diff --git a/app/(main)/[locale]/auth/callback/page.tsx b/app/(main)/[locale]/auth/callback/page.tsx
index 28a75200c..e9862713d 100644
--- a/app/(main)/[locale]/auth/callback/page.tsx
+++ b/app/(main)/[locale]/auth/callback/page.tsx
@@ -11,6 +11,7 @@ import { Loader2, AlertCircle } from "@/components/icons";
 import { Button } from "@/components/ui/button";
 import { useParams } from "next/navigation";
 import { buildMobileHandoffUrl, mobileHandoffTarget } from "@/lib/auth/mobile-handoff";
+import { confirmMobileHandoffCodeUrl } from "@/lib/auth/mobile-handoff-client";
 import { MobileHandoffConsent } from "@/components/auth/mobile-handoff-consent";
 
 function OAuthCallbackInner() {
@@ -22,7 +23,11 @@ function OAuthCallbackInner() {
   const [error, setError] = useState<string | null>(null);
   // A sign-in started for the mobile app, waiting for the user to confirm
   // the handoff (lib/auth/mobile-handoff.ts).
-  const [pendingHandoff, setPendingHandoff] = useState<{ url: string; username: string } | null>(null);
+  const [pendingHandoff, setPendingHandoff] = useState<{
+    url: string | null;
+    resolveUrl?: () => Promise<string | null>;
+    username: string;
+  } | null>(null);
   // The authorization code is single-use. React's development double effect
   // ran this twice: the second pass no longer saw the pairing re-auth flag
   // (the first had removed it) and spent the code on the login flow instead.
@@ -166,6 +171,27 @@ function OAuthCallbackInner() {
               return;
             }
             const data = await res.json();
+            if (typeof data.mobile_handoff_code === "string" && data.mobile_handoff_code) {
+              // The app sent a PKCE challenge: the server kept the tokens and
+              // the link carries only a one-time code (docs/mobile-handoff.md).
+              // The code starts working when the user confirms.
+              const handoffCode: string = data.mobile_handoff_code;
+              const handoffState = typeof data.mobile_state === "string" ? data.mobile_state : mobileState ?? "";
+              try {
+                sessionStorage.removeItem("mobile_redirect_uri");
+                sessionStorage.removeItem("mobile_state");
+              } catch { /* ignore */ }
+              setPendingHandoff({
+                url: null,
+                resolveUrl: () => confirmMobileHandoffCodeUrl({
+                  target: mobileRedirectUri,
+                  code: handoffCode,
+                  state: handoffState,
+                }),
+                username: typeof data.server_url === "string" ? data.server_url : "",
+              });
+              return;
+            }
             const serverUrl = data.server_url as string | undefined;
             const accessToken = data.access_token as string | undefined;
             const tokenEndpoint = data.token_endpoint as string | undefined;
@@ -235,7 +261,12 @@ function OAuthCallbackInner() {
       <div className="min-h-screen flex items-center justify-center bg-gradient-to-br from-background via-background to-muted/20">
         <MobileHandoffConsent
           url={pendingHandoff.url}
+          resolveUrl={pendingHandoff.resolveUrl ?? null}
           username={pendingHandoff.username}
+          onError={() => {
+            setPendingHandoff(null);
+            setError("token_exchange_failed");
+          }}
           onCancel={() => {
             setPendingHandoff(null);
             router.push(toRouterPath(`${getPathPrefix(params.locale as string)}/${params.locale}/login`));
diff --git a/app/(main)/[locale]/login/page.tsx b/app/(main)/[locale]/login/page.tsx
index e633a0ce4..dbe79acf4 100644
--- a/app/(main)/[locale]/login/page.tsx
+++ b/app/(main)/[locale]/login/page.tsx
@@ -33,7 +33,8 @@ import {
 } from "@/lib/auth/lite-oauth";
 import { toAsciiDomain } from "@/lib/idn";
 import { usePrefetchMailView } from "@/hooks/use-prefetch-mail-view";
-import { buildMobileHandoffUrl, mobileHandoffTarget, signedInLoginAction } from "@/lib/auth/mobile-handoff";
+import { buildMobileHandoffUrl, mobileCodeChallenge, mobileHandoffTarget, signedInLoginAction } from "@/lib/auth/mobile-handoff";
+import { requestMobileHandoffCodeUrl } from "@/lib/auth/mobile-handoff-client";
 import { MobileHandoffConsent } from "@/components/auth/mobile-handoff-consent";
 import { HelpLinks } from "@/components/layout/help-links";
 import { consumeSessionEnd, type SessionEndReason } from "@/lib/session-policy/sign-out-reason";
@@ -193,10 +194,25 @@ function LoginPageContent() {
   // Only the app's own callback link is a handoff target, and nothing is
   // handed over before the user confirms it (lib/auth/mobile-handoff.ts).
   const rawMobileState = searchParams.get("mobile_state") ?? "";
-  const mobileRedirectUri = mobileHandoffTarget(searchParams.get("mobile_redirect_uri"));
+  // App builds that send a PKCE challenge get a one-time code in the link
+  // instead of the sign-in (docs/mobile-handoff.md). A malformed challenge
+  // makes this no hand-off at all: falling back to the fragment would put
+  // the password in the link the app asked to keep it out of. Lite has no
+  // server to park the sign-in on and keeps handing over in the fragment.
+  const rawMobileCodeChallenge = IS_LITE ? "" : searchParams.get("mobile_code_challenge") ?? "";
+  const mobileChallenge = mobileCodeChallenge(rawMobileCodeChallenge);
+  const mobileRedirectUri = rawMobileCodeChallenge && !mobileChallenge
+    ? ""
+    : mobileHandoffTarget(searchParams.get("mobile_redirect_uri"));
   const mobileState = mobileRedirectUri ? rawMobileState : "";
   const isMobileHandoff = Boolean(mobileRedirectUri);
-  const [pendingHandoff, setPendingHandoff] = useState<{ url: string; username: string } | null>(null);
+  const [pendingHandoff, setPendingHandoff] = useState<{
+    url: string | null;
+    resolveUrl?: () => Promise<string | null>;
+    username: string;
+  } | null>(null);
+  // Parking the sign-in for the app failed (code hand-off only).
+  const [handoffError, setHandoffError] = useState<string | null>(null);
   usePrefetchMailView(!isMobileHandoff);
   const { login, loginWithToken, loginDemo, isLoading, error, clearError, isAuthenticated } = useAuthStore();
   const { theme, setTheme, initializeTheme } = useThemeStore(useShallow((s) => ({ theme: s.theme, setTheme: s.setTheme, initializeTheme: s.initializeTheme })));
@@ -266,6 +282,8 @@ function LoginPageContent() {
   const [oauthLoading, setOauthLoading] = useState(false);
   const [demoLoading, setDemoLoading] = useState(false);
 
+  // The auth store's error, else a failed hand-off to the app.
+  const shownError = error || handoffError;
   // The error box and the recent-accounts list, tied to the fields they
   // belong to so a screen reader reads them with the field.
   const formId = useId();
@@ -451,12 +469,28 @@ function LoginPageContent() {
     const payload = mobileHandoffPayloadRef.current;
     if (!payload) return;
     mobileHandoffPayloadRef.current = null;
+    setHandoffError(null);
+    if (mobileChallenge) {
+      // The app gets a one-time code: on confirmation the sign-in is parked
+      // on the server, and its 60 seconds start then.
+      setPendingHandoff({
+        url: null,
+        resolveUrl: () => requestMobileHandoffCodeUrl({
+          target: mobileRedirectUri,
+          challenge: mobileChallenge,
+          state: mobileState,
+          payload,
+        }),
+        username: payload.username,
+      });
+      return;
+    }
     // Shown to the user first; the navigation happens on their confirmation.
     setPendingHandoff({
       url: buildMobileHandoffUrl(mobileRedirectUri, { flow: "password", ...payload, state: mobileState }),
       username: payload.username,
     });
-  }, [mobileRedirectUri, mobileState]);
+  }, [mobileRedirectUri, mobileState, mobileChallenge]);
 
   useEffect(() => {
     if (!isAuthenticated) return;
@@ -490,6 +524,7 @@ function LoginPageContent() {
 
   useEffect(() => {
     clearError();
+    setHandoffError(null);
   }, [formData, clearError]);
 
   useEffect(() => {
@@ -591,7 +626,11 @@ function LoginPageContent() {
           server_id: selectedServer?.id,
           remember: persistProviderSession,
           ...(isMobileHandoff
-            ? { mobile_redirect_uri: mobileRedirectUri, mobile_state: mobileState }
+            ? {
+                mobile_redirect_uri: mobileRedirectUri,
+                mobile_state: mobileState,
+                ...(mobileChallenge ? { mobile_code_challenge: mobileChallenge } : {}),
+              }
             : {}),
         }),
       });
@@ -619,7 +658,7 @@ function LoginPageContent() {
     } catch {
       setOauthLoading(false);
     }
-  }, [params.locale, selectedServer?.id, persistProviderSession, isMobileHandoff, mobileRedirectUri, mobileState]);
+  }, [params.locale, selectedServer?.id, persistProviderSession, isMobileHandoff, mobileRedirectUri, mobileState, mobileChallenge]);
 
   useEffect(() => {
     // After signing out, show the login page rather than signing straight
@@ -1263,11 +1302,16 @@ function LoginPageContent() {
     <div className="min-h-screen flex flex-col items-center justify-center bg-gradient-to-br from-background via-muted/10 to-muted/30 relative px-4">
       <MobileHandoffConsent
         url={pendingHandoff?.url ?? null}
+        resolveUrl={pendingHandoff?.resolveUrl ?? null}
         username={pendingHandoff?.username ?? ""}
         onCancel={() => {
           setPendingHandoff(null);
           router.push('/');
         }}
+        onError={() => {
+          setPendingHandoff(null);
+          setHandoffError("server_error");
+        }}
       />
       {/* Theme toggle - top right, dropdown style; hidden while the theme policy locks the mode (#1280) */}
       <div className="absolute top-5 end-5" ref={themeMenuRef} hidden={colorModeLocked} suppressHydrationWarning>
@@ -1386,27 +1430,27 @@ function LoginPageContent() {
 
