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Express

This guide adds Bunkhouse to a server-rendered Express application. Every file shown below is part of a runnable example app that is tested end to end against the real Bunkhouse API and embed.

Tested setup

Section titled “Tested setup”
Framework Express 5.3.0, server-rendered EJS 7 views, express-session 1.19
Runtime Node.js 24 (v24.22.2 in the tested image)
Example apps/docs/integrations/express/ in the Bunkhouse repository

You need:

  • Your application’s exact origin registered on the site (for example https://app.example.com).
  • The site’s public key (pk_...).
  • For identity tokens (recommended): a signing secret (ss_...) and its key ID from the site’s Keys page.
  • For server mint instead: a mint key (sk_...).
  • Existing sign-in. The example uses express-session with two demo users; your own authentication takes its place. Bunkhouse only needs one thing from it: the signed-in user’s stable ID on the server.
File Purpose
src/config.ts Validates the public Bunkhouse configuration at startup
src/agent.ts GET /api/agent-identity and POST /api/agent-session
src/csp.ts Your Content-Security-Policy with the Bunkhouse sources added
views/agent-embed-*.ejs The script tags for one installation mode
public/agent-identity.js, public/agent-auth-headers.js Page hooks, loaded before the loader
Section titled “Identity token (recommended)”

Set the server environment. Only AGENT_PUBLIC_KEY and the origins ever reach the browser:

Terminal window
AGENT_PUBLIC_KEY=pk_...
AGENT_SIGNING_SECRET=ss_...        # server only
AGENT_SIGNING_KID=...              # key ID shown with the signing secret
AGENT_ALLOWED_ORIGINS='["https://app.example.com"]'
# AGENT_API_BASE and AGENT_EMBED_ORIGIN default to https://api.bunkhouse.ai and https://embed.bunkhouse.ai
src/config.ts
// Bunkhouse public configuration, validated once at startup. Signing and mint credentials are read
// only by the endpoints that use them, so each mode needs only its own secret.
function exactOrigin(name: string, value: string): string {
  const url = new URL(value);
  if (url.origin !== value) throw new Error(`${name} must be an exact origin like https://app.example.com`);
  return value;
}


const publicKey = process.env.AGENT_PUBLIC_KEY ?? "";
if (!publicKey.startsWith("pk_")) throw new Error("AGENT_PUBLIC_KEY must be the site's public key (pk_...)");


const origins: unknown = JSON.parse(process.env.AGENT_ALLOWED_ORIGINS ?? "[]");
if (!Array.isArray(origins) || origins.length === 0) throw new Error('AGENT_ALLOWED_ORIGINS must be a JSON array such as ["https://app.example.com"]');


export const agentConfig = {
  publicKey,
  allowedOrigins: origins.map((o) => exactOrigin("AGENT_ALLOWED_ORIGINS", String(o))),
  apiBase: exactOrigin("AGENT_API_BASE", process.env.AGENT_API_BASE ?? "https://api.bunkhouse.ai"),
  embedOrigin: exactOrigin("AGENT_EMBED_ORIGIN", process.env.AGENT_EMBED_ORIGIN ?? "https://embed.bunkhouse.ai"),
};

The router signs a ten-minute HS256 token for the user your session already authenticated. It rejects other methods (405), requests whose Sec-Fetch-Site is not same-origin (403) and signed-out users (401, never a login redirect), and marks every response Cache-Control: no-store. The subject comes only from the server session, never from the query, body or headers. The same file contains the server mint endpoint used by the alternative below.

src/agent.ts
// Bunkhouse endpoints: GET /api/agent-identity (identity token mode) and POST /api/agent-session
// (server mint mode). Both vouch only for the user your existing session already authenticated.
import { createHmac } from "node:crypto";
import { Router, type Request, type Response } from "express";
import { agentConfig } from "./config.ts";


