diff --git a/app-next/messages/de.json b/app-next/messages/de.json
index f8eac243..0547b33b 100644
--- a/app-next/messages/de.json
+++ b/app-next/messages/de.json
@@ -706,6 +706,12 @@
   },
   "auth": {
     "recommended": "EMPFOHLEN",
+    "signInRequired": {
+      "uploadDataset": "Sie müssen sich anmelden, um einen Datensatz hochzuladen",
+      "createTask": "Sie müssen sich anmelden, um eine Aufgabe zu definieren",
+      "createCollection": "Sie müssen sich anmelden, um eine Sammlung zu erstellen",
+      "default": "Sie müssen sich anmelden, um fortzufahren"
+    },
     "signIn": {
       "title": "Anmelden - OpenML",
       "description": "Melden Sie sich bei Ihrem OpenML-Konto an",
diff --git a/app-next/messages/en.json b/app-next/messages/en.json
index fa458f77..ecf2b6ad 100644
--- a/app-next/messages/en.json
+++ b/app-next/messages/en.json
@@ -689,6 +689,12 @@
   },
   "auth": {
     "recommended": "RECOMMENDED",
+    "signInRequired": {
+      "uploadDataset": "You need to sign in to upload a dataset",
+      "createTask": "You need to sign in to define a task",
+      "createCollection": "You need to sign in to create a collection",
+      "default": "You need to sign in to continue"
+    },
     "signIn": {
       "title": "Sign In - OpenML",
       "description": "Sign in to your OpenML account",
diff --git a/app-next/messages/fr.json b/app-next/messages/fr.json
index d72be02e..3a7abf1c 100644
--- a/app-next/messages/fr.json
+++ b/app-next/messages/fr.json
@@ -706,6 +706,12 @@
   },
   "auth": {
     "recommended": "RECOMMANDÉ",
+    "signInRequired": {
+      "uploadDataset": "Vous devez vous connecter pour télécharger un jeu de données",
+      "createTask": "Vous devez vous connecter pour définir une tâche",
+      "createCollection": "Vous devez vous connecter pour créer une collection",
+      "default": "Vous devez vous connecter pour continuer"
+    },
     "signIn": {
       "title": "Se connecter - OpenML",
       "description": "Connectez-vous à votre compte OpenML",
diff --git a/app-next/messages/nl.json b/app-next/messages/nl.json
index 7f4c61c4..86645523 100644
--- a/app-next/messages/nl.json
+++ b/app-next/messages/nl.json
@@ -706,6 +706,12 @@
   },
   "auth": {
     "recommended": "AANBEVOLEN",
+    "signInRequired": {
+      "uploadDataset": "U moet inloggen om een dataset te uploaden",
+      "createTask": "U moet inloggen om een taak te definiëren",
+      "createCollection": "U moet inloggen om een collectie aan te maken",
+      "default": "U moet inloggen om verder te gaan"
+    },
     "signIn": {
       "title": "Inloggen - OpenML",
       "description": "Log in op uw OpenML-account",
diff --git a/app-next/src/app/[locale]/(explore)/collections/create/page.tsx b/app-next/src/app/[locale]/(explore)/collections/create/page.tsx
new file mode 100644
index 00000000..f33adaf2
--- /dev/null
+++ b/app-next/src/app/[locale]/(explore)/collections/create/page.tsx
@@ -0,0 +1,26 @@
+import { getServerSession } from "next-auth";
+import { redirect } from "next/navigation";
+import { authOptions } from "@/app/api/auth/[...nextauth]/route";
+
+export default async function CollectionCreatePage({
+  params,
+}: {
+  params: Promise<{ locale: string }>;
+}) {
+  const { locale } = await params;
+  const session = await getServerSession(authOptions);
+
+  if (!session) {
+    redirect(
+      `/${locale}/auth/sign-in?reason=createCollection&callbackUrl=/${locale}/collections/create`
+    );
+  }
+
+  // TODO: Implement collection creation form
+  return (
+    <div className="container mx-auto px-4 py-16">
+      <h1 className="text-3xl font-bold">Create Collection</h1>
+      <p className="text-muted-foreground mt-2">Coming soon.</p>
+    </div>
+  );
+}
diff --git a/app-next/src/app/[locale]/(explore)/datasets/[id]/edit/page.tsx b/app-next/src/app/[locale]/(explore)/datasets/[id]/edit/page.tsx
index 0154c364..c64b66b9 100644
--- a/app-next/src/app/[locale]/(explore)/datasets/[id]/edit/page.tsx
+++ b/app-next/src/app/[locale]/(explore)/datasets/[id]/edit/page.tsx
@@ -30,7 +30,7 @@ export default async function DatasetEditPage({
   // Auth check — redirect to sign-in if not authenticated
   const session = await getServerSession(authOptions);
   if (!session?.user) {
-    redirect(`/auth/signin?callbackUrl=/datasets/${id}/edit`);
+    redirect(`/${locale}/auth/sign-in?reason=uploadDataset&callbackUrl=/${locale}/datasets/${id}/edit`);
   }
 
