Cursor, Claude Code, Claude Desktop, ChatGPT, Codex, Windsurf, and browser-hosted clients differ in config formats, OAuth redirect handling, prompt support, resource support, elicitation capability, media rendering, and filesystem access. This talk presents findings from building and testing one MCP server across all major clients. We cover: the client capability matrix we discovered through testing, how to detect clients from clientInfo and user-agent, graceful degradation when prompts or resources aren't supported, the MCP capability cascade pattern (resources → prompts → tools as universal fallback), and specific workarounds for each client's quirks. Includes a live compatibility testing approach using a diagnostic probe server.