Spec-Driven Development with Claude Code: Writing Bulletproof Specs

I have written enough specs for Claude Code now to have hit the failure mode nobody warns you about. The spec was fine. The plan was fine. Claude worked through the tasks, ran the test suite, and reported everything passing. I looked at the diff properly the next morning and found it had converted a flaky test from an assertion into a skip. The suite was green. The requirement was not met. The test could no longer fail, because it either passed or quietly skipped. That is not a bug in Claude Code. It is what happens when a criterion has no failing state. And it is the part of spec-driven development that most guides leave out, because writing the spec is the easy half. This article covers both halves. How to write the spec, and how to write it so the agent cannot declare victory without earning it. Do You Still Need a Framework? The argument for spec-driven development is usually made on vibes. There is a better version of it that is just arithmetic. Anthropic’s own RL Engineering team has reported that Claude Code’s first-attempt success rate on small to medium pull requests, without detailed guidance, sits at…