diff --git a/.env.example b/.env.example
index e472bba..e67bd67 100644
--- a/.env.example
+++ b/.env.example
@@ -33,7 +33,8 @@ MICROSOFT_CLIENT_SECRET=
 # Extra origins allowed to initiate auth flows (comma-separated).
 # NOTE: every https *.monashcoding.com subdomain is trusted automatically — you do NOT
 # need to list MAC apps here. Use this only for off-domain origins, e.g. local dev:
-#   TRUSTED_ORIGINS=http://localhost:3000
+#   TRUSTED_ORIGINS=http://localhost:3000,http://localhost:3001
+# Origins are exact: 127.0.0.1 and other ports must be listed separately if actually used.
 TRUSTED_ORIGINS=
 
 # JWT audience claim MAC apps verify against.
diff --git a/DEPLOY-dokploy.md b/DEPLOY-dokploy.md
index e74a475..e9d32ab 100644
--- a/DEPLOY-dokploy.md
+++ b/DEPLOY-dokploy.md
@@ -43,6 +43,9 @@ docker exec -it "$(docker ps -qf name=dokploy.1)" bash -c "pnpm run reset-passwo
   The compose only references `${...}`, so an empty Environment tab means empty values — e.g. a
   blank `POSTGRES_PASSWORD` makes Postgres refuse to start. Verify:
   - `BETTER_AUTH_URL=https://auth.monashcoding.com`
+  - `TRUSTED_ORIGINS=http://localhost:3000,http://localhost:3001` if both local app ports
+    are in use. Origins are exact, so only add `127.0.0.1` variants if developers actually
+    browse to those addresses.
   - `POSTGRES_PASSWORD` is **URL/shell-safe** (letters+digits only — no `$ @ : / #`).
   - No value is truncated when pasted (watch long ones like `GOOGLE_CLIENT_ID`, which must end
     in `.apps.googleusercontent.com`).
@@ -77,6 +80,36 @@ curl -s -X POST https://auth.monashcoding.com/api/auth/sign-in/social \
 ```
 Paste that returned `url` into a browser to walk the real Google consent → callback flow.
 
+### Verify localhost OAuth after this cookie-policy change
+
+The auth service deliberately issues Better Auth cookies with `HttpOnly; Secure;
+SameSite=None`. This is required because local MAC apps create the OAuth state cookie and
+later use the session through credentialed cross-site requests to the production auth
+origin. Better Auth's CSRF and trusted-origin validation remain enabled; do not replace the
+explicit `TRUSTED_ORIGINS` allowlist with `*`.
+
+After deploying:
+
+1. Confirm the Dokploy Environment tab contains every **actual** local origin, comma-separated
+   (normally `http://localhost:3000,http://localhost:3001`), then redeploy after any change.
+2. Use a Chrome Guest profile (or clear cookies for `auth.monashcoding.com`), open the local
+   app, enable **Preserve log** in DevTools Network, and start Google sign-in once.
+3. Inspect `POST /api/auth/sign-in/social`. It must return the exact requesting origin in
+   `Access-Control-Allow-Origin`, `Access-Control-Allow-Credentials: true`, and a redacted
+   state cookie equivalent to:
+   `__Secure-better-auth.state=<value>; Domain=.monashcoding.com; Path=/; HttpOnly; Secure; SameSite=None`.
+4. Confirm that cookie is stored under `https://auth.monashcoding.com` before leaving for
+   Google, then confirm the callback succeeds without `state_mismatch` and returns to the
+   allowlisted local callback URL.
+5. Repeat from both local ports that the apps actually use, then smoke-test the production
+   MAC Study and Job Board origins.
+
+`SameSite=None` cannot override a browser setting that blocks third-party cookies entirely.
+If DevTools says the cookie was blocked for that reason, the durable next step is a small
+allowlisted page on `auth.monashcoding.com` that starts OAuth while the auth domain is the
+top-level site. Do not work around that policy by disabling Better Auth's state-cookie,
+CSRF, or origin checks.
