diff --git a/frontend/src/app/api/auth/[...all]/route.ts b/frontend/src/app/api/auth/[...all]/route.ts
new file mode 100644
index 00000000..b7f0834d
--- /dev/null
+++ b/frontend/src/app/api/auth/[...all]/route.ts
@@ -0,0 +1,35 @@
+import { toNextJsHandler } from "better-auth/next-js";
+
+import { getIdentityAuthServices } from "@/modules/identity/auth";
+import { readIdentityConfig } from "@/modules/identity/config";
+import {
+  createHostIsolatedAuthHandlers,
+  type IdentityAuthHandlers,
+} from "@/modules/identity/host";
+
+export const dynamic = "force-dynamic";
+
+async function dispatch(
+  method: keyof IdentityAuthHandlers,
+  request: Request,
+): Promise<Response> {
+  const config = readIdentityConfig(process.env);
+  if (config.provider !== "self-hosted") {
+    return new Response("Not found", { status: 404 });
+  }
+
+  const services = getIdentityAuthServices();
+  const handlers = createHostIsolatedAuthHandlers(config, {
+    user: toNextJsHandler(services.user),
+    admin: toNextJsHandler(services.admin),
+  });
+  return handlers[method](request);
+}
+
+export function GET(request: Request): Promise<Response> {
+  return dispatch("GET", request);
+}
+
+export function POST(request: Request): Promise<Response> {
+  return dispatch("POST", request);
+}
diff --git a/frontend/src/modules/identity/auth-factory.ts b/frontend/src/modules/identity/auth-factory.ts
index 1f9693ab..dbd9f8ec 100644
--- a/frontend/src/modules/identity/auth-factory.ts
+++ b/frontend/src/modules/identity/auth-factory.ts
@@ -1,6 +1,6 @@
 import { createHmac } from "node:crypto";
 import type { Pool } from "pg";
-import type { BetterAuthOptions } from "better-auth";
+import { APIError, type BetterAuthOptions } from "better-auth";
 import { admin, emailOTP, type EmailOTPOptions } from "better-auth/plugins";
 
 import type { SelfHostedIdentityConfig } from "./config.ts";
@@ -114,7 +114,11 @@ export function buildAuthOptions({
             session: {
               create: {
                 async before(session: { userId: string }) {
-                  if (!(await authorizeAdminUser!(session.userId))) return false;
+                  if (!(await authorizeAdminUser!(session.userId))) {
+                    throw new APIError("FORBIDDEN", {
+                      message: "Administrator access required",
+                    });
+                  }
                 },
               },
             },
diff --git a/frontend/src/modules/identity/host.ts b/frontend/src/modules/identity/host.ts
new file mode 100644
index 00000000..7a7435da
--- /dev/null
+++ b/frontend/src/modules/identity/host.ts
@@ -0,0 +1,74 @@
+import type { SelfHostedIdentityConfig } from "./config.ts";
+import type { IdentitySurface } from "./contracts.ts";
+
+export type IdentityRequestHandler = (
+  request: Request,
+) => Response | Promise<Response>;
+
+export interface IdentityAuthHandlers {
+  GET: IdentityRequestHandler;
+  POST: IdentityRequestHandler;
+}
+
+function normalizeHost(value: string | null): string | null {
+  if (!value || value !== value.trim() || /[\s,@/\\]/.test(value)) return null;
+
+  try {
+    const url = new URL(`https://${value}`);
+    if (
+      url.username ||
+      url.password ||
+      url.pathname !== "/" ||
+      url.search ||
+      url.hash
+    ) {
+      return null;
+    }
+    return url.host.toLowerCase();
+  } catch {
+    return null;
+  }
+}
+
+export function resolveIdentitySurface(
+  hostHeader: string | null,
+  config: SelfHostedIdentityConfig,
+): IdentitySurface | null {
+  const host = normalizeHost(hostHeader);
+  if (!host) return null;
+
+  const userHost = new URL(config.userOrigin).host.toLowerCase();
+  const adminHost = new URL(config.adminOrigin).host.toLowerCase();
+  if (host === userHost) return "user";
+  if (host === adminHost) return "admin";
+  return null;
+}
+
+function isAdminEndpoint(request: Request): boolean {
+  try {
+    const path = decodeURIComponent(new URL(request.url).pathname);
+    return /^\/api\/auth\/+admin(?:\/|$)/i.test(path);
+  } catch {
+    return true;
+  }
+}
+
