diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index 45b7dfd..68909d4 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -15,6 +15,7 @@
         "react-dom": "latest",
         "react-quick-pinch-zoom": "latest",
         "recharts": "latest",
+        "sonner": "^2.0.7",
         "vite": "latest"
       },
       "devDependencies": {
@@ -3027,6 +3028,16 @@
       "dev": true,
       "license": "ISC"
     },
+    "node_modules/sonner": {
+      "version": "2.0.7",
+      "resolved": "https://package-mirror.liara.ir/repository/npm/sonner/-/sonner-2.0.7.tgz",
+      "integrity": "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
+        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
+      }
+    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://package-mirror.liara.ir/repository/npm/source-map-js/-/source-map-js-1.2.1.tgz",
diff --git a/frontend/package.json b/frontend/package.json
index f4d46a0..ffb53e5 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -17,7 +17,8 @@
     "react-quick-pinch-zoom": "latest",
     "recharts": "latest",
     "lucide-react": "latest",
-    "prop-types": "latest"
+    "prop-types": "latest",
+    "sonner": "^2.0.7"
   },
   "devDependencies": {
     "tailwindcss": "3.4.17",
diff --git a/frontend/src/App.jsx b/frontend/src/App.jsx
index 4c0b079..6b04d35 100644
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -1,9 +1,18 @@
 import { useEffect, useMemo, useState } from "react";
+import { Toaster, toast } from "sonner";
 import CanvasPane, { CanvasThumbnail } from "./components/CanvasPane.jsx";
 import Controls from "./components/Controls.jsx";
 import HistogramPanel from "./components/HistogramPanel.jsx";
 import { applyStateOperation, combineStates, deleteState, getOperations, listStates, uploadImage } from "./lib/api.js";
 
+function showErrorToast(error, fallback = "Something went wrong.") {
+  const message = error?.message || fallback;
+  toast.error(message, {
+    description: "Please check the selected image, operation, and parameters."
+  });
+  return message;
+}
+
 export default function App() {
   const [session, setSession] = useState(null);
   const [states, setStates] = useState([]);
@@ -26,7 +35,7 @@ export default function App() {
           setParams({ _repeat: 1, ...Object.fromEntries(Object.entries(first.params || {}).map(([key, schema]) => [key, schema.default])) });
         }
       })
-      .catch((error) => setStatus(error.message));
+      .catch((error) => setStatus(showErrorToast(error, "Unable to load operations.")));
   }, []);
 
   async function refreshStates(sessionId, nextActiveId = null) {
@@ -51,7 +60,7 @@ export default function App() {
       setTransform({ x: 0, y: 0, scale: 1 });
       setStatus(`${payload.width} x ${payload.height} ${payload.color_mode} image loaded as S0.`);
     } catch (error) {
-      setStatus(error.message);
+      setStatus(showErrorToast(error, "Unable to upload image."));
     } finally {
       setBusy(false);
     }
@@ -76,7 +85,7 @@ export default function App() {
       setSelectedStateIds([state.state_id]);
       setStatus(`${state.label} created.`);
     } catch (error) {
-      setStatus(error.message);
+      setStatus(showErrorToast(error, "Unable to apply operation."));
     } finally {
       setBusy(false);
     }
@@ -92,7 +101,7 @@ export default function App() {
       setSelectedStateIds([state.state_id]);
       setStatus(`${state.label} created.`);
     } catch (error) {
-      setStatus(error.message);
+      setStatus(showErrorToast(error, "Unable to combine states."));
     } finally {
       setBusy(false);
     }
@@ -117,7 +126,7 @@ export default function App() {
       setSelectedStateIds((current) => current.filter((id) => id !== state.state_id));
       setStatus(`${state.label} deleted.`);
     } catch (error) {
-      setStatus(error.message);
+      setStatus(showErrorToast(error, "Unable to delete state."));
     } finally {
       setBusy(false);
     }
@@ -131,6 +140,7 @@ export default function App() {
 
   return (
     <div className="flex h-screen overflow-hidden bg-zinc-950 text-zinc-100">
+      <Toaster richColors theme="dark" position="top-right" closeButton />
       <Controls
         operations={operations}
         selectedOperation={selectedOperation}
diff --git a/frontend/src/lib/api.js b/frontend/src/lib/api.js
index 199812d..08ab1d1 100644
--- a/frontend/src/lib/api.js
+++ b/frontend/src/lib/api.js
@@ -1,9 +1,25 @@
 const API_BASE = import.meta.env.VITE_API_BASE || "";
 
+function humanizeErrorDetail(detail) {
+  if (!detail) return "Request failed";
+  if (typeof detail === "string") return detail;
+  if (Array.isArray(detail)) return detail.map(humanizeErrorDetail).join(" ");
+  if (typeof detail === "object") {
+    return Object.entries(detail)
+      .map(([key, value]) => {
+        const message = humanizeErrorDetail(value);
+        return key === "non_field_errors" || key === "detail" ? message : `${key}: ${message}`;
+      })
+      .filter(Boolean)
+      .join(" ");
+  }
+  return String(detail);
+}
+
 async function parseResponse(response) {
   const payload = await response.json().catch(() => ({}));
   if (!response.ok) {
-    throw new Error(payload.detail || "Request failed");
+    throw new Error(humanizeErrorDetail(payload.detail || payload));
   }
   return payload;
 }