feat: implement legacyFetchSignInWithEmail behaviour for Angular and React

README.md

@@ -395,6 +395,57 @@ const ui = initializeUI({
 });
 ```
 
+#### `legacyFetchSignInWithEmail`
+
+The `legacyFetchSignInWithEmail` behavior augments OAuth `auth/account-exists-with-different-credential` flows by calling `fetchSignInMethodsForEmail(auth, email)` and storing the returned methods on the UI instance. In the packaged React and Angular screen components, this recovery state is rendered automatically as a modal on `SignInAuthScreen` and `OAuthScreen`.
+
+The original pending credential is still preserved, so after the user signs in with the correct method, Firebase UI can continue the existing linking flow.
+
+```ts
+import { legacyFetchSignInWithEmail } from '@firebase-oss/ui-core';
+
+const ui = initializeUI({
+  app,
+  behaviors: [legacyFetchSignInWithEmail()],
+});
+```
+
+If you want full control over the UI, hide the built-in recovery component on the screen and read the recovery state directly with `useLegacySignInRecovery()`:
+
+```tsx
+import { GitHubSignInButton, GoogleSignInButton, SignInAuthScreen, useLegacySignInRecovery } from '@firebase-oss/ui-react';
+
+function WrongProviderRecovery() {
+  const { recovery, clearRecovery } = useLegacySignInRecovery();
+
+  if (!recovery) {
+    return null;
+  }
+
+  return (
+    <div>
+      <p>You have previously signed in with a different method for {recovery.email}.</p>
+      {recovery.signInMethods.includes('google.com') && (
+        <GoogleSignInButton onSignIn={clearRecovery} />
+      )}
+      {recovery.signInMethods.includes('github.com') && (
+        <GitHubSignInButton onSignIn={clearRecovery} />
+      )}
+    </div>
+  );
+}
+
+export function CustomSignInScreen() {
+  return (
+    <SignInAuthScreen showLegacySignInRecovery={false}>
+      <WrongProviderRecovery />
+    </SignInAuthScreen>
+  );
+}
+```
+
+Angular apps can hide the built-in recovery UI with `showLegacySignInRecovery="false"` and read the same state with `injectLegacySignInRecovery()` / `injectClearLegacySignInRecovery()`.
+
 #### `oneTapSignIn`
 
 The `oneTapSignIn` behavior triggers the [Google One Tap](https://developers.google.com/identity/gsi/web/guides/features) experience to render.
@@ -1061,6 +1112,7 @@ By default, any missing translations will fallback to English if not specified.
   | onSignIn | `(user: User) => void?` | Callback when sign-in succeeds |
   | onForgotPasswordClick | `() => void?` | Callback when forgot password link is clicked |
   | onSignUpClick | `() => void?` | Callback when sign-up link is clicked |
+| showLegacySignInRecovery | `boolean?` | Whether to show the built-in legacy sign-in recovery UI |
 
   **`SignUpAuthScreen`**
 
@@ -1113,6 +1165,7 @@ By default, any missing translations will fallback to English if not specified.
   |------|:----:|-------------|
   | onSignIn | `(user: User) => void?` | Callback when sign-in succeeds |
   | children | `React.ReactNode?` | Child components |
+| showLegacySignInRecovery | `boolean?` | Whether to show the built-in legacy sign-in recovery UI |
 
   **`OAuthButton`**
 
@@ -1189,6 +1242,10 @@ By default, any missing translations will fallback to English if not specified.
   | asChild | `boolean?` | Render as child component using Slot |
   | ...props | `ComponentProps<"button">` | Standard button HTML attributes |
 
+  **`LegacySignInRecovery`**
+
+  Default component for displaying suggested previous sign-in methods from `legacyFetchSignInWithEmail`.
+
   **`Card`**
 
   Card container component.
@@ -1251,6 +1308,12 @@ By default, any missing translations will fallback to English if not specified.
 
   Returns `string | undefined`.
 
+  **`useLegacySignInRecovery`**
+
+  Gets the legacy sign-in recovery state populated by `legacyFetchSignInWithEmail`.
+
+  Returns `{ recovery: LegacySignInRecovery | undefined; clearRecovery: () => void }`.
+
   **`useSignInAuthFormSchema`**
 
   Creates a Zod schema for sign-in form validation.
@@ -1700,6 +1763,10 @@ By default, any missing translations will fallback to English if not specified.
 
   Screen component for email/password sign-in.
 
+  | Input | Type | Description |
+  |-------|:----:|-------------|
+  | showLegacySignInRecovery | `boolean` | Whether to show the built-in legacy sign-in recovery UI |
+
   | Output | Type | Description |
   |--------|:----:|-------------|
   | signIn | `EventEmitter<User>` | Emitted when sign-in succeeds |
@@ -1754,6 +1821,10 @@ By default, any missing translations will fallback to English if not specified.
 
   Screen component for OAuth provider sign-in.
