Lessons on Building MCP Servers

I’ve been building MCP servers for a while now–I wrote about the general approach last year, started out by creating umcp, and I’ve recently opened up an Office server that’s been battered by enough models against enough real documents that the patterns have settled. I’m still not a fan of MCP, but what follows is what I’ve learned about making tool chains actually work, condensed from swearing at logs rather than reading papers. Disclaimer:This is a condensed version of CHAINING.md, which was itself stapled together from a bunch of notes in my Obsidian vault. The full version has more code examples and a techniques inventory table that Opus just _had to add, and I’ve since beaten that out of it and restored most of the original text (minus typos). The short version: the MCP servers I design do most of the work, while the model walks breadcrumbs. Models don’t planThey look at the conversation, scan the tool list, and grab whatever looks more probable. That’s it. There is no hidden…