diff --git a/apps/web/src/lib/auth-cookie.test.ts b/apps/web/src/lib/auth-cookie.test.ts
new file mode 100644
index 0000000..71d7be0
--- /dev/null
+++ b/apps/web/src/lib/auth-cookie.test.ts
@@ -0,0 +1,97 @@
+import { describe, it, expect, beforeAll } from "vitest";
+import {
+  signSession,
+  verifySession,
+  COOKIE_NAME,
+  DEFAULT_TTL_SECONDS,
+  type SessionPayload,
+} from "./auth-cookie";
+
+const SECRET = "test-secret-not-used-anywhere-real";
+const NOW = 1_700_000_000; // 2023-11-14 — fixed clock for determinism
+
+beforeAll(() => {
+  process.env.AUTH_SECRET = SECRET;
+  process.env.OPERATOR_TOKEN_VERSION = "1";
+});
+
+const validPayload = (): SessionPayload => ({
+  userId: "11111111-1111-1111-1111-111111111111",
+  role: "admin",
+  iat: NOW,
+  exp: NOW + DEFAULT_TTL_SECONDS,
+  v: 1,
+});
+
+describe("auth-cookie", () => {
+  it("signSession + verifySession round-trips a valid payload", async () => {
+    const cookie = await signSession(validPayload(), SECRET);
+    const verified = await verifySession(cookie, SECRET, NOW);
+    expect(verified).toEqual(validPayload());
+  });
+
+  it("rejects when the payload portion has been tampered with", async () => {
+    const cookie = await signSession(validPayload(), SECRET);
+    // Flip the role to admin → user in the payload, keep the same signature.
+    const [, sig] = cookie.split(".");
+    const tampered = btoa(JSON.stringify({ ...validPayload(), role: "user" }))
+      .replace(/\+/g, "-")
+      .replace(/\//g, "_")
+      .replace(/=+$/, "") + "." + sig;
+    expect(await verifySession(tampered, SECRET, NOW)).toBeNull();
+  });
+
+  it("rejects when the signature has been tampered with", async () => {
+    const cookie = await signSession(validPayload(), SECRET);
+    const [payload] = cookie.split(".");
+    const tampered = payload + ".AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
+    expect(await verifySession(tampered, SECRET, NOW)).toBeNull();
+  });
+
+  it("rejects when verified with a different secret", async () => {
+    const cookie = await signSession(validPayload(), SECRET);
+    expect(await verifySession(cookie, "different-secret", NOW)).toBeNull();
+  });
+
+  it("rejects an expired cookie (exp <= now)", async () => {
+    const expired = { ...validPayload(), exp: NOW - 1 };
+    const cookie = await signSession(expired, SECRET);
+    expect(await verifySession(cookie, SECRET, NOW)).toBeNull();
+  });
+
+  it("rejects a cookie issued in the future beyond clock-skew tolerance", async () => {
+    const future = { ...validPayload(), iat: NOW + 120 };
+    const cookie = await signSession(future, SECRET);
+    expect(await verifySession(cookie, SECRET, NOW)).toBeNull();
+  });
+
+  it("accepts a cookie issued slightly in the future (within 60s skew)", async () => {
+    const future = { ...validPayload(), iat: NOW + 30 };
+    const cookie = await signSession(future, SECRET);
+    expect(await verifySession(cookie, SECRET, NOW)).not.toBeNull();
+  });
+
+  it("rejects a cookie whose v is stale (token-version bumped)", async () => {
+    const cookie = await signSession({ ...validPayload(), v: 1 }, SECRET);
+    process.env.OPERATOR_TOKEN_VERSION = "2";
+    expect(await verifySession(cookie, SECRET, NOW)).toBeNull();
+    process.env.OPERATOR_TOKEN_VERSION = "1";
+  });
+
+  it("rejects a cookie with an unknown role string", async () => {
+    const cookie = await signSession(
+      { ...validPayload(), role: "superadmin" as never },
+      SECRET,
+    );
+    expect(await verifySession(cookie, SECRET, NOW)).toBeNull();
+  });
+
+  it("rejects a cookie that doesn't have a '.' separator", async () => {
+    expect(await verifySession("not-a-cookie", SECRET, NOW)).toBeNull();
+    expect(await verifySession("", SECRET, NOW)).toBeNull();
+  });
+
+  it("exposes COOKIE_NAME as 'session'", () => {
+    expect(COOKIE_NAME).toBe("session");
+  });
+});
diff --git a/apps/web/src/lib/auth-cookie.ts b/apps/web/src/lib/auth-cookie.ts
new file mode 100644
index 0000000..c3928f9
--- /dev/null
+++ b/apps/web/src/lib/auth-cookie.ts
@@ -0,0 +1,119 @@
+/**
+ * Edge-runtime-safe HMAC-signed session cookie. Runs in middleware
+ * and Server Actions. NO database, NO bcrypt, NO Node-only APIs —
+ * pure Web Crypto so it survives Edge runtime.
