FPE Demo MCP is a lightweight MCP (Model Context Protocol) server that demonstrates authentication and format-preserving encryption using FF3 over decimal digits in a clear, readable implementation. The server exposes two tools via MCP JSON-RPC: fpe_encrypt, which takes a numeric string and returns a ciphertext payload prefixed with ENC_FPE:, and fpe_decrypt, which reverses it. FF3 FPE operates on radix-10 digits; input is normalized to digits before encryption and the resulting ciphertext is returned with a visually obvious ENC_FPE: prefix for logs and demos. The server supports multiple authentication modes: authless and debug for quick testing; test mode accepts either a shared secret or JWT; production mode requires JWT only. JWT signing uses HS256 by default, with optional issuer and audience validation. FPE configuration is provided via environment variables (FPE_KEY and FPE_TWEAK). The server can run via stdio (local) or HTTP transports, enabling use with local LLM clients or web-based playgrounds. A quick deploy flow is provided for DigitalOcean App Platform, giving a public URL for testing with web-based LLMs, while the local testing instructions show how to run the stdio and HTTP servers. The demo emphasizes learning, prototyping, and understanding MCP authentication and FF3 FPE integration.