             {/* Error Message. Asking for the code is the next step of a
                 sign-in that worked so far, not a failure (#1422). */}
-            {error && (
+            {shownError && (
               <div
                 className={cn(
                   "mb-5 p-3 rounded-xl border flex items-start gap-3",
-                  error === 'totp_required' ? "border-info/20 bg-info/5" : "border-destructive/20 bg-destructive/5",
+                  shownError === 'totp_required' ? "border-info/20 bg-info/5" : "border-destructive/20 bg-destructive/5",
                   shakeError && "animate-shake"
                 )}
                 data-testid="login-error"
-                role={error === 'totp_required' ? "status" : "alert"}
+                role={shownError === 'totp_required' ? "status" : "alert"}
               >
                 <div className={cn(
                   "w-10 h-10 rounded-full flex items-center justify-center flex-shrink-0 shadow-sm",
-                  error === 'totp_required' ? "bg-info/15 text-info" : "bg-destructive/15 text-destructive"
+                  shownError === 'totp_required' ? "bg-info/15 text-info" : "bg-destructive/15 text-destructive"
                 )}>
-                  {error === 'totp_required' ? <Info className="w-5 h-5" aria-hidden="true" /> : <AlertCircle className="w-5 h-5" aria-hidden="true" />}
+                  {shownError === 'totp_required' ? <Info className="w-5 h-5" aria-hidden="true" /> : <AlertCircle className="w-5 h-5" aria-hidden="true" />}
                 </div>
                 <div className="flex-1 min-w-0 self-center">
-                  <p id={errorId} className={cn("text-sm leading-relaxed", error === 'totp_required' ? "text-info" : "text-destructive")}>
-                    {error === 'invalid_credentials' && showTotpField && totpCode
+                  <p id={errorId} className={cn("text-sm leading-relaxed", shownError === 'totp_required' ? "text-info" : "text-destructive")}>
+                    {shownError === 'invalid_credentials' && showTotpField && totpCode
                       ? t('error.totp_invalid')
-                      : t(`error.${error}`) || t("error.generic")}
+                      : t(`error.${shownError}`) || t("error.generic")}
                   </p>
                 </div>
               </div>
diff --git a/app/api/auth/handoff/confirm/route.ts b/app/api/auth/handoff/confirm/route.ts
new file mode 100644
index 000000000..e909fb23b
--- /dev/null
+++ b/app/api/auth/handoff/confirm/route.ts
@@ -0,0 +1,29 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { confirmHandoff, isHandoffCode } from '@/lib/auth/handoff-store';
+import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
+
+// The SSO callback page calls this when the user confirms the hand-off to the
+// mobile app, right before it opens the app's link. /api/auth/sso/complete
+// parked the tokens before the confirmation was on screen; the code becomes
+// redeemable from here, for 60 seconds (lib/auth/handoff-store.ts). Nothing
+// secret comes back, and confirming a code only lets the holder of its
+// verifier redeem it.
+
+const NO_STORE = { 'Cache-Control': 'no-store' };
+
+export async function POST(request: NextRequest) {
+  const crossOrigin = rejectCrossOriginRequest(request);
+  if (crossOrigin) return crossOrigin;
+  try {
+    const body = await request.json().catch(() => null);
+    const code = body?.code;
+    if (!isHandoffCode(code) || !confirmHandoff(code)) {
+      return NextResponse.json({ error: 'invalid_code' }, { status: 400, headers: NO_STORE });
+    }
+    return NextResponse.json({ ok: true }, { headers: NO_STORE });
+  } catch (error) {
+    logger.error('Mobile hand-off confirm error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500, headers: NO_STORE });
+  }
+}
diff --git a/app/api/auth/handoff/create/route.ts b/app/api/auth/handoff/create/route.ts
new file mode 100644
index 000000000..ee9705a0c
--- /dev/null
+++ b/app/api/auth/handoff/create/route.ts
@@ -0,0 +1,59 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { isHandoffCodeChallenge, reserveHandoffCreate, stashHandoff } from '@/lib/auth/handoff-store';
+import { loginClientAddress } from '@/lib/auth/client-address';
+import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
+
+// Password sign-in hand-off to the mobile app, browser side. The login page
+// was opened by the app with a `mobile_code_challenge`; after the user signed
+// in and confirmed, it parks the sign-in here and opens the app's callback
+// link with the returned code instead of the password
+// (lib/auth/handoff-store.ts, docs/mobile-handoff.md).
+//
+// The page already checked the password against the mail server when it
+// signed in, and the old hand-off put whatever the page held into the link
+// unchecked too, so nothing is verified again here (a second login would
+// spend the account's failed-login allowance and a TOTP code twice). What
+// is parked only ever goes to whoever holds the verifier for the challenge.
+//
+// The SSO hand-off parks its tokens in /api/auth/sso/complete itself; they
+// never reach the browser.
+
+const NO_STORE = { 'Cache-Control': 'no-store' };
+
+function boundedString(value: unknown, max: number): value is string {
+  return typeof value === 'string' && value.length > 0 && value.length <= max;
+}
+
+export async function POST(request: NextRequest) {
+  const crossOrigin = rejectCrossOriginRequest(request);
+  if (crossOrigin) return crossOrigin;
+  try {
+    const body = await request.json().catch(() => null);
+    const challenge = body?.code_challenge;
+    const state = typeof body?.state === 'string' ? body.state : '';
+    if (
+      !isHandoffCodeChallenge(challenge)
+      || state.length > 512
+      || !boundedString(body?.server_url, 2048)
+      || !boundedString(body?.username, 512)
+      || !boundedString(body?.password, 4096)
+    ) {
+      return NextResponse.json({ error: 'invalid_request' }, { status: 400, headers: NO_STORE });
+    }
+
+    if (!reserveHandoffCreate(loginClientAddress(request))) {
+      return NextResponse.json({ error: 'too_many_requests' }, { status: 429, headers: NO_STORE });
+    }
+
+    const code = stashHandoff(
+      { flow: 'password', server_url: body.server_url, username: body.username, password: body.password },
+      { challenge, state },
+    );
+    logger.info('Mobile hand-off parked', { flow: 'password' });
+    return NextResponse.json({ code }, { headers: NO_STORE });
+  } catch (error) {
+    logger.error('Mobile hand-off create error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500, headers: NO_STORE });
+  }
+}
diff --git a/app/api/auth/handoff/redeem/route.ts b/app/api/auth/handoff/redeem/route.ts
new file mode 100644
index 000000000..fc6166af0
--- /dev/null
+++ b/app/api/auth/handoff/redeem/route.ts
@@ -0,0 +1,60 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import {
+  isHandoffCode,
+  isHandoffCodeVerifier,
+  redeemHandoff,
+  reserveHandoffRedeem,
+} from '@/lib/auth/handoff-store';
+import { loginClientAddress } from '@/lib/auth/client-address';
+import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
+
+// The mobile app's side of the browser sign-in hand-off. It POSTs the code
+// from its callback link with the PKCE verifier behind the challenge it sent
+// to the login page, and gets the sign-in the browser parked for it: the
+// same fields older builds read from the link's fragment
+// (lib/auth/handoff-store.ts, docs/mobile-handoff.md).
+//
+// The code and verifier are the only credentials, so this route is
+// unauthenticated (the app has no webmail cookies). Every failure is a 400
+// and spends the code: invalid_code (never issued or already used),
+// expired_code, invalid_verifier; invalid_request for a malformed body.
+
+const NO_STORE = { 'Cache-Control': 'no-store' };
+
+export async function POST(request: NextRequest) {
+  const crossOrigin = rejectCrossOriginRequest(request);
+  if (crossOrigin) return crossOrigin;
+  try {
+    const refund = reserveHandoffRedeem(loginClientAddress(request));
+    if (!refund) {
+      return NextResponse.json({ error: 'too_many_requests' }, { status: 429, headers: NO_STORE });
+    }
+
+    const body = await request.json().catch(() => null);
+    const code = body?.code;
+    const verifier = body?.code_verifier;
+    if (!isHandoffCode(code)) {
+      return NextResponse.json({ error: 'invalid_request' }, { status: 400, headers: NO_STORE });
+    }
+    if (!isHandoffCodeVerifier(verifier)) {
+      // Still spend the code: a malformed verifier is a wrong one.
+      redeemHandoff(code, '');
+      return NextResponse.json({ error: 'invalid_verifier' }, { status: 400, headers: NO_STORE });
+    }
+
+    const result = redeemHandoff(code, verifier);
+    if (!result.ok) {
+      const error = result.reason === 'verifier_mismatch' ? 'invalid_verifier' : `${result.reason}_code`;
+      logger.warn('Mobile hand-off redeem refused', { reason: result.reason });
+      return NextResponse.json({ error }, { status: 400, headers: NO_STORE });
+    }
+
+    refund();
+    logger.info('Mobile hand-off redeemed', { flow: result.bundle.flow });
+    return NextResponse.json({ ...result.bundle, state: result.state }, { headers: NO_STORE });
+  } catch (error) {
+    logger.error('Mobile hand-off redeem error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500, headers: NO_STORE });
+  }
+}
diff --git a/app/api/auth/sso/complete/route.ts b/app/api/auth/sso/complete/route.ts
index b4443e797..688e3db2d 100644
--- a/app/api/auth/sso/complete/route.ts
+++ b/app/api/auth/sso/complete/route.ts
@@ -9,6 +9,7 @@ import {
   hasClientSecret,
 } from '@/lib/oauth/token-exchange';
 import { buildRedeemBundle } from '@/lib/auth/pair-bundle';
+import { isHandoffCodeChallenge, stashHandoff } from '@/lib/auth/handoff-store';
 import { refreshTokenCookieName, refreshTokenServerCookieName } from '@/lib/oauth/tokens';
 import { getCookieOptions, getSignInCookieOptions } from '@/lib/oauth/cookie-config';
 import { storeIdToken } from '@/lib/oauth/end-session';
@@ -98,6 +99,9 @@ export async function POST(request: NextRequest) {
       typeof pending.mobile_redirect_uri === 'string' ? pending.mobile_redirect_uri : null;
     const mobileState = typeof pending.mobile_state === 'string' ? pending.mobile_state : null;
     const isMobileFlow = Boolean(mobileRedirectUri);
+    const mobileCodeChallenge = isHandoffCodeChallenge(pending.mobile_code_challenge)
+      ? pending.mobile_code_challenge
+      : null;
     const remember = pending.remember !== false;
 
