How censorship in WeChat works
Keyword censorship can be implemented in two ways: on the client-side (i.e., on the application itself) or on the server side (i.e., on a remote server). In a client-side implementation, all of the rules to perform censorship are inside of the application running on your device. Often the application has a built-in list of keywords that it uses to perform checks to determine if any of these keywords are present in your chat messages before your messages are sent. If your message contains a keyword from the list then the message is not sent. In a server-side implementation the rules to perform censorship are on a remote server. When a message is sent, it passes through the server that checks if banned keywords are present and, if detected, blocks the message.

Keyword filtering on WeChat is only enabled for users with accounts registered to mainland China phone numbers, and persists even if these users later link the account to an International number.

Keyword censorship is no longer transparent. In the past, users received notification when their message was blocked; now censorship of chat messages happens without any user notice.

More keywords are blocked on group chat, where messages can reach a larger audience, than one-to-one chat.
Keyword censorship is dynamic. Some keywords that triggered censorship in our original tests were later found to be permissible in later tests.  Some newfound censored keywords appear to have been added in response to current news events.

WeChat’s internal browser blocks China-based accounts from accessing a range of websites including gambling, Falun Gong, and media that report critically on China. Websites that are blocked for China accounts were fully accessible for International accounts, but there is intermittent blocking of gambling and pornography websites on International accounts.