diff --git a/apps/api/src/routes/auth.ts b/apps/api/src/routes/auth.ts
index 2f40d33..7d8cb3e 100644
--- a/apps/api/src/routes/auth.ts
+++ b/apps/api/src/routes/auth.ts
@@ -26,6 +26,7 @@ import { createFirebaseTokenVerifier, type TokenVerifier } from "../lib/token-ve
 /** Body-schema för Firebase-sessionväxling. */
 const firebaseSessionInputSchema = z.object({
  idToken: z.string().min(1),
+ displayName: z.string().optional(),
  locale: z.string().optional(),
 });
 
@@ -168,7 +169,7 @@ export async function authRoutes(app: FastifyInstance) {
  if (!user) {
  if (!identity.email) throw errors.badRequest("Firebase-token saknar e-postadress.");
  const locale = input.locale ?? "sv-SE";
- const displayName = identity.name ?? identity.email.split("@")[0] ?? "Användare";
+ const displayName = input.displayName ?? identity.name ?? identity.email.split("@")[0] ?? "Användare";
  [user] = await app.db
  .insert(schema.users)
  .values({
diff --git a/apps/mobile/src/app/(auth)/login.tsx b/apps/mobile/src/app/(auth)/login.tsx
index 023645d..9f84098 100644
--- a/apps/mobile/src/app/(auth)/login.tsx
+++ b/apps/mobile/src/app/(auth)/login.tsx
@@ -2,6 +2,7 @@ import { useState } from "react";
 import { Text, View } from "react-native";
 import { Link, Redirect, router } from "expo-router";
 import { api } from "@/lib/api";
+import { firebaseAuthProvider } from "@/lib/auth-provider/firebase";
 import { useAuth, type AuthUser } from "@/lib/auth";
 import { t } from "@/lib/i18n";
 import { Body, Button, Input, Screen, Spacer, Title } from "@/components/ui";
@@ -27,9 +28,12 @@ export default function LoginScreen() {
     setBusy(true);
     setError(null);
     try {
-      const result = await api<LoginResponse>("/v1/auth/login", {
+      await firebaseAuthProvider.signInWithEmail(email.trim(), password);
+      const idToken = await firebaseAuthProvider.getIdToken();
+      if (!idToken) throw new Error(t("common.error"));
+      const result = await api<LoginResponse>("/v1/auth/session", {
         method: "POST",
-        body: { email, password },
+        body: { idToken },
       });
       await setSession(
         { accessToken: result.accessToken, refreshToken: result.refreshToken },
diff --git a/apps/mobile/src/app/(auth)/register.tsx b/apps/mobile/src/app/(auth)/register.tsx
index b191476..d1a1082 100644
--- a/apps/mobile/src/app/(auth)/register.tsx
+++ b/apps/mobile/src/app/(auth)/register.tsx
@@ -2,6 +2,7 @@ import { useState } from "react";
 import { Text, View } from "react-native";
 import { Link, router } from "expo-router";
 import { api } from "@/lib/api";
+import { firebaseAuthProvider } from "@/lib/auth-provider/firebase";
 import { persistLanguageTag, useAuth, type AuthUser } from "@/lib/auth";
 import { detectDeviceLanguageTag, t } from "@/lib/i18n";
 import { localeDefaultsForRegion } from "@app/shared-types";
@@ -28,13 +29,15 @@ export default function RegisterScreen() {
     setBusy(true);
     setError(null);
     try {
-      const result = await api<RegisterResponse>("/v1/auth/register", {
+      await firebaseAuthProvider.signUpWithEmail(email.trim(), password);
+      const idToken = await firebaseAuthProvider.getIdToken();
+      if (!idToken) throw new Error(t("common.error"));
+      const result = await api<RegisterResponse>("/v1/auth/session", {
         method: "POST",
         body: {
+          idToken,
           displayName,
-          email,
-          password,
-          // D-031: enhetens språk – backend skapar locale-preferenser före välkomstmejlet.
+          // D-031: enhetens språk – backend skapar locale-preferenser.
           locale: detectDeviceLanguageTag() ?? "sv-SE",
         },
       });