diff --git a/fluxer_api/src/api/app/MiddlewarePipeline.ts b/fluxer_api/src/api/app/MiddlewarePipeline.ts
index 96cd460ea..742657d21 100644
--- a/fluxer_api/src/api/app/MiddlewarePipeline.ts
+++ b/fluxer_api/src/api/app/MiddlewarePipeline.ts
@@ -36,6 +36,20 @@ export function configureMiddleware(routes: HonoApp, options: MiddlewarePipeline
 	const resolvedHeader = resolveClientIpHeaderName(clientIpHeaderName);
 	routes.use('/webhooks/:webhook_id/:token', cors({origins: '*'}));
 	routes.use('/webhooks/:webhook_id/:token/messages/:message_id', cors({origins: '*'}));
+	routes.use(
+		'/.well-known/fluxer',
+		cors({
+			origins: '*',
+			methods: ['GET', 'HEAD', 'OPTIONS'],
+			allowedHeaders: [
+				HttpHeaders.ACCEPT,
+				HttpHeaders.CONTENT_TYPE,
+				HttpHeaders.IF_MODIFIED_SINCE,
+				HttpHeaders.IF_NONE_MATCH,
+			],
+			exposedHeaders: [HttpHeaders.ETAG, HttpHeaders.LAST_MODIFIED],
+		}),
+	);
 	applyMiddlewareStack(routes, {
 		requestId: {},
 		cors: {origins: corsOrigins, exposedHeaders: [HttpHeaders.X_FLUXER_VERSION]},
diff --git a/fluxer_api/src/api/instance/DiscoveryValidators.ts b/fluxer_api/src/api/instance/DiscoveryValidators.ts
new file mode 100644
index 000000000..0d545cbc8
--- /dev/null
+++ b/fluxer_api/src/api/instance/DiscoveryValidators.ts
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import {createHash} from 'node:crypto';
+import {entityTagMatches} from '../utils/EntityTag';
+
+export interface DiscoveryValidators {
+	etag: string;
+	lastModified: Date;
+}
+
+export function nextDiscoveryValidators(document: unknown, previous: DiscoveryValidators | null): DiscoveryValidators {
+	const etag = `"${createHash('sha256').update(JSON.stringify(document)).digest('hex')}"`;
+	if (previous !== null && previous.etag === etag) {
+		return previous;
+	}
+	return {etag, lastModified: new Date(Math.floor(Date.now() / 1000) * 1000)};
+}
+
+export function isDiscoveryNotModified(
+	validators: DiscoveryValidators,
+	ifNoneMatch: string | undefined,
+	ifModifiedSince: string | undefined,
+): boolean {
+	if (ifNoneMatch !== undefined) {
+		return entityTagMatches(ifNoneMatch, validators.etag);
+	}
+	if (ifModifiedSince === undefined) {
+		return false;
+	}
+	const since = Date.parse(ifModifiedSince);
+	return !Number.isNaN(since) && validators.lastModified.getTime() <= since;
+}
diff --git a/fluxer_api/src/api/instance/InstanceController.ts b/fluxer_api/src/api/instance/InstanceController.ts
index da8f6af4e..4d43fa7ea 100644
--- a/fluxer_api/src/api/instance/InstanceController.ts
+++ b/fluxer_api/src/api/instance/InstanceController.ts
@@ -13,8 +13,11 @@ import {RateLimitMiddleware} from '../middleware/RateLimitMiddleware';
 import {OpenAPI} from '../middleware/ResponseTypeMiddleware';
 import {RateLimitConfigs} from '../RateLimitConfig';
 import type {HonoEnv} from '../types/HonoEnv';
+import {type DiscoveryValidators, isDiscoveryNotModified, nextDiscoveryValidators} from './DiscoveryValidators';
 import type {InstanceCaptchaEffectiveConfig} from './InstanceConfigRepository';
 
+let discoveryValidators: DiscoveryValidators | null = null;
+
 function buildDiscoveryStaticInput(
 	gifService: GifService | undefined,
 	appPublic: InstanceAppPublic,
@@ -131,6 +134,18 @@ export function InstanceController(app: Hono<HonoEnv>) {
 					limits,
 				},
 			);
+			discoveryValidators = nextDiscoveryValidators(response, discoveryValidators);
+			ctx.header('ETag', discoveryValidators.etag);
+			ctx.header('Last-Modified', discoveryValidators.lastModified.toUTCString());
+			if (
+				isDiscoveryNotModified(
+					discoveryValidators,
+					ctx.req.header('If-None-Match'),
+					ctx.req.header('If-Modified-Since'),
+				)
+			) {
+				return ctx.body(null, 304);
+			}
 			return ctx.json(response);
 		},
 	);
diff --git a/fluxer_api/src/api/utils/EntityTag.ts b/fluxer_api/src/api/utils/EntityTag.ts
new file mode 100644
index 000000000..a4747b3e4
--- /dev/null
+++ b/fluxer_api/src/api/utils/EntityTag.ts
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+function stripWeakness(entityTag: string): string {
+	return entityTag.startsWith('W/') ? entityTag.slice(2) : entityTag;
+}
+
+export function entityTagMatches(ifNoneMatch: string, etag: string): boolean {
+	const target = stripWeakness(etag.trim());
+	return ifNoneMatch
+		.split(',')
+		.map((candidate) => candidate.trim())
+		.some((candidate) => candidate === '*' || stripWeakness(candidate) === target);
+}