diff --git a/packages/fxa-settings/src/lib/passkeys/signin-flow.test.tsx b/packages/fxa-settings/src/lib/passkeys/signin-flow.test.tsx
index 4c821d136a8..7a2728c986a 100644
--- a/packages/fxa-settings/src/lib/passkeys/signin-flow.test.tsx
+++ b/packages/fxa-settings/src/lib/passkeys/signin-flow.test.tsx
@@ -360,6 +360,32 @@ describe('usePasskeySignIn', () => {
     }
   );
 
+  it('sends service=sync for Sync integrations even when the service URL param is absent', async () => {
+    // Mobile Firefox omits service=sync and defaults via clientId, so
+    // getService() returns undefined; isSync() must still drive service=sync.
+    const { args, spies } = buildArgs({
+      integration: {
+        isSync: () => true,
+        isFirefoxNonSync: () => false,
+        getService: () => undefined,
+        type: IntegrationType.OAuthNative,
+        data: {},
+      } as unknown as PasskeySignInIntegration,
+    });
+
+    const { result } = renderHook(() => usePasskeySignIn(args));
+
+    await act(async () => {
+      await result.current.onClick();
+    });
+
+    expect(spies.completePasskeyAuthentication).toHaveBeenCalledWith(
+      MOCK_CREDENTIAL,
+      CHALLENGE,
+      { service: 'sync' }
+    );
+  });
+
   // Locks the contract: each DOMException name maps to its expected FTL id
   // AND its fallback string. Drift in either lands silently otherwise.
   // The fallback substrings are stable phrases pulled from webauthn-errors.ts
diff --git a/packages/fxa-settings/src/lib/passkeys/signin-flow.ts b/packages/fxa-settings/src/lib/passkeys/signin-flow.ts
index 0cb055f864f..f43813a8420 100644
--- a/packages/fxa-settings/src/lib/passkeys/signin-flow.ts
+++ b/packages/fxa-settings/src/lib/passkeys/signin-flow.ts
@@ -277,7 +277,9 @@ export function usePasskeySignIn({
         throw err;
       }
 
-      const service = integration.getService();
+      // Sync clients (notably mobile) often omit `service=sync` and default via
+      // clientId, leaving getService() undefined; isSync() is the reliable signal.
+      const service = integration.isSync() ? 'sync' : integration.getService();
       const completion = await authClient.completePasskeyAuthentication(
         credential,
         challengeOptions.challenge,
diff --git a/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.test.tsx b/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.test.tsx
index 6307f1db58c..a46fa5fe4ec 100644
--- a/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.test.tsx
+++ b/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.test.tsx
@@ -37,6 +37,7 @@ function createMockSyncIntegration() {
     type: IntegrationType.OAuthNative,
     getService: () => MozServices.FirefoxSync,
     isSync: () => true,
+    isFirefoxMobileClient: () => false,
     requiresKeys: () => true,
     wantsKeys: () => true,
     getCmsInfo: () => undefined,
@@ -199,6 +200,32 @@ describe('SigninPasskeyFallback container', () => {
       );
     });
 
+    it('navigates after reauth on desktop (performNavigation true)', async () => {
+      const { getByTestId } = render();
+      submitPassword(getByTestId);
+
+      await waitFor(() => {
+        expect(mockHandleNavigation).toHaveBeenCalledWith(
+          expect.objectContaining({ performNavigation: true })
+        );
+      });
+    });
+
+    it('skips navigation after reauth on mobile so Firefox drives Sync completion', async () => {
+      const mobileIntegration = {
+        ...createMockSyncIntegration(),
+        isFirefoxMobileClient: () => true,
+      } as unknown as Integration;
+      const { getByTestId } = render(mobileIntegration);
+      submitPassword(getByTestId);
+
+      await waitFor(() => {
+        expect(mockHandleNavigation).toHaveBeenCalledWith(
+          expect.objectContaining({ performNavigation: false })
+        );
+      });
+    });
+
     it('renders an error banner when sessionReauth throws', async () => {
       mockSessionReauth.mockRejectedValueOnce({
         errno: 103,
diff --git a/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.tsx b/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.tsx
index 5eb2e67d6aa..6a0e23003fd 100644
--- a/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.tsx
+++ b/packages/fxa-settings/src/pages/Signin/SigninPasskeyFallback/container.tsx
@@ -149,6 +149,9 @@ const SigninPasskeyFallbackContainer = ({
         queryParams: location.search,
         handleFxaLogin: true,
         handleFxaOAuthLogin: true,
+        // On mobile, navigating the WebView away leaves Sync paused; let Firefox
+        // drive completion via WebChannel instead.
+        performNavigation: !integration.isFirefoxMobileClient(),
       });
       if (navError) {
         GleanMetrics.passkeyEnterPassword.submitFrontendError({