diff --git a/api/budget.ts b/api/budget.ts
index 38eca1a..8dc7311 100644
--- a/api/budget.ts
+++ b/api/budget.ts
@@ -801,10 +801,49 @@ const FORMAT_LABEL: Record<ParsedBudget["format"], string> = {
   none: "No reconocido",
 };
 
+export const EXCEL_ONLY_MESSAGE = "Solo se aceptan archivos Excel (.xlsx o .xls).";
+export const BUDGET_FORMAT_MESSAGE =
+  "El Excel no tiene el formato de presupuesto esperado. Use la plantilla del panel o un archivo Opus/Neodata con columnas CLAVE, DESCRIPCION, UNIDAD, CANTIDAD y PRECIO.";
+
+export function excelFileTypeError(filename: string): string | null {
+  const name = filename.trim();
+  if (/\.(xlsx|xls)$/i.test(name)) return null;
+  const shown = name || "sin nombre";
+  return `${EXCEL_ONLY_MESSAGE} «${shown}» no es un Excel.`;
+}
+
+export function budgetReadErrorMessage(preview: { errors: { messages: string[] }[] }): string {
+  return preview.errors[0]?.messages?.[0] || BUDGET_FORMAT_MESSAGE;
+}
+
+function startsWith(bytes: Uint8Array, sig: number[]): boolean {
+  if (bytes.length < sig.length) return false;
+  return sig.every((value, i) => bytes[i] === value);
+}
+
+function unrecognizedBinaryMessage(bytes: Uint8Array): string | null {
+  if (startsWith(bytes, [0x25, 0x50, 0x44, 0x46])) {
+    return "El archivo es un PDF, no un Excel. Suba un .xlsx o .xls con el formato de presupuesto.";
+  }
+  if (startsWith(bytes, [0xff, 0xd8, 0xff]) || startsWith(bytes, [0x89, 0x50, 0x4e, 0x47])) {
+    return "El archivo es una imagen, no un Excel. Suba un .xlsx o .xls.";
+  }
+  return null;
+}
+
 function readBudgetSheet(bytes: Uint8Array): { sheet: string; rows: unknown[][] } | { error: string } {
-  const wb = XLSX.read(bytes, { type: "array" });
+  const binaryError = unrecognizedBinaryMessage(bytes);
+  if (binaryError) return { error: binaryError };
+  let wb: XLSX.WorkBook;
+  try {
+    wb = XLSX.read(bytes, { type: "array" });
+  } catch {
+    return {
+      error: "No se pudo abrir el archivo. Suba un Excel .xlsx o .xls con el formato de presupuesto (plantilla del panel u Opus/Neodata).",
+    };
+  }
   const sheet = wb.SheetNames.find((name) => /presupuesto/i.test(name)) || wb.SheetNames[0];
-  if (!sheet) return { error: "El archivo no tiene hojas" };
+  if (!sheet) return { error: "El archivo Excel no tiene hojas. Revise que no esté vacío o dañado." };
   return { sheet, rows: XLSX.utils.sheet_to_json(wb.Sheets[sheet], { header: 1, defval: "" }) as unknown[][] };
 }
 
