diff --git a/src/lib/api-client.ts b/src/lib/api-client.ts
index e0f9783..e3b5563 100644
--- a/src/lib/api-client.ts
+++ b/src/lib/api-client.ts
@@ -40,6 +40,64 @@ export class ApiError extends Error {
   }
 }
 
+/**
+ * Turn an arbitrary thrown value from an API mutation into a human-readable,
+ * actionable toast description. Deployment QA kept reporting generic
+ * "Submit failed" / "Upload failed" toasts — this helper surfaces the HTTP
+ * status and, when we recognise the code, a hint about what to do next
+ * (payload too big → ask ops to raise nginx, timeout → retry, etc.).
+ *
+ * @param err     the error thrown by an `api.*` call (usually an ApiError)
+ * @param fallback text to show if we can't classify the error
+ * @param context short context word used in the 413/504 hints
+ *                ("file", "request", "payload" …)
+ */
+export function describeApiError(
+  err: unknown,
+  fallback = "Something went wrong",
+  context = "request",
+): string {
+  const e = err as { status?: number; message?: string; data?: unknown } | null;
+  const status = e?.status;
+  // Try to extract a server-side error body even if the generic ApiError
+  // message wasn't populated (nginx 413/504 respond with an HTML page so
+  // `data.error` is empty and `message` looks like "413 ").
+  const serverMsg =
+    e?.data && typeof e.data === "object" && "error" in (e.data as object)
+      ? String((e.data as { error: unknown }).error || "").trim()
+      : "";
+
+  if (status === 413) {
+    return `The ${context} is too large for the server to accept (HTTP 413). Try a smaller file / fewer tasks, or ask an admin to raise the nginx \`client_max_body_size\` and Odoo \`limit_request\`.`;
+  }
+  if (status === 504) {
+    return `The server took too long to respond (HTTP 504). The ${context} may still be processing — wait a minute and reload before retrying.`;
+  }
+  if (status === 502) {
+    return "The backend is unreachable (HTTP 502). Odoo may be restarting — try again in a moment.";
+  }
+  if (status === 401) {
+    return "Your session expired — please sign in again.";
+  }
+  if (status === 403) {
+    return serverMsg || "You don't have permission to do that (HTTP 403).";
+  }
+  if (status === 404) {
+    return serverMsg || "The requested item no longer exists (HTTP 404).";
+  }
+  if (status === 422 || status === 400) {
+    return serverMsg || `The server rejected the ${context} as invalid (HTTP ${status}).`;
+  }
+  if (status && status >= 500) {
+    return `${serverMsg || "The server returned an error"} (HTTP ${status}). Check the Odoo logs for a stack trace.`;
+  }
+
+  // Network failure / fetch aborted / CORS — ApiError not thrown at all.
+  if (!status && e?.message) return e.message;
+
+  return fallback;
+}
+
 const ACCESS_KEY = "encoach_token";
 const REFRESH_KEY = "encoach_refresh_token";
 const EXP_KEY = "encoach_token_exp";
diff --git a/src/pages/GenerationPage.tsx b/src/pages/GenerationPage.tsx
index 28a260e..c3b4ece 100644
--- a/src/pages/GenerationPage.tsx
+++ b/src/pages/GenerationPage.tsx
@@ -61,7 +61,7 @@ import AiTipBanner from "@/components/ai/AiTipBanner";
 import { generationService } from "@/services/generation.service";
 import { mediaService, type Avatar } from "@/services/media.service";
 import { examsService } from "@/services/exams.service";
-import { api } from "@/lib/api-client";
+import { api, describeApiError } from "@/lib/api-client";
 import { useToast } from "@/hooks/use-toast";
 import type { ExamStructure, ExamStructureConfig } from "@/types";
 
@@ -807,7 +807,12 @@ export default function GenerationPage() {
       description: `Exam #${res.exam_id} created with status "${res.status}". ${res.status === "published" ? "The exam is now live." : "Pending approval."}`,
       duration: 8000,
     }),
-    onError: (err: Error) => toast({ variant: "destructive", title: "Submit failed", description: err.message }),
+    onError: (err) =>
+      toast({
+        variant: "destructive",
+        title: "Submit failed",
+        description: describeApiError(err, "Submit failed", "module"),
+      }),
   });
 
   const anyGenerating = generatePassageMut.isPending || generateExercisesMut.isPending ||
diff --git a/src/pages/admin/CustomExamCreate.tsx b/src/pages/admin/CustomExamCreate.tsx
index 7828d42..3245566 100644
--- a/src/pages/admin/CustomExamCreate.tsx
+++ b/src/pages/admin/CustomExamCreate.tsx
@@ -17,6 +17,7 @@ import { Checkbox } from "@/components/ui/checkbox";
 import { Label } from "@/components/ui/label";
 import { ScrollArea } from "@/components/ui/scroll-area";
 import { useCreateCustomExam, useSaveAsTemplate } from "@/hooks/queries/useExamTemplates";
+import { describeApiError } from "@/lib/api-client";
 import { useSubjects } from "@/hooks/queries/useAdaptive";
 import type { CustomExamCreateRequest, CustomScoringMethod } from "@/types";
 import { toast } from "sonner";
@@ -139,14 +140,11 @@ export default function CustomExamCreate() {
     return raw?.id ?? raw?.exam_id ?? raw?.data?.id ?? raw?.data?.exam_id;
   };
 