   const dataset = await fetchDataset(id);
@@ -39,12 +39,20 @@ export default async function DatasetEditPage({
   const userId = (session.user as { id?: string }).id;
   const isOwner = userId ? Number(userId) === dataset.uploader_id : false;
 
+  // Check whether the session has a valid OpenML API key
+  const hasApiKey = !!(session as { apikey?: string }).apikey;
+  // OAuth users created in local dev environments don't have a real OpenML API key
+  const isLocalUser =
+    (session.user as { isLocalUser?: boolean }).isLocalUser ?? false;
+
   return (
     <div className="container mx-auto max-w-3xl px-4 py-8 sm:px-6 lg:px-8">
       <DatasetEditForm
         datasetId={dataset.data_id}
         datasetName={dataset.name}
         isOwner={isOwner}
+        hasApiKey={hasApiKey}
+        isLocalUser={isLocalUser}
         initialValues={{
           description: dataset.description || "",
           creator: dataset.creator || "",
diff --git a/app-next/src/app/[locale]/(explore)/datasets/upload/page.tsx b/app-next/src/app/[locale]/(explore)/datasets/upload/page.tsx
new file mode 100644
index 00000000..1c5cf24f
--- /dev/null
+++ b/app-next/src/app/[locale]/(explore)/datasets/upload/page.tsx
@@ -0,0 +1,26 @@
+import { getServerSession } from "next-auth";
+import { redirect } from "next/navigation";
+import { authOptions } from "@/app/api/auth/[...nextauth]/route";
+
+export default async function DatasetUploadPage({
+  params,
+}: {
+  params: Promise<{ locale: string }>;
+}) {
+  const { locale } = await params;
+  const session = await getServerSession(authOptions);
+
+  if (!session) {
+    redirect(
+      `/${locale}/auth/sign-in?reason=uploadDataset&callbackUrl=/${locale}/datasets/upload`
+    );
+  }
+
+  // TODO: Implement dataset upload form
+  return (
+    <div className="container mx-auto px-4 py-16">
+      <h1 className="text-3xl font-bold">Upload Dataset</h1>
+      <p className="text-muted-foreground mt-2">Coming soon.</p>
+    </div>
+  );
+}
diff --git a/app-next/src/app/[locale]/(explore)/tasks/create/page.tsx b/app-next/src/app/[locale]/(explore)/tasks/create/page.tsx
new file mode 100644
index 00000000..73dbecf7
--- /dev/null
+++ b/app-next/src/app/[locale]/(explore)/tasks/create/page.tsx
@@ -0,0 +1,26 @@
+import { getServerSession } from "next-auth";
+import { redirect } from "next/navigation";
+import { authOptions } from "@/app/api/auth/[...nextauth]/route";
+
+export default async function TaskCreatePage({
+  params,
+}: {
+  params: Promise<{ locale: string }>;
+}) {
+  const { locale } = await params;
+  const session = await getServerSession(authOptions);
+
+  if (!session) {
+    redirect(
+      `/${locale}/auth/sign-in?reason=createTask&callbackUrl=/${locale}/tasks/create`
+    );
+  }
+
+  // TODO: Implement task creation form
+  return (
+    <div className="container mx-auto px-4 py-16">
+      <h1 className="text-3xl font-bold">Define Task</h1>
+      <p className="text-muted-foreground mt-2">Coming soon.</p>
+    </div>
+  );
+}
diff --git a/app-next/src/app/[locale]/(extra)/auth/sign-in/page.tsx b/app-next/src/app/[locale]/(extra)/auth/sign-in/page.tsx
index 6f444ca1..04df4b17 100644
--- a/app-next/src/app/[locale]/(extra)/auth/sign-in/page.tsx
+++ b/app-next/src/app/[locale]/(extra)/auth/sign-in/page.tsx
@@ -8,6 +8,7 @@ import {
   CardHeader,
   CardTitle,
 } from "@/components/ui/card";
+import { LogIn } from "lucide-react";
 
 export async function generateMetadata(): Promise<Metadata> {
   const t = await getTranslations("auth");
@@ -20,28 +21,50 @@ export async function generateMetadata(): Promise<Metadata> {
 
 export default async function SignInPage({
   params,
+  searchParams,
 }: {
   params: Promise<{ locale: string }>;
+  searchParams: Promise<{ reason?: string }>;
 }) {
   const { locale } = await params;
+  const { reason } = await searchParams;
   setRequestLocale(locale);
   const t = await getTranslations("auth");
 
