diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e1393dbef93..b2775938e04b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@
 - Enhance: `vmimiRelayTimelineImplemented` と `disableVmimiRelayTimeline` nodeinfo に追加しました
   - これによりサードパーティクライアントがVRTLの有無を認知できるようになりました。
 - Enhance: VRTL参加サーバーの取得に失敗したときのリトライの間隔を短く
+- Enhance: 応答がないインスタンスがいつから応答がなくなったかをクライアントに送信するようにしました
 - Fix: 自分自身に対するリプライがwithReplies = falseなVRTL/VSTLにて含まれていない問題を修正
 - チャート生成時にinstance.suspentionStateに置き換えられたinstance.isSuspendedが参照されてしまう問題を修正
 
diff --git a/packages/backend/src/core/entities/InstanceEntityService.ts b/packages/backend/src/core/entities/InstanceEntityService.ts
index 9117b1391481..93ee84061645 100644
--- a/packages/backend/src/core/entities/InstanceEntityService.ts
+++ b/packages/backend/src/core/entities/InstanceEntityService.ts
@@ -39,6 +39,7 @@ export class InstanceEntityService {
 			followingCount: instance.followingCount,
 			followersCount: instance.followersCount,
 			isNotResponding: instance.isNotResponding,
+			notRespondingSince: instance.notRespondingSince ? instance.notRespondingSince.toISOString() : null,
 			isSuspended: instance.suspensionState !== 'none',
 			suspensionState: instance.suspensionState,
 			isBlocked: this.utilityService.isBlockedHost(meta.blockedHosts, instance.host),
diff --git a/packages/backend/src/models/json-schema/federation-instance.ts b/packages/backend/src/models/json-schema/federation-instance.ts
index ed40d405c66d..fdf01315b597 100644
--- a/packages/backend/src/models/json-schema/federation-instance.ts
+++ b/packages/backend/src/models/json-schema/federation-instance.ts
@@ -41,6 +41,11 @@ export const packedFederationInstanceSchema = {
 			type: 'boolean',
 			optional: false, nullable: false,
 		},
+		notRespondingSince: {
+			type: 'string',
+			optional: false, nullable: true,
+			format: 'date-time',
+		},
 		isSuspended: {
 			type: 'boolean',
 			optional: false, nullable: false,
diff --git a/packages/misskey-js/src/autogen/types.ts b/packages/misskey-js/src/autogen/types.ts
index d56520b6ddf7..033d8dc1fa4e 100644
--- a/packages/misskey-js/src/autogen/types.ts
+++ b/packages/misskey-js/src/autogen/types.ts
@@ -4520,6 +4520,8 @@ export type components = {
       followingCount: number;
       followersCount: number;
       isNotResponding: boolean;
+      /** Format: date-time */
+      notRespondingSince: string | null;
       isSuspended: boolean;
       /** @enum {string} */
       suspensionState: 'none' | 'manuallySuspended' | 'goneSuspended' | 'autoSuspendedForNotResponding';