diff --git a/browser-host/README.md b/browser-host/README.md
index 1ed2d50e7d..1fe8eb0ecb 100644
--- a/browser-host/README.md
+++ b/browser-host/README.md
@@ -17,3 +17,19 @@ identity pin; never put a private key in environment configuration. Preserve
 supported browser capabilities while consolidating duplicate implementations.
 Use the native path for native acceptance and browser workflows to validate
 browser support; see the [consolidation guidance](../docs/contributing.md#broker-consolidation-and-completion).
+
+## Hosted browser previews
+
+`relayBrokerPlugin({ browserOrigin: "https://preview.example.org" })` accepts
+one explicit HTTPS browser origin through a trusted proxy. The listener must
+stay on loopback. This setting is an explicit development opt-in. The broker
+refuses it outside the Vite development server in `development` mode, including
+production mode and builds. The proxy must authenticate its users; the origin setting
+does not grant access or authenticate a remote client. Use a simulated identity
+and relay for shared UX previews.
+
+Set the origin in trusted server configuration, never from request headers.
+Origins with credentials, a path, a query, or a fragment are refused. Requests
+still require a loopback Host and matching Origin. Cross-site browser requests
+remain refused. Without this setting, the broker requires its HTTP loopback
+origin. Native and release hosts do not use this setting.
diff --git a/browser-host/broker-origin.test.mjs b/browser-host/broker-origin.test.mjs
new file mode 100644
index 0000000000..cbb0faaad5
--- /dev/null
+++ b/browser-host/broker-origin.test.mjs
@@ -0,0 +1,149 @@
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { request } from "node:http";
+import { getPublicKey } from "nostr-tools";
+import { createServer, resolveConfig } from "vite";
+import { expect, it } from "vitest";
+import { relayBrokerPlugin } from "./relay-broker.mjs";
+
+it("admits only the configured browser origin through a loopback proxy", async () => {
+  const directory = mkdtempSync(join(tmpdir(), "buzz-broker-origin-"));
+  const key = new Uint8Array(32).fill(1);
+  const browserOrigin = "https://preview.example.org";
+  const server = await createServer({
+    root: directory,
+    configFile: false,
+    logLevel: "silent",
+    optimizeDeps: { noDiscovery: true, include: [] },
+    server: { host: "127.0.0.1", port: 0, watch: null, hmr: false },
+    plugins: [
+      relayBrokerPlugin({
+        browserOrigin,
+        identity: () => key.slice(),
+        archiveFile: ":memory:",
+      }),
+    ],
+  });
+  try {
+    await server.listen();
+    const base = `http://127.0.0.1:${server.httpServer.address().port}`;
+    const allowed = await fetch(`${base}/api/relay/identity`, {
+      headers: { Origin: browserOrigin, "Sec-Fetch-Site": "same-origin" },
+    });
+    expect(allowed.status).toBe(200);
+    expect(await allowed.json()).toEqual({ viewer: getPublicKey(key) });
+    const post = await fetch(
+      `${base}/api/relay/prepare-remote-agent-authorization`,
+      {
+        method: "POST",
+        headers: {
+          Origin: browserOrigin,
+          "Sec-Fetch-Site": "same-origin",
+          "Content-Type": "application/json",
+        },
+        body: JSON.stringify({
+          owner: getPublicKey(key),
+          agentPubkey: getPublicKey(new Uint8Array(32).fill(2)),
+        }),
+      },
+    );
+    expect(post.status).toBe(200);
+    expect((await post.json()).slice(0, 2)).toEqual([
+      "auth",
+      getPublicKey(key),
+    ]);
+    for (const Origin of [
+      "https://other.example.org",
+      `${browserOrigin}.evil.example`,
+      "null",
+      base,
+    ]) {
+      const denied = await fetch(`${base}/api/relay/identity`, {
+        headers: { Origin },
+      });
+      expect(denied.status, Origin).toBe(403);
+    }
+    for (const headers of [
+      {},
+      { Origin: browserOrigin, "Sec-Fetch-Site": "cross-site" },
+    ]) {
+      const denied = await fetch(
+        `${base}/api/relay/prepare-remote-agent-authorization`,
+        {
+          method: "POST",
+          headers,
+          body: "{}",
+        },
+      );
+      expect(denied.status).toBe(403);
+    }
+    const hostStatus = await new Promise((resolve, reject) => {
+      const req = request(
+        `${base}/api/relay/identity`,
+        {
+          headers: { Origin: browserOrigin, Host: "other.example.org" },
+        },
