diff --git a/src/activitypub/contexts.js b/src/activitypub/contexts.js
new file mode 100644
index 0000000000..ebe3ddfe20
--- /dev/null
+++ b/src/activitypub/contexts.js
@@ -0,0 +1,77 @@
+'use strict';
+
+const winston = require('winston');
+
+const posts = require('../posts');
+
+const activitypub = module.parent.exports;
+const Contexts = module.exports;
+
+const acceptableTypes = ['Collection', 'CollectionPage', 'OrderedCollection', 'OrderedCollectionPage'];
+
+Contexts.get = async (uid, id) => {
+	let context;
+	let type;
+
+	try {
+		({ context } = await activitypub.get('uid', uid, id));
+		({ type } = await activitypub.get('uid', uid, context));
+	} catch (e) {
+		winston.verbose(`[activitypub/context] ${id} context not resolvable.`);
+		return false;
+	}
+
+	if (acceptableTypes.includes(type)) {
+		return context;
+	}
+
+	return false;
+};
+
+Contexts.getItems = async (uid, id, root = true) => {
+	winston.verbose(`[activitypub/context] Retrieving context ${id}`);
+	let { type, items, first, next } = await activitypub.get('uid', uid, id);
+	if (!acceptableTypes.includes(type)) {
+		return [];
+	}
+
+	if (items) {
+		items = (await Promise.all(items.map(async (item) => {
+			const { type, id } = await activitypub.helpers.resolveLocalId(item);
+			const pid = type === 'post' && id ? id : item;
+			const postData = await posts.getPostData(pid);
+			if (postData) {
+				// Already cached
+				return postData;
+			}
+
+			// No local copy, fetch from source
+			try {
+				const object = await activitypub.get('uid', uid, pid);
+				winston.verbose(`[activitypub/context] Retrieved ${pid}`);
+				return await activitypub.mocks.post(object);
+			} catch (e) {
+				// Unresolvable, either temporariliy or permanent, ignore for now.
+				winston.verbose(`[activitypub/context] Cannot retrieve ${id}`);
+				return null;
+			}
+		}))).filter(Boolean);
+		winston.verbose(`[activitypub/context] Found ${items.length} items.`);
+	}
+
+	const chain = new Set(items || []);
+	if (!next && root && first) {
+		next = first;
+	}
+
+	if (next) {
+		winston.verbose('[activitypub/context] Fetching next page...');
+		Array
+			.from(await Contexts.getItems(uid, next, false))
+			.forEach((item) => {
+				chain.add(item);
+			});
+	}
+
+	return chain;
+};
diff --git a/src/activitypub/helpers.js b/src/activitypub/helpers.js
index 71639298fb..1721ce40a7 100644
--- a/src/activitypub/helpers.js
+++ b/src/activitypub/helpers.js
@@ -134,7 +134,6 @@ Helpers.resolveLocalId = async (input) => {
 				activityData = { activity, data, timestamp };
 			}
 
-			// https://bb.devnull.land/cid/2#activity/follow/activitypub@community.nodebb.org│
 			switch (prefix) {
 				case 'uid':
 					return { type: 'user', id: value, ...activityData };
diff --git a/src/activitypub/index.js b/src/activitypub/index.js
index 21aa47a492..3fa7055ee8 100644
--- a/src/activitypub/index.js
+++ b/src/activitypub/index.js
@@ -40,6 +40,7 @@ ActivityPub.helpers = require('./helpers');
 ActivityPub.inbox = require('./inbox');
 ActivityPub.mocks = require('./mocks');
 ActivityPub.notes = require('./notes');
+ActivityPub.contexts = require('./contexts');
 ActivityPub.actors = require('./actors');
 ActivityPub.instances = require('./instances');
 
diff --git a/src/activitypub/notes.js b/src/activitypub/notes.js
index 794d0660b8..2209dda8f4 100644
--- a/src/activitypub/notes.js
+++ b/src/activitypub/notes.js
@@ -43,13 +43,23 @@ Notes.assert = async (uid, input, options = { skipChecks: false }) => {
 		return null;
 	}
 
-	const chain = Array.from(await Notes.getParentChain(uid, input));
+	let chain;
+	const context = await activitypub.contexts.get(uid, id);
+	if (context) {
+		chain = Array.from(await activitypub.contexts.getItems(uid, context));
+	} else {
+		// Fall back to inReplyTo traversal
+		chain = Array.from(await Notes.getParentChain(uid, input));
+	}
 	if (!chain.length) {
 		unlock(id);
 		return null;
 	}
 
-	const mainPost = chain[chain.length - 1];
+	// Reorder chain items by timestamp
+	chain = chain.sort((a, b) => a.timestamp - b.timestamp);
+
+	const mainPost = chain[0];
 	let { pid: mainPid, tid, uid: authorId, timestamp, name, content, _activitypub } = mainPost;
 	const hasTid = !!tid;
 
@@ -133,7 +143,7 @@ Notes.assert = async (uid, input, options = { skipChecks: false }) => {
 			.filter(o => o.type === 'Hashtag' && !systemTags.includes(o.name.slice(1)))
 			.map(o => o.name.slice(1));
 
-		if (maxTags && tags.length > maxTags) {
+		if (tags.length > maxTags) {
 			tags.length = maxTags;
 		}
 
@@ -152,10 +162,9 @@ Notes.assert = async (uid, input, options = { skipChecks: false }) => {
 			Notes.updateLocalRecipients(mainPid, { to, cc }),
 			posts.attachments.update(mainPid, attachment),
 		]);
-		unprocessed.pop();
+		unprocessed.shift();
 	}
 
-	unprocessed.reverse();
 	for (const post of unprocessed) {
 		const { to, cc, attachment } = post._activitypub;
 
diff --git a/src/controllers/activitypub/actors.js b/src/controllers/activitypub/actors.js
index 4391f03cee..bcd8709b74 100644
--- a/src/controllers/activitypub/actors.js
+++ b/src/controllers/activitypub/actors.js
@@ -101,7 +101,8 @@ Actors.topic = async function (req, res, next) {
 		url: `${nconf.get('url')}/topic/${slug}`,
 		name,
 		type: paginate && items ? 'OrderedCollectionPage' : 'OrderedCollection',
-		audience: `${nconf.get('url')}/category/${cid}`,
+		attributedTo: `${nconf.get('url')}/category/${cid}`,
+		audience: cid !== -1 ? `${nconf.get('url')}/category/${cid}/followers` : undefined,
 		totalItems: postcount,
 	};
 
diff --git a/src/posts/create.js b/src/posts/create.js
index 2c23560266..af1ea92c25 100644
--- a/src/posts/create.js
+++ b/src/posts/create.js
@@ -82,6 +82,10 @@ module.exports = function (Posts) {
 	}
 
 	async function checkToPid(toPid, uid) {
+		if (!utils.isNumber(toPid)) {
+			return;
+		}
+
 		const [toPost, canViewToPid] = await Promise.all([
 			Posts.getPostFields(toPid, ['pid', 'deleted']),
 			privileges.posts.can('posts:view_deleted', toPid, uid),