     if (!codeVerifier || !redirectUri) {
@@ -158,6 +162,24 @@ export async function POST(request: NextRequest) {
         trusted: true,
       }, webmailBaseFromCallback(redirectUri));
       if (bundle.flow !== 'oauth') throw new Error('unexpected bundle');
+      if (mobileCodeChallenge) {
+        // The app sent a PKCE challenge: park the tokens and give the browser
+        // only a one-time code for the link (lib/auth/handoff-store.ts). The
+        // tokens never reach the page. The code starts working when the user
+        // confirms the hand-off (/api/auth/handoff/confirm).
+        const handoffCode = stashHandoff(bundle, {
+          challenge: mobileCodeChallenge,
+          state: mobileState ?? '',
+          awaitConfirm: true,
+        });
+        logger.info('Mobile hand-off parked', { flow: 'oauth' });
+        return NextResponse.json({
+          mobile_handoff_code: handoffCode,
+          server_url: bundle.server_url,
+          mobile_redirect_uri: mobileRedirectUri,
+          mobile_state: mobileState,
+        });
+      }
       return NextResponse.json({
         access_token: bundle.access_token,
         expires_in: bundle.expires_in,
diff --git a/app/api/auth/sso/start/route.ts b/app/api/auth/sso/start/route.ts
index a667a763b..43729c012 100644
--- a/app/api/auth/sso/start/route.ts
+++ b/app/api/auth/sso/start/route.ts
@@ -12,7 +12,7 @@ import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { hasSessionSecret } from '@/lib/auth/session-secret';
 import { configManager } from '@/lib/admin/config-manager';
 import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
-import { mobileHandoffTarget } from '@/lib/auth/mobile-handoff';
+import { isMobileCodeChallenge, mobileHandoffTarget } from '@/lib/auth/mobile-handoff';
 
 const SSO_PENDING_COOKIE = 'sso_pending';
 const SSO_PENDING_MAX_AGE = 300; // 5 minutes
@@ -51,6 +51,7 @@ export async function POST(request: NextRequest) {
       server_id: bodyServerId,
       mobile_redirect_uri: rawMobileRedirectUri,
       mobile_state: rawMobileState,
+      mobile_code_challenge: rawMobileCodeChallenge,
       purpose: rawPurpose,
       slot: rawSlot,
       remember,
@@ -74,6 +75,15 @@ export async function POST(request: NextRequest) {
     if (mobileRedirectUri && !mobileHandoffTarget(mobileRedirectUri)) {
       return NextResponse.json({ error: 'Invalid mobile_redirect_uri' }, { status: 400 });
     }
+    // An app build that sends a PKCE challenge gets a one-time code instead
+    // of the tokens (lib/auth/handoff-store.ts). A malformed one is refused
+    // rather than ignored, which would hand the tokens over in the link.
+    const mobileCodeChallenge = mobileRedirectUri && rawMobileCodeChallenge != null && rawMobileCodeChallenge !== ''
+      ? rawMobileCodeChallenge
+      : null;
+    if (mobileCodeChallenge !== null && !isMobileCodeChallenge(mobileCodeChallenge)) {
+      return NextResponse.json({ error: 'Invalid mobile_code_challenge' }, { status: 400 });
+    }
 
     const cookieStore = await cookies();
 
@@ -125,6 +135,7 @@ export async function POST(request: NextRequest) {
       ...(serverId ? { server_id: serverId } : {}),
       ...(mobileRedirectUri ? { mobile_redirect_uri: mobileRedirectUri } : {}),
       ...(mobileState ? { mobile_state: mobileState } : {}),
+      ...(mobileCodeChallenge ? { mobile_code_challenge: mobileCodeChallenge } : {}),
       // The re-auth completes into a grant for the phone paired to this slot.
       ...(isReauth ? { purpose: 'reauth', slot: reauthSlot } : {}),
       // "Remember me" left unticked: /complete writes cookies that end with
diff --git a/components/auth/__tests__/mobile-handoff-consent.test.tsx b/components/auth/__tests__/mobile-handoff-consent.test.tsx
index cbdb6cf22..c4d2243d0 100644
--- a/components/auth/__tests__/mobile-handoff-consent.test.tsx
+++ b/components/auth/__tests__/mobile-handoff-consent.test.tsx
@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { fireEvent, render, screen } from '@testing-library/react';
+import { act, fireEvent, render, screen } from '@testing-library/react';
 
 vi.mock('next-intl', () => ({
   useTranslations: () => (key: string, values?: Record<string, string>) =>
@@ -52,3 +52,45 @@ describe('MobileHandoffConsent', () => {
     expect(screen.queryByText('mobile_handoff.title')).toBeNull();
   });
 });
+
+describe('MobileHandoffConsent with a code hand-off', () => {
+  // docs/mobile-handoff.md: the code only goes live when the user confirms,
+  // so the link is resolved on confirmation, not before.
+  const CODE_URL = 'bulwarkmobile://auth/callback?code=abc&state=m-1';
+
+  it('makes the code live on confirmation and opens the link it gets', async () => {
+    let resolve!: (url: string | null) => void;
+    const resolveUrl = vi.fn(() => new Promise<string | null>((r) => { resolve = r; }));
+    const onCancel = vi.fn();
+    render(<MobileHandoffConsent url={null} resolveUrl={resolveUrl} username="alice@example.org" onCancel={onCancel} />);
+    expect(screen.getByText('mobile_handoff.title')).toBeTruthy();
+    expect(resolveUrl).not.toHaveBeenCalled();
+
+    fireEvent.click(screen.getByText('mobile_handoff.confirm'));
+    expect(resolveUrl).toHaveBeenCalledTimes(1);
+    // Confirming is not declining, even though the dialog closes itself.
+    expect(onCancel).not.toHaveBeenCalled();
+    expect(replaced).toEqual([]);
+
+    await act(async () => { resolve(CODE_URL); });
+    expect(replaced).toEqual([CODE_URL]);
+  });
+
+  it('reports a failure and opens nothing', async () => {
+    const onError = vi.fn();
+    const onCancel = vi.fn();
+    render(
+      <MobileHandoffConsent
+        url={null}
+        resolveUrl={async () => null}
+        username="alice@example.org"
+        onCancel={onCancel}
+        onError={onError}
+      />,
+    );
+    await act(async () => { fireEvent.click(screen.getByText('mobile_handoff.confirm')); });
+    expect(onError).toHaveBeenCalledTimes(1);
+    expect(onCancel).not.toHaveBeenCalled();
+    expect(replaced).toEqual([]);
+  });
+});
diff --git a/components/auth/mobile-handoff-consent.tsx b/components/auth/mobile-handoff-consent.tsx
index cc0a00266..39033e317 100644
--- a/components/auth/mobile-handoff-consent.tsx
+++ b/components/auth/mobile-handoff-consent.tsx
@@ -1,15 +1,24 @@
 "use client";
 
