If you’ve ever watched SaltyBet and wondered “how is this even possible” — how can Goku fight Shrek fight a literal refrigerator on a 24/7 Twitch stream — the answer is MUGEN.
MUGEN (pronounced “moo-gen”) is a free, customizable 2D fighting game engine that’s been around since 1999. The name comes from the Japanese word 無限, meaning “limitless” or “infinity.” It’s also technically an acronym, but here’s the thing: even the creators forgot what it stands for. We love that.
In this post, we break down what MUGEN is, how it works, the wild story behind its creators, and why it’s the engine that makes SaltyBet possible.
MUGEN was created by a group called Elecbyte — three University of Michigan electrical engineering and computer science students who built it because, in their words, “there weren’t any good commercial fighting games on the PC at the time.” So they made their own engine. Respect.
The first public beta dropped on July 27, 1999. It was a DOS application, written in C, and it let anyone create custom fighting game characters with their own sprites, moves, sounds, and AI behavior. The idea was simple: build a fighting game where the community creates the content. 26 years later, that’s exactly what happened.
Here’s the wild part: Elecbyte’s founders were anonymous for decades. They went by aliases — “Akito,” “Geki,” and “Admin” — and disappeared from the internet around 2002. The community spent years trying to figure out who they were. It wasn’t until researchers dug through archived websites, copyright notices, and Michigan business records that the connection to the University of Michigan was confirmed. To this day, the individual names haven’t been publicly disclosed. The creators of one of the most influential fan game engines ever made are essentially ghosts.
| Year | What Happened |
|---|---|
| ~1997-98 | Elecbyte starts building MUGEN at the University of Michigan |
| 1999 | First public DOS beta released (July 27) |
| 2001 | DOS development ends, Elecbyte switches to Linux |
| 2002 | Last official release before Elecbyte vanishes |
| ~2003 | A Windows version gets leaked (“WinMUGEN”), becomes the community standard |
| 2009 | New team with source code access releases Version 1.0 RC1 |
| 2011 | Version 1.0 stable released (January 18) |
| 2013 | Version 1.1 Beta released (August) — SaltyBet launches around the same time |
That last line isn’t a coincidence. SaltyBet started streaming AI vs AI MUGEN fights right around when the 1.1 beta dropped in 2013. The engine had just gotten a major update, the community was active, and someone had the brilliant idea to stream it 24/7 with virtual currency betting. The rest is history — and 570,000+ recorded matches later, here we are.
MUGEN is essentially a framework. The engine handles the fighting game mechanics — health bars, rounds, hit detection, physics — and the community creates everything else. Characters, stages, music, menus, all of it.
The engine uses a 7-button system: A, B, C, X, Y, Z, and Start. Typically that maps to three punches, three kicks, and a taunt, similar to a Capcom-style six-button fighter. But since every character is custom-made, creators can map those buttons to whatever they want.
Every character is a folder containing a set of files:
| File | What It Does |
|---|---|
.def | The definition file — the character’s ID card. Links everything together |
.sff | Sprites — all the visual frames for every animation |
.snd | Sound effects — hits, voice lines, special moves |
.cns | States and logic — the character’s behavior, stats, and how moves work |
.cmd | Commands — what button combinations trigger which moves |
.air | Animations — how sprites sequence together for each action |
This is what makes MUGEN so powerful. If you can draw sprites and write state logic, you can create any character you want. A frame-accurate recreation of Ryu from Street Fighter III? Sure. An original character with completely custom mechanics? Absolutely. A sentient car that shoots lasers? Yeah, that exists too.
This is the part that matters most for SaltyBet, because every match is AI vs AI. No humans involved.
MUGEN has three types of AI:
Default AI — When a character has no custom AI coded, MUGEN literally mashes random buttons on their behalf. This is as chaotic as it sounds. The character might throw out moves, might jump around, might just stand there. If you’ve ever watched a SaltyBet P-tier match where a character seems confused about life, this is probably why.
Custom AI — The character creator writes specific AI logic into the .cns file that tells the character when to block, when to attack, which moves to use at which ranges, and how to respond to the opponent’s actions. Good custom AI is the difference between a character that dominates S-tier and one that gets stuck in P-tier.
Neural Network AI — Some advanced creators have experimented with machine learning-trained AI for their characters, though this is rare.
The quality of a character’s AI is one of the biggest factors in SaltyBet outcomes. A beautifully drawn character with terrible AI will lose to an ugly sprite with smart AI every single time. This is why “never judge a character by its looks” is unwritten rule #1 on SaltyBet.
