The Witcher 3 devs had to practically remake the game engine to make official modding possible

The Witcher 3: Wild Hunt is still going. Nearly 10 years into its life, The Witcher 3 remains a popular canvas for modders adding new quests and locations, custom gameplay tuning, and the occasional oddity like Geralt on ice skates. For most of the last decade the mod scene had no official support from CD Projekt, but in May 2024 CD Projekt finally launched REDkit, a set of modding tools that enables more streamlined creation and integration of mods.

Why did that take so long? As CD Projekt explained in a panel at this year's Game Developers Conference, it had to essentially remake major portions of The Witcher 3's RED Engine over 10 months to ensure that the REDKit would work for audiences

"What works for internal dev may not work for modders," said publishing producer Oleg Shatulo. In addition to streamlining the technical interfaces and workflow within the tools, CDPR had to add critical options like Projects, a feature used to organize individual modding programs, which didn't exist within the original engine.

Back in 2015 CD Projekt said it had no plans to release modding tools as substantial as REDKit. But it's fair to say no one at the company anticipated The Witcher 3 would remain as popular as it has.

"We were starting to shift our internal focus to other projects slowly, so as our final goodbye to The Witcher 3, we wanted to extend its life by empowering the modding community so that they can keep improving and expanding the game," said Shatulo. "So it made perfect sense for us to create an official modding tool that we call REDkit."

For many years, CD Projekt Red has had experience collaborating with modders—players who go the extra mile tuning their games by learning the same programming and design skills the developers have. In some cases, modders from the Witcher community have landed jobs with CD Projekt Red or established support studios—which is how the GDC panel's second speaker, Bence Hambalkó, got his start in game development. He eventually became the CEO of Yigsoft, a support studio that's contributed to The Witcher 3 and Cyberpunk 2077.

Shatulo and Hambalkó tackled three particular challenge areas in giving the player community access to the core tools used to make The Witcher 3: Technical, dependencies, and documentation—all of which created a confluence of issues that would potentially make REDkit unwieldy for the public.

They also had to clean up years of code that CD Projekt had never expected anyone outside the company to see. In one slide from the panel, the developers showed a set of in-jokes and joke images that the devs would leave each other hidden in the game engine—which included Geralt wearing a Cyberpunk-style outfit, but made in the style of The Witcher.

"Some examples we found were Geralt with a Cyberpunk jacket, ASCII art rewriting the code, and some designers [leaving messages] arguing how to write correct gameplay code within the game engine itself," said the CEO of Yigsoft. "So you never know what you can find, and you really have to be careful because you might miss something you don't want the public to see." They created some automated tools to find some of those problem files, but also had to perform the "really painful" process of checking the code manually.

The team had to make REDKit more easily customizable for users so that they could import their own data and files, which would often be created with different tools than the original developers had used. Modders can use Maya to record their lines of dialogue and rig animations for characters' voice performances, for instance. In a showcase for the revised voice asset and automated motion capture, they used a Captain Picard monologue from the film Star Trek: First Contact but with Geralt's head to show how adaptive the REDKit assets can be.

Hambalkó said the response was positive—and overwhelming—when the mod tools were released in 2024. "To reflect from someone like me, who used to be involved in the modding community: It's amazing to see all the debug tools available and have just such a depth of access to our models," said Hambalkó. "We got feedback from some players that "this is not a modding tool, this is an engine, you guys are crazy." So we were super happy about how it turned out."

The Witcher 3: Wild Hunt saw a suite of updates and DLC expansions throughout its post-launch life, but with the full release of REDKit, the goal was to give tools to players that would make modding not only more manageable, but also easier to share. Alongside it CD Projekt launched an ambitious video series on YouTube that gave modders tips and instructions on how to use the tools.

"After adapting the engine, working through the dependency challenges, building the documentation, testing, finally releasing it, we can clearly say that it was all worth it," said Shatulo. "We set out to give modders the tools to create something amazing, to push the boundaries of what's possible with the game, and to keep the game alive for years to come. Seeing the community's response and the incredible mods they have made was incredibly rewarding. And hopefully, it will inspire a new generation of game developers who now have access to a tool like this."