diff --git a/src/activitypub/jobs.js b/src/activitypub/jobs.js
index 1ba0dd57ce..57c2e51221 100644
--- a/src/activitypub/jobs.js
+++ b/src/activitypub/jobs.js
@@ -5,6 +5,8 @@ const { CronJob } = require('cron');
 
 const db = require('../database');
 const meta = require('../meta');
+const topics = require('../topics');
+const utils = require('../utils');
 const activitypub = module.parent.exports;
 
 const Jobs = module.exports;
@@ -34,7 +36,11 @@ Jobs.start = () => {
 
 	new CronJob('0 * * * * *', async () => {
 		await tryCronJob(retryFailedMessages);
-	}, null, true, null, null, false);
+	}, null, true, null, null, false); // change last argument to true for debugging
+
+	new CronJob('15 * * * *', async () => {
+		await tryCronJob(backfill);
+	}, null, true, null, null, false); // change last argument to true for debugging
 };
 
 async function retryFailedMessages() {
@@ -87,3 +93,22 @@ async function retryFailedMessages() {
 		db.deleteAll(queueIdsToRemove.map(id => `ap:retry:queue:${id}`)),
 	]);
 }
+
+async function backfill() {
+	const start = 0;
+	const stop = meta.config.topicsPerPage - 1;
+	const sorted = await topics.getSortedTopics({
+		term: 'day',
+		sort: 'posts',
+		uid: 0,
+		start,
+		stop,
+	});
+
+	// Remote mainPids only
+	const pids = sorted.topics
+		.map(({ mainPid }) => mainPid)
+		.filter(pid => !utils.isNumber(pid));
+
+	await activitypub.notes.backfill(pids);
+}
diff --git a/src/activitypub/notes.js b/src/activitypub/notes.js
index a66adb0dc6..01ead62530 100644
--- a/src/activitypub/notes.js
+++ b/src/activitypub/notes.js
@@ -16,11 +16,19 @@ const user = require('../user');
 const topics = require('../topics');
 const posts = require('../posts');
 const api = require('../api');
+const ttlCache = require('../cache/ttl');
+const websockets = require('../socket.io');
 const utils = require('../utils');
 
 const activitypub = module.parent.exports;
 const Notes = module.exports;
 
+const backfillCache = ttlCache({
+	name: 'ap-backfill-cache',
+	max: 500,
+	ttl: 1000 * 60 * 2, // 2 minutes
+});
+
 Notes._normalizeTags = async (tag, cid) => {
 	const systemTags = (meta.config.systemTags || '').split(',');
 	const maxTags = await categories.getCategoryField(cid, 'maxTags');
@@ -255,17 +263,31 @@ Notes.assert = async (uid, input, options = { skipChecks: false }) => {
 			}
 		}
 
+		let added = [];
 		await Promise.all(unprocessed.map(async (post) => {
 			const { to, cc } = post._activitypub;
 
 			try {
-				await topics.reply(post);
+				const postData = await topics.reply(post);
+				added.push(postData);
 				await Notes.updateLocalRecipients(post.pid, { to, cc });
 			} catch (e) {
 				activitypub.helpers.log(`[activitypub/notes.assert] Could not add reply (${post.pid}): ${e.message}`);
 			}
 		}));
 
+		if (added.length) {
+			// Because replies are added in parallel, `index` is calculated incorrectly
+			added = added
+				.sort((a, b) => a.timestamp - b.timestamp)
+				.map((post, idx) => {
+					post.index = post.index - idx;
+					return post;
+				})
+				.reverse();
+			websockets.in(`topic_${tid}`).emit('event:new_post', { posts: added });
+		}
+
 		await Notes.syncUserInboxes(tid, uid);
 
 		if (crosspostCid) {
@@ -584,6 +606,22 @@ Notes.getCategoryFollowers = async (cid) => {
 	return uids;
 };
 
+Notes.backfill = async (pids) => {
+	if (!Array.isArray(pids)) {
+		pids = [pids];
+	}
+
+	return Promise.all(pids.map(async (pid) => {
+		if (backfillCache.has(pid)) {
+			console.log('cache hit, not proactively backfilling');
+			return;
+		}
+
+		await Notes.assert(0, pid, { skipChecks: 1 });
+		backfillCache.set(pid, 1);
+	}));
+};
+
 Notes.announce = {};
 
 Notes.announce.list = async ({ pid, tid }) => {
diff --git a/src/controllers/topics.js b/src/controllers/topics.js
index 08958ce730..74ec15c80f 100644
--- a/src/controllers/topics.js
+++ b/src/controllers/topics.js
@@ -14,6 +14,7 @@ const helpers = require('./helpers');
 const pagination = require('../pagination');
 const utils = require('../utils');
 const analytics = require('../analytics');
+const activitypub = require('../activitypub');
 
 const topicsController = module.exports;
 
@@ -142,11 +143,18 @@ topicsController.get = async function getTopic(req, res, next) {
 		res.locals.linkTags.push(rel);
 	});
 
-	if (meta.config.activitypubEnabled && postAtIndex) {
-		// Include link header for richer parsing
-		const { pid } = postAtIndex;
-		const href = utils.isNumber(pid) ? `${nconf.get('url')}/post/${pid}` : pid;
-		res.set('Link', `<${href}>; rel="alternate"; type="application/activity+json"`);
+	if (meta.config.activitypubEnabled) {
+		if (postAtIndex) {
+			// Include link header for richer parsing
+			const { pid } = postAtIndex;
+			const href = utils.isNumber(pid) ? `${nconf.get('url')}/post/${pid}` : pid;
+			res.set('Link', `<${href}>; rel="alternate"; type="application/activity+json"`);
+		}
+
+		if (!utils.isNumber(topicData.mainPid)) {
+			// not awaited on purpose so topic loading is not blocked
+			activitypub.notes.backfill(topicData.mainPid);
+		}
 	}
 
 	res.render('topic', topicData);