diff --git a/packages/backend/src/core/RemoteUserResolveService.ts b/packages/backend/src/core/RemoteUserResolveService.ts
index 098b5e1706..a2f1b73cdb 100644
--- a/packages/backend/src/core/RemoteUserResolveService.ts
+++ b/packages/backend/src/core/RemoteUserResolveService.ts
@@ -74,7 +74,7 @@ export class RemoteUserResolveService {
 		if (user == null) {
 			const self = await this.resolveSelf(acctLower);
 
-			if (self.href.startsWith(this.config.url)) {
+			if (this.utilityService.isUriLocal(self.href)) {
 				const local = this.apDbResolverService.parseUri(self.href);
 				if (local.local && local.type === 'users') {
 					// the LR points to local
diff --git a/packages/backend/src/core/activitypub/ApRendererService.ts b/packages/backend/src/core/activitypub/ApRendererService.ts
index 5617a29bab..e2cac87ba1 100644
--- a/packages/backend/src/core/activitypub/ApRendererService.ts
+++ b/packages/backend/src/core/activitypub/ApRendererService.ts
@@ -27,6 +27,7 @@ import type { UsersRepository, UserProfilesRepository, NotesRepository, DriveFil
 import { bindThis } from '@/decorators.js';
 import { CustomEmojiService } from '@/core/CustomEmojiService.js';
 import { IdService } from '@/core/IdService.js';
+import { UtilityService } from '@/core/UtilityService.js';
 import { JsonLdService } from './JsonLdService.js';
 import { ApMfmService } from './ApMfmService.js';
 import { CONTEXT } from './misc/contexts.js';
@@ -61,6 +62,7 @@ export class ApRendererService {
 		private apMfmService: ApMfmService,
 		private mfmService: MfmService,
 		private idService: IdService,
+		private utilityService: UtilityService,
 	) {
 	}
 
@@ -574,7 +576,7 @@ export class ApRendererService {
 
 	@bindThis
 	public renderUndo(object: string | IObject, user: { id: MiUser['id'] }): IUndo {
-		const id = typeof object !== 'string' && typeof object.id === 'string' && object.id.startsWith(this.config.url) ? `${object.id}/undo` : undefined;
+		const id = typeof object !== 'string' && typeof object.id === 'string' && this.utilityService.isUriLocal(object.id) ? `${object.id}/undo` : undefined;
 
 		return {
 			type: 'Undo',