Custom MCP connector: browser stuck in re-login loop after successful OAuth callback

Description

After adding a custom MCP connector in Claude Desktop, the OAuth flow completes successfully but the browser remains stuck on the identity provider's "Trying to sign you back in" page, spinning indefinitely. The browser tab never closes or shows a completion message.

This is a regression — the same flow worked correctly ~2-3 days ago.

Reproduction

Tested with two independent MCP servers to confirm this is a Claude Desktop issue, not server-specific:

Server 1: Custom Google OAuth MCP server

1. Settings → Connectors → Add custom connector
2. URL: a custom Streamable HTTP MCP server with Google OAuth 2.0 (FastMCP v3, deployed on Cloud Run)
3. Authenticate with Google → browser stuck on "Trying to sign you back in to [server]"

Server 2: Linear MCP (third-party, for comparison)

1. Settings → Connectors → Add custom connector
2. URL: https://mcp.linear.app/mcp
3. Approve → Log in to Linear via Google → browser stuck on "Trying to sign you back in to Linear"

Both servers show identical behavior: browser stuck on Google's re-login page after OAuth callback.

Key evidence: OAuth flow succeeds server-side

Server logs confirm the full OAuth flow completes successfully:

GET /oauth/callback → 302 Found (authorization code exchanged)
POST /token → 200 OK (access token issued)
POST /mcp → 200 OK (MCP session established)
Processing ListToolsRequest ✓
Processing ListResourcesRequest ✓
Processing ListPromptsRequest ✓

The MCP connection is fully established — tools are available in Claude Desktop. Only the browser tab fails to close/redirect.

Tested browsers

• Chrome (default browser): stuck
• Safari (changed default browser): stuck

Workaround

Manually close the browser tab. The MCP connection works correctly.

Environment

• macOS 26.4.0 (Intel i9 MacBook Pro)
• Claude Desktop (latest, updated March 2026)
• Both Chrome 146 and Safari tested