@@ -863,7 +902,7 @@ export function previewBudgetExcel(bytes: Uint8Array): BudgetPreview {
   const iva = Math.round(subtotal * BUDGET_IVA * 100) / 100;
   const errors = parsed.items.length
     ? []
-    : [{ row: 0, messages: ["No se encontraron partidas. Revise que el Excel tenga Código/Concepto o la plantilla WBS."] }];
+    : [{ row: 0, messages: [BUDGET_FORMAT_MESSAGE] }];
   return {
     format: parsed.format,
     format_label: FORMAT_LABEL[parsed.format],
@@ -902,7 +941,7 @@ export async function importBudgetExcel(database: Db, projectId: number, bytes:
       chapters: 0,
       inserted: 0,
       skipped: parsed.skipped,
-      errors: [{ row: 0, messages: ["No se encontraron partidas. Use la plantilla o un Excel con CLAVE, DESCRIPCION, UNIDAD, CANTIDAD y PRECIO."] }],
+      errors: [{ row: 0, messages: [BUDGET_FORMAT_MESSAGE] }],
     };
   }
   await replaceBudgetFromParsed(database, projectId, parsed);
diff --git a/api/budget_test.ts b/api/budget_test.ts
index b728114..a946ec1 100644
--- a/api/budget_test.ts
+++ b/api/budget_test.ts
@@ -1,6 +1,13 @@
 import { assertEquals, assertStringIncludes } from "jsr:@std/assert@1";
 import * as XLSX from "xlsx";
-import { buildBudgetTemplate, parseBudgetRows, parseMoney, previewBudgetExcel } from "./budget.ts";
+import {
+  BUDGET_FORMAT_MESSAGE,
+  buildBudgetTemplate,
+  excelFileTypeError,
+  parseBudgetRows,
+  parseMoney,
+  previewBudgetExcel,
+} from "./budget.ts";
 
 Deno.test("parsea partidas estilo CLAVE DESCRIPCION UNIDAD", () => {
   const parsed = parseBudgetRows([
@@ -96,3 +103,25 @@ Deno.test("preview de plantilla no guarda y arma arbol", () => {
   assertEquals(preview.tree[0]?.children.length, 2);
   assertEquals(preview.totals.item_count, 4);
 });
+
+Deno.test("rechaza archivos que no son Excel por nombre", () => {
+  assertEquals(excelFileTypeError("obra.pdf")?.includes(".xlsx"), true);
+  assertEquals(excelFileTypeError("notas.docx")?.includes("«notas.docx»"), true);
+  assertEquals(excelFileTypeError("presupuesto.xlsx"), null);
+  assertEquals(excelFileTypeError("PRESUPUESTO.XLS"), null);
+});
+
+Deno.test("preview de PDF explica que no es Excel", () => {
+  const preview = previewBudgetExcel(new Uint8Array([0x25, 0x50, 0x44, 0x46, 0x2d, 0x31, 0x2e, 0x34]));
+  assertEquals(preview.totals.item_count, 0);
+  assertStringIncludes(preview.errors[0]?.messages[0] || "", "PDF");
+});
+
+Deno.test("preview de Excel sin columnas de presupuesto explica el formato", () => {
+  const wb = XLSX.utils.book_new();
+  XLSX.utils.book_append_sheet(wb, XLSX.utils.aoa_to_sheet([["Nombre", "Notas"], ["Juan", "sin partidas"]]), "Hoja1");
+  const bytes = XLSX.write(wb, { type: "array", bookType: "xlsx" });
+  const preview = previewBudgetExcel(bytes instanceof Uint8Array ? bytes : new Uint8Array(bytes));
+  assertEquals(preview.totals.item_count, 0);
+  assertEquals(preview.errors[0]?.messages[0], BUDGET_FORMAT_MESSAGE);
+});
diff --git a/api/main.ts b/api/main.ts
index 4617023..3800628 100644
--- a/api/main.ts
+++ b/api/main.ts
@@ -73,6 +73,8 @@ import { registerIamRoutes } from "./iam_http.ts";
 import { createSubcompany, listCompanies, resolveCompany, updateCompany, companyById } from "./companies.ts";
 import {
   buildBudgetTemplate,
+  budgetReadErrorMessage,
+  excelFileTypeError,
   exportBudgetWorkbook,
   previewBudgetExcel,
   importBudgetExcel,
@@ -125,6 +127,17 @@ async function rpcDocumentForDownload(
   return { doc, envelope };
 }
 
