diff --git a/src/index.ts b/src/index.ts
index 5be2ebd..ffb01cb 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -522,12 +522,14 @@ async function startMessageLoop(): Promise<void> {
       for (const msg of messages) {
         try {
           await processMessage(msg);
+          // Only advance timestamp after successful processing for at-least-once delivery
+          lastTimestamp = msg.timestamp;
+          saveState();
         } catch (err) {
-          logger.error({ err, msg: msg.id }, 'Error processing message');
+          logger.error({ err, msg: msg.id }, 'Error processing message, will retry');
+          // Stop processing this batch - failed message will be retried next loop
+          break;
         }
-        // Advance timestamp after each message to avoid reprocessing on retry
-        lastTimestamp = msg.timestamp;
-        saveState();
       }
     } catch (err) {
       logger.error({ err }, 'Error in message loop');