diff --git a/frontend/src/lib/api/endpoints/auth.test.ts b/frontend/src/lib/api/endpoints/auth.test.ts
index 69b48e35..43570ef3 100644
--- a/frontend/src/lib/api/endpoints/auth.test.ts
+++ b/frontend/src/lib/api/endpoints/auth.test.ts
@@ -1,14 +1,37 @@
 import { it, expect, vi, beforeEach, afterEach } from 'vitest';
 vi.mock('$lib/api/client', () => ({ apiFetch: vi.fn(), apiJson: vi.fn() }));
 vi.mock('$lib/api/csrf', () => ({ getCsrfHeaders: () => ({}) }));
+// Mock the OPAQUE WASM client so `login()`'s silent-migration hook can
+// exercise the wire path (params + register handshake) without touching
+// real WASM in jsdom.
+vi.mock('@serenity-kit/opaque', () => ({
+	ready: Promise.resolve(),
+	client: {
+		startRegistration: vi.fn(() => ({
+			clientRegistrationState: 'STATE-R',
+			registrationRequest: 'REQ-R'
+		})),
+		finishRegistration: vi.fn(() => ({
+			registrationRecord: 'RECORD-R',
+			exportKey: 'EK',
+			serverStaticPublicKey: 'SPK'
+		}))
+	}
+}));
 import { apiFetch, apiJson } from '$lib/api/client';
 import * as auth from './auth';
+import { __resetOpaqueParamsCache } from './opaque';
 const f = apiFetch as unknown as ReturnType<typeof vi.fn>;
 const j = apiJson as unknown as ReturnType<typeof vi.fn>;
 // Several auth probes use the raw global fetch (NOT apiFetch) on purpose.
 const okRes = { ok: true, status: 200, json: async () => ({}) };
 beforeEach(() => {
 	vi.clearAllMocks();
+	// login() dynamically imports the OPAQUE client and calls
+	// syncOpaqueEnvelope, which caches /params results in a
+	// module-level singleton. Reset it so each test's mock
+	// responses drive a fresh /params fetch.
+	__resetOpaqueParamsCache();
 	f.mockResolvedValue(okRes);
 	j.mockResolvedValue({});
 	vi.stubGlobal('fetch', vi.fn().mockResolvedValue(okRes));
@@ -42,3 +65,83 @@ it('tryRefresh returns false when the refresh fails', async () => {
 	);
 	await expect(auth.tryRefresh()).resolves.toBe(false);
 });
+
+// ── Phase 2: silent OPAQUE migration on legacy login ─────────────────
+//
+// `login()` MUST trigger `syncOpaqueEnvelope(password)` after a
+// successful POST /api/auth/login response, so users pick up an OPAQUE
+// envelope automatically over time. Verified here by observing the
+// wire calls made after the login POST — a successful login without
+// a follow-up `/api/auth/opaque/*` fetch is a regression that would
+// silently break Phase 3's cutover assumption ("most active users
+// have an envelope on file by the time the SPA flips to OPAQUE").
+it('login triggers OPAQUE silent-migration on success', async () => {
+	// Login POST → 200 AuthResponse
+	// Then syncOpaqueEnvelope runs: GET /params → POST register/start → POST register/finish
+	f.mockResolvedValueOnce({
+		ok: true,
+		status: 200,
+		statusText: 'OK',
+		json: async () => ({
+			user: { id: 'u1', email: 'a@x.test' },
+			access_token: 'at',
+			refresh_token: 'rt',
+			token_type: 'Bearer',
+			expires_in: 3600
+		})
+	})
+		.mockResolvedValueOnce({
+			ok: true,
+			status: 200,
+			statusText: 'OK',
+			json: async () => ({
+				enabled: true,
+				ciphersuiteVersion: 1,
+				ksf: { memoryKib: 8, iterations: 1, parallelism: 1 }
+			})
+		})
+		.mockResolvedValueOnce({
+			ok: true,
+			status: 200,
+			statusText: 'OK',
+			json: async () => ({ registrationResponse: 'RESP-R' })
+		})
+		.mockResolvedValueOnce({ ok: true, status: 204, json: async () => ({}) });
+
+	const authResponse = await auth.login('alice@example.com', 'correct horse battery staple');
+	expect(authResponse.access_token).toBe('at');
+
+	// 4 apiFetch calls: login + params + register/start + register/finish
+	const urls = f.mock.calls.map((c: unknown[]) => c[0] as string);
+	expect(urls).toContain('/api/auth/login');
+	expect(urls).toContain('/api/auth/opaque/params');
+	expect(urls).toContain('/api/auth/opaque/register/start');
+	expect(urls).toContain('/api/auth/opaque/register/finish');
+	// Login POST must come FIRST — silent migration only runs on a
+	// session that's already been established.
+	expect(urls[0]).toBe('/api/auth/login');
+});
+
+it('login returns AuthResponse even when silent-migration fails (non-fatal)', async () => {
+	// Login POST succeeds; the follow-up /params returns 500. Silent
+	// migration swallows the error (console.warn) — the login itself
+	// still returns success to the caller, so the user reaches their
+	// session and the envelope gets retried on next login.
+	f.mockResolvedValueOnce({
+		ok: true,
+		status: 200,
+		statusText: 'OK',
+		json: async () => ({
+			user: { id: 'u1', email: 'a@x.test' },
+			access_token: 'at',
+			refresh_token: 'rt',
+			token_type: 'Bearer',
+			expires_in: 3600
+		})
+	}).mockResolvedValueOnce({ ok: false, status: 500, json: async () => ({}) });
+
+	const consoleSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+	const authResponse = await auth.login('alice@example.com', 'pw');
+	expect(authResponse.access_token).toBe('at');
+	consoleSpy.mockRestore();
+});
diff --git a/frontend/src/lib/api/endpoints/auth.ts b/frontend/src/lib/api/endpoints/auth.ts
index 7d1ea762..b2c045e3 100644
--- a/frontend/src/lib/api/endpoints/auth.ts
+++ b/frontend/src/lib/api/endpoints/auth.ts
@@ -77,7 +77,31 @@ export async function login(emailOrUsername: string, password: string): Promise<
 		const { errorType, message } = await parseErrorBody(res);
 		throw new ApiError(res.status, res.statusText, '/api/auth/login', errorType, message);
 	}
-	return (await res.json()) as AuthResponse;
+	const auth = (await res.json()) as AuthResponse;
+
+	// ── OPAQUE silent migration (Phase 2) ──────────────────────────────
+	// After a successful legacy password login, transparently mint an
+	// OPAQUE envelope for the same passphrase. Users pick up the new
+	// auth path over time, one login at a time, with zero UX change —
+	// by the time Phase 3 flips the SPA to OPAQUE-first and Phase 4
+	// refuses legacy for migrated users, most active accounts already
+	// have an envelope on file.
+	//
+	// The freshly-issued session cookie is already active in the
+	// browser at this point (Set-Cookie from the POST response), so
+	// the session-authenticated register endpoints are reachable
+	// straight away. `syncOpaqueEnvelope` no-ops when OPAQUE is
+	// disabled server-side (mode=off) and swallows any error — a
+	// failure here just leaves the envelope stale, and the NEXT
+	// legacy login retries the same hook.
+	//
+	// Dynamic import keeps the ~200 KiB `@serenity-kit/opaque` WASM
+	// bundle out of pre-login-page bundles — it loads only for users
+	// who actually reach a successful login.
+	const { syncOpaqueEnvelope } = await import('$lib/api/endpoints/opaque');
+	await syncOpaqueEnvelope(password);
+
+	return auth;
 }
 
 export interface OidcProviders {