diff --git a/src/categories/recentreplies.js b/src/categories/recentreplies.js
index 8e21647c70..92eed3be56 100644
--- a/src/categories/recentreplies.js
+++ b/src/categories/recentreplies.js
@@ -191,10 +191,15 @@ module.exports = function (Categories) {
 
 			const bulkRemove = [];
 			const bulkAdd = [];
+			const bulkIncr = [];
 			postData.forEach((post) => {
 				bulkRemove.push([`cid:${oldCid}:uid:${post.uid}:pids`, post.pid]);
 				bulkRemove.push([`cid:${oldCid}:uid:${post.uid}:pids:votes`, post.pid]);
 				bulkAdd.push([`cid:${cid}:uid:${post.uid}:pids`, post.timestamp, post.pid]);
+				bulkIncr.push(
+					[`uid:${post.uid}:cids`, -1, oldCid],
+					[`uid:${post.uid}:cids`, 1, cid],
+				);
 				if (post.votes > 0 || post.votes < 0) {
 					bulkAdd.push([`cid:${cid}:uid:${post.uid}:pids:votes`, post.votes, post.pid]);
 				}
@@ -207,6 +212,7 @@ module.exports = function (Categories) {
 				db.sortedSetAdd(`cid:${cid}:pids`, timestamps, postsToReAdd.map(p => p.pid)),
 				db.sortedSetRemoveBulk(bulkRemove),
 				db.sortedSetAddBulk(bulkAdd),
+				db.sortedSetIncrByBulk(bulkIncr),
 			]);
 		}, { batch: 500 });
 	};
diff --git a/src/controllers/accounts/posts.js b/src/controllers/accounts/posts.js
index 9ce048bd28..1535ed0737 100644
--- a/src/controllers/accounts/posts.js
+++ b/src/controllers/accounts/posts.js
@@ -31,7 +31,8 @@ const templateToData = {
 		noItemsFoundKey: '[[user:has-no-posts]]',
 		crumb: '[[global:posts]]',
 		getSets: async function (callerUid, userData) {
-			const cids = await categories.getCidsByPrivilege('categories:cid', callerUid, 'topics:read');
+			let cids = await db.getSortedSetRangeByScore(`uid:${userData.uid}:cids`, 0, -1, 1, '+inf');
+			cids = await privileges.categories.filterCids('topics:read', cids, callerUid);
 			return cids.map(c => `cid:${c}:uid:${userData.uid}:pids`);
 		},
 	},
@@ -143,7 +144,8 @@ const templateToData = {
 		noItemsFoundKey: '[[user:has-no-topics]]',
 		crumb: '[[global:topics]]',
 		getSets: async function (callerUid, userData) {
-			const cids = await categories.getCidsByPrivilege('categories:cid', callerUid, 'topics:read');
+			let cids = await db.getSortedSetRangeByScore(`uid:${userData.uid}:cids`, 0, -1, 1, '+inf');
+			cids = await privileges.categories.filterCids('topics:read', cids, callerUid);
 			return cids.map(c => `cid:${c}:uid:${userData.uid}:tids`);
 		},
 	},
diff --git a/src/database/helpers.js b/src/database/helpers.js
index eb8206f783..d66bfb87a6 100644
--- a/src/database/helpers.js
+++ b/src/database/helpers.js
@@ -42,3 +42,21 @@ helpers.globToRegex = function (match) {
 	}
 	return _match;
 };
+
+helpers.aggregateIncrByBulk = function (data) {
+	const buckets = Object.create(null);
+
+	for (const [key, incr, val] of data) {
+		buckets[key] = buckets[key] || {};
+		buckets[key][val] = (buckets[key][val] || 0) + incr;
+	}
+
+	const result = [];
+	for (const [key, vals] of Object.entries(buckets)) {
+		for (const [val, incr] of Object.entries(vals)) {
+			result.push([key, incr, val]);
+		}
+	}
+
+	return result;
+};
diff --git a/src/database/mongo/sorted.js b/src/database/mongo/sorted.js
index c6f1ee90ce..71c4f4c273 100644
--- a/src/database/mongo/sorted.js
+++ b/src/database/mongo/sorted.js
@@ -459,16 +459,35 @@ module.exports = function (module) {
 	};
 
