Lessons from building 20 MCP Apps in 2 days

Short summary

MCP Apps extend the MCP spec to let tools return rich UI resources rendered inline in chat interfaces. After building 20 in two days: organize UI code server-side using Vite, design responsive layouts for variable host widths (ChatGPT browser-wide, Claude chat-panel-narrow, VS Code sidebar-constrained, mobile-small), never collect secrets in sandboxed iframes, and always ensure text responses work standalone for unsupporting clients.

• •MCP Apps let tools return rich UI resources inline in chat, now supported by Claude, ChatGPT, and VS Code
• •Bundle UI code server-side using Vite; design responsive layouts for variable host widths across clients
• •Never store API keys or secrets in iframes; always ship complete text fallback for clients without MCP App support

Generated with AI, which can make mistakes.