+import { useRef } from "react";
 import { useTranslations } from "next-intl";
 import { ConfirmDialog } from "@/components/ui/confirm-dialog";
 
 interface MobileHandoffConsentProps {
   /** The app link the browser will open, with the credential material in its fragment. */
   url: string | null;
+  /**
+   * For the code hand-off (docs/mobile-handoff.md): makes the code live on
+   * confirmation and returns the link to open, or null when that failed.
+   * Shows the dialog on its own, without `url`.
+   */
+  resolveUrl?: (() => Promise<string | null>) | null;
   /** The account being handed over, for the explanation. */
   username: string;
-  /** The user stays in the webmail instead. */
+  /** The user stays in the webmail instead (also runs after the link opened). */
   onCancel: () => void;
+  /** `resolveUrl` returned null: nothing was handed over. */
+  onError?: () => void;
 }
 
 /**
@@ -20,13 +29,32 @@ interface MobileHandoffConsentProps {
  * `bulwarkmobile` scheme on the device receives whatever is handed over
  * (lib/auth/mobile-handoff.ts). Nothing leaves without this confirmation.
  */
-export function MobileHandoffConsent({ url, username, onCancel }: MobileHandoffConsentProps) {
+export function MobileHandoffConsent({ url, resolveUrl, username, onCancel, onError }: MobileHandoffConsentProps) {
   const t = useTranslations("login");
+  // The dialog closes itself right after confirming; while the code is being
+  // made live, that close must not count as declining.
+  const resolving = useRef(false);
   return (
     <ConfirmDialog
-      isOpen={!!url}
-      onClose={onCancel}
+      isOpen={!!url || !!resolveUrl}
+      onClose={() => {
+        if (!resolving.current) onCancel();
+      }}
       onConfirm={() => {
+        if (resolveUrl) {
+          if (resolving.current) return;
+          resolving.current = true;
+          void resolveUrl().catch(() => null).then((resolved) => {
+            resolving.current = false;
+            if (!resolved) {
+              onError?.();
+              return;
+            }
+            window.location.replace(resolved);
+            onCancel();
+          });
+          return;
+        }
         if (url) window.location.replace(url);
       }}
       title={t("mobile_handoff.title")}
diff --git a/docs/mobile-handoff.md b/docs/mobile-handoff.md
new file mode 100644
index 000000000..1a38fb9ec
--- /dev/null
+++ b/docs/mobile-handoff.md
@@ -0,0 +1,126 @@
+# Mobile sign-in hand-off with a one-time code
+
+The Bulwark mobile app can sign in through the webmail's login page in the
+system browser. Until now the page opened the app's callback link with the
+sign-in in the fragment (`flow=password` with the password, or `flow=oauth`
+with the tokens). Any app on the device can register the `bulwarkmobile`
+scheme, so that link can be caught by someone else.
+
+With a PKCE challenge the link carries only a one-time code. The sign-in
+stays on the webmail server until the app redeems the code with the matching
+verifier. Without a challenge the page behaves as before, so older app builds
+keep working.
+
+## 1. Open the login page
+
+```
+GET <webmail>/<locale>/login
+    ?mobile_redirect_uri=bulwarkmobile://auth/callback
+    &mobile_state=<opaque, up to 512 chars>
+    &mobile_code_challenge=<S256 challenge>
+```
+
+- `mobile_code_challenge` is `BASE64URL(SHA256(code_verifier))` without
+  padding, exactly 43 characters (RFC 7636, method S256 only). The verifier is
+  43 to 128 characters from `A-Z a-z 0-9 - . _ ~`. Use a fresh verifier for
+  every sign-in.
+- A challenge that is present but malformed makes the page ignore the
+  hand-off: it acts as a normal sign-in page and gives the app nothing.
+- `mobile_redirect_uri` must be exactly `bulwarkmobile://auth/callback`, as
+  before.
+
+This works for the password form and for SSO/OAuth sign-in (the page routes
+mobile OAuth through the server-side SSO flow). Bulwark Lite has no server to
+keep the sign-in on; it ignores the challenge and still uses the fragment.
+
+## 2. The callback link
+
+After the user signs in and confirms the hand-off, the browser opens:
+
+```
+bulwarkmobile://auth/callback?code=<64 lowercase hex chars>&state=<mobile_state>
+```
+
+The parameters are in the query string, not the fragment. There is no
+`flow`, no server URL and no secret in the link. Check `state` against the
+value you sent before redeeming.
+
+A link with a fragment (`#flow=...`) means the webmail is older than this
+change, or is a Lite build; handle it as before.
+
+## 3. Redeem the code
+
+```
+POST <webmail>/api/auth/handoff/redeem
+Content-Type: application/json
+
+{ "code": "<code from the link>", "code_verifier": "<verifier>" }
+```
+
+The code is valid for 60 seconds from the moment the user confirmed the
+hand-off in the browser, and works once. Send no `Origin` or
+`Sec-Fetch-Site` header (native HTTP clients don't), or the same-origin check
+answers 403.
+
+### Success: 200
+
+The same fields the fragment used to carry, as JSON. `state` is included so
+existing fragment parsing can be reused.
+
+```json
+{
+  "flow": "password",
+  "server_url": "https://mail.example.org",
+  "username": "alice@example.org",
+  "password": "...",
+  "state": "<mobile_state>"
+}
+```
+
+```json
+{
+  "flow": "oauth",
+  "server_url": "https://mail.example.org",
+  "access_token": "...",
+  "refresh_token": "...",
+  "expires_in": 3540,
+  "token_endpoint": "https://mail.example.org/auth/token",
+  "client_id": "bulwark-webmail",
+  "state": "<mobile_state>"
+}
+```
+
+- `expires_in` is a number (the fragment had a string) and counts from the
+  redeem, not from the sign-in.
+- `refresh_token` and `expires_in` are optional, as before.
+- `token_endpoint` may point at the webmail's token proxy
+  (`<webmail>/api/auth/pair/token`), with a sealed `refresh_token`, exactly
+  like the old fragment and the QR pairing bundle.
+
+### Errors
+
+Every failure spends the code. Start a new sign-in after any of them.
+
+| Status | `error`             | Meaning                                         |
+| ------ | ------------------- | ----------------------------------------------- |
+| 400    | `invalid_request`   | Body is not JSON or `code` is malformed         |
+| 400    | `invalid_code`      | Never issued, already redeemed or tried         |
+| 400    | `expired_code`      | Confirmed more than 60 seconds ago              |
+| 400    | `invalid_verifier`  | Verifier malformed or does not match            |
+| 429    | `too_many_requests` | More than 10 failed redeems from this address in 15 minutes |
+| 403    |                     | Cross-origin browser request                    |
+
+All responses carry `Cache-Control: no-store`.
+
+## Webmail internals
+
+- Store: `lib/auth/handoff-store.ts` (in-process, like the QR pairing store,
+  so single-instance deployments or sticky sessions).
+- Password flow: when the user confirms, the login page parks the sign-in at
+  `POST /api/auth/handoff/create` and opens the link with the code it gets.
+- SSO flow: `POST /api/auth/sso/start` takes `mobile_code_challenge` with the
+  other mobile parameters. `POST /api/auth/sso/complete` parks the tokens
+  itself and returns only `mobile_handoff_code`, so the tokens never reach
+  the browser. That code waits up to five minutes for the user; when they
+  confirm, the callback page calls `POST /api/auth/handoff/confirm`, which
+  starts its 60 seconds, and opens the link.
diff --git a/lib/__tests__/auth-routes-same-origin.test.ts b/lib/__tests__/auth-routes-same-origin.test.ts
index cdd6901a0..82b2d2796 100644
--- a/lib/__tests__/auth-routes-same-origin.test.ts
+++ b/lib/__tests__/auth-routes-same-origin.test.ts
@@ -219,6 +219,9 @@ const MUTATING_HANDLERS: Array<[string, string]> = [
   ['@/app/api/auth/pair/create/route', 'POST'],
   ['@/app/api/auth/pair/redeem/route', 'POST'],
   ['@/app/api/auth/pair/token/route', 'POST'],
+  ['@/app/api/auth/handoff/create/route', 'POST'],
+  ['@/app/api/auth/handoff/confirm/route', 'POST'],
+  ['@/app/api/auth/handoff/redeem/route', 'POST'],
   ['@/app/api/auth/sso/start/route', 'POST'],
   ['@/app/api/auth/sso/complete/route', 'POST'],
   ['@/app/api/auth/reauth/sso/complete/route', 'POST'],
diff --git a/lib/__tests__/mobile-handoff-client.test.ts b/lib/__tests__/mobile-handoff-client.test.ts
new file mode 100644
index 000000000..67caf953c
--- /dev/null
+++ b/lib/__tests__/mobile-handoff-client.test.ts
@@ -0,0 +1,61 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+// The browser side of the code hand-off (docs/mobile-handoff.md): parking a
+// password sign-in, and confirming an SSO one, each give the app link with
+// only the code. A failure gives no link at all, never the old fragment.
+
+const apiFetch = vi.fn();
+vi.mock('@/lib/browser-navigation', () => ({
+  apiFetch: (...args: unknown[]) => apiFetch(...args),
+}));
+
+import { confirmMobileHandoffCodeUrl, requestMobileHandoffCodeUrl } from '@/lib/auth/mobile-handoff-client';
+
+const TARGET = 'bulwarkmobile://auth/callback';
+const CODE = 'ab'.repeat(32);
+const CHALLENGE = 'E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM';
+const payload = { server_url: 'https://mail.example.org', username: 'alice@example.org', password: 'secret' };
+
+function respond(status: number, body: unknown) {
+  apiFetch.mockResolvedValueOnce(new Response(JSON.stringify(body), { status }));
+}
+
+beforeEach(() => {
+  apiFetch.mockReset();
+});
+
+describe('requestMobileHandoffCodeUrl', () => {
+  it('parks the sign-in and links the app with the code only', async () => {
+    respond(200, { code: CODE });
+    const url = await requestMobileHandoffCodeUrl({ target: TARGET, challenge: CHALLENGE, state: 'm-1', payload });
+    expect(url).toBe(`${TARGET}?code=${CODE}&state=m-1`);
+    expect(url).not.toContain('secret');
+    const [path, init] = apiFetch.mock.calls[0];
+    expect(path).toBe('/api/auth/handoff/create');
+    expect(JSON.parse(init.body)).toEqual({ code_challenge: CHALLENGE, state: 'm-1', ...payload });
+  });
+
+  it('gives no link when the server refuses', async () => {
+    respond(429, { error: 'too_many_requests' });
+    expect(await requestMobileHandoffCodeUrl({ target: TARGET, challenge: CHALLENGE, state: 'm-1', payload })).toBeNull();
+  });
+
+  it('gives no link when the request fails', async () => {
+    apiFetch.mockRejectedValueOnce(new Error('offline'));
+    expect(await requestMobileHandoffCodeUrl({ target: TARGET, challenge: CHALLENGE, state: 'm-1', payload })).toBeNull();
+  });
+});
+
+describe('confirmMobileHandoffCodeUrl', () => {
+  it('confirms the code and links the app with it', async () => {
+    respond(200, { ok: true });
+    expect(await confirmMobileHandoffCodeUrl({ target: TARGET, code: CODE, state: 'm-1' })).toBe(`${TARGET}?code=${CODE}&state=m-1`);
+    expect(apiFetch.mock.calls[0][0]).toBe('/api/auth/handoff/confirm');
+    expect(JSON.parse(apiFetch.mock.calls[0][1].body)).toEqual({ code: CODE });
