diff --git a/exporter/file.ts b/exporter/file.ts
index 8a2ac70..1916db2 100644
--- a/exporter/file.ts
+++ b/exporter/file.ts
@@ -10,6 +10,15 @@ type FileExporterType = {
   data: VsBattle;
 };
 
+/**
+ * Don't save url in exported file
+ */
+function replacer(key: string, value: unknown): unknown {
+  return ["url", "maskImageUrl", "overlayImageUrl"].includes(key)
+    ? undefined
+    : value;
+}
+
 /**
  * Exporter to file.
  *
@@ -26,6 +35,10 @@ export class FileExporter implements BattleExporter<VsBattle> {
       fullId.slice(fullId.length - 52, fullId.length - 37),
     );
 
+    if (!/\d{8}T\d{6}/.test(ts)) {
+      throw new Error("Invalid battle ID");
+    }
+
     return `${ts}Z.json`;
   }
   async exportBattle(battle: VsBattle) {
@@ -42,7 +55,10 @@ export class FileExporter implements BattleExporter<VsBattle> {
       data: battle,
     };
 
-    await Deno.writeTextFile(filepath, JSON.stringify(body));
+    await Deno.writeTextFile(
+      filepath,
+      JSON.stringify(body, replacer),
+    );
   }
   async notExported(list: string[]): Promise<string[]> {
     const out: string[] = [];