[BUG] Zapier MCP OAuth loop — client stuck on "needs authentication" while Zapier dashboard shows connected

Preflight Checklist

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

What's Wrong?

Zapier MCP cannot complete authentication from Claude Code. /mcp persistently shows "zapier · needs authentication." Selecting Authenticate opens the Zapier sign-in, which completes in the browser, but the client never receives the callback. It returns to the same sign-in link and shows the "I just connected to Zapier, use the onboarding skill" prompt instead of flipping to connected.

State mismatch: mcp.zapier.com shows the server as connected with apps authorized, while Claude Code shows "needs authentication." The two disagree.

This is isolated to Zapier. My other MCP connectors (Exa, Gmail, Google Calendar, Google Drive, Higgsfield) all authenticate and work normally.

I reported this roughly a week ago as well (don't have the issue number to hand); it remains unresolved, so I'm filing fresh with fuller detail.

What Should Happen?

After completing the Zapier sign-in in the browser, /mcp should show "zapier · connected" and expose the configured tools.

Error Messages/Logs

Steps to Reproduce

1. Run /mcp in Claude Code
2. Select "zapier" (shows "needs authentication")
3. Select Authenticate
4. Complete the Zapier sign-in in the browser through to the success page
5. Client returns to the same sign-in link and shows the onboarding prompt
6. Run /mcp again — Zapier still shows "needs authentication"
7. Loops indefinitely on retry

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

• Claude Code: v2.1.203, native install, win32-x64

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

What I already ruled out (per Zapier's "sign-in completes in browser but client does not connect" troubleshooting article):

• - Retried in a clean incognito window with all extensions/ad-blocker disabled — same loop
• Let the client drive the sign-in; did not pre-open any Zapier link
• Tried the Continue-with-Google auth path
• On the latest Claude Code (2.1.203)
• Confirmed via /mcp that config resolves to a single Zapier entry (not a duplicate-server conflict)