# Pi

> Source: `src/content/docs/agent-os/agents/pi.mdx`
> Canonical URL: https://rivet.dev/docs/agent-os/agents/pi
> Description: Run the Pi coding agent inside a VM with extensions and custom configuration.

---
## Quick start

```ts @nocheck server.ts
import { agentOs } from "rivetkit/agent-os";
import { setup } from "rivetkit";
import common from "@rivet-dev/agent-os-common";
import pi from "@rivet-dev/agent-os-pi";

const vm = agentOs({
  options: { software: [common, pi] },
});

export const registry = setup({ use: { vm } });
registry.start();
```

```ts @nocheck client.ts
import { createClient } from "rivetkit/client";
import type { registry } from "./server";

const client = createClient<typeof registry>("http://localhost:6420");
const agent = client.vm.getOrCreate(["my-agent"]);

const session = await agent.createSession("pi", {
  env: { ANTHROPIC_API_KEY: process.env.ANTHROPIC_API_KEY! },
});

const { text } = await agent.sendPrompt(
  session.sessionId,
  "What files are in the current directory?",
);
console.log(text);
```

Read [Sessions](/docs/agent-os/sessions) first for session options, streaming events, prompts, and lifecycle management.

## Extensions

Pi supports [extensions](https://github.com/badlogic/pi-mono/tree/main/packages/coding-agent/examples/extensions) that let you register custom tools, modify the system prompt, and hook into agent lifecycle events. Write a `.js` file into the VM's extensions directory before creating a session and Pi discovers it automatically.

Pi scans two directories for `.js` extension files:

| Directory | Scope |
|-----------|-------|
| `~/.pi/agent/extensions/` | Global — applies to all Pi sessions |
| `<cwd>/.pi/extensions/` | Project — applies only when cwd matches |

```ts @nocheck
const extensionCode = `
module.exports = function(pi) {
  // Modify the system prompt before each agent turn
  pi.on("before_agent_start", async (event) => {
    return {
      systemPrompt: event.systemPrompt +
        "\\n\\nAlways respond in formal English."
    };
  });
};
`;

// Write the extension before creating the session
await vm.mkdir("/home/user/.pi/agent/extensions", { recursive: true });
await vm.writeFile("/home/user/.pi/agent/extensions/formal.js", extensionCode);

// Pi discovers the extension automatically
const { sessionId } = await vm.createSession("pi", {
  env: { ANTHROPIC_API_KEY: process.env.ANTHROPIC_API_KEY },
});
```

See the [Pi extension documentation](https://github.com/badlogic/pi-mono/tree/main/packages/coding-agent/examples/extensions) for the full extension API.

_Source doc path: /docs/agent-os/agents/pi_
