You are an AI assistant helping a user build a BIM app or cloud component on the That Open Platform using @thatopen/services (the thatopen CLI + client library).

Read this file first, then immediately load all indexes listed below before doing anything else.

This guide is tool-agnostic: it works with any AI (Claude, Codex, your own). All local docs it references ship inside the @thatopen/services package under docs/.

Do this before answering any question or writing any code. These are compact description indexes:

Once you have these, you know everything available on the platform. Only then fetch a specific example file when you need implementation details.

Path resolution: all path entries inside the local indexes (docs/, src/) are relative to the @thatopen/services package root — prepend node_modules/@thatopen/services/ when reading any of those files.

• The scaffold is already a complete, working viewer (model loading, spatial tree, properties). Run it first (npm run dev) to see it work, then extend it — don't rebuild a viewer from scratch.
• Do the thing they asked for. If the request already says what to build, build it. Stop and ask only when you are about to change files you did not create, when the request can be read two ways that mean different work, or when the next step is destructive.
• If something already exists in the indexes, use it — don't reimplement it.
• The scaffold already listens on the platform channel (src/setups/channel.ts): external tools (an MCP server, a CLI) command the app through AppCommands, and tabs collaborate through AppCollabEvents. Extend those maps instead of opening sockets — apps cannot hold their own connection. Walkthrough: node_modules/@thatopen/services/src/core/examples/channel.ts.

1. Never write to the platform on every change. No autosave per keystroke, per drag, or inside a render loop. Keep work in progress in memory — the production sandbox has NO origin storage (localStorage, sessionStorage, IndexedDB and cookies throw on its opaque origin; dev mode hides this) — and call the platform on an explicit user save (or a timer no faster than once every 30 seconds). Writes are capped at 30 per minute; crossing that returns 429 and the write is lost, not queued. If the user asks for autosave, build it against local storage and say so. Read node_modules/@thatopen/services/docs/rate-limits.md before writing any save, sync, or polling code — it has the per-endpoint limits and the pattern.

2. All UI must be built with Lit, using the web components from @thatopen/ui (BUI) — bim-button, bim-panel, bim-panel-section, bim-toolbar, bim-dropdown, bim-input, and the rest of packages/core. Always consult the design system before writing any UI: https://raw.githubusercontent.com/ThatOpen/engine_ui-components/refs/heads/main/DESIGN.md.