diff --git a/server/service/agent-tool/executors/document/document.ts b/server/service/agent-tool/executors/document/document.ts
index 874fd5a..15e0102 100644
--- a/server/service/agent-tool/executors/document/document.ts
+++ b/server/service/agent-tool/executors/document/document.ts
@@ -29,6 +29,10 @@ export const documentInputSchema = z.object({
 
 type DocumentInput = z.infer<typeof documentInputSchema>;
 
+function toISO(date: Date | null | undefined): string | null {
+  return date ? date.toISOString() : null;
+}
+
 export const documentExecutor: ToolExecutor<DocumentToolConfig> = {
   buildInputSchema(_config: DocumentToolConfig): JSONSchema7 {
     return {
@@ -129,8 +133,8 @@ export const documentExecutor: ToolExecutor<DocumentToolConfig> = {
               title: doc.title,
               summary: doc.summary,
               tags: deserializeTags(doc.tags),
-              createdAt: doc.createdAt,
-              updatedAt: doc.updatedAt,
+              createdAt: toISO(doc.createdAt),
+              updatedAt: toISO(doc.updatedAt),
             },
             metadata: { durationMs: Date.now() - start },
           };
@@ -160,8 +164,8 @@ export const documentExecutor: ToolExecutor<DocumentToolConfig> = {
             title: doc.title,
             summary: doc.summary,
             tags: deserializeTags(doc.tags),
-            createdAt: doc.createdAt,
-            updatedAt: doc.updatedAt,
+            createdAt: toISO(doc.createdAt),
+            updatedAt: toISO(doc.updatedAt),
           };
           if (data.fullContent) {
             result.content = doc.content;
@@ -211,8 +215,8 @@ export const documentExecutor: ToolExecutor<DocumentToolConfig> = {
               title: updated.title,
               summary: updated.summary,
               tags: deserializeTags(updated.tags),
-              createdAt: updated.createdAt,
-              updatedAt: updated.updatedAt,
+              createdAt: toISO(updated.createdAt),
+              updatedAt: toISO(updated.updatedAt),
             },
             metadata: { durationMs: Date.now() - start },
           };
@@ -260,8 +264,8 @@ export const documentExecutor: ToolExecutor<DocumentToolConfig> = {
                 title: item.title,
                 summary: item.summary,
                 tags: deserializeTags(item.tags),
-                createdAt: item.createdAt,
-                updatedAt: item.updatedAt,
+                createdAt: toISO(item.createdAt),
+                updatedAt: toISO(item.updatedAt),
               })),
               total: result.total,
               page: result.page,