# Yun Build Prototype > Build a throwaway prototype to answer a design question by making it concrete and runnable instead of arguing it on paper — a hand-driven state model, or rival UI variants side by side. Use when a decision hangs on "does this logic/state model feel right" or "what should this look like", or the user wants to sanity-check a shape by running it before committing; reach for it over conversation when arguing on paper isn't settling the question. Also when another skill needs a design question resolved with a concrete artifact. - Skill: `matiinchauspe/yun-build-prototype` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add matiinchauspe/yun-build-prototype` - Raw SKILL.md: https://api.skillmd.com/api/skills/matiinchauspe/yun-build-prototype/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: matiinchauspe (https://skillmd.com/u/matiinchauspe) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/matiinchauspe/yun-build-prototype --- # yun-build-prototype A prototype is **throwaway** code that answers a question — and the question decides its shape. It raises the fidelity of a decision by making something concrete to react to, instead of arguing the shape in the abstract. The prototype is disposable from the first line; only the answer it settles is kept. ## Route by the question Pick the branch by **which question** is being answered — getting this wrong wastes the whole prototype. Read the question off the user's prompt, the surrounding code, or by asking if the user is around. - **"Does this logic / state model feel right?"** — build it as a hand-driven state model. Read `LOGIC.md` for how. - **"What should this look like?"** — build it as rival UI variants side by side. Read `UI.md` for how. If the question is genuinely ambiguous and the user isn't reachable, default by the surrounding code — a backend module leans logic, a page or component leans UI — and state the assumption at the top of the prototype. This step only chooses the path; the build happens in the branch file, and the skill finishes at capture. Done when the branch is chosen and its file loaded. ## Shared rules — both branches - **Throwaway from day one, marked as such.** Locate it beside the module or page it prototypes for, but name it so any reader sees at a glance it is a prototype. - **One command to run.** Whatever the project's task runner already supports; the user starts it without thinking. - **No persistence.** State lives in memory. Persistence is the thing the prototype is *checking*, not something it should lean on — add a scratch store only when the question itself is about persistence, and name it so it screams "wipe me". - **Least code that runs.** The point is to learn something fast, so stop at runnable: no tests, no abstractions, no error handling beyond what keeps it on its feet. - **Surface the state.** After every action or variant switch, render the full relevant state — the whole value is the human seeing what happened. ## Capture when done The prototype has done its job the moment the question is answered; now keep the answer and discard the scaffold. - **Fold the validated decision into the real code** — only the decision, never the prototype itself. Each branch's file says what "the decision" is for its shape. - **Park the prototype out of the way.** Commit the full prototype on an isolated throwaway branch, off the mainline, so the losing variants and the throwaway shell can't rot into the real code. The branch is the archive — it outlives any worktree it was built in. - **Persist the answer.** Persist a short digest — the verdict and the question it settled — through `skills/_shared/memory-convention.md`, scoped to the project, so a later session recalls what this prototype decided without re-deriving it. Done when the code reflects the answer to the question, the prototype is committed off the mainline, and its digest is persisted through the memory convention. ## Attribution Adapted for this workspace from **Matt Pocock's `prototype`** (github.com/mattpocock/skills, MIT). Changes: switched to model-invocation with harness trigger phrasing; the two branches stay as **disclosed reference files** (`LOGIC.md` / `UI.md`) because they are mutually exclusive paths — a run loads only the branch it takes; capture is rebound to the harness seams that exist today — the prototype is parked on a throwaway branch off the mainline and its verdict `persist`ed through `memory-convention` — rather than the source's "commit + leave a context pointer on the implementation issue", since the harness has no ticket-resolve seam yet. The question-routes-the-shape spine, the pure-module / throwaway-shell split, and the structurally-different-variants rule are preserved.