+async function readExcelUpload(
+  c: { req: { formData: () => Promise<FormData> } },
+): Promise<{ file: File; bytes: Uint8Array } | { error: string }> {
+  const form = await c.req.formData();
+  const file = form.get("file");
+  if (!(file instanceof File)) return { error: "Seleccione un archivo Excel (.xlsx o .xls)." };
+  const typeError = excelFileTypeError(file.name);
+  if (typeError) return { error: typeError };
+  return { file, bytes: new Uint8Array(await file.arrayBuffer()) };
+}
+
 async function scopedCompany(
   db: Db,
   id: number,
@@ -738,40 +751,59 @@ app.get("/v1/projects/:id/budget/export", ...requireCoreAuth, requirePermission(
 });
 
 app.post("/v1/budget/preview", ...requireCoreAuth, requirePermission("budget.create"), async (c) => {
-  const form = await c.req.formData();
-  const file = form.get("file");
-  if (!(file instanceof File)) return c.json({ error: "Archivo Excel requerido" }, 400);
-  const bytes = new Uint8Array(await file.arrayBuffer());
-  const preview = previewBudgetExcel(bytes);
-  if (preview.errors.length && !preview.items.length) return c.json(preview, 400);
+  const upload = await readExcelUpload(c);
+  if ("error" in upload) {
+    return respondApiError(c, "VALIDATION", upload.error, { route: routeLabel(c) });
+  }
+  const preview = previewBudgetExcel(upload.bytes);
+  if (preview.errors.length && !preview.items.length) {
+    return respondApiError(c, "VALIDATION", budgetReadErrorMessage(preview), {
+      route: routeLabel(c),
+      filename: upload.file.name,
+      format: preview.format,
+    }, preview.errors.flatMap((row) => row.messages));
+  }
   return c.json(preview);
 });
 
 app.post("/v1/projects/:id/budget/preview", ...requireCoreAuth, requirePermission("budget.create"), async (c) => {
   const id = Number(c.req.param("id"));
   if (!await denyUnlessProjectScope(c, id)) return;
-  const form = await c.req.formData();
-  const file = form.get("file");
-  if (!(file instanceof File)) return c.json({ error: "Archivo Excel requerido" }, 400);
+  const upload = await readExcelUpload(c);
+  if ("error" in upload) {
+    return respondApiError(c, "VALIDATION", upload.error, { route: routeLabel(c), project_id: id });
+  }
   const db = c.get("db");
   if (!await projectById(db, id)) return c.json({ error: "Proyecto no encontrado" }, 404);
-  const bytes = new Uint8Array(await file.arrayBuffer());
-  const preview = previewBudgetExcel(bytes);
-  if (preview.errors.length && !preview.items.length) return c.json(preview, 400);
+  const preview = previewBudgetExcel(upload.bytes);
+  if (preview.errors.length && !preview.items.length) {
+    return respondApiError(c, "VALIDATION", budgetReadErrorMessage(preview), {
+      route: routeLabel(c),
+      project_id: id,
+      filename: upload.file.name,
+      format: preview.format,
+    }, preview.errors.flatMap((row) => row.messages));
+  }
   return c.json(preview);
 });
 
 app.post("/v1/projects/:id/budget/import", ...requireCoreAuth, requirePermission("budget.update"), async (c) => {
   const id = Number(c.req.param("id"));
   if (!await denyUnlessProjectScope(c, id)) return;
-  const form = await c.req.formData();
-  const file = form.get("file");
-  if (!(file instanceof File)) return c.json({ error: "Archivo Excel requerido" }, 400);
+  const upload = await readExcelUpload(c);
+  if ("error" in upload) {
+    return respondApiError(c, "VALIDATION", upload.error, { route: routeLabel(c), project_id: id });
+  }
   const db = c.get("db");
   if (!await projectById(db, id)) return c.json({ error: "Proyecto no encontrado" }, 404);
-  const bytes = new Uint8Array(await file.arrayBuffer());
-  const report = await importBudgetExcel(db, id, bytes);
-  if (report.errors.length && !report.inserted) return c.json(report, 400);
+  const report = await importBudgetExcel(db, id, upload.bytes);
+  if (report.errors.length && !report.inserted) {
+    return respondApiError(c, "VALIDATION", budgetReadErrorMessage(report), {
+      route: routeLabel(c),
+      project_id: id,
+      filename: upload.file.name,
+    }, report.errors.flatMap((row) => row.messages));
+  }
   return c.json(report);
 });
 