+  });
+
+  it('gives no link for a code that is gone', async () => {
+    respond(400, { error: 'invalid_code' });
+    expect(await confirmMobileHandoffCodeUrl({ target: TARGET, code: CODE, state: 'm-1' })).toBeNull();
+  });
+});
diff --git a/lib/__tests__/mobile-handoff-code-routes.test.ts b/lib/__tests__/mobile-handoff-code-routes.test.ts
new file mode 100644
index 000000000..e56c1f1c9
--- /dev/null
+++ b/lib/__tests__/mobile-handoff-code-routes.test.ts
@@ -0,0 +1,227 @@
+// @vitest-environment node
+import { createHash } from 'crypto';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest } from 'next/server';
+
+// The code hand-off to the mobile app (docs/mobile-handoff.md): the browser
+// parks the sign-in (/api/auth/handoff/create, or /api/auth/sso/complete plus
+// /api/auth/handoff/confirm), the app link carries only a code, and the app
+// redeems it with its PKCE verifier (/api/auth/handoff/redeem). Wrong
+// verifier, reuse and expiry are 400s, and every attempt spends the code.
+
+vi.mock('@/lib/logger', () => ({
+  logger: { warn: () => {}, error: () => {}, info: () => {}, debug: () => {} },
+}));
+
+import { HANDOFF_CONFIRM_TTL_MS, HANDOFF_TTL_MS, resetHandoffStoreForTests, stashHandoff } from '@/lib/auth/handoff-store';
+import { POST as createPOST } from '@/app/api/auth/handoff/create/route';
+import { POST as confirmPOST } from '@/app/api/auth/handoff/confirm/route';
+import { POST as redeemPOST } from '@/app/api/auth/handoff/redeem/route';
+
+const WEBMAIL = 'https://webmail.example';
+const VERIFIER = 'v'.repeat(43) + '-._~0123456789';
+const CHALLENGE = createHash('sha256').update(VERIFIER).digest('base64url');
+const OTHER_VERIFIER = 'w'.repeat(64);
+
+// Each test talks from its own address, so the per-client limits of one test
+// don't leak into the next.
+let ip = 0;
+let clientIp = '';
+
+async function call(handler: (req: NextRequest) => Promise<Response>, path: string, body: unknown) {
+  const res = await handler(new NextRequest(`${WEBMAIL}${path}`, {
+    method: 'POST',
+    headers: { 'content-type': 'application/json', 'x-forwarded-for': clientIp },
+    body: typeof body === 'string' ? body : JSON.stringify(body),
+  }));
+  return { status: res.status, body: await res.json(), cacheControl: res.headers.get('cache-control') };
+}
+
+const create = (body: unknown) => call(createPOST, '/api/auth/handoff/create', body);
+const confirm = (body: unknown) => call(confirmPOST, '/api/auth/handoff/confirm', body);
+const redeem = (body: unknown) => call(redeemPOST, '/api/auth/handoff/redeem', body);
+
+const PASSWORD_SIGN_IN = {
+  code_challenge: CHALLENGE,
+  state: 'm-1',
+  server_url: 'https://mail.example.org',
+  username: 'alice@example.org',
+  password: 'secret',
+};
+
+beforeEach(() => {
+  resetHandoffStoreForTests();
+  ip += 1;
+  clientIp = `203.0.113.${ip}`;
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe('password hand-off', () => {
+  it('gives the app the sign-in for the code and the right verifier, once', async () => {
+    const created = await create(PASSWORD_SIGN_IN);
+    expect(created.status).toBe(200);
+    expect(created.body.code).toMatch(/^[0-9a-f]{64}$/);
+    expect(JSON.stringify(created.body)).not.toContain('secret');
+    expect(created.cacheControl).toBe('no-store');
+
+    const first = await redeem({ code: created.body.code, code_verifier: VERIFIER });
+    expect(first.status).toBe(200);
+    expect(first.body).toEqual({
+      flow: 'password',
+      server_url: 'https://mail.example.org',
+      username: 'alice@example.org',
+      password: 'secret',
+      state: 'm-1',
+    });
+    expect(first.cacheControl).toBe('no-store');
+
+    const again = await redeem({ code: created.body.code, code_verifier: VERIFIER });
+    expect(again.status).toBe(400);
+    expect(again.body).toEqual({ error: 'invalid_code' });
+  });
+
+  it('spends the code on a wrong verifier', async () => {
+    const { body } = await create(PASSWORD_SIGN_IN);
+    const wrong = await redeem({ code: body.code, code_verifier: OTHER_VERIFIER });
+    expect(wrong.status).toBe(400);
+    expect(wrong.body).toEqual({ error: 'invalid_verifier' });
+
+    const right = await redeem({ code: body.code, code_verifier: VERIFIER });
+    expect(right.status).toBe(400);
+    expect(right.body.password).toBeUndefined();
+  });
+
+  it('spends the code on a malformed verifier', async () => {
+    const { body } = await create(PASSWORD_SIGN_IN);
+    const wrong = await redeem({ code: body.code, code_verifier: 'short' });
+    expect(wrong.status).toBe(400);
+    expect(wrong.body).toEqual({ error: 'invalid_verifier' });
+    expect((await redeem({ code: body.code, code_verifier: VERIFIER })).status).toBe(400);
+  });
+
+  it('expires the code after 60 seconds', async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    const { body } = await create(PASSWORD_SIGN_IN);
+    vi.setSystemTime(Date.now() + HANDOFF_TTL_MS + 1);
+    const late = await redeem({ code: body.code, code_verifier: VERIFIER });
+    expect(late.status).toBe(400);
+    expect(late.body).toEqual({ error: 'expired_code' });
+  });
+
+  it('still redeems just before the 60 seconds are up', async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    const { body } = await create(PASSWORD_SIGN_IN);
+    vi.setSystemTime(Date.now() + HANDOFF_TTL_MS - 1000);
+    expect((await redeem({ code: body.code, code_verifier: VERIFIER })).status).toBe(200);
+  });
+
+  it.each([
+    ['no challenge', { ...PASSWORD_SIGN_IN, code_challenge: undefined }],
+    ['a plain challenge', { ...PASSWORD_SIGN_IN, code_challenge: 'abc' }],
+    ['a padded challenge', { ...PASSWORD_SIGN_IN, code_challenge: `${CHALLENGE.slice(0, 42)}=` }],
+    ['no password', { ...PASSWORD_SIGN_IN, password: '' }],
+    ['no username', { ...PASSWORD_SIGN_IN, username: undefined }],
+    ['no server', { ...PASSWORD_SIGN_IN, server_url: undefined }],
+    ['a body that is not JSON', 'code_challenge=x'],
+  ])('refuses to park %s', async (_label, body) => {
+    const res = await create(body);
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({ error: 'invalid_request' });
+  });
+
+  it('limits how many sign-ins one client parks', async () => {
+    for (let i = 0; i < 10; i++) expect((await create(PASSWORD_SIGN_IN)).status).toBe(200);
+    expect((await create(PASSWORD_SIGN_IN)).status).toBe(429);
+  });
+});
+
+describe('redeem', () => {
+  it.each([
+    ['no code', { code_verifier: VERIFIER }],
+    ['uppercase hex', { code: 'AB'.repeat(32), code_verifier: VERIFIER }],
+    ['a body that is not JSON', 'code=x'],
+  ])('refuses %s', async (_label, body) => {
+    const res = await redeem(body);
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({ error: 'invalid_request' });
+  });
+
+  it('answers invalid_code for a code nobody issued', async () => {
+    const res = await redeem({ code: 'ab'.repeat(32), code_verifier: VERIFIER });
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({ error: 'invalid_code' });
+  });
+
+  it('stops a client after ten failed redeems, and a success costs nothing', async () => {
+    const { body } = await create(PASSWORD_SIGN_IN);
+    expect((await redeem({ code: body.code, code_verifier: VERIFIER })).status).toBe(200);
+    for (let i = 0; i < 10; i++) {
+      expect((await redeem({ code: 'cd'.repeat(32), code_verifier: VERIFIER })).status).toBe(400);
+    }
+    expect((await redeem({ code: 'cd'.repeat(32), code_verifier: VERIFIER })).status).toBe(429);
+  });
+});
+
+describe('SSO hand-off', () => {
+  const bundle = {
+    flow: 'oauth' as const,
+    server_url: 'https://mail.example.org',
+    access_token: 'at',
+    refresh_token: 'rt',
+    expires_in: 3600,
+    token_endpoint: 'https://mail.example.org/auth/token',
+    client_id: 'webmail',
+  };
+
+  it('redeems only after the user confirmed, for 60 seconds from then', async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    const start = Date.now();
+    const code = stashHandoff(bundle, { challenge: CHALLENGE, state: 'm-2', awaitConfirm: true });
+
+    // The user reads the confirmation for a few minutes.
+    vi.setSystemTime(start + HANDOFF_CONFIRM_TTL_MS - 1000);
+    expect((await confirm({ code })).status).toBe(200);
+    vi.setSystemTime(start + HANDOFF_CONFIRM_TTL_MS + HANDOFF_TTL_MS - 2000);
+
+    const res = await redeem({ code, code_verifier: VERIFIER });
+    expect(res.status).toBe(200);
+    expect(res.body).toMatchObject({ flow: 'oauth', access_token: 'at', refresh_token: 'rt', state: 'm-2' });
+    // Counted from when the tokens were issued, not from the redeem.
+    expect(res.body.expires_in).toBe(3600 - Math.floor((HANDOFF_CONFIRM_TTL_MS + HANDOFF_TTL_MS - 2000) / 1000));
+  });
+
+  it('refuses and spends a code the user has not confirmed', async () => {
+    const code = stashHandoff(bundle, { challenge: CHALLENGE, state: 'm-2', awaitConfirm: true });
+    const early = await redeem({ code, code_verifier: VERIFIER });
+    expect(early.status).toBe(400);
+    expect(early.body).toEqual({ error: 'invalid_code' });
+    expect((await confirm({ code })).status).toBe(400);
+  });
+
+  it('expires 60 seconds after the confirmation', async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    const code = stashHandoff(bundle, { challenge: CHALLENGE, state: 'm-2', awaitConfirm: true });
+    expect((await confirm({ code })).status).toBe(200);
+    vi.setSystemTime(Date.now() + HANDOFF_TTL_MS + 1);
+    const late = await redeem({ code, code_verifier: VERIFIER });
+    expect(late.status).toBe(400);
+    expect(late.body).toEqual({ error: 'expired_code' });
+  });
+
+  it('cannot be confirmed once the five minutes to confirm are over', async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    const code = stashHandoff(bundle, { challenge: CHALLENGE, state: 'm-2', awaitConfirm: true });
+    vi.setSystemTime(Date.now() + HANDOFF_CONFIRM_TTL_MS + 1);
+    expect((await confirm({ code })).status).toBe(400);
+  });
+
+  it('confirms a code once', async () => {
+    const code = stashHandoff(bundle, { challenge: CHALLENGE, state: 'm-2', awaitConfirm: true });
+    expect((await confirm({ code })).status).toBe(200);
+    expect((await confirm({ code })).status).toBe(400);
+    expect((await confirm({ code: 'nope' })).status).toBe(400);
+  });
+});
diff --git a/lib/__tests__/mobile-handoff.test.ts b/lib/__tests__/mobile-handoff.test.ts
index 565eea8ff..bc616c2aa 100644
--- a/lib/__tests__/mobile-handoff.test.ts
+++ b/lib/__tests__/mobile-handoff.test.ts
@@ -1,5 +1,12 @@
 import { describe, expect, it } from 'vitest';
-import { MOBILE_HANDOFF_REDIRECT_URI, buildMobileHandoffUrl, mobileHandoffTarget, signedInLoginAction } from '@/lib/auth/mobile-handoff';
+import {
+  MOBILE_HANDOFF_REDIRECT_URI,
+  buildMobileHandoffCodeUrl,
+  buildMobileHandoffUrl,
+  mobileCodeChallenge,
+  mobileHandoffTarget,
+  signedInLoginAction,
+} from '@/lib/auth/mobile-handoff';
 