+        (response) => {
+          response.resume();
+          response.on("end", () => resolve(response.statusCode));
+        },
+      );
+      req.on("error", reject);
+      req.end();
+    });
+    expect(hostStatus).toBe(403);
+  } finally {
+    await server.close();
+    rmSync(directory, { recursive: true, force: true });
+  }
+});
+
+it.each([
+  ["serve", "production"],
+  ["build", "development"],
+  ["build", "production"],
+])("refuses a hosted origin for %s in %s mode", async (command, mode) => {
+  await expect(
+    resolveConfig(
+      {
+        configFile: false,
+        mode,
+        plugins: [
+          relayBrokerPlugin({
+            browserOrigin: "https://preview.example.org",
+            identity: () => {
+              throw new Error("Identity read");
+            },
+          }),
+        ],
+      },
+      command,
+      mode,
+    ),
+  ).rejects.toThrow(
+    "browserOrigin requires the Vite development server in development mode",
+  );
+});
+
+it.each([
+  "*",
+  "http://preview.example.org",
+  "https://user:pass@preview.example.org",
+  "https://preview.example.org/path",
+  "https://preview.example.org/?query",
+  "https://preview.example.org/#fragment",
+  "",
+])(
+  "refuses invalid browser origin %s before reading identity",
+  (browserOrigin) => {
+    expect(() =>
+      relayBrokerPlugin({
+        browserOrigin,
+        identity: () => {
+          throw new Error("Identity read");
+        },
+      }),
+    ).toThrow("browserOrigin must be an HTTPS origin");
+  },
+);
diff --git a/browser-host/relay-broker.d.mts b/browser-host/relay-broker.d.mts
index 353212fc92..e53c0149c3 100644
--- a/browser-host/relay-broker.d.mts
+++ b/browser-host/relay-broker.d.mts
@@ -2,6 +2,7 @@ import type { Plugin } from "vite";
 export function relayBrokerPlugin(options?: {
   archiveFile?: string;
   authorizedViewer?: string | undefined;
+  browserOrigin?: string | undefined;
   agentLibrary?: () => Promise<
     import("../src/features/agents/library").AgentLibrary
   >;
diff --git a/browser-host/relay-broker.mjs b/browser-host/relay-broker.mjs
index eab68ca6fa..431d066942 100644
--- a/browser-host/relay-broker.mjs
+++ b/browser-host/relay-broker.mjs
@@ -848,6 +848,7 @@ const json = (res, code, body) => {
 /** @returns {import('vite').Plugin} */
 export function relayBrokerPlugin({
   authorizedViewer,
+  browserOrigin,
   relayUrl,
   communityAliases,
   identity = () => loadIdentity(authorizedViewer),
@@ -860,11 +861,38 @@ export function relayBrokerPlugin({
   // Tests that hold EOSE on purpose lengthen this real-time deadline.
   liveSetupTimeoutMs,
 } = {}) {
+  const publicOrigin =
+    browserOrigin === undefined
+      ? undefined
+      : typeof browserOrigin === "string"
+        ? URL.parse(browserOrigin)
+        : null;
+  if (
+    browserOrigin !== undefined &&
+    (publicOrigin?.protocol !== "https:" ||
+      publicOrigin.username ||
+      publicOrigin.password ||
+      publicOrigin.pathname !== "/" ||
+      publicOrigin.search ||
+      publicOrigin.hash)
+  )
+    throw new Error(
+      "browserOrigin must be an HTTPS origin without credentials, path, query, or fragment.",
+    );
   const aliases = parseCommunityAliases(communityAliases);
   const defaultRelay = relayUrl?.trim() ? relayOrigin(relayUrl) : undefined;
   return {
     name: "buzz-relay-broker",
     configResolved(config) {
+      if (
+        publicOrigin &&
+        (config.command !== "serve" ||
+          config.mode !== "development" ||
+          config.isProduction)
+      )
+        throw new Error(
+          "browserOrigin requires the Vite development server in development mode.",
+        );
       // Request headers express browser origin intent, not remote-client
       // authentication. Check the effective listener, including CLI overrides,
       // before configureServer loads the signing identity.
@@ -1010,7 +1038,8 @@ export function relayBrokerPlugin({
           else if (res.statusCode >= 400) log.warn(line);
           else log.debug(line);
         });
-        const origin = `http://${req.headers.host ?? ""}`;
+        const origin =
+          publicOrigin?.origin ?? `http://${req.headers.host ?? ""}`;
         // Same-origin browser access only; trusted plugins/local processes are not sandboxed.
         if (!/^(localhost|127\.0\.0\.1):\d+$/.test(req.headers.host ?? ""))
           return json(res, 403, { error: "Host rejected" });