Anything. That’s not an exaggeration. The MUGEN community has created:
The MUGEN Database wiki alone documents over 4,400 pages of content. Multiple download sites host thousands of characters and stages. SaltyBet’s own roster has over 10,000 unique fighters, all sourced from the MUGEN community.
MUGEN also supports custom stages (backgrounds with their own animations and music), screenpacks (complete UI themes that change the look and feel of the character select screen, menus, and HUD), and lifebars (custom health bar designs).
SaltyBet uses MUGEN’s built-in Watch mode — an AI vs AI spectator mode where you can pit characters against each other and just… watch. SaltyBet took that concept, streamed it on Twitch, added virtual currency betting, and created one of the most oddly addictive corners of the internet.
Every fight you see on SaltyBet is two MUGEN characters with their built-in AI fighting each other. The outcomes are determined entirely by how those characters were coded — their moves, their stats, their AI logic. This is why tracking historical performance data matters so much. A character’s AI doesn’t change between matches (unless the SaltyBet admins update the roster), so past performance is genuinely predictive of future results.
This is also what we built SaltyTrack around. Our Chrome extension uses 570,000+ historical matches to show you win rates, head-to-head records, and AI-powered predictions. When the data says a fighter has an 89% win rate across 200+ matches, that’s not a hot streak — that’s a well-coded AI doing its thing consistently. The data doesn’t lie.
MUGEN has one of the longest-running fan communities in gaming. Some of the major hubs:
The community has been creating content for 25+ years. Characters range from pixel-perfect professional quality to intentionally absurd meme-tier creations. Both have their place, and honestly, both show up on SaltyBet regularly.
One question that comes up: is all of this legal?
MUGEN itself is free software — always has been. The engine is totally legal to download and use. The gray area is in the characters, since many are based on copyrighted properties (Street Fighter characters, Dragon Ball Z characters, Marvel characters, etc.).
In practice, IP holders have generally turned a blind eye to MUGEN content as long as it stays non-commercial. Nobody is selling these characters, nobody is profiting off the sprites themselves. It’s fan-created content for a fan community, and most rights holders treat it the same way they treat fan art or fan fiction.
There have been a few exceptions. French Bread (the studio behind Melty Blood and Under Night In-Birth) explicitly asked the MUGEN community to stop using their character assets. And Elecbyte’s own license prohibits using the engine for commercial purposes — when a Taiwanese company tried selling MUGEN content, they ran into legal trouble.
But for the average MUGEN user creating and sharing characters for free? 25+ years and counting with no issues.
MUGEN is free to download from elecbyte.com. From there, you can:
chars/ directoryselect.def fileThe default install comes with Kung Fu Man, MUGEN’s built-in example character. He’s basic but functional — the “Hello World” of fighting game characters. From there, you build out your roster however you want.
Fair warning: once you start customizing, it’s hard to stop. There’s a reason the community has been doing this for over two decades.
What does MUGEN stand for?
MUGEN is technically an acronym, but even Elecbyte has said they forgot what the letters stand for. The name also comes from the Japanese word 無限, meaning “limitless” or “infinity.” We think that meaning is more fitting anyway.
Is MUGEN free?
Yes, completely free. Always has been. You can download it from elecbyte.com.
Is MUGEN still being updated?
The most recent official release is Version 1.1 Beta 1 from August 2013. The engine itself hasn’t been updated since, but the community continues to create new characters, stages, and tools actively. Some community-driven forks and tools extend the engine’s capabilities.
Can I make my own character?
Yes! You’ll need sprite artwork, knowledge of the CNS scripting language for states and AI, and patience. The community has extensive tutorials. It’s not trivial — a polished character can take months of work — but it’s absolutely doable.
How does SaltyBet use MUGEN?
SaltyBet runs MUGEN in Watch mode (AI vs AI) and streams it 24/7 on Twitch. Two characters are selected, their built-in AI fights each other, and viewers bet virtual currency on the outcome. SaltyBet’s roster includes over 10,000 MUGEN characters across five tiers.
The best way to experience MUGEN is to watch it live on saltybet.com.
If you want an edge while you’re at it, grab the SaltyTrack Chrome extension. Free stats, AI predictions, and head-to-head records for every MUGEN fighter in the SaltyBet roster. We’ve analyzed 570,000+ matches so you don’t have to bet blind.
SaltyBet uses virtual currency only. No real money is wagered or exchanged. SaltyTrack is not affiliated with SaltyBet or Elecbyte.