/** Your existing authentication: the signed-in user of this request, or null. */
export type CurrentUser = (req: Request) => { id: string } | null;


const base64url = (value: object) => Buffer.from(JSON.stringify(value)).toString("base64url");


/** HS256 identity token: kid header, sub = stable opaque user ID, aud = public key, 10 minutes. */
export function signIdentityToken(userId: string, secret: string, kid: string): string {
  const iat = Math.floor(Date.now() / 1000);
  const header = base64url({ alg: "HS256", typ: "JWT", kid });
  const payload = base64url({ sub: userId, aud: agentConfig.publicKey, iat, exp: iat + 600 });
  const signature = createHmac("sha256", secret).update(`${header}.${payload}`).digest("base64url");
  return `${header}.${payload}.${signature}`;
}


const methodNotAllowed = (allow: string) => (_req: Request, res: Response) => {
  res.set({ Allow: allow, "Cache-Control": "no-store" }).status(405).json({ error: "method_not_allowed" });
};


function notConfigured(res: Response, variable: string) {
  console.error(`Bunkhouse: ${variable} is not set`);
  res.status(503).json({ error: "not_configured" });
}


export function agentRouter(currentUser: CurrentUser): Router {
  const router = Router();


  router
    .route("/api/agent-identity")
    .get((req, res) => {
      res.set("Cache-Control", "no-store");
      if (req.get("sec-fetch-site") !== "same-origin") return res.status(403).json({ error: "forbidden" });
      const user = currentUser(req);
      if (!user) return res.status(401).json({ error: "unauthorized" });
      const { AGENT_SIGNING_SECRET: secret, AGENT_SIGNING_KID: kid } = process.env;
      if (!secret) return notConfigured(res, "AGENT_SIGNING_SECRET");
      if (!kid) return notConfigured(res, "AGENT_SIGNING_KID");
      res.json({ identityToken: signIdentityToken(String(user.id), secret, kid) });
    })
    .all(methodNotAllowed("GET"));


  router
    .route("/api/agent-session")
    .post(async (req, res) => {
      res.set("Cache-Control", "no-store");
      // The embed's POST carries no CSRF token; these three checks are this route's CSRF defense.
      const origin = req.get("origin");
      if (!origin || !agentConfig.allowedOrigins.includes(origin) || req.get("x-agent-embed") !== "1" || req.get("sec-fetch-site") !== "same-origin") {
        return res.status(403).json({ error: "forbidden" });
      }
      const user = currentUser(req);
      if (!user) return res.status(401).json({ error: "unauthorized" });
      const key = process.env.AGENT_EMBED_SECRET_KEY;
      if (!key) return notConfigured(res, "AGENT_EMBED_SECRET_KEY");
      try {
        const upstream = await fetch(`${agentConfig.apiBase}/v1/embed/sessions`, {
          method: "POST",
          headers: { Authorization: `Bearer ${key}`, "Content-Type": "application/json" },
          body: JSON.stringify({ publicKey: agentConfig.publicKey, mode: "signed", userId: String(user.id), origin }),
          signal: AbortSignal.timeout(15_000),
        });
        const body = Buffer.from(await upstream.arrayBuffer());
        res.status(upstream.status).type(upstream.headers.get("content-type") ?? "application/json").send(body);
      } catch {
        res.status(502).json({ error: "session_service_unavailable" });
      }
    })
    .all(methodNotAllowed("POST"));


  return router;
}

Mount it after your session middleware. currentUser is the seam to your authentication: return the signed-in user, or null.

src/server.ts (excerpt)
/** The signed-in user of this request (your existing authentication), or null. */
const currentUser = (req: Request) => req.session.user ?? null;


// Bunkhouse endpoints, behind the session middleware so they see the signed-in user.
app.use(agentRouter(currentUser));

