Adding a custom MCP server to Claude and ChatGPT

A custom MCP connection can let a chat assistant query a specific data source. In his July 2026 walkthrough, Simon Willison connected Claude and ChatGPT to a server offering read-only SQL queries against a copy of his blog database.

The two interfaces exposed the connection differently. His Claude setup used a custom connector; his ChatGPT setup required developer mode, plugin registration and activation in the chat. The example shows why a server address alone was insufficient: the chat application also needed to register and enable the tools. His test used a server without authentication.