Stack Overflow connector: auth fails immediately with generic "unknown error" — never reaches consent screen, no error ID

## MCP server URL or name
Stack Overflow (mcp.stackoverflow.com) — official directory-listed connector

## How are you connecting to this server?
From browsing the MCP server directory

## Where does the issue occur?
When pressing the Connect button (auth never reaches an authorization/consent screen at all)

## When did you last reproduced this?
2026-08-19, reproduced 3 times same day: once via Claude Code CLI health check, once via automated browser navigation to the generated auth-start URL, once by manually opening the same URL directly in my own browser. All three attempts failed identically.

## Browser and OS
Chrome on Windows 11 (manual repro); also reproduced via Claude Code CLI 2.1.229 on Windows 11 (non-browser path)

## Describe the issue
The Stack Overflow connector cannot complete authentication. Three independent reproduction paths all fail at the earliest possible point, before any Stack Overflow-specific login or consent screen ever renders:

1. `claude mcp list` (CLI health check) reports the connector as failed to connect.
2. `claude mcp login "claude.ai Stack Overflow"` opens a browser to a generated auth-start URL (`https://claude.ai/api/organizations/{org_id}/mcp/start-auth/mcpsrv_01BHEL7sHPfTLArmjA2iZp7D?product_surface=claude-vscode`). Navigating to that URL — both via an automated browser tool and manually in my own Chrome — immediately server-side-redirects to `https://claude.ai/settings/mcp/auth_error` with a generic "Authentication Error: An unknown error occurred during authentication." No consent/authorization screen is ever shown; the failure happens before that step.
3. The `/mcp` slash-command reconnect flow reports `Failed to reconnect to claude.ai Stack Overflow: HTTP 429`.

This connector has never once completed a working session — I have made zero real Stack Overflow API calls (so the documented 100-calls/day beta quota is not a factor).

All 8 other `claude.ai`-hosted connectors on the same account (Gmail, Drive, Calendar, Notion, Granola, Atlassian Rovo, Mermaid Chart, Shopify) connect and respond to live tool calls without issue, ruling out a general account/network/proxy problem — this looks isolated to this specific connector instance.

## Error details
```
Browser (claude.ai/settings/mcp/auth_error):
"Authentication Error: An unknown error occurred during authentication."
(No error ID / ofid shown on the page.)

CLI (`claude mcp list` / `/mcp`):
claude.ai Stack Overflow: https://mcp.stackoverflow.com - × Failed to connect —
HTTP 429: Error POSTing to endpoint: {"jsonrpc":"2.0","id":0,"error":
{"code":-32600,"message":"Anthropic Proxy: Invalid content from server","data":null}}

/mcp reconnect:
Failed to reconnect to claude.ai Stack Overflow: HTTP 429
```

## Expected behavior
Connecting should either succeed, or fail with a specific, actionable error and a traceable reference/error ID — not a generic "unknown error" with no diagnostic path, and not different, seemingly-unrelated error shapes across the CLI vs. browser paths for what appears to be the same underlying failure.

## Additional context
- Server's own repo (`stackexchange/stack-mcp`) has GitHub issues disabled, so there's no way to check/report this from the Stack Overflow side — filing here since the failure appears to originate in Claude's own connector-initiation flow, before any request would reach Stack Overflow's server.
- The generic `-32600 "Anthropic Proxy: Invalid content from server"` error string also appears in unrelated prior reports against completely different servers (#51812 — Linear, #55839 — Pendo), suggesting it's a catch-all error surfaced by the proxy for several distinct underlying failures, not specific diagnostic information.
- Also related in symptom shape (connector registration failing before a consent screen / before any authenticated request is sent) to #429, #880, #291, #810 — though those are all self-hosted custom connectors, this is a first-party directory-listed one.