+export function createHostIsolatedAuthHandlers(
+  config: SelfHostedIdentityConfig,
+  handlers: Record<IdentitySurface, IdentityAuthHandlers>,
+): IdentityAuthHandlers {
+  const dispatch =
+    (method: keyof IdentityAuthHandlers): IdentityRequestHandler =>
+    async (request) => {
+      const surface = resolveIdentitySurface(request.headers.get("host"), config);
+      if (!surface) {
+        return new Response("Unrecognized identity host", { status: 421 });
+      }
+      if (surface === "user" && isAdminEndpoint(request)) {
+        return new Response("Not found", { status: 404 });
+      }
+      return handlers[surface][method](request);
+    };
+
+  return { GET: dispatch("GET"), POST: dispatch("POST") };
+}
diff --git a/frontend/src/modules/identity/session.ts b/frontend/src/modules/identity/session.ts
new file mode 100644
index 00000000..0c2fd6e1
--- /dev/null
+++ b/frontend/src/modules/identity/session.ts
@@ -0,0 +1,80 @@
+import type { IdentitySession, IdentityUser } from "./contracts.ts";
+
+interface RawIdentitySession {
+  session: { expiresAt: Date | string };
+  user: {
+    id: string;
+    email: string;
+    emailVerified: boolean;
+    name: string;
+    image?: string | null;
+    role?: string | null;
+  };
+}
+
+export interface IdentitySessionReader {
+  getSession(input: { headers: Headers }): Promise<RawIdentitySession | null>;
+}
+
+export class IdentityAuthorizationError extends Error {
+  constructor(
+    message: string,
+    readonly status: 401 | 403,
+  ) {
+    super(message);
+    this.name = "IdentityAuthorizationError";
+  }
+}
+
+function parseRoles(role: string | null | undefined): string[] {
+  const roles = (role ?? "user")
+    .split(",")
+    .map((value) => value.trim())
+    .filter(Boolean);
+  return [...new Set(roles.length ? roles : ["user"])];
+}
+
+export async function readIdentitySession(
+  reader: IdentitySessionReader,
+  requestHeaders: Headers,
+): Promise<IdentitySession | null> {
+  const value = await reader.getSession({ headers: requestHeaders });
+  if (!value) return null;
+
+  const expiresAt = new Date(value.session.expiresAt);
+  if (!Number.isFinite(expiresAt.getTime())) {
+    throw new Error("identity session has an invalid expiry");
+  }
+
+  return {
+    expiresAt,
+    user: {
+      id: value.user.id,
+      email: value.user.email.trim().toLowerCase(),
+      emailVerified: value.user.emailVerified,
+      name: value.user.name,
+      image: value.user.image ?? null,
+      role: parseRoles(value.user.role),
+    },
+  };
+}
+
+export async function requireIdentityUser(
+  reader: IdentitySessionReader,
+  requestHeaders: Headers,
+): Promise<IdentityUser> {
+  const session = await readIdentitySession(reader, requestHeaders);
+  if (!session) throw new IdentityAuthorizationError("Authentication required", 401);
+  return session.user;
+}
+
+export async function requireIdentityAdmin(
+  reader: IdentitySessionReader,
+  requestHeaders: Headers,
+): Promise<IdentityUser> {
+  const user = await requireIdentityUser(reader, requestHeaders);
+  if (!user.role.includes("admin")) {
+    throw new IdentityAuthorizationError("Administrator access required", 403);
+  }
+  return user;
+}
diff --git a/frontend/tests/identity-auth-factory.test.ts b/frontend/tests/identity-auth-factory.test.ts
index 0f8e3aea..5e4bf342 100644
--- a/frontend/tests/identity-auth-factory.test.ts
+++ b/frontend/tests/identity-auth-factory.test.ts
@@ -145,7 +145,10 @@ test("admin surface disables sign-up and rejects non-admin session creation", as
     createdAt: new Date(),
     updatedAt: new Date(),
   };
-  assert.equal(await before(session, null), false);
+  await assert.rejects(
+    before(session, null),
+    /Administrator access required/,
+  );
   assert.equal(
     await before({ ...session, userId: "admin-user-id" }, null),
     undefined,
diff --git a/frontend/tests/identity-auth-integration.test.ts b/frontend/tests/identity-auth-integration.test.ts
new file mode 100644
index 00000000..1021f569
--- /dev/null
+++ b/frontend/tests/identity-auth-integration.test.ts
@@ -0,0 +1,155 @@
