diff --git a/packages/shared-auth-ui/src/stores/createManaAuthStore.svelte.ts b/packages/shared-auth-ui/src/stores/createManaAuthStore.svelte.ts
index 9635d8b5c..0ec2436d8 100644
--- a/packages/shared-auth-ui/src/stores/createManaAuthStore.svelte.ts
+++ b/packages/shared-auth-ui/src/stores/createManaAuthStore.svelte.ts
@@ -186,11 +186,11 @@ export function createManaAuthStore(config: ManaAuthStoreConfig = {}) {
 			return authService.isPasskeyAvailable();
 		},
 
-		async signInWithPasskey() {
+		async signInWithPasskey(options?: { conditional?: boolean }) {
 			const authService = getAuthService();
 			if (!authService) return { success: false, error: 'Auth not available on server' };
 			try {
-				const result = await authService.signInWithPasskey();
+				const result = await authService.signInWithPasskey(options);
 				if (!result.success)
 					return { success: false, error: result.error || 'Passkey authentication failed' };
 				user = await authService.getUserFromToken();
diff --git a/packages/shared-auth-ui/src/types.ts b/packages/shared-auth-ui/src/types.ts
index 017e34da2..fd70c73ed 100644
--- a/packages/shared-auth-ui/src/types.ts
+++ b/packages/shared-auth-ui/src/types.ts
@@ -35,13 +35,33 @@ export interface AuthServiceInterface {
 	forgotPassword(email: string): Promise<AuthResult>;
 }
 
+/**
+ * Structured error code for an auth operation. Frontend should branch on
+ * this rather than parsing the human-readable `error` string, which is
+ * locale-dependent.
+ */
+export type AuthErrorCode =
+	| 'INVALID_CREDENTIALS'
+	| 'EMAIL_NOT_VERIFIED'
+	| 'RATE_LIMITED'
+	| 'ACCOUNT_LOCKED'
+	| 'NETWORK_ERROR'
+	| 'UNKNOWN';
+
 /**
  * Result from auth operations
  */
 export interface AuthResult {
 	success: boolean;
+	/** Human-readable, possibly localized error message */
 	error?: string;
+	/** Stable, locale-independent error code for branching */
+	errorCode?: AuthErrorCode;
 	needsVerification?: boolean;
+	/** Set when sign-in succeeded but a 2FA challenge must be completed */
+	twoFactorRedirect?: boolean;
+	/** Seconds until the user may retry, set on RATE_LIMITED / ACCOUNT_LOCKED */
+	retryAfter?: number;
 }
 
 /**
diff --git a/packages/shared-auth/src/core/authService.ts b/packages/shared-auth/src/core/authService.ts
index f45967b2e..d31184c16 100644
--- a/packages/shared-auth/src/core/authService.ts
+++ b/packages/shared-auth/src/core/authService.ts
@@ -436,9 +436,14 @@ export function createAuthService(config: AuthServiceConfig): AuthServiceInterfa
 		},
 
 		/**
-		 * Sign in with a passkey
+		 * Sign in with a passkey.
+		 *
+		 * Pass `{ conditional: true }` to use the WebAuthn Conditional UI flow,
+		 * where the browser surfaces passkeys directly inside the email autofill
+		 * dropdown instead of opening a modal. The host MUST verify
+		 * `PublicKeyCredential.isConditionalMediationAvailable()` first.
 		 */
-		async signInWithPasskey(): Promise<AuthResult> {
+		async signInWithPasskey(options: { conditional?: boolean } = {}): Promise<AuthResult> {
 			try {
 				const { startAuthentication } = await import('@simplewebauthn/browser');
 				const storage = getStorageAdapter();
@@ -454,10 +459,13 @@ export function createAuthService(config: AuthServiceConfig): AuthServiceInterfa
 					return { success: false, error: err.message || 'Failed to get authentication options' };
 				}
 
-				const { options, challengeId } = await optionsRes.json();
+				const { options: webauthnOptions, challengeId } = await optionsRes.json();
 
 				// Step 2: Authenticate via browser WebAuthn API
-				const credential = await startAuthentication({ optionsJSON: options });
+				const credential = await startAuthentication({
+					optionsJSON: webauthnOptions,
+					useBrowserAutofill: options.conditional === true,
+				});
 
 				// Step 3: Send credential to server for verification
 				const verifyRes = await fetch(`${baseUrl}${endpoints.passkeyAuthVerify}`, {
diff --git a/packages/shared-auth/src/types/index.ts b/packages/shared-auth/src/types/index.ts
index 2a78e6a01..f57bf412b 100644
--- a/packages/shared-auth/src/types/index.ts
+++ b/packages/shared-auth/src/types/index.ts
@@ -211,7 +211,7 @@ export interface AuthServiceInterface {
 	// Passkeys
 	isPasskeyAvailable(): boolean;
 	registerPasskey(friendlyName?: string): Promise<AuthResult>;
-	signInWithPasskey(): Promise<AuthResult>;
+	signInWithPasskey(options?: { conditional?: boolean }): Promise<AuthResult>;
 	listPasskeys(): Promise<any[]>;
 	deletePasskey(passkeyId: string): Promise<AuthResult>;
 	renamePasskey(passkeyId: string, friendlyName: string): Promise<AuthResult>;