+  // Map reason param to translated message
+  const reasonMessages: Record<string, string> = {
+    uploadDataset: t("signInRequired.uploadDataset"),
+    createTask: t("signInRequired.createTask"),
+    createCollection: t("signInRequired.createCollection"),
+  };
+  const reasonMessage = reason
+    ? (reasonMessages[reason] ?? t("signInRequired.default"))
+    : null;
+
   return (
     <div className="container mx-auto flex min-h-[calc(100vh-200px)] items-center justify-center px-4 py-16">
-      <Card className="w-full max-w-md border-slate-200 bg-white shadow-lg dark:border-slate-700 dark:bg-slate-800">
-        <CardHeader className="text-center">
-          <CardTitle className="text-3xl text-slate-800 dark:text-white">
-            {t("signIn.welcome")}
-          </CardTitle>
-          <CardDescription className="text-base text-slate-500 dark:text-slate-400">
-            {t("signIn.subtitle")}
-          </CardDescription>
-        </CardHeader>
-        <CardContent>
-          <AuthTabs />
-        </CardContent>
-      </Card>
+      <div className="w-full max-w-md space-y-4">
+        {/* Contextual message shown when redirected from a protected action */}
+        {reasonMessage && (
+          <div className="flex items-center gap-3 rounded-lg border border-slate-200 bg-slate-800 px-4 py-3 text-slate-200 dark:border-slate-950 dark:bg-slate-950 dark:text-slate-200">
+            <LogIn className="size-5 shrink-0" />
+            <p className="text-sm font-medium">{reasonMessage}</p>
+          </div>
+        )}
+        <Card className="border-slate-200 bg-white shadow-lg dark:border-slate-700 dark:bg-slate-800">
+          <CardHeader className="text-center">
+            <CardTitle className="text-3xl text-slate-800 dark:text-white">
+              {t("signIn.welcome")}
+            </CardTitle>
+            <CardDescription className="text-base text-slate-600 dark:text-slate-300">
+              {t("signIn.subtitle")}
+            </CardDescription>
+          </CardHeader>
+          <CardContent>
+            <AuthTabs />
+          </CardContent>
+        </Card>
+      </div>
     </div>
   );
 }
diff --git a/app-next/src/app/api/datasets/[id]/edit/route.ts b/app-next/src/app/api/datasets/[id]/edit/route.ts
index d73b8bb6..bc5e5546 100644
--- a/app-next/src/app/api/datasets/[id]/edit/route.ts
+++ b/app-next/src/app/api/datasets/[id]/edit/route.ts
@@ -72,8 +72,12 @@ export async function POST(
     if (!response.ok) {
       const text = await response.text();
       console.error(`OpenML API error editing dataset ${id}:`, text);
+      const message =
+        response.status === 401 || response.status === 403
+          ? "Your API key is not accepted by the OpenML server. If you are using a local test account, dataset editing is not supported — only real OpenML accounts can save changes."
+          : "Failed to save changes. Please try again.";
       return NextResponse.json(
-        { error: "Failed to save changes. Please try again." },
+        { error: message },
         { status: response.status },
       );
     }
diff --git a/app-next/src/components/dataset/dataset-edit-form.tsx b/app-next/src/components/dataset/dataset-edit-form.tsx
index 4a4270e5..eaba012f 100644
--- a/app-next/src/components/dataset/dataset-edit-form.tsx
+++ b/app-next/src/components/dataset/dataset-edit-form.tsx
@@ -2,6 +2,7 @@
 
 import { useState } from "react";
 import { useRouter } from "next/navigation";
+import { useLocale } from "next-intl";
 import Link from "next/link";
 import { ArrowLeft, Save, Loader2, AlertTriangle } from "lucide-react";
 import { Button } from "@/components/ui/button";
@@ -9,11 +10,14 @@ import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { Textarea } from "@/components/ui/textarea";
+import { useToast } from "@/hooks/use-toast";
 