 	module.sortedSetIncrByBulk = async function (data) {
+		if (!Array.isArray(data) || !data.length) {
+			return [];
+		}
+		const aggregated = dbHelpers.aggregateIncrByBulk(data);
 		const bulk = module.client.collection('objects').initializeUnorderedBulkOp();
-		data.forEach((item) => {
+		aggregated.forEach((item) => {
 			bulk.find({ _key: item[0], value: helpers.valueToString(item[2]) })
 				.upsert()
 				.update({ $inc: { score: parseFloat(item[1]) } });
 		});
-		await bulk.execute();
+
+		try {
+			await bulk.execute();
+		} catch (err) {
+			// retry failed e11000 operations
+			if (err.code === 11000 || (err.writeErrors && err.writeErrors.some(e => e.code === 11000))) {
+				const failedIndices = err.writeErrors.map(e => e.index);
+				const retryData = failedIndices.map(idx => aggregated[idx]);
+				await Promise.all(retryData.map(
+					item => module.sortedSetIncrBy(item[0], item[1], item[2])
+				));
+			} else {
+				throw err;
+			}
+		}
+
 		const result = await module.client.collection('objects').find({
-			_key: { $in: _.uniq(data.map(i => i[0])) },
-			value: { $in: _.uniq(data.map(i => i[2])) },
+			_key: { $in: _.uniq(aggregated.map(i => i[0])) },
+			value: { $in: _.uniq(aggregated.map(i => i[2])) },
 		}, {
 			projection: { _id: 0, _key: 1, value: 1, score: 1 },
 		}).toArray();
diff --git a/src/database/postgres/sorted.js b/src/database/postgres/sorted.js
index 351fe3e059..6332907e43 100644
--- a/src/database/postgres/sorted.js
+++ b/src/database/postgres/sorted.js
@@ -1,8 +1,10 @@
 'use strict';
 
 module.exports = function (module) {
-	const helpers = require('./helpers');
 	const util = require('util');
+
+	const helpers = require('./helpers');
+	const dbHelpers = require('../helpers');
 	const Cursor = require('pg-cursor');
 	Cursor.prototype.readAsync = util.promisify(Cursor.prototype.read);
 	const sleep = util.promisify(setTimeout);
@@ -547,18 +549,19 @@ RETURNING "score" s`,
 	};
 
 	module.sortedSetIncrByBulk = async function (data) {
-		if (!data.length) {
+		if (!Array.isArray(data) || !data.length) {
 			return [];
 		}
 
+		const aggregated = dbHelpers.aggregateIncrByBulk(data);
 		return await module.transaction(async (client) => {
-			await helpers.ensureLegacyObjectsType(client, data.map(item => item[0]), 'zset');
+			await helpers.ensureLegacyObjectsType(client, aggregated.map(item => item[0]), 'zset');
 
 			const values = [];
 			const queryParams = [];
 			let paramIndex = 1;
 
-			data.forEach(([key, increment, value]) => {
+			aggregated.forEach(([key, increment, value]) => {
 				value = helpers.valueToString(value);
 				increment = parseFloat(increment);
 				values.push(key, value, increment);
diff --git a/src/database/redis/sorted.js b/src/database/redis/sorted.js
index 8433133f15..a29ef75a20 100644
--- a/src/database/redis/sorted.js
+++ b/src/database/redis/sorted.js
@@ -261,8 +261,12 @@ module.exports = function (module) {
 	};
 
 	module.sortedSetIncrByBulk = async function (data) {
+		if (!Array.isArray(data) || !data.length) {
+			return [];
+		}
+		const aggregated = dbHelpers.aggregateIncrByBulk(data);
 		const multi = module.client.multi();
-		data.forEach((item) => {
+		aggregated.forEach((item) => {
 			multi.zIncrBy(item[0], item[1], String(item[2]));
 		});
 		const result = await multi.exec();
diff --git a/src/posts/delete.js b/src/posts/delete.js
index f6b7aafb37..9235f37111 100644
--- a/src/posts/delete.js
+++ b/src/posts/delete.js
@@ -96,14 +96,19 @@ module.exports = function (Posts) {
 