 
+  | Input | Type | Description |
+  |-------|:----:|-------------|
+  | showLegacySignInRecovery | `boolean` | Whether to show the built-in legacy sign-in recovery UI |
+
   | Output | Type | Description |
   |--------|:----:|-------------|
   | onSignIn | `EventEmitter<User>` | Emitted when OAuth sign-in succeeds |
@@ -1904,6 +1975,12 @@ By default, any missing translations will fallback to English if not specified.
 
   Component that displays redirect errors from Firebase UI authentication flow.
 
+  **`LegacySignInRecoveryComponent`**
+
+  Selector: `fui-legacy-sign-in-recovery`
+
+  Default component for displaying suggested previous sign-in methods from `legacyFetchSignInWithEmail`.
+
   **`ContentComponent`**
 
   Selector: `fui-content`
@@ -1922,6 +1999,18 @@ By default, any missing translations will fallback to English if not specified.
 
   Returns `Signal<string \| undefined>`.
 
+  **`injectLegacySignInRecovery`**
+
+  Injects the legacy sign-in recovery state from the UI store as a signal.
+
+  Returns `Signal<LegacySignInRecovery \| undefined>`.
+
+  **`injectClearLegacySignInRecovery`**
+
+  Injects a callback that clears the current legacy sign-in recovery state.
+
+  Returns `() => void`.
+
   **`injectTranslation`**
 
   Injects a translated string for a given category and key.

examples/react/src/firebase/firebase.ts

@@ -16,7 +16,7 @@
 
 "use client";
 
-import { countryCodes, initializeUI, oneTapSignIn } from "@firebase-oss/ui-core";
+import { countryCodes, initializeUI, legacyFetchSignInWithEmail, oneTapSignIn } from "@firebase-oss/ui-core";
 import { getApps, initializeApp } from "firebase/app";
 import { connectAuthEmulator, getAuth } from "firebase/auth";
 
@@ -30,6 +30,7 @@ export const ui = initializeUI({
   app: firebaseApp,
   behaviors: [
     // autoAnonymousLogin(),
+    legacyFetchSignInWithEmail(),
     oneTapSignIn({
       clientId: "616577669988-led6l3rqek9ckn9t1unj4l8l67070fhp.apps.googleusercontent.com",
     }),

examples/react/src/routes.ts

@@ -1,6 +1,7 @@
 import SignInAuthScreenPage from "./screens/sign-in-auth-screen";
 import SignInAuthScreenWithHandlersPage from "./screens/sign-in-auth-screen-w-handlers";
 import SignInAuthScreenWithOAuthPage from "./screens/sign-in-auth-screen-w-oauth";
+import LegacyRecoveryDemoPage from "./screens/legacy-recovery-demo";
 import SignUpAuthScreenPage from "./screens/sign-up-auth-screen";
 import SignUpAuthScreenWithHandlersPage from "./screens/sign-up-auth-screen-w-handlers";
 import SignUpAuthScreenWithOAuthPage from "./screens/sign-up-auth-screen-w-oauth";
@@ -32,6 +33,12 @@ export const routes = [
     path: "/screens/sign-in-auth-screen-w-oauth",
     component: SignInAuthScreenWithOAuthPage,
   },
+  {
+    name: "Legacy Recovery Demo",
+    description: "Use this screen to test wrong-provider recovery for email/password and OAuth attempts.",
+    path: "/screens/legacy-recovery-demo",
+    component: LegacyRecoveryDemoPage,
+  },
   {
     name: "Sign Up Screen",
     description: "A sign up screen with email and password.",

examples/react/src/screens/legacy-recovery-demo.tsx

@@ -0,0 +1,60 @@
+/**
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  AppleSignInButton,
+  FacebookSignInButton,
+  GitHubSignInButton,
+  GoogleSignInButton,
+  MicrosoftSignInButton,
+  SignInAuthScreen,
+  TwitterSignInButton,
+  YahooSignInButton,
+} from "@firebase-oss/ui-react";
+import { useNavigate } from "react-router";
+
+export default function LegacyRecoveryDemoPage() {
+  const navigate = useNavigate();
+
+  return (
+    <div className="space-y-6">
+      <div className="max-w-sm mx-auto pt-10 text-sm text-gray-700 dark:text-gray-300 space-y-3">
+        <p className="font-medium text-base text-black dark:text-white">Legacy recovery demo</p>
+        <p>Use this screen to test wrong-provider recovery with both email/password and OAuth attempts.</p>
+        <p>
+          Suggested flow: create an account with Google first, sign out, then come back here and try the same email with
+          with email/password or another provider like GitHub.
+        </p>
+      </div>
+
+      <SignInAuthScreen
+        onSignIn={() => {
+          navigate("/");
+        }}
+      >
+        <div className="space-y-2">
+          <GoogleSignInButton />
+          <FacebookSignInButton />
+          <AppleSignInButton />
+          <GitHubSignInButton />
+          <MicrosoftSignInButton />
+          <TwitterSignInButton />
+          <YahooSignInButton />
+        </div>
+      </SignInAuthScreen>
+    </div>
+  );
+}