+import assert from "node:assert/strict";
+import { spawnSync } from "node:child_process";
+import { fileURLToPath } from "node:url";
+import test from "node:test";
+import { toNextJsHandler } from "better-auth/next-js";
+
+import {
+  createIdentityAuthServices,
+  createIdentityPool,
+} from "../src/modules/identity/auth.ts";
+import type { SelfHostedIdentityConfig } from "../src/modules/identity/config.ts";
+import { FakeEmailOtpSender } from "../src/modules/identity/email/fake-email-otp-sender.ts";
+import { createHostIsolatedAuthHandlers } from "../src/modules/identity/host.ts";
+import { startPostgresFixture } from "./helpers/postgres-fixture.ts";
+
+const runnerPath = fileURLToPath(
+  new URL("../scripts/db-migrate.mjs", import.meta.url),
+);
+const migrationsDirectory = fileURLToPath(
+  new URL("../db/migrations", import.meta.url),
+);
+
+function request(
+  host: string,
+  path: string,
+  body: Record<string, unknown>,
+): Request {
+  return new Request(`https://${host}${path}`, {
+    method: "POST",
+    headers: {
+      "content-type": "application/json",
+      host,
+      origin: `https://${host}`,
+    },
+    body: JSON.stringify(body),
+  });
+}
+
+test("Better Auth completes OTP sign-in against the migrated identity schema with isolated cookies", async () => {
+  const fixture = startPostgresFixture();
+  const migration = spawnSync(process.execPath, [runnerPath], {
+    encoding: "utf8",
+    env: {
+      ...process.env,
+      MIGRATIONS_DIRECTORY: migrationsDirectory,
+      SCHEMA_DATABASE_URL: fixture.connectionUrl(
+        "schema_owner",
+        "schema-owner-test-password",
+      ),
+    },
+  });
+  assert.equal(migration.status, 0, migration.stderr);
+
+  const config: SelfHostedIdentityConfig = {
+    provider: "self-hosted",
+    databaseUrl: fixture.connectionUrl(
+      "identity_runtime",
+      "identity-runtime-test-password",
+    ),
+    userOrigin: "https://staging.jyotisha.chat",
+    adminOrigin: "https://admin.staging.jyotisha.chat",
+    userSecret: "user-secret-that-is-at-least-32-bytes-long",
+    adminSecret: "admin-secret-that-is-at-least-32-bytes-long",
+    resendApiKey: "re_test",
+    resendFrom: "Jyotisha <login@staging.jyotisha.chat>",
+  };
+  const sender = new FakeEmailOtpSender();
+  const pool = createIdentityPool(config.databaseUrl);
+  const services = createIdentityAuthServices(config, {
+    pool,
+    emailSender: sender,
+  });
+  const handlers = createHostIsolatedAuthHandlers(config, {
+    user: toNextJsHandler(services.user),
+    admin: toNextJsHandler(services.admin),
+  });
+
+  try {
+    const userSend = await handlers.POST(
+      request(
+        "staging.jyotisha.chat",
+        "/api/auth/email-otp/send-verification-otp",
+        { email: "person@example.com", type: "sign-in" },
+      ),
+    );
+    assert.equal(userSend.status, 200, await userSend.text());
+    assert.equal(sender.messages.length, 1);
+
+    const userSignIn = await handlers.POST(
+      request("staging.jyotisha.chat", "/api/auth/sign-in/email-otp", {
+        email: "person@example.com",
+        otp: sender.messages[0].otp,
+      }),
+    );
+    const userCookie = userSignIn.headers.get("set-cookie") ?? "";
+    assert.equal(userSignIn.status, 200, await userSignIn.text());
+    assert.match(userCookie, /jyotisha-user\.session_token=/);
+    assert.doesNotMatch(userCookie, /jyotisha-admin/);
+    assert.match(userCookie, /HttpOnly/i);
+    assert.match(userCookie, /Secure/i);
+    assert.match(userCookie, /SameSite=Lax/i);
+    assert.doesNotMatch(userCookie, /Domain=/i);
+    assert.equal(fixture.psql("select count(*) from identity.users"), "1");
+    assert.equal(fixture.psql("select count(*) from identity.sessions"), "1");
+
+    const adminSend = await handlers.POST(
+      request(
+        "admin.staging.jyotisha.chat",
+        "/api/auth/email-otp/send-verification-otp",
+        { email: "person@example.com", type: "sign-in" },
+      ),
+    );
+    assert.equal(adminSend.status, 200, await adminSend.text());
+    const deniedAdminSignIn = await handlers.POST(
+      request(