In the page head, define the identity hook and then the loader, before your application’s own scripts. The hook is an external same-origin script, so no inline script needs a CSP exception:

views/agent-embed-identity.ejs
<%# Identity token mode (recommended): the hook first, then the loader, before the app's scripts. %>
<script src="/agent-identity.js" nonce="<%= nonce %>"></script>
<script src="<%= agent.embedOrigin %>/embed.js" nonce="<%= nonce %>"
  data-public-key="<%= agent.publicKey %>" data-hints-url="/agent-hints.json"></script>
public/agent-identity.js
// Identity token mode: the embed calls this hook whenever it starts a session for the signed-in user.
window.AgentEmbedConfig = {
  ...window.AgentEmbedConfig,
  getIdentityToken: async () => {
    const res = await fetch("/api/agent-identity", { credentials: "same-origin", cache: "no-store" });
    if (!res.ok) throw new Error("Not signed in");
    return (await res.json()).identityToken;
  },
};
views/layout.ejs (excerpt)
<% if (csrfToken) { %><meta name="csrf-token" content="<%= csrfToken %>" /><% } %>
<!-- Page hooks merge into window.AgentEmbedConfig before the loader runs. -->
<script src="/agent-auth-headers.js" nonce="<%= nonce %>"></script>
<%- include(`agent-embed-${mode}`) %>
<!-- The application's own JavaScript comes after the loader. -->
<script src="/app.js" nonce="<%= nonce %>" defer></script>

The example layout includes one of the three agent-embed-* partials, selected by a test-only ?mode= switch. Your application uses exactly one.

Server mint alternative

Section titled “Server mint alternative”

Instead of the identity hook, add data-session-endpoint to the loader. Do not keep agent-identity.js on the page: when getIdentityToken is defined it takes precedence.

views/agent-embed-mint.ejs
<%# Server mint mode: no identity hook; the embed posts to the same-origin session endpoint. %>
<script src="<%= agent.embedOrigin %>/embed.js" nonce="<%= nonce %>"
  data-public-key="<%= agent.publicKey %>" data-hints-url="/agent-hints.json"
  data-session-endpoint="/api/agent-session"></script>

POST /api/agent-session in src/agent.ts above accepts only requests with an Origin from AGENT_ALLOWED_ORIGINS, X-Agent-Embed: 1 and Sec-Fetch-Site: same-origin (403 otherwise), authenticates the session (401), then creates the session with the mint key and returns the API’s status and body unchanged with Cache-Control: no-store. If the API is unreachable it returns 502 without details. Set AGENT_EMBED_SECRET_KEY=sk_... on the server; the identity endpoint does not need it.

Express has no built-in CSRF middleware. If you use one (such as csrf-csrf), exempt only /api/agent-session: the embed’s POST carries no CSRF token, and the three header checks are that route’s protection.

Anonymous alternative

Section titled “Anonymous alternative”

Anonymous sessions need only the loader. Enable anonymous access for the site in the dashboard first and set its monthly token and spend limits; usage is charged to your account’s credit and stops at a limit. Anyone who can load your page can start one, so it is not proof of identity.

views/agent-embed-anonymous.ejs
<%# Anonymous mode (enable it for the site first): the loader alone. %>
<script src="<%= agent.embedOrigin %>/embed.js" nonce="<%= nonce %>"
  data-public-key="<%= agent.publicKey %>" data-hints-url="/agent-hints.json"></script>

Sign-in, sign-out and navigation

Section titled “Sign-in, sign-out and navigation”

The example signs in and out with ordinary form posts that redirect back to the same page. Every account change therefore loads a new document, and a new document starts a new embed with no session, so the previous user’s session can never carry over. Session middleware is configured with HttpOnly, SameSite cookies, and the session is regenerated on login:

src/server.ts (excerpt)
app.use(
  session({
    name: "example.sid",
    secret: SESSION_SECRET,
    resave: false,
    saveUninitialized: false,
    // Secure everywhere except the loopback HTTP test origin.
    cookie: { httpOnly: true, sameSite: "lax", secure: APP_ORIGIN.startsWith("https://") },
  }),
);

If your Express app changes the signed-in user without a full page load (a fetch login, a single-page frontend), call window.AgentEmbed?.resetSession() right after the login, logout or account switch succeeds and before the new user’s requests. The React, Vue and Angular guides use this backend that way. Render the loader once in the layout, not in per-page templates.

