Runtimes.

The Node.js backend is the only runtime currently wired. It embeds libnode (V8 + Node) inside the Paper JVM by way of the Rust loader, which uses Panama FFM to keep both sides in the same address space. Your script runs in a real Node environment — fs, http, process, and the npm ecosystem are all available — but the upcalls to Bukkit don't go over a socket. They're direct synchronous calls through V8 → Rust → JVM.

// Plain Node-side work happens normally.
import fs from "node:fs";
import { z } from "zod";

const envSchema = z.object({ DATABASE_URI: z.string() });
const env = envSchema.parse(loadEnvFile());

// Bukkit work hits the JVM through FFM.
@Listener
export class Pulse {
  @EventHandler(Events.PlayerJoinEvent)
  onJoin(e: PlayerJoinEvent) {
    const player = e.getPlayer();        // live ref, not a snapshot
    rune.runOnMain(() => {                // schedule on the main thread
      player.sendMessage("welcome.");
    });
  }
}

Wasmtime is included as a second runtime backend. The plumbing works — a script declared with language = "wasm" in rune.toml routes to the Wasm backend instead of Node — but the host bindings exposed to Wasm guests are still being designed. The published Wasm interface lives in wit/ in the Rune repo and uses the Component Model. Treat this as preview surface; the WIT will move before 1.0.

The roadmap covers three more guests. Python and Lua get native embeddings (CPython and LuaJIT respectively, both via FFM, same pattern as Node). Rust gets the easier path: you compile to Wasm and the existing Wasmtime runtime hosts it. The host API is language-agnostic by design; what your Rune sees is the same across guests.

Each backend implements a single trait, LanguageRuntime, in crates/rune-host-api. The trait says: given a path to a script, load it; given a query callback, register it; given a reload signal, tear down and rebuild. Everything else — event dispatch, command registration, store persistence — is host-side machinery routed back to the guest via callbacks. New backends register themselves in the loader; nothing in the Paper plugin changes.