Architecture

Repository layout

index.ts          the adapter: options + adapt()
templates/        runtime entrypoints, compiled into the app
├── index.ts      Bun.serve() setup, graceful shutdown
├── handler.ts    static files, health endpoint, SvelteKit SSR
├── env.ts        env() lookup with envPrefix
└── healthcheck.ts  the healthcheck binary
internal.d.ts     types for the virtual modules templates import
examples/         compiled-app and native-addon (compile: false)
test/             integration tests (build + run the examples)

What adapt() does

1. Writes client assets to out/client/ and prerendered pages to out/prerendered/ (compressing them if precompress is on).
2. Writes the SvelteKit server and a manifest.js into .svelte-kit/adapter-bun/.
3. Copies templates/ next to it, swapping the placeholder tokens — ENV, HANDLER, MANIFEST, SERVER become relative imports; ENV_PREFIX, BUILD_OPTIONS, SERVE_OPTIONS become JSON literals.
4. Prepends import "./server/instrumentation.server.js" to the entrypoint if the app has one, so it loads before anything else.
5. Runs Bun.build on templates/index.ts — compile to a binary, or a plain index.js when compile: false — and compiles healthcheck.ts separately.

Templates ship as raw .ts; Bun transpiles them during the compile, so the published package has no build step for them.

Request flow

handler.ts answers, in order:

1. GET on the health path → JSON status.
2. A prerendered page or client asset, when serveAssets is on.
3. Everything else → server.respond(), with the URL rebuilt from ORIGIN or the forwarded headers and getClientAddress() wired to ADDRESS_HEADER or the socket.

platform is { request, server } — the raw request and the Bun.Server.

Finding assets at runtime

A compiled binary's import.meta.dir is virtual, so assets are located from dirname(process.execPath). A plain index.js uses import.meta.dir instead, since process.execPath is the bun binary there. ASSETS_DIR overrides both.

Loading under Node

index.ts only touches Bun inside adapt(). Tooling that loads svelte.config.js under Node — svelte-check, the language server, svelte-kit sync — works without Bun.