diff --git a/web-panel/components/BudgetImportDialog.vue b/web-panel/components/BudgetImportDialog.vue
index 146d63d..87f9845 100644
--- a/web-panel/components/BudgetImportDialog.vue
+++ b/web-panel/components/BudgetImportDialog.vue
@@ -9,16 +9,17 @@
     :content-style="{ overflow: 'auto', maxHeight: 'calc(100vh - 9rem)' }"
   >
     <Message severity="info" :closable="false">
-      Suba el Excel de Opus/Neodata, uno plano o la plantilla del panel. Primero se muestra lo encontrado;
+      Suba un Excel .xlsx o .xls (Opus/Neodata, plano o la plantilla del panel). Primero se muestra lo encontrado;
       nada se guarda hasta que confirme. Reemplazará el presupuesto de <strong>{{ projectName }}</strong>.
     </Message>
 
     <div class="flex flex-wrap align-items-center gap-2 mt-3">
       <Button label="Descargar plantilla" icon="pi pi-download" outlined :loading="templateLoading" @click="downloadTemplate" />
-      <input ref="fileInput" type="file" accept=".xlsx,.xls,application/vnd.ms-excel,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" class="hidden-file" @change="onNativeFile" />
+      <input ref="fileInput" type="file" :accept="EXCEL_ACCEPT" class="hidden-file" @change="onNativeFile" />
       <Button label="Seleccionar Excel" icon="pi pi-plus" :disabled="busy" @click="pickFile" />
       <span v-if="pending" class="muted">{{ pending.name }}</span>
     </div>
+    <div class="muted mt-2">Solo archivos Excel (.xlsx o .xls). Otros tipos se rechazan antes de subirlos.</div>
     <ProgressBar v-if="busy" mode="indeterminate" class="mt-3" style="height: 4px" />
 
     <template v-if="preview">
@@ -82,6 +83,8 @@
 
 <script setup lang="ts">
 import type { TreeNode } from "primevue/treenode";
+import { displayErrorMessage } from "~/composables/api-response";
+import { EXCEL_ACCEPT, excelOnlyMessage, isExcelSpreadsheet } from "~/utils/excelFile";
 
 type PreviewNode = {
   wbs: string;
@@ -172,10 +175,21 @@ function onNativeFile(event: Event) {
   if (file) readFile(file);
 }
 
+function showReadError(message: string, summary = "No se pudo importar el presupuesto") {
+  error.value = message;
+  toast.add({ severity: "error", summary, detail: message, life: 5000 });
+}
+
 async function readFile(file: File) {
   if (!file) return;
   if (!props.projectId) {
-    error.value = "Seleccione una obra en el árbol antes de importar.";
+    showReadError("Seleccione una obra en el árbol antes de importar.");
+    return;
+  }
+  if (!isExcelSpreadsheet(file)) {
+    pending.value = null;
+    preview.value = null;
+    showReadError(excelOnlyMessage(file.name), "Archivo no permitido");
     return;
   }
   reading.value = true;
@@ -187,13 +201,18 @@ async function readFile(file: File) {
     body.append("file", file);
     preview.value = await api<Preview>(`/v1/projects/${props.projectId}/budget/preview`, { method: "POST", body });
     if (preview.value.errors?.length && !preview.value.totals.item_count) {
-      error.value = preview.value.errors[0]?.messages?.[0] || "No se pudo leer el Excel";
+      const message = preview.value.errors[0]?.messages?.[0] || "El Excel no tiene el formato de presupuesto esperado.";
+      pending.value = null;
+      preview.value = null;
+      showReadError(message, "Formato de Excel no válido");
     }
   } catch (err) {
     pending.value = null;
-    const detail = err as { data?: { errors?: { messages: string[] }[] } };
-    error.value = detail.data?.errors?.[0]?.messages?.[0] || (err instanceof Error ? err.message : "No se pudo leer el Excel");
-    toast.add({ severity: "error", summary: error.value, life: 3500 });
+    const detail = err as { data?: unknown };
+    showReadError(
+      displayErrorMessage(detail.data ?? err, err instanceof Error ? err.message : "No se pudo leer el Excel"),
+      "Formato de Excel no válido",
+    );
   } finally {
     reading.value = false;
   }
@@ -217,8 +236,9 @@ async function confirmSave() {
     emit("imported");
     visibleModel.value = false;
   } catch (err) {
-    error.value = err instanceof Error ? err.message : "No se pudo guardar";
-    toast.add({ severity: "error", summary: error.value, life: 3500 });
+    const detail = err as { data?: unknown };
+    error.value = displayErrorMessage(detail.data ?? err, err instanceof Error ? err.message : "No se pudo guardar el presupuesto");
+    toast.add({ severity: "error", summary: "No se pudo guardar el presupuesto", detail: error.value, life: 5000 });
   } finally {
     saving.value = false;
   }
diff --git a/web-panel/composables/api-response.ts b/web-panel/composables/api-response.ts
index 786c95e..8c445c2 100644
--- a/web-panel/composables/api-response.ts
+++ b/web-panel/composables/api-response.ts
@@ -66,14 +66,43 @@ export function unwrapApiPayload<T>(body: unknown): T {
   } as T;
 }
 
