[BUG] Google Cloud BigQuery / Google Compute Engine connectors — persistent redirect_uri_mismatch (Error 400)

### MCP server URL or name

Google Cloud BigQuery

### How are you connecting to this server?

From browsing the MCP server directory

### Where does the issue occur?

When pressing the Connect button

### When did you last reproduce this?

2026-09-02, 6:57 AM EDT

### Browser and OS

Reproduced via claude.ai. Tried on Samsung Internet (Android phone) and Safari (iPad) — same "Failed to start MCP authorization" error on both browsers and both devices.

### Describe the issue

Both the Google Cloud BigQuery and Google Compute Engine connectors fail to authorize, both producing the identical error. Current error shown: "Failed to start MCP authorization" (toast notification, no error ID displayed, tapping the message does not reveal further detail). Reproducible on both connectors, both currently unusable.
Previously reported at anthropics/claude-code#90669 under a different error description (redirect_uri_mismatch, Error 400), closed by the github-actions bot as out of scope for that repo ("this doesn't appear to be about Claude Code"). Not clear whether the underlying issue changed since that report or the same root cause is now surfacing with different error text. Refiling here per that repo's guidance pointing MCP/connector issues to claude-ai-mcp.

### Error details

```shell
"Failed to start MCP authorization" — no error ID shown alongside it.
```

### Expected behavior

Successful OAuth completion and a working connection to the Google Cloud BigQuery and Google Compute Engine connectors.

### Additional context

Prior report: https://github.com/anthropics/claude-code/issues/90669 (closed without review, wrong repo)

Start by reproducing the failure from the MCP server directory's Connect button for the Google Cloud BigQuery and Google Compute Engine connectors, then compare it with the prior redirect_uri_mismatch report in anthropics/claude-code#90669. Done means OAuth completes successfully and both connectors establish working connections; the payload names no source file or test.

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