Check your license!
By operating the original client, you might have agreed to a license, the EULA. Check that license for what it says about you being allowed to do with the product.
In a recent case of Bungie vs. Elite Boss Tech, a US court found in a default judgment that a certain cheat software that would interact with both the game's client and the company's servers among other things did...
- constitute copyright infringement [as a derivative work]
- was a breach of contract [formed by the EULA and agreeing to the Terms of Service for the game]
- was interference with Contractual allegations [of other users that obtained the software]
However, not all internet games or experiences are locked down like that. Other companies do offer an open API set or terms under which a client's software may be modified. As a very generous example, LindenLabs does for its SecondLife Virtual World, for which they provide not just the framework, but also the terms under which you are allowed to make a third party Viewer
Sometimes, the Terms of service are rather hidden. For example, the Chinese Go platform Fox Weiqi operates in china. To get to the terms of service, you need to download the free client, go to Settings (via the cog), then choose 野狐围棋用户协议, which is Chinese for Wild Fox Go User Agreement. This links to https://edu.foxwq.com/complex/useragreement.html. The user agreement is of course in Chinese, but Google Translate manages to get that to English. It contains the following clause:
7.2 Unless permitted by law or with the written permission of Yehu, you shall not engage in the following acts during the use of this software:
( 1 ) Delete the copyright information on the software and its copies;
( 2 ) Reverse engineer, reverse assemble, reverse compile the software, or try to find the source code of the software in other ways;
( 6 ) Log in or use Yehu Go and its services through third-party software, plug-ins, plug-ins, systems not authorized by Yehu, or make, publish, and disseminate the above tools;
( 7 ) Interfering with the software and its components, modules, and data by itself or authorizing others or third-party software
While it takes a court to see if the terms hold water and are actually enforceable because of how the EULA is offered, the face value of those clauses is, that it is forbidden unless you have a specific law that allows such or you obtain written permission.
With that kind of possible liability that might cost millions, there might be serious legal problems. Read your license agreement and terms of service and consult a lawyer.