+function firstListedError(value: object): string | null {
+  if (!("errors" in value) || value.errors == null) return null;
+  const errors = (value as { errors: unknown }).errors;
+  if (typeof errors === "string" && errors.trim()) return errors;
+  if (!Array.isArray(errors)) return null;
+  for (const item of errors) {
+    if (typeof item === "string" && item.trim()) return item;
+    if (item && typeof item === "object" && "messages" in item) {
+      const msgs = (item as { messages: unknown }).messages;
+      if (Array.isArray(msgs)) {
+        const first = msgs.find((msg) => typeof msg === "string" && msg.trim());
+        if (typeof first === "string") return first;
+      }
+    }
+  }
+  return null;
+}
+
 /** Muestra el mensaje detallado de la API o front sin reescribirlo. */
 export function displayErrorMessage(value: unknown, fallback = "Ocurrió un error"): string {
-  if (isApiEnvelope(value)) return value.message;
-  if (value && typeof value === "object" && "message" in value) {
-    return String((value as { message: unknown }).message);
+  if (isApiEnvelope(value)) {
+    const listed = firstListedError(value);
+    const message = value.message?.trim();
+    if (message) return message;
+    if (listed) return listed;
   }
-  if (value && typeof value === "object" && "error" in value) {
-    return String((value as { error: unknown }).error);
+  if (value && typeof value === "object") {
+    if ("message" in value) {
+      const message = String((value as { message: unknown }).message || "").trim();
+      if (message) return message;
+    }
+    if ("error" in value) {
+      const error = String((value as { error: unknown }).error || "").trim();
+      if (error) return error;
+    }
+    const listed = firstListedError(value);
+    if (listed) return listed;
   }
-  return fallback;
+  return fallback || "Ocurrió un error";
 }
diff --git a/web-panel/pages/proyectos.vue b/web-panel/pages/proyectos.vue
index a23d71c..c9747bc 100644
--- a/web-panel/pages/proyectos.vue
+++ b/web-panel/pages/proyectos.vue
@@ -124,7 +124,7 @@
               />
               <div v-if="readingBudget" class="muted mt-1">Leyendo Excel…</div>
               <div v-else-if="presupuestoFile" class="muted mt-1">{{ presupuestoFile.name }}</div>
-              <div v-else class="muted mt-1">PDF o Excel. Si es Excel, el monto de contrato se calcula solo.</div>
+              <div v-else class="muted mt-1">PDF para el expediente, o Excel (.xlsx / .xls) para cargar partidas y el monto.</div>
             </div>
           </FormField>
         </div>
@@ -142,6 +142,8 @@
 import FormField from "~/components/padron/FormField.vue";
 import type { CompanyOption } from "~/types/padron";
 import { companyLabel } from "~/types/padron";
+import { displayErrorMessage } from "~/composables/api-response";
+import { isExcelSpreadsheet } from "~/utils/excelFile";
 
