BigQuery MCP connector OAuth flow hangs — infinite loading on consent callback

### MCP server URL

https://bigquery.googleapis.com/mcp

### Where does the issue occur?

When auth finishes and redirects back to Claude.ai

### Transport used

streamable-http

### Client registration type

Static

### SSE only: Does your server return the expected status code for POST requests?

Not applicable (my server does not use SSE)

### When did you last reproduce this?

February 24, 2026, approximately 5:00-6:00 PM GMT-3

### Browser and OS

Chrome and MacOS

### Describe the issue

Attempting to connect the Google BigQuery MCP server (https://bigquery.googleapis.com/mcp) via a custom connector on claude.ai. OAuth client is a Web Application created in Google Cloud Console with redirect URI https://claude.ai/api/mcp/auth_callback. Using a Google Workspace account.

Flow: Click "Connect" on the BigQuery connector in Claude → Google OAuth consent screen appears correctly → After clicking "Allow", the page hangs indefinitely with infinite loading. It never redirects back to Claude.
If I refresh the page during the hang, it redirects to Claude chat and shows a toast error: "Error connecting to the MCP server. Please confirm that you have permission to access the service, that you're using the correct credentials, and that your server handles auth correctly."
**Error reference ID**: e24245bbec1f9918

Steps tried:
- Verified OAuth consent screen has BigQuery scope (bigquery)
- Verified redirect URI is exactly https://claude.ai/api/mcp/auth_callback
- Verified Google Workspace admin allows third-party apps (API controls → "Allow users to access any third-party apps")
- Added user as test user in OAuth consent screen Audience settings
- Revoked app access at myaccount.google.com/permissions and re-authenticated
- Removed and re-added connector multiple times
- Tried with only one client secret enabled
- BigQuery API is enabled on the project

### Issue details

```shell
Described above
```

### Expected behavior

MCP working

### Logs from your server

```shell

```

### Additional context

_No response_