Codex

Quick start

server.tsclient.ts

server.ts

import { agentOS, setup } from "@rivet-dev/agentos";
import codex from "@agentos-software/codex";

const vm = agentOS({ software: [codex] });

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

client.ts

import { createClient } from "@rivet-dev/agentos/client";
import type { registry } from "./server";

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

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

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

Read Sessions first for session options, streaming events, prompts, and lifecycle management.

See Full Example

LLM Credentials

Set the relevant variable(s) on the session’s env, sourced from your server’s environment:

• OPENAI_API_KEY — OpenAI API key (built-in openai provider).
• OPENAI_BASE_URL — route through a gateway or OpenAI-compatible endpoint.
• Custom providers — defined in ~/.codex/config.toml; each provider’s env_key names the variable Codex reads for its key (e.g. AZURE_OPENAI_API_KEY, MISTRAL_API_KEY).

See LLM Credentials, and Codex’s config reference for details.

Skills

Codex discovers SKILL.md files from its skills directory. Write the skill into the VM before creating a session and Codex loads it automatically.

const skill = `---
name: commit-style
description: How to write commit messages in this project.
---

Write commit messages in the imperative mood and keep the subject under 50 characters.
`;

// Write the skill before creating the session
await agent.mkdir("/home/agentos/.codex/skills/commit-style", { recursive: true });
await agent.writeFile("/home/agentos/.codex/skills/commit-style/SKILL.md", skill);

// Codex discovers the skill automatically
const session = await agent.createSession("codex", {
  env: { OPENAI_API_KEY: process.env.OPENAI_API_KEY! },
});

MCP servers

Expose extra tools to the agent by passing mcpServers to createSession. Both local child-process servers and remote URLs are supported.

const session = await agent.createSession("codex", {
  env: { OPENAI_API_KEY: process.env.OPENAI_API_KEY! },
  mcpServers: [
    {
      type: "local",
      command: "npx",
      args: ["-y", "@modelcontextprotocol/server-filesystem", "/home/agentos"],
      env: {},
    },
    {
      type: "remote",
      url: "https://mcp.example.com/sse",
      headers: { Authorization: "Bearer my-token" },
    },
  ],
});

Note

Pre-install npx-launched servers. A local server started with npx -y … writes install progress to stdout on its first run, which corrupts the MCP stdio handshake (you’ll see Connection closed). Pre-install it in the VM so npx is silent — await agent.exec("npm install -g @modelcontextprotocol/server-filesystem") before the session — or pin the package and point command at the installed binary.

Customizing the agent

Codex is a built-in agent, but it’s just a software package under the hood. To ship your own ACP adapter, swap the underlying agent SDK, or register a tweaked build as a new agent, see Custom Agents.