 // The login page handed the typed password (and the SSO callback the tokens)
 // to any `bulwarkmobile://` URL named in the query string, with nothing on
@@ -59,3 +66,26 @@ describe('signedInLoginAction', () => {
     expect(signedInLoginAction({ isMobileHandoff: false, isAddAccountMode: true, hasHandoffPayload: false })).toBe('none');
   });
 });
+
+describe('code hand-off link', () => {
+  // docs/mobile-handoff.md: an app that sent a challenge gets a link with
+  // the one-time code and its state, in the query, and nothing else.
+  it('carries only the code and the state', () => {
+    const url = new URL(buildMobileHandoffCodeUrl(MOBILE_HANDOFF_REDIRECT_URI, 'ab'.repeat(32), 'st&ate=1'));
+    expect(`${url.protocol}//${url.host}${url.pathname}`).toBe(MOBILE_HANDOFF_REDIRECT_URI);
+    expect([...url.searchParams.keys()]).toEqual(['code', 'state']);
+    expect(url.searchParams.get('code')).toBe('ab'.repeat(32));
+    expect(url.searchParams.get('state')).toBe('st&ate=1');
+    expect(url.hash).toBe('');
+  });
+
+  it('accepts only an S256 challenge', () => {
+    const challenge = 'E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM';
+    expect(mobileCodeChallenge(challenge)).toBe(challenge);
+    expect(mobileCodeChallenge(`${challenge}=`)).toBe('');
+    expect(mobileCodeChallenge(challenge.slice(1))).toBe('');
+    expect(mobileCodeChallenge(`${challenge.slice(1)}+`)).toBe('');
+    expect(mobileCodeChallenge('')).toBe('');
+    expect(mobileCodeChallenge(null)).toBe('');
+  });
+});
diff --git a/lib/__tests__/sso-complete-route.test.ts b/lib/__tests__/sso-complete-route.test.ts
index c4e163448..8de15ddf5 100644
--- a/lib/__tests__/sso-complete-route.test.ts
+++ b/lib/__tests__/sso-complete-route.test.ts
@@ -37,6 +37,8 @@ vi.mock('@/lib/oauth/token-exchange', () => ({
 import { encryptPayload } from '@/lib/auth/crypto';
 import { openPhoneRefreshToken } from '@/lib/auth/pair-bundle';
 import { POST } from '@/app/api/auth/sso/complete/route';
+import { createHash } from 'crypto';
+import { confirmHandoff, redeemHandoff, resetHandoffStoreForTests } from '@/lib/auth/handoff-store';
 
 function pending(extra: Record<string, unknown> = {}) {
   jar.set('sso_pending', encryptPayload({
@@ -59,6 +61,7 @@ async function complete(extra: Record<string, unknown> = {}) {
 