 	async function deleteFromTopicUserNotification(postData) {
 		const bulkRemove = [];
+		const bulkIncr = [];
 		postData.forEach((p) => {
 			bulkRemove.push([`tid:${p.tid}:posts`, p.pid]);
 			bulkRemove.push([`tid:${p.tid}:posts:votes`, p.pid]);
 			bulkRemove.push([`uid:${p.uid}:posts`, p.pid]);
 			bulkRemove.push([`cid:${p.cid}:uid:${p.uid}:pids`, p.pid]);
 			bulkRemove.push([`cid:${p.cid}:uid:${p.uid}:pids:votes`, p.pid]);
+			bulkIncr.push([`uid:${p.uid}:cids`, -1, p.cid]);
 		});
-		await db.sortedSetRemoveBulk(bulkRemove);
+		await Promise.all([
+			db.sortedSetRemoveBulk(bulkRemove),
+			db.sortedSetIncrByBulk(bulkIncr),
+		]);
 
 		const localCount = postData.filter(p => utils.isNumber(p.pid)).length;
 		const incrObjectBulk = [['global', { postCount: -localCount }]];
diff --git a/src/posts/user.js b/src/posts/user.js
index d096b0fa50..cdb9836bdb 100644
--- a/src/posts/user.js
+++ b/src/posts/user.js
@@ -150,6 +150,7 @@ module.exports = function (Posts) {
 
 		const bulkRemove = [];
 		const bulkAdd = [];
+		const bulkIncr = [];
 		let repChange = 0;
 		const postsByUser = {};
 		postData.forEach((post, i) => {
@@ -164,6 +165,11 @@ module.exports = function (Posts) {
 			if (post.votes > 0 || post.votes < 0) {
 				bulkAdd.push([`cid:${post.cid}:uid:${toUid}:pids:votes`, post.votes, post.pid]);
 			}
+
+			bulkIncr.push(
+				[`uid:${post.uid}:cids`, -1, post.cid],
+				[`uid:${toUid}:cids`, 1, post.cid],
+			);
 			postsByUser[post.uid] = postsByUser[post.uid] || [];
 			postsByUser[post.uid].push(post);
 		});
@@ -172,6 +178,7 @@ module.exports = function (Posts) {
 			db.setObjectField(pids.map(pid => `post:${pid}`), 'uid', toUid),
 			db.sortedSetRemoveBulk(bulkRemove),
 			db.sortedSetAddBulk(bulkAdd),
+			db.sortedSetIncrByBulk(bulkIncr),
 			user.incrementUserReputationBy(toUid, repChange),
 			handleMainPidOwnerChange(postData, toUid),
 			updateTopicPosters(postData, toUid),
diff --git a/src/topics/fork.js b/src/topics/fork.js
index fc8b030f63..22903d112e 100644
--- a/src/topics/fork.js
+++ b/src/topics/fork.js
@@ -169,6 +169,10 @@ module.exports = function (Topics) {
 			db.sortedSetRemove(removeFrom, postData.pid),
 			db.sortedSetAdd(`cid:${topicData[1].cid}:pids`, postData.timestamp, postData.pid),
 			db.sortedSetAdd(`cid:${topicData[1].cid}:uid:${postData.uid}:pids`, postData.timestamp, postData.pid),
+			db.sortedSetIncrByBulk([
+				[`uid:${postData.uid}:cids`, -1, topicData[0].cid],
+				[`uid:${postData.uid}:cids`, 1, topicData[1].cid],
+			]),
 		];
 		if (postData.votes > 0 || postData.votes < 0) {
 			tasks.push(db.sortedSetAdd(`cid:${topicData[1].cid}:uid:${postData.uid}:pids:votes`, postData.votes, postData.pid));
diff --git a/src/upgrades/4.11.0/backfill-user-cids.js b/src/upgrades/4.11.0/backfill-user-cids.js
new file mode 100644
index 0000000000..58db2fc194
--- /dev/null
+++ b/src/upgrades/4.11.0/backfill-user-cids.js
@@ -0,0 +1,38 @@
+'use strict';
+
+const db = require('../../database');
+const posts = require('../../posts');
+const batch = require('../../batch');
+
+module.exports = {
+	name: 'Backfill user posted categories',
+	timestamp: Date.UTC(2026, 2, 20),
+	method: async function () {
+		const { progress } = this;
+		await batch.processSortedSet('posts:pid', async (pids) => {
+			const postData = await db.getObjectsFields(pids.map(pid => `post:${pid}`), ['uid']);
+			const cids = await posts.getCidsByPids(pids);
+			const uidPostCountByCid = Object.create(null);
+			postData.forEach((post, idx) => {
+				const cid = cids[idx];
+				uidPostCountByCid[post.uid] = uidPostCountByCid[post.uid] || {};
+				uidPostCountByCid[post.uid][cid] = (uidPostCountByCid[post.uid][cid] || 0) + 1;
+			});
+			const bulkIncr = [];
+			Object.keys(uidPostCountByCid).forEach((uid) => {
+				Object.keys(uidPostCountByCid[uid]).forEach((cid) => {
+					bulkIncr.push([`uid:${uid}:cids`, uidPostCountByCid[uid][cid], cid]);
+				});
+			});
+
+			if (bulkIncr.length) {
+				await db.sortedSetIncrByBulk(bulkIncr);
+			}
+
+			progress.incr(pids.length);
+		}, {
+			batch: 500,
+			progress,
+		});
+	},
+};
diff --git a/src/user/delete.js b/src/user/delete.js
index 5c15c5c660..b13c197f7f 100644
--- a/src/user/delete.js
+++ b/src/user/delete.js
@@ -126,6 +126,7 @@ module.exports = function (User) {
 			`user:${uid}:usernames`,
 			`user:${uid}:emails`,
 			`uid:${uid}:topics`, `uid:${uid}:posts`,
+			`uid:${uid}:cids`,
 			`uid:${uid}:chats`, `uid:${uid}:chats:unread`,
 			`uid:${uid}:chat:rooms`,
 			`uid:${uid}:chat:rooms:unread`,
diff --git a/src/user/posts.js b/src/user/posts.js
index 30ddeaadfc..c8be6f81cd 100644
--- a/src/user/posts.js
+++ b/src/user/posts.js
@@ -101,6 +101,7 @@ module.exports = function (User) {
 			`uid:${postData.uid}:posts`,
 			`cid:${postData.cid}:uid:${postData.uid}:pids`,
 		], postData.timestamp, postData.pid);
+		await db.sortedSetIncrBy(`uid:${postData.uid}:cids`, 1, postData.cid);
 		await User.updatePostCount(postData.uid);
 	};
 
