> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zap.wzrd.tech/llms.txt
> Use this file to discover all available pages before exploring further.

# Kernel and plugins

> The Zap kernel composes runtimes from plugins with reversible effects, services, and events.

`@wzrdtech/zap-kernel` is the composition layer under every Zap runtime. A runtime is a tree of plugins mounted into a **context**; each plugin can register services, subscribe to events, and acquire resources through reversible effects.

## Plugin shape

A plugin is `{ name, inject, apply(ctx, config) }`. `apply` receives the context and registers what the plugin provides:

* **Services** — named capabilities other plugins consume (for example `sandbox`, `memory`, `gateway`, `mediafs`, `pay`).
* **Events** — typed pub/sub across the plugin tree.
* **Effects** — every acquisition (a sandbox, socket, watcher, timer, meter reservation, hosted port, file lock) is registered inside `ctx.effect(setup → dispose)`, so tearing down a context releases everything it acquired.

## Every plugin registers an inverse

Disposal is a contract, not a convention: fork/dispose cycles must leave zero live handles. The build runs leak tests that fork and dispose contexts a thousand times and fail on any leaked resource.

## Two kernel instances

A composed runtime involves two kernels:

* the **caller kernel** on your machine (or the control plane), which resolves the plugin graph, drives runs, and proxies events; and
* the **in-VM kernel** inside the tenant sandbox, which executes lanes, tools, and the agent host.

No model loop ever runs outside the tenant VM; the control plane only proxies.

## Composing

```ts theme={null}
import { compose } from "@wzrdtech/zap-runtime";
import { box } from "@wzrdtech/zap-sandbox";
import { openviking } from "@wzrdtech/zap-runtime/memory";

await compose([box({ template: "zap-heavy" }), openviking()]);
```

Or declaratively in `Runtime.md`, resolved by [`zap compose`](/reference/cli#runtime-workflow). Both forms produce the same deterministic plugin tree and lock hash.