 type Project = Record<string, unknown> & {
   id: number;
@@ -289,9 +291,7 @@ function onPickContrato(event: { files: File[] }) {
 }
 
 function isExcelFile(file: File) {
-  const name = file.name.toLowerCase();
-  return name.endsWith(".xlsx") || name.endsWith(".xls") ||
-    file.type.includes("spreadsheet") || file.type.includes("excel");
+  return isExcelSpreadsheet(file);
 }
 
 function moneyMx(value: number) {
@@ -302,7 +302,8 @@ async function onPickPresupuesto(event: { files: File[] }) {
   const file = event.files?.[0] || null;
   presupuestoFile.value = file;
   budgetHint.value = "";
-  if (!file || !isExcelFile(file)) return;
+  if (!file) return;
+  if (!isExcelFile(file)) return;
   readingBudget.value = true;
   try {
     const body = new FormData();
@@ -310,6 +311,7 @@ async function onPickPresupuesto(event: { files: File[] }) {
     const preview = await api<{
       totals: { subtotal: number; iva: number; total: number; item_count: number };
       format_label?: string;
+      errors?: { messages: string[] }[];
     }>("/v1/budget/preview", { method: "POST", body });
     const subtotal = Number(preview.totals?.subtotal || 0);
     if (subtotal > 0) {
@@ -321,14 +323,23 @@ async function onPickPresupuesto(event: { files: File[] }) {
         detail: `Subtotal del presupuesto: ${moneyMx(subtotal)}`,
         life: 2800,
       });
+    } else {
+      const message = preview.errors?.[0]?.messages?.[0]
+        || "El Excel no tiene el formato de presupuesto esperado. Use la plantilla del panel u Opus/Neodata.";
+      toast.add({
+        severity: "warn",
+        summary: "Excel sin partidas",
+        detail: message,
+        life: 4500,
+      });
     }
   } catch (error) {
-    const detail = error as { data?: { error?: string } };
+    const detail = error as { data?: unknown };
     toast.add({
-      severity: "warn",
+      severity: "error",
       summary: "No se pudo leer el Excel",
-      detail: detail.data?.error || (error instanceof Error ? error.message : "Revise el formato"),
-      life: 3500,
+      detail: displayErrorMessage(detail.data ?? error, error instanceof Error ? error.message : "Revise que sea .xlsx o .xls con el formato de presupuesto"),
+      life: 4500,
     });
   } finally {
     readingBudget.value = false;
@@ -367,12 +378,12 @@ async function uploadDocs(projectId: number) {
         });
       }
     } catch (error) {
-      const detail = error as { data?: { error?: string } };
+      const detail = error as { data?: unknown };
       toast.add({
         severity: "warn",
         summary: "Documento guardado, pero no se importaron partidas",
-        detail: detail.data?.error || (error instanceof Error ? error.message : "Revise el Excel"),
-        life: 4000,
+        detail: displayErrorMessage(detail.data ?? error, error instanceof Error ? error.message : "Revise el formato del Excel"),
+        life: 4500,
       });
     }
   }
diff --git a/web-panel/utils/excelFile.ts b/web-panel/utils/excelFile.ts
new file mode 100644
index 0000000..750d8ba
--- /dev/null
+++ b/web-panel/utils/excelFile.ts
@@ -0,0 +1,13 @@
+export const EXCEL_ACCEPT = ".xlsx,.xls,application/vnd.ms-excel,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
+
+export function isExcelSpreadsheet(file: File): boolean {
+  const name = file.name.toLowerCase();
+  return name.endsWith(".xlsx") || name.endsWith(".xls");
+}
+
+export function excelOnlyMessage(fileName?: string): string {
+  const name = fileName?.trim();
+  return name
+    ? `Solo se aceptan archivos Excel (.xlsx o .xls). «${name}» no es un Excel.`
+    : "Solo se aceptan archivos Excel (.xlsx o .xls).";
+}