 beforeEach(() => {
   jar.clear();
+  resetHandoffStoreForTests();
   tokenEndpoint = 'https://idp.example.net/token';
   confidential = false;
   exchangeCodeForTokens.mockReset().mockResolvedValue({ access_token: 'at', refresh_token: 'rt', expires_in: 3600 });
@@ -145,3 +148,53 @@ describe('/api/auth/sso/complete for the mobile app handoff', () => {
     expect(openPhoneRefreshToken(res.body.refresh_token)?.tokenEndpoint).toBe('https://idp.example.net/token');
   });
 });
+
+describe('/api/auth/sso/complete for an app that sent a PKCE challenge', () => {
+  // docs/mobile-handoff.md: the tokens stay on the server and the page gets
+  // only a one-time code for the app link.
+  const VERIFIER = 'x'.repeat(50);
+  const CHALLENGE = createHash('sha256').update(VERIFIER).digest('base64url');
+
+  it('returns a code instead of the tokens, and the app redeems the tokens with it', async () => {
+    tokenEndpoint = 'https://mail.example.org/auth/token';
+    pending({
+      redirect_uri: 'https://webmail.example/mail/en/auth/callback',
+      mobile_redirect_uri: 'bulwarkmobile://auth/callback',
+      mobile_state: 'm-1',
+      mobile_code_challenge: CHALLENGE,
+    });
+    const res = await complete();
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual({
+      mobile_handoff_code: expect.stringMatching(/^[0-9a-f]{64}$/),
+      server_url: 'https://mail.example.org',
+      mobile_redirect_uri: 'bulwarkmobile://auth/callback',
+      mobile_state: 'm-1',
+    });
+    expect(JSON.stringify(res.body)).not.toMatch(/"at"|"rt"/);
+    expect(jar.has('jmap_rt')).toBe(false);
+
+    expect(confirmHandoff(res.body.mobile_handoff_code)).toBe(true);
+    const redeemed = redeemHandoff(res.body.mobile_handoff_code, VERIFIER);
+    expect(redeemed).toMatchObject({
+      ok: true,
+      state: 'm-1',
+      bundle: {
+        flow: 'oauth',
+        access_token: 'at',
+        refresh_token: 'rt',
+        token_endpoint: 'https://mail.example.org/auth/token',
+        client_id: 'webmail',
+        server_url: 'https://mail.example.org',
+      },
+    });
+  });
+
+  it('keeps answering with the tokens for an app that sent no challenge', async () => {
+    tokenEndpoint = 'https://mail.example.org/auth/token';
+    pending({ mobile_redirect_uri: 'bulwarkmobile://auth/callback', mobile_state: 'm-1' });
+    const res = await complete();
+    expect(res.body.access_token).toBe('at');
+    expect(res.body.mobile_handoff_code).toBeUndefined();
+  });
+});
diff --git a/lib/__tests__/sso-start-route.test.ts b/lib/__tests__/sso-start-route.test.ts
index fc8b57896..9805d5f0d 100644
--- a/lib/__tests__/sso-start-route.test.ts
+++ b/lib/__tests__/sso-start-route.test.ts
@@ -22,8 +22,12 @@ vi.mock('@/lib/logger', () => ({
   logger: { warn: vi.fn(), error: vi.fn(), debug: vi.fn(), info: vi.fn() },
 }));
 
+const encrypted = vi.hoisted(() => ({ last: null as Record<string, unknown> | null }));
 vi.mock('@/lib/auth/crypto', () => ({
-  encryptPayload: () => 'encrypted',
+  encryptPayload: (payload: Record<string, unknown>) => {
+    encrypted.last = payload;
+    return 'encrypted';
+  },
 }));
 
