Making a Mod

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This article is designed to help you make a Source Engine MOD. First, it has some advice on starting a MOD, and assembling a team. Following that is a collection of helpful tips on figuring out the game design for your MOD. Finally, it has a step-by-step guide on how to tie up the loose ends and finish your MOD, which is actually the hardest part of MOD making.

Starting Out

Lets start by looking at how to assemble a team. The guiding rule here is to Keep it small. Managing a team of people is a full-time job, even when all the team resides in the same building. If you’re dealing with an on-line team, you can easily spend all your time managing it, and that means you won’t be spending any time on making your MOD. Adding more people to the team doesn’t mean more work will get done. The more people you have, the more time spent managing them. Team Fortress’s core team was 3 people. Counter-Strike’s core team was 1 person.

When looking for team members, try to only hire people you absolutely cannot survive without. Your first instinct might be to hire anyone who can code, or model, or make maps, and so on. But, for your first version, you probably won’t need more than 1 person for each area of your MOD (code, sound, models, maps). You may not even need any new models, sounds, or maps. Don’t hire anyone until you’ve seen examples of their work. Make sure they’ve actually finished things. If they’re a modeler who’s done 20 models, but they’re all half-finished, you don’t want them.

MOD Design

As a MOD author, the most useful question you can ask yourself is "Why should someone play my MOD?" It's a hard question to answer truthfully, but if you can answer it well, you're on the right track. Think about what other MODs are out there, and what they offer. Does your MOD offer something new to the players? Is what you offer enough to entice players who are busy playing other MODs? Even if you can't answer this question, just thinking about it will probably help your MOD.

You have power commercial developers don't: You don't have to worry about the commercial viability of new gameplay styles. Commercial developers have to worry about appealing to retail, breaking even, and other nasty things, which is why most games are slight modifications on already proven gameplay. But you don't. You can try out truly new gameplay ideas that just might become the Next Big Thing. This is your edge over commercial developers. Make your job easier by concentrating on this edge, and don't spend your time trying to compete in the areas that commercial products are strong in. Most MODs can't compete on a content level (maps, models, sounds, etc) with commercial products...they've got teams of artists with years of experience. Beat them with your gameplay. Players will play a MOD that has very little in the way of new content, but has really fun gameplay. Something many people don't realize is that Team Fortress 1 had almost no new art for a year after it was first released.

You have another power over commercial developers. You can release much, much faster and more often than they can. We've summarized this MOD development philosophy with the phrase, "Release soon, Release often." Commercial developers work for 2--Strike didn't have half of the features they have now. The CS team released a high quality, but not big MOD. Over the past year, they've been regularly adding more and more features and, in response, their player base has just continued to grow and grow.

"Different is NOT always Better." When thinking about your game design, don't fall into the trap of believing that "Different is Better." There's no reason to rewrite the shotgun code and have a new shotgun model if it doesn't impact your game in any interesting way. Keep in mind the first question, "Why should someone play my MOD?" The answer, "My MOD has a new combat system, and a new movement system," isn't necessarily a good answer. So your combat system is different that Half-Life's...OK…but is it better? Does it make your MOD more fun to play? Does a new movement system make the game more fun? Player's are used to existing systems, and making them learn another one needs to be worth it for them. So before you think about changing something, make sure you know you're changing it for the better, and that it'll make your MOD more fun. Don't be afraid to just leave something the same as it was in HL.

Create realistic goals for yourself. Think about how long it takes a commercial developer to make an FPS shooter with 10 weapons. If your MOD is going to have 40 weapons, you're making life really hard for yourself. The thing to keep in mind here is "Quality over Quantity." Players would far prefer to have 10 unique, well balanced, and fun to use weapons than 40 unbalanced weapons, some of which are slightly tweaked versions of others.

Don't be afraid to cut content and features. If the MOD looks like it's never going to be finished, or there's some content that you don't think meets the quality of the rest of the MOD, then start cutting. During the development of HL at least 30% of the original features in the design were cut because it became obvious they were unattainable in our timeline, or because we decided they weren't worth their development time. As we said above, "Quality over Quantity." Players would prefer having 3 really good, well play-tested maps over 10 untested ones, and it'll give your MOD a reputation for quality content. Don't let the world see your worst stuff.

Understand the engine. You really should read the documentation included in the SDK. The thing you'll learn most by doing so isn't whether you can do X with the engine, but rather how X should be done so it works well. You can make a gun that fires 50 rockets, but if you don't understand the way the engine works, you might do it in a way that significantly increases the network traffic your MOD uses. This is important for everyone in your MOD. If your mapmakers don't understand the engine, they'll make huge maps without any thought for how much network data will be sent to the players in them, and everyone will blame your code for being too network intensive. If you're a programmer, it's a good idea to join to HL Coders mailing list, where you'll be able to talk to many other MOD programmers, and a few Valve employees as well. The mailing list has archives going back a long way, which contain a lot of useful solutions to common MOD problems.