+ */
+
+export const COOKIE_NAME = "session";
+export const DEFAULT_TTL_SECONDS = 30 * 86400; // 30 days
+export const CLOCK_SKEW_SECONDS = 60;
+
+export type Role = "admin" | "user";
+
+export interface SessionPayload {
+  userId: string;
+  role: Role;
+  iat: number;
+  exp: number;
+  v: number;
+}
+
+function isValidPayload(x: unknown): x is SessionPayload {
+  if (typeof x !== "object" || x === null) return false;
+  const o = x as Record<string, unknown>;
+  return (
+    typeof o.userId === "string" &&
+    (o.role === "admin" || o.role === "user") &&
+    typeof o.iat === "number" &&
+    typeof o.exp === "number" &&
+    typeof o.v === "number"
+  );
+}
+
+function b64urlEncode(bytes: Uint8Array): string {
+  let s = "";
+  for (const b of bytes) s += String.fromCharCode(b);
+  return btoa(s).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
+}
+
+function b64urlDecode(str: string): Uint8Array {
+  const pad = str.length % 4 === 0 ? "" : "=".repeat(4 - (str.length % 4));
+  const s = atob(str.replace(/-/g, "+").replace(/_/g, "/") + pad);
+  const out = new Uint8Array(s.length);
+  for (let i = 0; i < s.length; i++) out[i] = s.charCodeAt(i);
+  return out;
+}
+
+async function importKey(secret: string): Promise<CryptoKey> {
+  return crypto.subtle.importKey(
+    "raw",
+    new TextEncoder().encode(secret),
+    { name: "HMAC", hash: "SHA-256" },
+    false,
+    ["sign", "verify"],
+  );
+}
+
+/** Constant-time compare on byte arrays. Returns true iff equal. */
+function timingSafeEqual(a: Uint8Array, b: Uint8Array): boolean {
+  if (a.length !== b.length) return false;
+  let diff = 0;
+  for (let i = 0; i < a.length; i++) diff |= a[i]! ^ b[i]!;
+  return diff === 0;
+}
+
+export async function signSession(
+  payload: SessionPayload,
+  secret: string,
+): Promise<string> {
+  const json = JSON.stringify(payload);
+  const payloadEnc = b64urlEncode(new TextEncoder().encode(json));
+  const key = await importKey(secret);
+  const sigBytes = new Uint8Array(
+    await crypto.subtle.sign("HMAC", key, new TextEncoder().encode(payloadEnc)),
+  );
+  return `${payloadEnc}.${b64urlEncode(sigBytes)}`;
+}
+
+export async function verifySession(
+  cookie: string,
+  secret: string,
+  now: number = Math.floor(Date.now() / 1000),
+): Promise<SessionPayload | null> {
+  if (!cookie || typeof cookie !== "string") return null;
+  const dot = cookie.indexOf(".");
+  if (dot <= 0 || dot === cookie.length - 1) return null;
+  const payloadEnc = cookie.slice(0, dot);
+  const sigEnc = cookie.slice(dot + 1);
+
+  let sigBytes: Uint8Array;
+  try {
+    sigBytes = b64urlDecode(sigEnc);
+  } catch {
+    return null;
+  }
+
+  const key = await importKey(secret);
+  const expected = new Uint8Array(
+    await crypto.subtle.sign("HMAC", key, new TextEncoder().encode(payloadEnc)),
+  );
+  if (!timingSafeEqual(sigBytes, expected)) return null;
+
+  let json: string;
+  let payload: unknown;
+  try {
+    json = new TextDecoder().decode(b64urlDecode(payloadEnc));
+    payload = JSON.parse(json);
+  } catch {
+    return null;
+  }
+  if (!isValidPayload(payload)) return null;
+
+  if (payload.exp <= now) return null;
+  if (payload.iat > now + CLOCK_SKEW_SECONDS) return null;
+
+  const expectedV = Number(process.env.OPERATOR_TOKEN_VERSION ?? "1");
+  if (payload.v !== expectedV) return null;
+
+  return payload;
+}