diff --git a/frontend/index.html b/frontend/index.html
index 4313922..81f2911 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -367,6 +367,9 @@ <h5 class="modal-title d-flex align-items-center gap-2">
                                 </div>
                             </div>
                             <input type="file" class="form-control form-control-sm" id="fileInput" accept=".csv">
+                            <div class="form-text small mt-1">
+                                <i class="bi bi-info-circle"></i> Max 20 MB &middot; runs in your browser
+                            </div>
                             <div id="uploadPreview" class="mt-2" style="display: none;">
                                 <small class="text-muted">Preview:</small>
                                 <div class="table-responsive" style="max-height: 150px;">
diff --git a/frontend/js/app.js b/frontend/js/app.js
index 0641d05..4bbeacd 100644
--- a/frontend/js/app.js
+++ b/frontend/js/app.js
@@ -444,7 +444,7 @@ async function uploadFile() {
     try {
         const file = fileInput.files[0];
         if (file.size > 20 * 1024 * 1024) {
-            throw new Error('File exceeds 20 MB cap — Pyodide runs out of memory on large CSVs.');
+            throw new Error('File is too large for the browser runtime — please subsample first (max 20 MB).');
         }
         const buf = new Uint8Array(await file.arrayBuffer());
         const data = await pyCallBinary('upload_csv', buf, [file.name]);