fix(coding-agent): avoid empty compaction summaries

closes #4811

packages/coding-agent/src/core/compaction/compaction.ts

@@ -698,6 +698,10 @@ export function prepareCompaction(
 		}
 	}
 
+	if (messagesToSummarize.length === 0 && turnPrefixMessages.length === 0) {
+		return undefined;
+	}
+
 	// Extract file operations from messages and previous compaction
 	const fileOps = extractFileOperations(messagesToSummarize, pathEntries, prevCompactionIndex);