We see a lot of MODs that start out strong, produce a lot of great looking content, and never quite make the last step of getting it into the player’s hands. This section will help you get into a release mode where you're driving towards producing a releasable version of your MOD.

We chose five weeks as a starting estimate of the time it'll take to get from normal development mode to a shippable version. It's likely you'll get better, and hence faster, at this with successive releases. If your MOD is larger in scope, or your team is substantially international, then it is likely to take more than five weeks, though the steps will be similar to the following.. If possible, try and get the team to commit a few hours of every day to the MOD for this period of time. If some team members can't do that, you're probably better off removing them from the shipping process. Get them to hand their part to someone else on the team who can put in the required effort. Shipping a product, even a small product, is hard and requires a substantial commitment.

There are many things in this section that might sound harsh or rigid. This is unfortunate, but a reflection on how hard a process this is. The advice here is a summation of lessons learned in the shipping of many products, and most of it a result of painful mistakes that set back our release dates. When you wonder whether a particular piece of advice here is necessary, it's possible that we once added weeks to our release date because we didn't take it.

This is also something that prospective employers are extremely interested in. It’s one thing to see that a MOD maker has produced a bunch of cool stuff, it's another thing entirely to see that they produced some cool stuff and actually shipped it out and people played it. The coolest map/model/code/sound/etc. in the world is useless if you couldn't go the last mile and ship it.

Fear not, this gets a lot easier once you've been through it a couple of times... by the third or fourth release of your MOD, you'll be an expert!

Five Weeks Out

Centralize Ownership

You should designate a single member of your MOD team as the Shipping Leader (SL). This person will drive progress on the MOD for the next five weeks. All changes made to the MOD from now on should occur only at the request of the SL, and all requests for changes should be funneled through this person. No team member should make any changes, no matter how minor, to the MOD unless the SL has requested that they make a particular change. This doesn't mean the rest of the team are losing control of the MOD... the SL is still a part of the team, and will be listening to all feedback. The point of the SL is to ensure that all changes to the MOD are going through a single person. This avoids problems such as a mapmaker breaking the game by making a last minute change because he didn't realize something else had changed in the game code. The SL will know the state of every component in the game (code, maps, models, textures, etc) at all times throughout the next 5 weeks to ensure this never happens. Choosing the SL isn't easy. Here are a few tips: - Don't immediately assume the person who's currently running the MOD is the best choice for SL, especially if the MOD has been worked on for months and hasn't got any closer to being released. - Game Coders are probably the best choice. As the shipping process comes to an end, most fixes will be made in the game code. - The SL should be highly motivated, disciplined, organized, and as ego-less as possible. The SL will need to be able to commit five weeks of his or her life to this process. - The SL should be able to make global decisions for the MOD. The SL should understand that this often requires cutting features and content in order to ship

Establish a Build Process

You need to create a process by which you build your MOD. Building is the process of taking all your work and producing an installable, working version of the MOD (generally in the form of an install file). This should be done exclusively by the SL for the next five weeks, and the SL should have a strict process that is always followed. Creating a strict process for this will ensure you don't waste hours tracking down bugs that are simply a result of someone building in a different way than the previous person. The SL should maintain the final/release candidate version of the MOD from now on. All changes should be sent to him, and he should incorporate them into his copy of the MOD one by one and with a full understanding of the impact of the changes on all parts of the MOD. Don't forget to backup your code and content regularly! The SL should build the MOD every day for playtests. More on that later.

Feature Locking

At this point in the shipping process, five weeks out, you should also be feature locked. This means you shouldn't be adding any new features to your MOD whatsoever. If part of your team is not involved in shipping but wants to continue working on developing the MOD, they should be working in a separate content and code database. Most source code control packages allow for branching of code in this fashion. (Yes, we strongly recommend that you use some form of source control ). Every change made to the MOD from now on should be a bug fix. The SL should ensure this. Even if a coder thinks of another cool feature that they say will only take them 10 minutes to code, do not let them add it in. Even if the coder sends the code, finished and bug-free, to the SL, do not add it to the MOD. Save it for the next version. A healthy attitude for the SL to have is that every change to the MOD from now on will add two days to the release date.


From now on, you should be running playtests every day, or every second day if that's too much. Playtests should be based on installable versions of the game, built by the SL. Don't let team members play from their personal versions of the game... everyone should be running a version of the MOD installed from a build sent out by the SL (that’s what your viewing public will be installing and using, that’s what you should be testing). You'll waste many hours on finding bugs caused by incompatible versions if you don't do this. To make this easier, the MOD must be kept in a playable state at all times. Get very, very worried for every day the MOD isn't playable. If a coder or mapmaker makes a change that breaks the MOD, think about it carefully before incorporating it into the SL's build. How long will the game remain unplayable? How many playtests will you miss? How many team members won't be able to work because the MOD isn't running? Not breaking the MOD should be religion for the team. When you do playtest, make sure as many of your team members are playing as possible. Everyone working on the game should be playing it regularly. Make sure you have some external players as well. Turn on server console logging (set "log" to "on" in the server.cfg file). This will dump all the output of the server into a file in the gamedir/logs directory ( the name of the file will match the date). Whenever any player in the game spots a bug, have them use their "say" key to say "BUG: description of bug". Then, when the game is over, you can open up the log file and get all the bugs out of it by searching for the word "BUG."

