APIs don't make good MCP tools
The Model Context Protocol (MCP) is a pretty big deal these days. It’s become the de facto standard for giving LLMs access to tools that someone else wrote, which, of course, turns them into agents. But writing tools for a new MCP server is hard, and so people often propose auto-converting existing APIs into MCP tools; typically using OpenAPI metadata (1, 2).
Infamously, VS Code has a hard limit of 128 tools - but many models struggle with accurate tool calling well before that number. Also, each tool and its description takes up valuable context window space.
Most web APIs weren’t designed with these constraints in mind! It’s fine to have umpteen APIs for a single product area when those APIs are called from code, but if each of those APIs is mapped to an MCP tool the results might not be great.
MCP tools designed from the ground up are typically much more flexible than individual web APIs, with each tool being able to do the work of several individual APIs.