+
 ---
 
 ## Troubleshooting: 404 / hanging requests
diff --git a/auth/package.json b/auth/package.json
index 9354050..6300a8a 100644
--- a/auth/package.json
+++ b/auth/package.json
@@ -10,6 +10,7 @@
     "dev": "tsx watch src/server.ts",
     "build": "tsc",
     "start": "node dist/server.js",
+    "test": "tsx --test src/auth.test.ts",
     "typecheck": "tsc --noEmit",
     "lint": "biome check src",
     "lint:fix": "biome check src --write",
diff --git a/auth/src/auth.test.ts b/auth/src/auth.test.ts
new file mode 100644
index 0000000..5e3aaad
--- /dev/null
+++ b/auth/src/auth.test.ts
@@ -0,0 +1,36 @@
+import assert from "node:assert/strict";
+import { after, test } from "node:test";
+
+// auth.ts constructs the Drizzle adapter at import time. postgres.js connects lazily, so
+// this non-routable URL is sufficient for inspecting the generated cookie policy without
+// querying a database or making any network request.
+process.env.DATABASE_URL = "postgres://test:test@127.0.0.1:1/test";
+process.env.BETTER_AUTH_URL = "https://auth.monashcoding.com";
+process.env.BETTER_AUTH_SECRET = "test-only-secret-that-is-at-least-32-characters";
+process.env.GOOGLE_CLIENT_ID = "test-google-client-id";
+process.env.GOOGLE_CLIENT_SECRET = "test-google-client-secret";
+process.env.MICROSOFT_CLIENT_ID = "test-microsoft-client-id";
+process.env.MICROSOFT_CLIENT_SECRET = "test-microsoft-client-secret";
+
+const [{ auth }, { client }] = await Promise.all([import("./auth.js"), import("./db.js")]);
+
+after(async () => {
+  await client.end({ timeout: 0 });
+});
+
+test("OAuth state and session cookies support credentialed localhost requests", async () => {
+  const context = await auth.$context;
+  const stateCookie = context.createAuthCookie("state", { maxAge: 300 });
+  const sessionCookie = context.authCookies.sessionToken;
+
+  for (const cookie of [stateCookie, sessionCookie]) {
+    assert.equal(cookie.attributes.httpOnly, true);
+    assert.equal(cookie.attributes.secure, true);
+    assert.equal(cookie.attributes.sameSite, "none");
+    assert.equal(cookie.attributes.domain, ".monashcoding.com");
+    assert.equal(cookie.attributes.path, "/");
+  }
+
+  assert.equal(stateCookie.name, "__Secure-better-auth.state");
+  assert.equal(stateCookie.attributes.maxAge, 300);
+});
diff --git a/auth/src/auth.ts b/auth/src/auth.ts
index b0f0cdc..233fee6 100644
--- a/auth/src/auth.ts
+++ b/auth/src/auth.ts
@@ -8,7 +8,8 @@
  *
  * Option shapes here were verified against the installed better-auth@1.6.x types
  * (jwt plugin `jwt.definePayload`/`expirationTime`, `socialProviders.microsoft.tenantId`,
- * `advanced.crossSubDomainCookies`, `user.additionalFields`, `databaseHooks`).
+ * `advanced.crossSubDomainCookies`/`defaultCookieAttributes`,
+ * `user.additionalFields`, `databaseHooks`).
  */
 import { betterAuth } from "better-auth";
 import { drizzleAdapter } from "better-auth/adapters/drizzle";
@@ -18,6 +19,7 @@ import { claimsForEmail } from "./roster/lookup.js";
 import { schema } from "./schema.js";
 
 const baseURL = process.env.BETTER_AUTH_URL ?? "http://localhost:3000";
+const isSecureAuthOrigin = baseURL.startsWith("https://");
 const audience = process.env.JWT_AUDIENCE ?? "mac-suite";
 
 // Any https app on a *.monashcoding.com subdomain is trusted by default. Better Auth
@@ -171,6 +173,19 @@ export const auth = betterAuth({
       enabled: true,
       domain: ".monashcoding.com",
     },
+
+    // Local MAC apps start OAuth and fetch their resulting session/token from this
+    // production auth origin. Those are cross-site credentialed requests, so Better
+    // Auth's SameSite=Lax default prevents both the short-lived signed state cookie and
+    // the resulting session cookie from being stored/sent. SameSite=None requires Secure
+    // on the deployed HTTPS origin; retain Lax/non-Secure for a plain-HTTP local auth server.
+    // HttpOnly keeps the cookies inaccessible to application JavaScript. Better Auth's CSRF
+    // and trusted-origin checks remain enabled and CORS only reflects allowed origins.
+    defaultCookieAttributes: {
+      httpOnly: true,
+      secure: isSecureAuthOrigin,
+      sameSite: isSecureAuthOrigin ? "none" : "lax",
+    },
   },
 
   plugins: [