diff --git a/test/database/sorted.js b/test/database/sorted.js
index fde2bafb0b..f1eca291bf 100644
--- a/test/database/sorted.js
+++ b/test/database/sorted.js
@@ -2,6 +2,7 @@
 
 const assert = require('assert');
 const db = require('../mocks/databasemock');
+const utils = require('../../src/utils');
 
 describe('Sorted Set methods', () => {
 	before(async () => {
@@ -1070,31 +1071,19 @@ NUMERIC)-- WsPn&query[cid]=-1&parentCid=0&selectedCids[]=-1&privilege=topics:rea
 		});
 	});
 
-	describe('sortedSetIncrBy()', () => {
-		it('should create a sorted set with a field set to 1', (done) => {
-			db.sortedSetIncrBy('sortedIncr', 1, 'field1', function (err, newValue) {
-				assert.equal(err, null);
-				assert.equal(arguments.length, 2);
-				assert.strictEqual(newValue, 1);
-				db.sortedSetScore('sortedIncr', 'field1', (err, score) => {
-					assert.equal(err, null);
-					assert.strictEqual(score, 1);
-					done();
-				});
-			});
+	describe('sortedSetIncrBy()/sortedSetIncrByBulk()', () => {
+		it('should create a sorted set with a field set to 1', async () => {
+			const newValue = await db.sortedSetIncrBy('sortedIncr', 1, 'field1');
+			assert.strictEqual(newValue, 1);
+			const score = await db.sortedSetScore('sortedIncr', 'field1');
+			assert.strictEqual(score, 1);
 		});
 
