The capstone brief: design, build, defend
The closing lesson of the track, and the one with homework that matters. There is no new material: the capstone is the seven previous lessons exercised at once, on a system the reader chooses, sized deliberately small.
The capability: after this lesson, the reader has designed, built, and defended a complete agentic system of their own, and holds a reusable defense format for every future design review.
The assignment. One clear job, a project memory file that makes standards durable, one custom tool on a small MCP server with a five-part description, structured output whose schema keeps the truth legal, one must-hold rule enforced in structure, and one escalation path with an explicit criterion. Two agents allowed only if the work genuinely splits.
The three phases. Design on one page (the job, the path, the rules, the seams, the desk). Build in Claude Code, configuration first, testing each piece where it is weakest. Then the defense: one page for a skeptical senior colleague stating where judgment lives, what was traded away, what breaks first, and what was deliberately not built.
The assessment. A ten-point rubric weighted toward failure handling, plus the comparison that closes the track: the reader’s lesson 1 paper sketch read against the finished design. The distance between the two pages is the measure of what the track taught.
Why the defense is the deliverable. Anyone can assemble an agent from tutorials. The professional act is stating the trade-offs in writing, costs admitted, weakest point named, refused features listed. That page is the shape of a real design review, rehearsed on a system small enough to understand completely.