diff --git a/server/api/public/comments-create-body.test.ts b/server/api/public/comments-create-body.test.ts
index 1051403..8119aeb 100644
--- a/server/api/public/comments-create-body.test.ts
+++ b/server/api/public/comments-create-body.test.ts
@@ -25,6 +25,21 @@ describe("parsePublicPostCreateCommentBody", () => {
       body: "hello",
     });
   });
+
+  test("keeps guestIsAnonymous as undefined when field is missing", () => {
+    const parsed = parsePublicPostCreateCommentBody({
+      guestDisplayName: "访客C",
+      guestEmail: undefined,
+      body: "legacy",
+    });
+    expect(parsed).toEqual({
+      parentId: null,
+      guestDisplayName: "访客C",
+      guestEmail: undefined,
+      guestIsAnonymous: undefined,
+      body: "legacy",
+    });
+  });
 });
 
 describe("parseUnlistedCreateCommentBody", () => {
diff --git a/server/api/public/comments-create-body.ts b/server/api/public/comments-create-body.ts
index 1b467b8..824eec4 100644
--- a/server/api/public/comments-create-body.ts
+++ b/server/api/public/comments-create-body.ts
@@ -10,7 +10,7 @@ export type ParsedCreateCommentInput = {
   parentId: number | null;
   guestDisplayName?: string;
   guestEmail?: string;
-  guestIsAnonymous: boolean;
+  guestIsAnonymous?: boolean;
   body: string;
 };
 
@@ -22,7 +22,7 @@ export function parsePublicPostCreateCommentBody(body: CreateCommentRequestBody)
     parentId: parseParentId(body.parentId),
     guestDisplayName: typeof body.guestDisplayName === "string" ? body.guestDisplayName : undefined,
     guestEmail: typeof body.guestEmail === "string" ? body.guestEmail : undefined,
-    guestIsAnonymous: body.guestIsAnonymous === true,
+    guestIsAnonymous: typeof body.guestIsAnonymous === "boolean" ? body.guestIsAnonymous : undefined,
     body: body.body,
   };
 }
@@ -35,7 +35,7 @@ export function parseUnlistedCreateCommentBody(body: CreateCommentRequestBody):
     parentId: parseParentId(body.parentId),
     guestDisplayName: typeof body.guestDisplayName === "string" ? body.guestDisplayName : undefined,
     guestEmail: typeof body.guestEmail === "string" ? body.guestEmail : undefined,
-    guestIsAnonymous: body.guestIsAnonymous === true,
+    guestIsAnonymous: typeof body.guestIsAnonymous === "boolean" ? body.guestIsAnonymous : undefined,
     body: body.body,
   };
 }