Bugs /Changes

The SL should maintain a complete list of all bugs and changes, and their current status. Preferably this should be done using some kind of true database. E-mail is totally insufficient for tracking bugs; it’s just too easy for items to drop of the first page of a user’s mailbox, etc. After each playtest, the bugs and necessary changes from the log file should be added to the list by the SL, and assigned to team members. When a team member has fixed a bug / change, they should submit the new content to the SL, who should verify that it is fixed and then update the status on the bug list. The bug list is a fantastic tool to evaluate how well you are progressing. It can be used to find out who is overloaded with work, who is underloaded, who is not fixing his bugs, which area of your MOD is farthest from completion, and so on. Don't remove anything from the bug list, even when it has been fixed (though you should mark it as fixed in some way, of course). It's very useful to see what bugs have been fixed throughout the history of the project. Something might regress, re-creating a bug, and knowing who fixed it last time makes it easy to ask them what caused it. At the end of the project, you should be able to see every bug fixed and every change made in your MOD for the entire shipping process. The SL shouldn't allow any bug fix or change into the SL's master copy of the game unless the bug / change has been detailed in the bug list. There is software that will help you create and maintain a bug list. Alternatively, a spreadsheet will work just fine. Again, e-mail is a bad choice.

Cut / Defer Broken Features

The hardest, nastiest, and unfortunately most necessary part of shipping is the act of being realistic and cutting features. We have a saying at Valve that everyone will have their favorite feature cut from the game. While it's not true, it does help everyone prepare themselves for the fact that they will have features they like-- or that they spent some to a lot of time on--cut. Your game simply cannot have every cool feature and still ship in a reasonable time frame. The SL should make decisions about what to finish and what to cut, based on how far along in the release process you are. The closer you get to releasing, the more you should think about each bug as you find it. Is the bug in a feature that absolutely must be in this version? How many days will it take to fix this feature? Can this feature be cut, or deferred to a later version?

Work Smart, Not Hard

As we’ve said over and over again, the shipping process is hard, and it's even harder if you don't think carefully about what to work on. Working a lot is no substitute for carefully choosing what to fix, what to defer, and what to cut out altogether. The SL should be extremely careful about which bugs / changes should be worked on, and by whom. Don't spend a week fixing a minor problem in a feature just because the feature is cool. Fix crash bugs (showstoppers). Fix bugs that utterly prevent you from shipping the game. Fix bugs that are preventing other team members from fixing their bugs. The SL should develop categories for bugs to aid in making the right decisions. A good level of granularity is Must Fix, Severe, Medium, Minor, Zero, Deferred. As the project gets closer to shipping, the SL should be carefully evaluating every bug that shows up. Remember, every bug that's fixed creates more playtesting, and usually more bugs. If you are two weeks from your release date, and you've got a bug that will take someone three days and 500 lines of code to fix, you're not going to make that release date unless you cut or defer that portion of the game.

Three Weeks Out

Content Locked

By now you should aim to be content complete. This means that all content in the game is in a locked state, except for the game code itself. All the maps, models, textures, sounds, HUD art, launcher art, and so on should be finished and in the SL's master copy.

Shutting Down

This was mentioned at the five week mark, but it's even more important now. The SL is the only person who should be touching the master copy of the game, and he should simply be rolling in the bug fixes from the coders, who should be fixing only the bugs the SL hands them.


The MOD should be being playtested every day, for at least two hours. Between now and when you ship, you want as many people as possible hammering away at your MOD. It’s too late now to make any major game design changes – don’t even be tempted.

One Week Out

No last minute changes

The SL should be evaluating every change that has to be made, and deciding whether they should be deferred to the next version. Again, a healthy way to think about it is that every single change, even if it's a single line of code, will add two days to the release date.

2 day Safe period

Once every bug that is going to be fixed has been fixed, and everything else has been deferred, you're not done. Now you wait at least 48 hours, during which time you should playtest like crazy. Try to get everyone hammering away at the game for as much time as possible. If you find any more bugs that have to be fixed, fix them, and start the 48 hours again. If your MOD passes 48 hours of heavy playtesting without finding any new issues, you're ready to release!


So you’ve released, the players love it, and web pages everywhere are talking about how much fun your MOD is. Whether you’re done now is up to you. From our experiences in the on-line multiplayer field, a MOD only stays popular as long as it’s supported. No matter how great your MOD is, it’s not going to garner really significant player numbers with its first version. Player numbers are grown over time through repeated releases of new content, bug fixes, and of course, community support. Both Counter-Strike and Team Fortress started out small and grew over time. Each time they released a new version, more players tried them out and started playing them. Knowing what to fix, what to change, and how to listen to your community is a continual learning process.

Good luck!