I Wrote Integration Tests for My MCP Failure Library. Here's the Pattern That Caught 3 Hidden Bugs.

I'd been shipping fixes to my MCP failure library for weeks. Every release felt solid. Then I've written one real integration test - and three "fixed" bugs weren't fixed at all.

The Problem

My failure library is an MCP server that stores crash patterns so AI agents can warn each other before hitting the same bug twice. Think of it as shared memory for tool errors: agent A hits a timeout talking to an external API, records the failure pattern, and agent B checks the library before making the same call. It works - when the server is running and the database is primed.

But getting it to work reliably in production was a different story. I'd been testing each component in isolation: the SQLite layer with unit tests, the MCP transport with mock clients, the retry logic with controlled timeouts. Each passed with flying colors. I had 90%+ coverage.

And yet, in production, things broke every other day. A fix that worked locally would fail in CI. A feature that passed every unit test would crash the first time two agents used it simultaneously.