diff --git a/src/types/index.ts b/src/types/index.ts
index 0942af8..919430d 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -71,6 +71,7 @@ export interface WebSubSubscription {
   callbackUrl: string;
   secret?: string;
   expiresAt: number; // Unix timestamp ms
+  format?: "rss" | "atom";
 }
 
 // Declare KVNamespace for TypeScript