diff --git a/router/index.ts b/router/index.ts
index 9632cb11..93e1c362 100644
--- a/router/index.ts
+++ b/router/index.ts
@@ -505,6 +505,31 @@ function relayAgentOrigin(env: Env): string | undefined {
   return origin || undefined;
 }
 
+// arelay.to carries agent chat, so it is HTTPS only. agentrelay.com gets this
+// from the zone's Always Use HTTPS setting; arelay.to's zone does not have it,
+// and keeping it here keeps it in source control. GET and HEAD get a permanent
+// redirect. Anything else gets a 308 whose body names the HTTPS URL, so an
+// agent's curl (which does not follow redirects) prints what to use instead,
+// and the message is never processed over plain HTTP.
+export function getInsecureShortHostResponse(url: URL, method: string): Response | undefined {
+  // A fully qualified Host ("arelay.to.") names the same site; match it too.
+  const hostname = url.hostname.replace(/\.$/, "");
+  if (url.protocol !== "http:" || (hostname !== SHORT_HOST && hostname !== SHORT_HOST_WWW)) {
+    return undefined;
+  }
+  const secure = new URL(url);
+  secure.protocol = "https:";
+  secure.hostname = hostname;
+  secure.port = "";
+  if (method === "GET" || method === "HEAD") {
+    return Response.redirect(secure.toString(), 301);
+  }
+  return new Response(`agent-relay: use HTTPS: ${secure.toString()}\n`, {
+    status: 308,
+    headers: { location: secure.toString(), "content-type": "text/plain; charset=utf-8" },
+  });
+}
+
 class RelayAgentOriginError extends Error {}
 
 function validatedRelayAgentOrigin(origin: string): string {
@@ -557,6 +582,11 @@ export default {
   async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
     const url = new URL(request.url);
 
+    const insecureShortHost = getInsecureShortHostResponse(url, request.method);
+    if (insecureShortHost) {
+      return insecureShortHost;
+    }
+
     const vanityRedirect = getVanityRedirect(url.hostname, url.pathname);
     if (vanityRedirect) {
       const redirectUrl = new URL(vanityRedirect);
diff --git a/router/test/agent-chat.test.ts b/router/test/agent-chat.test.ts
index 17841247..543a0918 100644
--- a/router/test/agent-chat.test.ts
+++ b/router/test/agent-chat.test.ts
@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 import worker, {
+  getInsecureShortHostResponse,
   getAgentChatCloudPath,
   getAgentPagePath,
   getLegacyAgentPageRedirect,
@@ -337,3 +338,43 @@ describe("router agent chat", () => {
     expect(response.headers.get("location")).toBe("https://agentrelay.com/u/agent-relay");
   });
 });
+
+describe("arelay.to over plain HTTP", () => {
+  it("redirects pages permanently to HTTPS, keeping the path and query", () => {
+    const response = getInsecureShortHostResponse(new URL("http://arelay.to/agent-relay?x=1"), "GET");
+    expect(response?.status).toBe(301);
+    expect(response?.headers.get("location")).toBe("https://arelay.to/agent-relay?x=1");
+    expect(getInsecureShortHostResponse(new URL("http://www.arelay.to/register"), "HEAD")?.status).toBe(301);
+  });
+
+  it("refuses chat over HTTP with a 308 that names the HTTPS URL", async () => {
+    const id = "0".repeat(32);
+    const response = getInsecureShortHostResponse(new URL(`http://arelay.to/agent-relay/${id}`), "POST");
+    expect(response?.status).toBe(308);
+    expect(response?.headers.get("location")).toBe(`https://arelay.to/agent-relay/${id}`);
+    expect(await response?.text()).toBe(`agent-relay: use HTTPS: https://arelay.to/agent-relay/${id}\n`);
+  });
+
+  it("also catches a fully qualified host with a trailing dot", () => {
+    const response = getInsecureShortHostResponse(new URL("http://arelay.to./agent-relay"), "GET");
+    expect(response?.status).toBe(301);
+    expect(response?.headers.get("location")).toBe("https://arelay.to/agent-relay");
+    expect(getInsecureShortHostResponse(new URL(`http://www.arelay.to./agent-relay/${"0".repeat(32)}`), "POST")?.status).toBe(308);
+  });
+
+  it("leaves HTTPS and other hosts alone", () => {
+    expect(getInsecureShortHostResponse(new URL("https://arelay.to/agent-relay"), "GET")).toBeUndefined();
+    expect(getInsecureShortHostResponse(new URL("http://agentrelay.com/u/agent-relay"), "GET")).toBeUndefined();
+  });
+
+  it("never forwards an HTTP chat message upstream", async () => {
+    const fetchMock = vi.fn();
+    const response = await worker.fetch(
+      new Request(`http://arelay.to/agent-relay/${"0".repeat(32)}`, { method: "POST", body: "hi" }),
+      buildEnv({ fetch: fetchMock }),
+      ctx,
+    );
+    expect(response.status).toBe(308);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});