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/<redacted>/mcp?mcp_server_id=<redacted>&mcp_url=<redacted>&toolbox_mcp_server_id=<redacted>
(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.