 interface DatasetEditFormProps {
   datasetId: number;
   datasetName: string;
   isOwner: boolean;
+  hasApiKey: boolean;
+  isLocalUser: boolean;
   initialValues: {
     description: string;
     creator: string;
@@ -33,29 +37,27 @@ export function DatasetEditForm({
   datasetId,
   datasetName,
   isOwner,
+  hasApiKey,
+  isLocalUser,
   initialValues,
   features,
 }: DatasetEditFormProps) {
   const router = useRouter();
+  const locale = useLocale();
+  const { toast } = useToast();
   const [values, setValues] = useState(initialValues);
   const [saving, setSaving] = useState(false);
-  const [error, setError] = useState<string | null>(null);
-  const [success, setSuccess] = useState(false);
 
   const handleChange = (
     field: keyof typeof values,
     value: string,
   ) => {
     setValues((prev) => ({ ...prev, [field]: value }));
-    setError(null);
-    setSuccess(false);
   };
 
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     setSaving(true);
-    setError(null);
-    setSuccess(false);
 
     try {
       const res = await fetch(`/api/datasets/${datasetId}/edit`, {
@@ -72,14 +74,21 @@ export function DatasetEditForm({
         throw new Error(data.error || `Failed to save (${res.status})`);
       }
 
-      setSuccess(true);
-      // Redirect back to dataset page after short delay
+      toast({
+        title: "Changes saved",
+        description: "Redirecting back to dataset...",
+      });
+
       setTimeout(() => {
-        router.push(`/datasets/${datasetId}`);
+        router.push(`/${locale}/datasets/${datasetId}`);
         router.refresh();
       }, 1500);
     } catch (err) {
-      setError(err instanceof Error ? err.message : "Failed to save changes");
+      toast({
+        title: "Failed to save",
+        description: err instanceof Error ? err.message : "Failed to save changes",
+        variant: "destructive",
+      });
     } finally {
       setSaving(false);
     }
@@ -90,7 +99,7 @@ export function DatasetEditForm({
       {/* Header */}
       <div className="mb-8">
         <Link
-          href={`/datasets/${datasetId}`}
+          href={`/${locale}/datasets/${datasetId}`}
           className="text-muted-foreground hover:text-foreground mb-4 inline-flex items-center gap-1.5 text-sm transition-colors"
         >
           <ArrowLeft className="h-4 w-4" />
@@ -103,16 +112,18 @@ export function DatasetEditForm({
         </p>
       </div>
 
-      {/* Status messages */}
-      {error && (
-        <div className="mb-6 flex items-center gap-2 rounded-lg border border-red-200 bg-red-50 px-4 py-3 text-sm text-red-700 dark:border-red-900/50 dark:bg-red-950/30 dark:text-red-400">
-          <AlertTriangle className="h-4 w-4 shrink-0" />
-          {error}
-        </div>
-      )}
-      {success && (
-        <div className="mb-6 rounded-lg border border-green-200 bg-green-50 px-4 py-3 text-sm text-green-700 dark:border-green-900/50 dark:bg-green-950/30 dark:text-green-400">
-          Changes saved successfully! Redirecting...
+      {/* Warning: user has no valid OpenML API key (e.g. local dev account) */}
+      {(!hasApiKey || isLocalUser) && (
+        <div className="mb-6 flex items-start gap-3 rounded-lg border border-amber-200 bg-amber-50 px-4 py-3 text-amber-800 dark:border-amber-900 dark:bg-amber-950/30 dark:text-amber-300">
+          <AlertTriangle className="mt-0.5 size-5 shrink-0" />
+          <div className="space-y-1">
+            <p className="text-sm font-medium">Saving is unavailable in this environment</p>
+            <p className="text-xs">
+              {isLocalUser
+                ? "This account was created locally and does not have a valid OpenML API key. Dataset edits cannot be saved to the OpenML backend in a local development environment."
+                : "Your session does not include an OpenML API key. Saving changes requires signing in with a valid OpenML account."}
+            </p>
+          </div>
         </div>
       )}
 
@@ -284,12 +295,12 @@ export function DatasetEditForm({
 
       {/* Actions */}
       <div className="flex items-center justify-between">
-        <Link href={`/datasets/${datasetId}`}>
+        <Link href={`/${locale}/datasets/${datasetId}`}>
           <Button type="button" variant="outline">
             Cancel
           </Button>
         </Link>
-        <Button type="submit" disabled={saving} className="gap-2">
+        <Button type="submit" disabled={saving || !hasApiKey || isLocalUser} className="gap-2" title={(!hasApiKey || isLocalUser) ? "Saving is not available in this environment" : undefined}>
           {saving ? (
             <>
               <Loader2 className="h-4 w-4 animate-spin" />