# Build-time discovery
> How @svelte-lean/vite scans static markers, injects register modules, writes the manifest and build report, and why there is no MutationObserver.
Page: https://svelte-lean.vercel.app/docs/architecture/build-time-discovery · Markdown: https://svelte-lean.vercel.app/docs/architecture/build-time-discovery.md · Index: https://svelte-lean.vercel.app/llms.txt
The build finds static markers in your markup and imports the matching registration modules; the runtime finds each root on its first event.
## Two plugins
- `svelte-lean:scan` (`enforce: 'pre'`) reads each included `.svelte` source for static `data-slean="…"` markers.
- `svelte-lean:inject` (`enforce: 'post'`) appends one import per registration module to the compiled file.
`vite.config.ts`
```ts
// vite.config.ts
import { sveltekit } from '@sveltejs/kit/vite';
import { svelteLean } from '@svelte-lean/vite';
import { defineConfig } from 'vite';
export default defineConfig({
plugins: [svelteLean(), sveltekit()]
});
```
`+page.svelte, compiled`
```js
// What the compiled module of that page receives from the plugin, appended after the
// compiled code (imports are hoisted, so it evaluates first):
import '@svelte-lean/primitives/tabs/register';
```
- Imports are sorted, de-duplicated and appended, so source maps stay valid; hoisting runs them before the component code. The source markup is never rewritten.
- A hundred files with tabs still yield one module in the graph; registration is idempotent, so a manual import next to an injected one changes nothing.
- Works before or after `sveltekit()`; put it first so the scanner sees the source before any preprocessor.
## Static-only contract
Discovered: `data-slean="tabs"`, `data-slean='tabs'`, `data-slean=tabs`. An expression is not; it is recorded in the manifest and reported once per file with its position and the fix.
```svelte
…
…
```
`Build warning`
```bash
Dynamic behavior identifiers cannot be auto-discovered.
Found:
src/lib/Panel.svelte:12:2 data-slean={kind}
Use a static identifier such as data-slean="tabs", or import the registration module
manually (for example import '@svelte-lean/primitives/tabs/register').
```
- The scanner is lexical: it skips `