+        "admin.staging.jyotisha.chat",
+        "/api/auth/sign-in/email-otp",
+        { email: "person@example.com", otp: sender.messages[1].otp },
+      ),
+    );
+    assert.equal(deniedAdminSignIn.status, 403);
+    assert.equal(deniedAdminSignIn.headers.has("set-cookie"), false);
+    assert.equal(fixture.psql("select count(*) from identity.sessions"), "1");
+
+    fixture.psqlAs(
+      "identity_runtime",
+      "identity-runtime-test-password",
+      "update identity.users set role = 'user,admin' where email = 'person@example.com'",
+    );
+    const promotedSend = await handlers.POST(
+      request(
+        "admin.staging.jyotisha.chat",
+        "/api/auth/email-otp/send-verification-otp",
+        { email: "person@example.com", type: "sign-in" },
+      ),
+    );
+    assert.equal(promotedSend.status, 200, await promotedSend.text());
+    const adminSignIn = await handlers.POST(
+      request(
+        "admin.staging.jyotisha.chat",
+        "/api/auth/sign-in/email-otp",
+        { email: "person@example.com", otp: sender.messages[2].otp },
+      ),
+    );
+    const adminCookie = adminSignIn.headers.get("set-cookie") ?? "";
+    assert.equal(adminSignIn.status, 200, await adminSignIn.text());
+    assert.match(adminCookie, /jyotisha-admin\.session_token=/);
+    assert.doesNotMatch(adminCookie, /jyotisha-user/);
+    assert.doesNotMatch(adminCookie, /Domain=/i);
+    assert.equal(fixture.psql("select count(*) from identity.sessions"), "2");
+  } finally {
+    await pool.end();
+    fixture.stop();
+  }
+});
diff --git a/frontend/tests/identity-host-routing.test.ts b/frontend/tests/identity-host-routing.test.ts
new file mode 100644
index 00000000..3cca107f
--- /dev/null
+++ b/frontend/tests/identity-host-routing.test.ts
@@ -0,0 +1,130 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import type { SelfHostedIdentityConfig } from "../src/modules/identity/config.ts";
+import {
+  createHostIsolatedAuthHandlers,
+  resolveIdentitySurface,
+} from "../src/modules/identity/host.ts";
+
+const config: SelfHostedIdentityConfig = {
+  provider: "self-hosted",
+  databaseUrl: "postgresql://identity_runtime:test@postgres:5432/jyotisha",
+  userOrigin: "https://staging.jyotisha.chat",
+  adminOrigin: "https://admin.staging.jyotisha.chat",
+  userSecret: "user-secret-that-is-at-least-32-bytes-long",
+  adminSecret: "admin-secret-that-is-at-least-32-bytes-long",
+  resendApiKey: "re_test",
+  resendFrom: "Jyotisha <login@staging.jyotisha.chat>",
+};
+
+test("identity surface matching is exact, case-insensitive, and port-normalized", () => {
+  assert.equal(resolveIdentitySurface("staging.jyotisha.chat", config), "user");
+  assert.equal(resolveIdentitySurface("STAGING.JYOTISHA.CHAT:443", config), "user");
+  assert.equal(
+    resolveIdentitySurface("admin.staging.jyotisha.chat", config),
+    "admin",
+  );
+
+  for (const host of [
+    null,
+    "",
+    "evil-staging.jyotisha.chat",
+    "staging.jyotisha.chat.evil.example",
+    "staging.jyotisha.chat,evil.example",
+    "staging.jyotisha.chat/path",
+    "user@staging.jyotisha.chat",
+    " staging.jyotisha.chat",
+  ]) {
+    assert.equal(resolveIdentitySurface(host, config), null, String(host));
+  }
+});
+
+test("auth route dispatches only to the exact matching host", async () => {
+  const calls: string[] = [];
+  const handlers = createHostIsolatedAuthHandlers(config, {
+    user: {
+      GET: async () => {
+        calls.push("user:get");
+        return new Response("user");
+      },
+      POST: async () => {
+        calls.push("user:post");
+        return new Response("user");
+      },
+    },
+    admin: {
+      GET: async () => {
+        calls.push("admin:get");
+        return new Response("admin");
+      },
+      POST: async () => {
+        calls.push("admin:post");
+        return new Response("admin");
+      },
+    },
+  });
+
+  const userResponse = await handlers.POST(
+    new Request("https://internal/api/auth/email-otp/send-verification-otp", {
+      method: "POST",
+      headers: { host: "staging.jyotisha.chat" },
+    }),
