diff --git a/src/lib/prompt-filename.ts b/src/lib/prompt-filename.ts
new file mode 100644
index 0000000..5dcaee2
--- /dev/null
+++ b/src/lib/prompt-filename.ts
@@ -0,0 +1,17 @@
+/**
+ * Prompts the user for a filename before download. Returns the chosen base
+ * name (without extension) sanitized for filesystem use, or null if cancelled.
+ *
+ * @param defaultName Suggested base name (no extension)
+ * @param ext Extension shown to the user, e.g. "pdf", "docx", "csv"
+ */
+export function promptFilename(defaultName: string, ext: string): string | null {
+  const sanitize = (s: string) => s.replace(/[\\/:*?"<>|]+/g, "_").trim();
+  const cleanDefault = sanitize(defaultName).replace(new RegExp(`\\.${ext}$`, "i"), "") || "Document";
+  const input = typeof window !== "undefined"
+    ? window.prompt(`File name (.${ext})`, cleanDefault)
+    : cleanDefault;
+  if (input === null) return null; // user cancelled
+  const cleaned = sanitize(input).replace(new RegExp(`\\.${ext}$`, "i"), "");
+  return cleaned || cleanDefault;
+}