diff --git a/packages/backend/src/server/api/endpoints/notes/translate.ts b/packages/backend/src/server/api/endpoints/notes/translate.ts
index 39119bc206..1566a8404b 100644
--- a/packages/backend/src/server/api/endpoints/notes/translate.ts
+++ b/packages/backend/src/server/api/endpoints/notes/translate.ts
@@ -10,22 +10,28 @@ import { NoteEntityService } from '@/core/entities/NoteEntityService.js';
 import { HttpRequestService } from '@/core/HttpRequestService.js';
 import { GetterService } from '@/server/api/GetterService.js';
 import { RoleService } from '@/core/RoleService.js';
-import { ApiError } from '../../error.js';
-import { MiMeta } from '@/models/_.js';
+import type { MiMeta, MiNote } from '@/models/_.js';
 import { DI } from '@/di-symbols.js';
+import { CacheService } from '@/core/CacheService.js';
+import { hasText } from '@/models/Note.js';
+import { ApiLoggerService } from '@/server/api/ApiLoggerService.js';
+import { ApiError } from '../../error.js';
 
 export const meta = {
 	tags: ['notes'],
 
+	// TODO allow unauthenticated if default template allows?
+	//   Maybe a value 'optional' that allows unauthenticated OR a token w/ appropriate role.
+	//   This will allow unauthenticated requests without leaking post data to restricted clients.
 	requireCredential: true,
 	kind: 'read:account',
 
 	res: {
 		type: 'object',
-		optional: true, nullable: false,
+		optional: false, nullable: false,
 		properties: {
-			sourceLang: { type: 'string' },
-			text: { type: 'string' },
+			sourceLang: { type: 'string', optional: true, nullable: false },
+			text: { type: 'string', optional: true, nullable: false },
 		},
 	},
 
@@ -45,6 +51,11 @@ export const meta = {
 			code: 'CANNOT_TRANSLATE_INVISIBLE_NOTE',
 			id: 'ea29f2ca-c368-43b3-aaf1-5ac3e74bbe5d',
 		},
+		translationFailed: {
+			message: 'Failed to translate note. Please try again later or contact an administrator for assistance.',
+			code: 'TRANSLATION_FAILED',
+			id: '4e7a1a4f-521c-4ba2-b10a-69e5e2987b2f',
+		},
 	},
 
 	// 10 calls per 5 seconds
@@ -73,6 +84,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		private getterService: GetterService,
 		private httpRequestService: HttpRequestService,
 		private roleService: RoleService,
+		private readonly cacheService: CacheService,
+		private readonly loggerService: ApiLoggerService,
 	) {
 		super(meta, paramDef, async (ps, me) => {
 			const policies = await this.roleService.getUserPolicies(me.id);
@@ -89,8 +102,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				throw new ApiError(meta.errors.cannotTranslateInvisibleNote);
 			}
 
-			if (note.text == null) {
-				return;
+			if (!hasText(note)) {
+				return {};
 			}
 
 			const canDeeplFree = this.serverSettings.deeplFreeMode && !!this.serverSettings.deeplFreeInstance;
@@ -101,13 +114,32 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			let targetLang = ps.targetLang;
 			if (targetLang.includes('-')) targetLang = targetLang.split('-')[0];
 
+			let response = await this.cacheService.getCachedTranslation(note, targetLang);
+			if (!response) {
+				response = await this.fetchTranslation(note, targetLang);
+				if (!response) {
+					throw new ApiError(meta.errors.translationFailed);
+				}
+
+				await this.cacheService.setCachedTranslation(note, targetLang, response);
+			}
+			return response;
+		});
+	}
+
+	private async fetchTranslation(note: MiNote & { text: string }, targetLang: string) {
+		// Load-bearing try/catch - removing this will shift indentation and cause ~80 lines of upstream merge conflicts
+		try {
+			// Ignore deeplFreeInstance unless deeplFreeMode is set
+			const deeplFreeInstance = this.serverSettings.deeplFreeMode ? this.serverSettings.deeplFreeInstance : null;
+
 			// DeepL/DeepLX handling
-			if (canDeepl) {
+			if (this.serverSettings.deeplAuthKey || deeplFreeInstance) {
 				const params = new URLSearchParams();
 				if (this.serverSettings.deeplAuthKey) params.append('auth_key', this.serverSettings.deeplAuthKey);
 				params.append('text', note.text);
 				params.append('target_lang', targetLang);
-				const endpoint = canDeeplFree ? this.serverSettings.deeplFreeInstance as string : this.serverSettings.deeplIsPro ? 'https://api.deepl.com/v2/translate' : 'https://api-free.deepl.com/v2/translate';
+				const endpoint = deeplFreeInstance ?? this.serverSettings.deeplIsPro ? 'https://api.deepl.com/v2/translate' : 'https://api-free.deepl.com/v2/translate';
 
 				const res = await this.httpRequestService.send(endpoint, {
 					method: 'POST',
@@ -151,8 +183,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			}
 
 			// LibreTranslate handling
-			if (canLibre) {
-				const res = await this.httpRequestService.send(this.serverSettings.libreTranslateURL as string, {
+			if (this.serverSettings.libreTranslateURL) {
+				const res = await this.httpRequestService.send(this.serverSettings.libreTranslateURL, {
 					method: 'POST',
 					headers: {
 						'Content-Type': 'application/json',
@@ -182,8 +214,10 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					text: json.translatedText,
 				};
 			}
+		} catch (e) {
+			this.loggerService.logger.error('Unhandled error from translation API: ', { e });
+		}
 
-			return;
-		});
+		return null;
 	}
 }
diff --git a/packages/misskey-js/src/autogen/types.ts b/packages/misskey-js/src/autogen/types.ts
index 2b4c39c280..271a0d0b52 100644
--- a/packages/misskey-js/src/autogen/types.ts
+++ b/packages/misskey-js/src/autogen/types.ts
@@ -28709,15 +28709,11 @@ export type operations = {
       200: {
         content: {
           'application/json': {
-            sourceLang: string;
-            text: string;
+            sourceLang?: string;
+            text?: string;
           };
         };
       };
-      /** @description OK (without any results) */
-      204: {
-        content: never;
-      };
       /** @description Client error */
       400: {
         content: {