Auth headers, user IDs and CSP

Section titled “Auth headers, user IDs and CSP”

The assistant calls your API with the page’s cookies; supply any extra header your API requires with getAuthHeaders. The example’s session owns a random CSRF token, rendered into a <meta> tag for signed-in users and checked on every write:

public/agent-auth-headers.js
// The app's own CSRF token (rendered by the server for the signed-in session) for the assistant's calls
// to this app's API. Values never reach the model or the sandbox.
window.AgentEmbedConfig = {
  ...window.AgentEmbedConfig,
  getAuthHeaders: () => {
    const token = document.querySelector('meta[name="csrf-token"]')?.content;
    return token ? { "X-CSRF-Token": token } : {};
  },
};

The hook merges into window.AgentEmbedConfig, so it works with the identity hook in either order. Its values never reach the model or the sandbox.

Send a stable, opaque user ID (the example’s fixture-alice), not a name or email address.

Add the Bunkhouse sources to your existing policy rather than replacing it. The example sets a fresh nonce per response and puts it on its script tags; no 'unsafe-inline' or 'unsafe-eval' is needed:

src/csp.ts
// The app's existing Content-Security-Policy with the Bunkhouse sources added: the embed origin to
// script-src and frame-src, the API to connect-src. A fresh nonce per response (res.locals.cspNonce)
// goes on the page's script tags.
import { randomBytes } from "node:crypto";
import type { NextFunction, Request, Response } from "express";
import { agentConfig } from "./config.ts";


export function contentSecurityPolicy(_req: Request, res: Response, next: NextFunction) {
  const nonce = randomBytes(16).toString("base64");
  res.locals.cspNonce = nonce;
  res.set(
    "Content-Security-Policy",
    [
      "default-src 'self'",
      `script-src 'self' 'nonce-${nonce}' ${agentConfig.embedOrigin}`,
      `style-src 'self' 'nonce-${nonce}'`,
      `frame-src ${agentConfig.embedOrigin}`,
      `connect-src 'self' ${agentConfig.apiBase}`,
      "img-src 'self' data:",
      "object-src 'none'",
      "base-uri 'self'",
      "form-action 'self'",
      "frame-ancestors 'self'",
    ].join("; "),
  );
  next();
}

Verify

Section titled “Verify”

From a checkout of the Bunkhouse repository, with Docker and local Supabase running:

Terminal window
node tests/framework-integrations/run.ts express

The run builds the example image, starts it against a fresh local site and drives the real pane in Chromium: identity, mint and anonymous sessions, endpoint guards, sign-out and account switches, discovery of the page’s startup request, a CSRF-protected write through getAuthHeaders, CSP and secret checks. In your own app, sign in and open the Assistant launcher: the first message creates a signed session.

Symptom Check
/api/agent-identity returns 401 The router is mounted before your session middleware, or currentUser does not find the user.
403 from either endpoint The request is cross-site or missing Sec-Fetch-Site; for mint, Origin must exactly match AGENT_ALLOWED_ORIGINS and the loader must carry data-session-endpoint. A CSRF middleware must exempt only /api/agent-session.
503 The signing secret, key ID or mint key for the mode you use is not set.
The pane reports a CSP block Add the embed origin to script-src and frame-src and the API to connect-src.
Startup requests are not discovered The loader must come before your application’s scripts in the head.
Two launchers Render the loader once, in the layout.
A secret in page source Only AGENT_PUBLIC_KEY and origins belong in views; keep ss_ and sk_ values in server environment variables.
Section titled “Related”