# App Builder > Create and iteratively improve local web products from natural-language requests, then prepare them to run as Rudder Apps. Use Rudder's maintained scaffold for new websites and preserve the existing stack for Next.js, React, Vue, Astro, SvelteKit, Nuxt, and other local web projects. - Skill: `undertone0809/app-builder` (Agent Skill, multi-file: 64 files) - Install (CLI): `npx skillmds@latest add undertone0809/app-builder` - Raw SKILL.md: https://api.skillmd.com/api/skills/undertone0809/app-builder/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - Author: undertone0809 (https://skillmd.com/u/undertone0809) - Updated: 2026-09-22 - Page: https://skillmd.com/skills/undertone0809/app-builder --- # App Builder Create or improve a useful local web product without making a non-technical user choose a framework, database, package manager, or process topology. A Rudder App is an ordinary local website presented as an App inside Rudder. This Skill owns requirements, source changes, and product verification. Rudder Desktop owns folder selection, launch-definition review, process lifecycle, loopback attestation, and embedded opening. ## Build Workflow 1. Capture a compact brief in the conversation: user/job, primary workflow, essential screens, durable data, external integrations, constraints, and exclusions. Ask at most three grouped questions, and only when an answer materially changes behavior, real-data safety, or external authority. 2. Choose the source path: - For a new App, use the maintained scaffold in `assets/scaffold/`. Prefer the Apps workspace flow when it is available. Otherwise run `scripts/scaffold.mjs` with an explicit workspace root and target. Never recreate the foundation from memory or overwrite a non-empty directory. - For an existing web project, inspect its current framework, package manager, development script, data boundary, and tests before editing. Preserve its stack and conventions; never replace it with the maintained scaffold merely to make it a Rudder App. When the originating prompt includes `Rudder App handoff`, retain its App ID for the completion protocol below. Do not guess an App ID or use one from a different conversation. 3. Read [references/scaffold-contract.md](references/scaffold-contract.md) before changing the runtime, database foundation, manifest, health endpoint, import/export contract, or background-job runner. 4. Implement the user's domain model and workflows. Keep the generated project conventional and independently runnable. 5. Follow [references/data-safety.md](references/data-safety.md) whenever an app already has data, imports user data, changes its schema, or needs a real-data diagnosis. 6. Follow [references/design-guidelines.md](references/design-guidelines.md) for visible UI work. For a new App, also read [references/rudder-ui-preset.md](references/rudder-ui-preset.md) and start from the versioned Rudder UI preset already present in the maintained scaffold. Keep its semantic tokens and source-owned shadcn primitives unless the brief explicitly requires a different product identity. For an existing project, preserve its design system by default; align it to Rudder only when the user asks for that change. Prefer coherent workflow screens over a generic dashboard assembled from decorative cards. 7. Run migrations against development or snapshot data, then run typecheck, unit tests, build, and relevant app E2E tests. Review the final source and rendered product for correctness, regressions, usability, data safety, and evidence integrity; resolve every blocking finding before handoff. 8. Prepare the website for its Rudder Apps handoff: - A new managed App created from Apps Home keeps its typed manifest. After the completion gate passes, report `verified_source_ready` with the originating App ID so Rudder Desktop can verify, start, and open that App automatically. Do not load the same source through the manual Local App path. - An independently authored existing project keeps a direct supported development script. Add the `package.json` `rudder` readiness or open-path fields only when discovery cannot infer them safely, then direct the operator to **Apps + > Add local web project** to select the folder and review the launch definition. The Agent writes and verifies source; Desktop alone owns runtime admission and process start. Do not invent a public URL, tunnel, cloud deployment, or unreviewed launch command. 9. Read [references/verification.md](references/verification.md), then verify the rendered app with Rudder Browser. Exercise the main workflow plus at least one production-shaped edge case and inspect console errors. 10. Materialize final screenshots and validation evidence in the originating Chat or Run. A localhost URL or tool-only screenshot is not durable output. 11. For an originating managed App handoff, use the injected run context without printing or requesting credentials: - Run `node scripts/report-build-status.mjs building ` when implementation begins. - Run `node scripts/report-build-status.mjs verified_source_ready ` only after every Completion Gate item passes. This is the machine-visible signal that authorizes Desktop to verify, start, and open the exact assigned source root. - Run `node scripts/report-build-status.mjs failed ` when work ends in a terminal failure. Do not emit `verified_source_ready` for a question, partial implementation, canceled run, missing evidence, or blocker. Finish by telling the user the App is opening and that they can continue improving it in this Chat. If the status command fails, report that failure honestly instead of claiming the App opened. ## Real Data Decision - New app with no user data: use the scaffold's development database and synthetic fixtures. - UI or ordinary logic change on an app with user data: verify against a temporary database snapshot by default. - Diagnosis that genuinely depends on user records: ask whether to use the original database, a snapshot, or a redacted copy. State that relevant data may be sent to the configured model provider. - Schema change: snapshot first and rehearse the migration on the snapshot. Applying it to user data is an application-specific action and requires explicit user intent. - A direct operation on the formal database requires explicit user intent. Never claim the Agent is sandboxed from files it can access. Use product controls and least-data workflow choices; explain the boundary honestly. ## Runtime Boundary - App source, data, builds, and execution stay on the user's device. - The app may use outbound APIs only when requested and configured. - Background tasks implemented by the App run only with that managed App process. V1 does not provide a separate Rudder job scheduler or catch-up UI. - Never place secret values in source, `rudder.app.json`, tool arguments, Chat, screenshots, test fixtures, or logs. V1 does not provide a Rudder Secret binding UI, so ask the user how the App should obtain any required integration credential. - Opening Chat, Apps Home, or a saved view must not passively start an app. A run-scoped `verified_source_ready` handoff may start and open its managed App. - Cloud builds, hosted runtimes, public preview links, custom domains, managed cloud databases, and cross-device synchronization are outside this skill. ## Completion Gate Do not call the app complete until: - source and any required manifest or `package.json` Rudder configuration are durable; - typecheck, unit tests, and build pass; - blocking code and product review findings are resolved; - the reviewed readiness path is ready on an attested loopback process; - Browser verification covers the primary workflow and a relevant edge case; - the versioned Rudder UI preset check passes for a new maintained App; - persistence behavior is verified against development or snapshot data; - current screenshot evidence is attached to the work; - user data was not modified without explicit intent; and - stop/cleanup leaves no owned process behind.