 vi.mock('@/lib/oauth/pkce-server', () => ({
@@ -134,3 +138,44 @@ describe('sso start route', () => {
     expect(url.searchParams.has('max_age')).toBe(false);
   });
 });
+
+describe('sso start for the mobile app', () => {
+  const CHALLENGE = 'E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM';
+  const mobile = (extra: Record<string, unknown> = {}) => callRoute({
+    redirect_uri: 'https://mail.example.com/auth/callback',
+    mobile_redirect_uri: 'bulwarkmobile://auth/callback',
+    mobile_state: 'm-1',
+    ...extra,
+  });
+
+  beforeEach(() => {
+    encrypted.last = null;
+  });
+
+  it('keeps the PKCE challenge of the app for the complete route', async () => {
+    const { status } = await mobile({ mobile_code_challenge: CHALLENGE });
+    expect(status).toBe(200);
+    expect(encrypted.last).toMatchObject({
+      mobile_redirect_uri: 'bulwarkmobile://auth/callback',
+      mobile_state: 'm-1',
+      mobile_code_challenge: CHALLENGE,
+    });
+  });
+
+  it('refuses a malformed challenge instead of falling back to handing over tokens', async () => {
+    const { status } = await mobile({ mobile_code_challenge: 'plain-text' });
+    expect(status).toBe(400);
+    expect(encrypted.last).toBeNull();
+  });
+
+  it('starts as before without a challenge', async () => {
+    const { status } = await mobile();
+    expect(status).toBe(200);
+    expect(encrypted.last).not.toHaveProperty('mobile_code_challenge');
+  });
+
+  it('drops a challenge sent without the app callback', async () => {
+    await callRoute({ redirect_uri: 'https://mail.example.com/auth/callback', mobile_code_challenge: CHALLENGE });
+    expect(encrypted.last).not.toHaveProperty('mobile_code_challenge');
+  });
+});
diff --git a/lib/auth/handoff-store.ts b/lib/auth/handoff-store.ts
new file mode 100644
index 000000000..57ef77fb1
--- /dev/null
+++ b/lib/auth/handoff-store.ts
@@ -0,0 +1,184 @@
+import { createHash, randomBytes, timingSafeEqual } from 'crypto';
+import type { RedeemBundle } from '@/lib/auth/pair-bundle';
+import { isMobileCodeChallenge } from '@/lib/auth/mobile-handoff';
+import { createFailureBudget } from '@/lib/auth/verify-budget';
+
+// One-time codes for the browser sign-in hand-off to the mobile app
+// (lib/auth/mobile-handoff.ts, docs/mobile-handoff.md). The app opens the
+// login page with a PKCE `mobile_code_challenge`; once the user has signed in
+// and confirmed, the sign-in (an OAuth bundle, or username + password) is
+// parked here and the browser opens the app's callback link with nothing but
+// a random code. The app redeems that code at /api/auth/handoff/redeem with
+// the verifier behind its challenge. A custom-scheme link can be caught by
+// another app on the device; the code alone is worthless to it.
+//
+// A code can be redeemed for 60 seconds from the moment the user confirms,
+// and only once. Any redeem attempt spends it, including one with the wrong
+// verifier, so a code that leaked cannot be tried twice.
+//
+// The password flow parks on confirmation (the page holds the password), so
+// its code is live at once. The SSO flow parks in /api/auth/sso/complete,
+// before the user has seen the confirmation: that code waits, unusable, for
+// up to five minutes until the page confirms it (/api/auth/handoff/confirm),
+// and its 60 seconds start then.
+//
+// Spent and expired records stay a while without their sign-in, so a late
+// redeem is told "expired" rather than "invalid".
+//
+// Storage is an in-process Map pinned on globalThis, like the pairing store
+// (lib/auth/pairing-store.ts), with the same single-instance assumption.
+
+export const HANDOFF_TTL_MS = 60 * 1000;
+/** How long an SSO sign-in waits for the user to confirm the hand-off. */
+export const HANDOFF_CONFIRM_TTL_MS = 5 * 60 * 1000;
+const TOMBSTONE_MS = 10 * 60 * 1000;
+const CODE_BYTES = 32; // 256 bits
+/** Records at most; the oldest gives way when a new one comes in. */
+const MAX_RECORDS = 2000;
+
+/** RFC 7636 §4.1: 43 to 128 unreserved characters. */
+const CODE_VERIFIER_RE = /^[A-Za-z0-9._~-]{43,128}$/;
+const CODE_RE = /^[0-9a-f]{64}$/;
+
+export const isHandoffCodeChallenge = isMobileCodeChallenge;
+
+export function isHandoffCodeVerifier(value: unknown): value is string {
+  return typeof value === 'string' && CODE_VERIFIER_RE.test(value);
+}
+
+export function isHandoffCode(value: unknown): value is string {
+  return typeof value === 'string' && CODE_RE.test(value);
+}
+
+interface HandoffRecord {
+  /** Null once redeemed (or tried) or expired. */
+  bundle: RedeemBundle | null;
+  state: string;
+  challenge: string;
+  /** Whether the user confirmed the hand-off; only confirmed codes redeem. */
+  confirmed: boolean;
+  issuedAt: number;
+  expiresAt: number;
+  forgetAt: number;
+}
+
+export type HandoffRedeemResult =
+  | { ok: true; bundle: RedeemBundle; state: string }
+  | { ok: false; reason: 'invalid' | 'expired' | 'verifier_mismatch' };
+
+const STORE_KEY = '__bulwarkMobileHandoffStore';
+
+function store(): Map<string, HandoffRecord> {
+  const g = globalThis as typeof globalThis & { [STORE_KEY]?: Map<string, HandoffRecord> };
+  if (!g[STORE_KEY]) g[STORE_KEY] = new Map();
+  return g[STORE_KEY];
+}
+
+// Wipe sign-ins past their expiry and forget tombstones past theirs. Called
+// on every operation, so memory stays bounded without a timer.
+function sweep(now: number): void {
+  const codes = store();
+  for (const [code, record] of codes) {
+    if (record.expiresAt <= now) record.bundle = null;
+    if (record.forgetAt <= now) codes.delete(code);
+  }
+}
+
+function s256(verifier: string): string {
+  return createHash('sha256').update(verifier, 'ascii').digest('base64url');
+}
+
+/**
+ * Park a sign-in for the app; returns the code the browser hands over. With
+ * `awaitConfirm` the code only becomes redeemable through confirmHandoff.
+ */
+export function stashHandoff(
+  bundle: RedeemBundle,
+  options: { challenge: string; state: string; awaitConfirm?: boolean },
+  now = Date.now(),
+): string {
+  sweep(now);
+  const codes = store();
+  // Map keeps insertion order, so the first key is the oldest.
+  while (codes.size >= MAX_RECORDS) {
+    const oldest = codes.keys().next().value;
+    if (oldest === undefined) break;
+    codes.delete(oldest);
+  }
+  const code = randomBytes(CODE_BYTES).toString('hex');
+  const expiresAt = now + (options.awaitConfirm ? HANDOFF_CONFIRM_TTL_MS : HANDOFF_TTL_MS);
+  codes.set(code, {
+    bundle,
+    state: options.state,
+    challenge: options.challenge,
+    confirmed: !options.awaitConfirm,
+    issuedAt: now,
+    expiresAt,
+    forgetAt: expiresAt + TOMBSTONE_MS,
+  });
+  return code;
+}
+
+/**
+ * The user confirmed the hand-off of a code parked with `awaitConfirm`: it
+ * can be redeemed for the next 60 seconds. False when there is nothing (left)
+ * to confirm.
+ */
+export function confirmHandoff(code: string, now = Date.now()): boolean {
+  sweep(now);
+  const record = store().get(code);
+  if (!record || !record.bundle || record.confirmed) return false;
+  record.confirmed = true;
+  record.expiresAt = now + HANDOFF_TTL_MS;
+  record.forgetAt = record.expiresAt + TOMBSTONE_MS;
+  return true;
+}
+
+/**
+ * Hand out the sign-in behind `code` when `verifier` matches its challenge.
+ * The code is spent by the attempt, whatever its outcome. `expires_in` of an
+ * OAuth bundle counts down from when it was parked.
+ */
+export function redeemHandoff(code: string, verifier: string, now = Date.now()): HandoffRedeemResult {
+  sweep(now);
+  const record = store().get(code);
+  if (!record) return { ok: false, reason: 'invalid' };
+  const bundle = record.bundle;
+  record.bundle = null;
+  if (!bundle) return { ok: false, reason: record.expiresAt <= now ? 'expired' : 'invalid' };
+  if (!record.confirmed) return { ok: false, reason: 'invalid' };
+  const expected = Buffer.from(record.challenge, 'ascii');
+  const actual = Buffer.from(s256(verifier), 'ascii');
+  if (expected.length !== actual.length || !timingSafeEqual(expected, actual)) {
+    return { ok: false, reason: 'verifier_mismatch' };
+  }
+  if (bundle.flow === 'oauth' && typeof bundle.expires_in === 'number') {
+    const elapsed = Math.floor((now - record.issuedAt) / 1000);
+    return { ok: true, bundle: { ...bundle, expires_in: Math.max(0, bundle.expires_in - elapsed) }, state: record.state };
+  }
+  return { ok: true, bundle, state: record.state };
+}
+
+/** Test hook: forget everything. */
+export function resetHandoffStoreForTests(): void {
+  store().clear();
+}
+
+// Per-client limits on the routes. Codes carry 256 bits and die on the first
+// attempt, so guessing is hopeless either way; these keep one client from
+// filling the store or hammering the redeem route.
+const createBudget = createFailureBudget({ clientMax: 10, globalMax: Infinity });
+const redeemBudget = createFailureBudget({ clientMax: 10, globalMax: Infinity });
+
+/** Count one parked sign-in for `client`; false when it is over its limit. */
+export function reserveHandoffCreate(client: string, now = Date.now()): boolean {
+  return createBudget.reserve(client, now) !== null;
+}
+
+/**
+ * Reserve one redeem attempt for `client`: null when it is over its limit,
+ * otherwise a refund to call when the redeem succeeded (only failures count).
+ */
+export function reserveHandoffRedeem(client: string, now = Date.now()): (() => void) | null {
+  return redeemBudget.reserve(client, now);
+}
diff --git a/lib/auth/mobile-handoff-client.ts b/lib/auth/mobile-handoff-client.ts
new file mode 100644
index 000000000..9730bcce4
--- /dev/null
+++ b/lib/auth/mobile-handoff-client.ts
@@ -0,0 +1,52 @@
+import { apiFetch } from '@/lib/browser-navigation';
+import { buildMobileHandoffCodeUrl } from '@/lib/auth/mobile-handoff';
+
+/**
+ * Park a password sign-in for the mobile app (/api/auth/handoff/create) and
+ * return the app link that carries only the one-time code. Null when the
+ * server would not take it; the caller must not fall back to putting the
+ * password in the link, which is what the app's challenge asked to avoid.
+ */
+export async function requestMobileHandoffCodeUrl(opts: {
+  target: string;
+  challenge: string;
+  state: string;
+  payload: { server_url: string; username: string; password: string };
+}): Promise<string | null> {
+  try {
+    const res = await apiFetch('/api/auth/handoff/create', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ code_challenge: opts.challenge, state: opts.state, ...opts.payload }),
+    });
+    if (!res.ok) return null;
+    const data = (await res.json().catch(() => null)) as { code?: unknown } | null;
+    if (typeof data?.code !== 'string' || !data.code) return null;
+    return buildMobileHandoffCodeUrl(opts.target, data.code, opts.state);
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Confirm an SSO hand-off the server parked (/api/auth/handoff/confirm) and
+ * return the app link with its code. Null when the code is gone (it waited
+ * too long); the user has to start the sign-in in the app again.
+ */
+export async function confirmMobileHandoffCodeUrl(opts: {
+  target: string;
+  code: string;
+  state: string;
+}): Promise<string | null> {
+  try {
+    const res = await apiFetch('/api/auth/handoff/confirm', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ code: opts.code }),
+    });
+    if (!res.ok) return null;
+    return buildMobileHandoffCodeUrl(opts.target, opts.code, opts.state);
+  } catch {
+    return null;
+  }
+}
diff --git a/lib/auth/mobile-handoff.ts b/lib/auth/mobile-handoff.ts
index 820403d2e..5356600bc 100644
--- a/lib/auth/mobile-handoff.ts
+++ b/lib/auth/mobile-handoff.ts
@@ -32,6 +32,28 @@ export function buildMobileHandoffUrl(target: string, fragment: Record<string, s
   return `${target}#${new URLSearchParams(fragment).toString()}`;
 }
 
+/**
+ * App builds that send `mobile_code_challenge` (PKCE S256: the base64url
+ * SHA-256 of a verifier only the app knows) get no credentials in the link.
+ * The sign-in is parked on the server (lib/auth/handoff-store.ts) and the
+ * link carries a one-time code the app redeems with its verifier at
+ * /api/auth/handoff/redeem. Older builds send no challenge and keep getting
+ * the fragment above. Contract: docs/mobile-handoff.md.
+ */
+export function isMobileCodeChallenge(value: unknown): value is string {
+  return typeof value === 'string' && /^[A-Za-z0-9_-]{43}$/.test(value);
+}
+
+/** The challenge when `raw` is a well-formed S256 challenge, else ''. */
+export function mobileCodeChallenge(raw: string | null | undefined): string {
+  return isMobileCodeChallenge(raw) ? raw : '';
+}
+
+/** The URL the browser hands to an app that sent a challenge: code and state only. */
+export function buildMobileHandoffCodeUrl(target: string, code: string, state: string): string {
+  return `${target}?${new URLSearchParams({ code, state }).toString()}`;
+}
+
 /**
  * What the login page does once the browser is signed in:
  *  - `handoff`: a mobile hand-off whose sign-in just happened here; show the
diff --git a/scripts/lite/lib.mjs b/scripts/lite/lib.mjs
index ccfd84fee..2f9186cf7 100644
--- a/scripts/lite/lib.mjs
+++ b/scripts/lite/lib.mjs
@@ -100,6 +100,11 @@ export const LITE_API_STRING_ALLOWLIST = [
   "/api/auth/reauth/sso/complete",
   "/api/auth/pair/create",
   "/api/auth/pair/status",
+  // Mobile sign-in hand-off with a code (login page, SSO callback) - skipped
+  // in Lite, which has no server to park the sign-in on and hands over in
+  // the fragment.
+  "/api/auth/handoff/create",
+  "/api/auth/handoff/confirm",
   "/api/auth/totp-token-exchange",
   "/api/auth/session",
   "/api/auth/verify",