

2.1.112

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

No response

[github-actions]

Closing for now — inactive for too long. Please open a new issue if this is still relevant.

[github-actions]

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.