Cowork: a custom MCP connector fails to connect with 502 CLIENT_HTTP_NOT_IMPLEMENTED from the MCP proxy, and only that connector, for a whole session

**Surface:** Claude Cowork on the web, cloud session, Max plan (individual), 2026-09-14.

One custom MCP connector out of ~25 fails at session start and stays unavailable for the life of the session, while every other connector on the same account connects and reconnects normally. The session reminder reports:

```
Gmail_Multi (502): "SdkHttpError dialing
https://api.anthropic.com/v2/ccr-sessions//mcp?mcp_server_id=&mcp_url=&toolbox_mcp_server_id=
(CLIENT_HTTP_NOT_IMPLEMENTED)"
```

Notes that may narrow it:

- The failure is at the **Anthropic MCP proxy hop** (`api.anthropic.com/v2/ccr-sessions/.../mcp`), not at the server: the same server is reachable and healthy from other sessions on this account.
- `CLIENT_HTTP_NOT_IMPLEMENTED` suggests a transport/method the proxy did not implement for that upstream rather than an upstream error, but nothing in the session surfaces more than this line.
- Other connectors disconnected and reconnected several times during the same session (normal churn, announced and recovered). This one never recovered across several hours.
- There is no way to retry a single connector from inside a session: `RefreshMcpTools` re-reads tool lists over existing connections and does not re-dial, so a connector that failed to connect at session start is unavailable until the user restarts the session.

**Two asks:**

1. The 502/CLIENT_HTTP_NOT_IMPLEMENTED path should be retried or surfaced with the upstream status, rather than parking one connector for the session.
2. A way to re-dial a named MCP server from inside a running session, so a transient failure at start does not cost the whole session's access to that tool set.