+  );
+  assert.equal(await userResponse.text(), "user");
+
+  const adminResponse = await handlers.GET(
+    new Request("https://internal/api/auth/get-session", {
+      headers: { host: "admin.staging.jyotisha.chat" },
+    }),
+  );
+  assert.equal(await adminResponse.text(), "admin");
+  assert.deepEqual(calls, ["user:post", "admin:get"]);
+});
+
+test("unknown hosts fail closed before an auth handler reads cookies", async () => {
+  let calls = 0;
+  const handler = async () => {
+    calls += 1;
+    return new Response("unexpected");
+  };
+  const handlers = createHostIsolatedAuthHandlers(config, {
+    user: { GET: handler, POST: handler },
+    admin: { GET: handler, POST: handler },
+  });
+
+  const response = await handlers.GET(
+    new Request("https://internal/api/auth/get-session", {
+      headers: {
+        host: "staging.jyotisha.chat.evil.example",
+        cookie: "jyotisha-admin.session_token=attacker-controlled",
+      },
+    }),
+  );
+
+  assert.equal(response.status, 421);
+  assert.equal(calls, 0);
+  assert.equal(response.headers.has("set-cookie"), false);
+});
+
+test("user host cannot reach Better Auth admin endpoints", async () => {
+  let calls = 0;
+  const handler = async () => {
+    calls += 1;
+    return new Response("unexpected");
+  };
+  const handlers = createHostIsolatedAuthHandlers(config, {
+    user: { GET: handler, POST: handler },
+    admin: { GET: handler, POST: handler },
+  });
+
+  const response = await handlers.POST(
+    new Request("https://internal/api/auth/admin/set-role", {
+      method: "POST",
+      headers: { host: "staging.jyotisha.chat" },
+    }),
+  );
+
+  assert.equal(response.status, 404);
+  assert.equal(calls, 0);
+});
diff --git a/frontend/tests/identity-session.test.ts b/frontend/tests/identity-session.test.ts
new file mode 100644
index 00000000..162bf43f
--- /dev/null
+++ b/frontend/tests/identity-session.test.ts
@@ -0,0 +1,77 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  IdentityAuthorizationError,
+  readIdentitySession,
+  requireIdentityAdmin,
+  requireIdentityUser,
+  type IdentitySessionReader,
+} from "../src/modules/identity/session.ts";
+
+function readerFor(role: string | null): IdentitySessionReader {
+  return {
+    async getSession() {
+      if (role === null) return null;
+      return {
+        session: { expiresAt: new Date("2030-01-01T00:00:00.000Z") },
+        user: {
+          id: "018f4e6d-7a11-7000-8000-000000000001",
+          email: "Person@Example.com",
+          emailVerified: true,
+          name: "Person",
+          image: null,
+          role,
+        },
+      };
+    },
+  };
+}
+
+test("identity session mapper returns a narrow normalized DTO", async () => {
+  const session = await readIdentitySession(readerFor("user,admin"), new Headers());
+
+  assert.deepEqual(session, {
+    expiresAt: new Date("2030-01-01T00:00:00.000Z"),
+    user: {
+      id: "018f4e6d-7a11-7000-8000-000000000001",
+      email: "person@example.com",
+      emailVerified: true,
+      name: "Person",
+      image: null,
+      role: ["user", "admin"],
+    },
+  });
+  assert.equal("token" in (session ?? {}), false);
+});
+
+test("require user rejects a missing server-side session", async () => {
+  await assert.rejects(
+    requireIdentityUser(readerFor(null), new Headers({ cookie: "present=1" })),
+    (error: unknown) => {
+      assert.ok(error instanceof IdentityAuthorizationError);
+      assert.equal(error.status, 401);
+      return true;
+    },
+  );
+});
+
+test("require admin checks persisted session roles, not cookie presence", async () => {
+  await assert.rejects(
+    requireIdentityAdmin(
+      readerFor("user"),
+      new Headers({ cookie: "jyotisha-admin.session_token=present" }),
+    ),
+    (error: unknown) => {
+      assert.ok(error instanceof IdentityAuthorizationError);
+      assert.equal(error.status, 403);
+      return true;
+    },
+  );
+
+  const admin = await requireIdentityAdmin(
+    readerFor("user,admin"),
+    new Headers(),
+  );
+  assert.deepEqual(admin.role, ["user", "admin"]);
+});