Which part of your agent setup breaks every time you switch machines?

Claude Code, OpenCode, Codex and Cursor each keep their own MCP servers, skills, prompts and keys. A line that keeps coming up: "I use fewer tools because I don't want to migrate config."

What is the one file or setting that always breaks when you move to a new machine or a new agent? And if you have a setup that actually keeps everything in sync, share it, someone here needs it.