-		it('should increment a field of a sorted set by 5', (done) => {
-			db.sortedSetIncrBy('sortedIncr', 5, 'field1', function (err, newValue) {
-				assert.equal(err, null);
-				assert.equal(arguments.length, 2);
-				assert.strictEqual(newValue, 6);
-				db.sortedSetScore('sortedIncr', 'field1', (err, score) => {
-					assert.equal(err, null);
-					assert.strictEqual(score, 6);
-					done();
-				});
-			});
+		it('should increment a field of a sorted set by 5', async () => {
+			const newValue = await db.sortedSetIncrBy('sortedIncr', 5, 'field1');
+			assert.strictEqual(newValue, 6);
+			const score = await db.sortedSetScore('sortedIncr', 'field1');
+			assert.strictEqual(score, 6);
 		});
 
 		it('should increment fields of sorted sets with a single call', async () => {
@@ -1122,12 +1111,27 @@ NUMERIC)-- WsPn&query[cid]=-1&parentCid=0&selectedCids[]=-1&privilege=topics:rea
 			);
 		});
 
+		it('should increment the same zset twice', async () => {
+			const zset = utils.generateUUID();
+			const value1 = utils.generateUUID();
+			const value2 = utils.generateUUID();
+			await db.sortedSetIncrByBulk([
+				[zset, 1, value1],
+				[zset, 1, value2],
+			]);
+			const scores = await Promise.all([
+				db.sortedSetScore(zset, value1),
+				db.sortedSetScore(zset, value2),
+			]);
+			assert.deepStrictEqual(scores, [1, 1]);
+		});
+
 		it('should increment the same field', async () => {
-			const data1 = await db.sortedSetIncrByBulk([
+			await db.sortedSetIncrByBulk([
 				['sortedIncrBulk5', 5, 'value5'],
 			]);
 
-			const data2 = await db.sortedSetIncrByBulk([
+			await db.sortedSetIncrByBulk([
 				['sortedIncrBulk5', 5, 'value5'],
 			]);
 			assert.deepStrictEqual(
@@ -1137,6 +1141,41 @@ NUMERIC)-- WsPn&query[cid]=-1&parentCid=0&selectedCids[]=-1&privilege=topics:rea
 				],
 			);
 		});
+
+		it('should return empty array', async function () {
+			const zset = utils.generateUUID();
+			const response = await db.sortedSetIncrByBulk(zset, []);
+			assert(Array.isArray(response));
+			assert.strictEqual(response.length, 0);
+		});
+
+		it('should aggregate increments to the same key/value pair', async function () {
+			const zset = utils.generateUUID();
+			await db.sortedSetIncrByBulk([
+				[zset, 1, 'baz'],
+				[zset, 1, 'baz'],
+				[zset, 7, 'baz'],
+				[zset, 1, 'foo'],
+				[zset, 3, 'foo'],
+				[zset, 4, 'foo'],
+				[zset, 2, 'fizz'],
+				[zset, 1, 'fizz'],
+				[zset, -3, 'fizz'],
+			]);
+			const score = await db.sortedSetScores(zset, ['foo', 'baz', 'fizz']);
+			assert.deepStrictEqual(score, [8, 9, 0]);
+		});
+
+		it('should handle parallel increments with same key/value pairs', async function () {
+			const zset = utils.generateUUID();
+			await Promise.all([
+				db.sortedSetIncrByBulk([[zset, 1, 'baz']]),
+				db.sortedSetIncrByBulk([[zset, 1, 'baz']]),
+				db.sortedSetIncrByBulk([[zset, 1, 'baz']]),
+			]);
+			const score = await db.sortedSetScore(zset, 'baz');
+			assert.deepStrictEqual(score, 3);
+		});
 	});
 
 
diff --git a/test/topics.js b/test/topics.js
index f67e0fc40e..767af84901 100644
--- a/test/topics.js
+++ b/test/topics.js
@@ -1425,10 +1425,13 @@ describe('Topic\'s', () => {
 			const unreadTids = await topics.getUnreadTids({ cid: 0, uid: uid });
 
 			await sleep(2000);
-			const _unreadTids = await topics.getUnreadTids({ cid: 0, uid: uid });
+			const [_unreadTids, topicData] = await Promise.all([
+				topics.getUnreadTids({ cid: 0, uid: uid }),
+				topics.getTopicData(result.topicData.tid),
+			]);
 			assert(
 				!unreadTids.includes(result.topicData.tid),
-				JSON.stringify({ unreadTids, _unreadTids, tid: result.topicData.tid })
+				JSON.stringify({ unreadTids, _unreadTids, topic: topicData })
 			);
 		});
 	});