-  const describeError = (err: unknown, fallback: string): string => {
-    const e = err as { status?: number; message?: string };
-    if (e?.status === 413) return "Payload too large (HTTP 413). Please reduce the number of questions per section and try again.";
-    if (e?.status === 504) return "Server took too long to respond (HTTP 504). Please retry in a moment.";
-    if (e?.status === 401) return "Session expired — please sign in again.";
-    if (e?.message) return `${fallback}: ${e.message}`;
-    return fallback;
-  };
+  // Thin wrapper around the shared helper so the existing call-sites below
+  // keep working; the helper lives in api-client so every page gets the same
+  // actionable 413/504/401 messaging.
+  const describeError = (err: unknown, fallback: string): string =>
+    describeApiError(err, fallback, "exam");
 
   const onSaveDraft = async () => {
     // Validate everything before firing the request — previously the Save
diff --git a/src/pages/admin/ResourceManager.tsx b/src/pages/admin/ResourceManager.tsx
index 2e3d404..4317ab1 100644
--- a/src/pages/admin/ResourceManager.tsx
+++ b/src/pages/admin/ResourceManager.tsx
@@ -17,6 +17,7 @@ import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { resourcesService } from "@/services/resources.service";
 import { coursewareService } from "@/services/courseware.service";
 import { taxonomyService } from "@/services/taxonomy.service";
+import { describeApiError } from "@/lib/api-client";
 import { useToast } from "@/hooks/use-toast";
 import { TaxonomyCascade } from "@/components/TaxonomyCascade";
 import type { Resource, ResourceTag } from "@/types";
@@ -274,17 +275,16 @@ export default function ResourceManager() {
   const uploadMutation = useMutation({
     mutationFn: (formData: FormData) => resourcesService.upload(formData),
     onSuccess: () => { qc.invalidateQueries({ queryKey: ["resources"] }); toast({ title: "Resource uploaded" }); setShowUpload(false); setUploadTagIds([]); setUploadSubjectId("none"); setUploadDomainId("none"); setUploadTopicIds([]); setUploadObjectiveIds([]); },
-    onError: (err: unknown) => {
-      // Surface status / server message so users can tell 413 (file too big)
-      // from 401 (session expired) from 500 (server error) without opening
-      // devtools. Default "Upload failed" hid real failures during QA.
-      const e = err as { status?: number; message?: string };
-      const status = e?.status;
-      let description = e?.message || "Upload failed";
-      if (status === 413) description = "File is too large for the server (HTTP 413). Ask an admin to raise nginx/odoo upload limits.";
-      else if (status === 401) description = "Session expired — please sign in again.";
-      else if (status === 504) description = "Server timed out while saving the file (HTTP 504). Try a smaller file or retry.";
-      toast({ title: "Upload failed", description, variant: "destructive" });
+    onError: (err) => {
+      // describeApiError surfaces status code + actionable next step so QA /
+      // ops can tell 413 (payload limit) from 504 (timeout) from 401 (session
+      // expired) without opening DevTools. Default "Upload failed" was
+      // reported as non-actionable during the Apr-2026 deployment.
+      toast({
+        title: "Upload failed",
+        description: describeApiError(err, "Upload failed", "file"),
+        variant: "destructive",
+      });
     },
   });
 
diff --git a/src/pages/teacher/TeacherLibrary.tsx b/src/pages/teacher/TeacherLibrary.tsx
index 78303ff..11c1748 100644
--- a/src/pages/teacher/TeacherLibrary.tsx
+++ b/src/pages/teacher/TeacherLibrary.tsx
@@ -15,6 +15,7 @@ import {
 } from "lucide-react";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { resourcesService } from "@/services/resources.service";
+import { describeApiError } from "@/lib/api-client";
 import { coursewareService } from "@/services/courseware.service";
 import { taxonomyService } from "@/services/taxonomy.service";
 import { useToast } from "@/hooks/use-toast";
@@ -116,14 +117,12 @@ export default function TeacherLibrary() {
       setUploadTopicIds([]);
       setUploadObjectiveIds([]);
     },
-    onError: (err: unknown) => {
-      const e = err as { status?: number; message?: string };
-      const status = e?.status;
-      let description = e?.message || "Upload failed";
-      if (status === 413) description = "File is too large for the server (HTTP 413). Try a smaller file or ask an admin to raise the upload limit.";
-      else if (status === 401) description = "Session expired — please sign in again.";
-      else if (status === 504) description = "Server timed out while saving the file (HTTP 504). Try a smaller file or retry.";
-      toast({ title: "Upload failed", description, variant: "destructive" });
+    onError: (err) => {
+      toast({
+        title: "Upload failed",
+        description: describeApiError(err, "Upload failed", "file"),
+        variant: "destructive",
+      });
     },
   });