Valve's Cancelled Portal Prequel F-Stop Leaks in Playable Form

Short answer: F-Stop, the Portal prequel Valve built and shelved in 2007, can now be played start to almost finish. The last missing pieces, its map source files and source code, were posted to the Valve Cut Content Discord server at the end of October 2026. Combined with game assets from an earlier leak, fans have assembled a build Kotaku calls essentially 99 percent complete. It is not an official release, and Valve has said nothing.

The key facts at a glance

What F-Stop was

In 2007, Valve split into five small teams for a three-month exercise that Gabe Newell later called the Directed Design Experiments. F-Stop was the fifth project: a prequel to Portal built around an experimental Aperture Science camera.

The idea was simple to explain and odd to play. You take a photo of an object, it disappears into the picture, and you put it back into the world later, sometimes bigger or smaller, to get past a puzzle. Fans have compared it to the 2019 indie game Superliminal.

Kotaku reports that Newell liked it a lot, but playtesters were less keen. Valve dropped it and reused some of its characters, dialogue and ideas in Portal 2. A first detailed look at the concept surfaced in early 2020, which is when most players first heard the name.

How the pieces came together

This was not one leak but three.

1. Late August: roughly 13 terabytes of old Steam depot data leaked, covering much of Valve's catalogue from 2003 to 2013. It contained most of F-Stop's art and sound, but not its code or its maps.
2. Source code: posted to the Valve Cut Content Discord, a community that digs into unreleased Valve material.
3. Map source files: posted to the same server. Valve calls this folder "mapsrc."

Dataminer Gabe Follower summed it up on October 31: with everything combined, F-Stop "is fully playable with all code, assets, and now maps available to the public." Kotaku says the Valve Cut Content team checked the map files and confirmed they are genuine.

One detail stands out. According to the server's moderators, as quoted by Kotaku, the person holding the map files first asked for $1,000, then gave them away for free.

What it is like to play

Going by gameplay videos, VGTimes says the build can be beaten in under an hour. Engadget describes it as "largely stable" but still short on textures and dialogue, which is what you would expect from a three-month prototype that was never finished.

What happens next

Valve has not commented, and it has never said it plans to release F-Stop. The build is made from leaked Valve code and data, so Valve could act against copies that spread. For players who want something legitimate, an independent team called IronStone Collective is working on Stop The F, its own take on the idea.

Common mistakes

• Calling F-Stop a Portal sequel. It was planned as a prequel, and much of it fed into Portal 2.
• Calling this an official release. It is a fan assembly of leaked files.
• Treating "99 percent complete" as a finished game. It refers to how much of the original prototype has been recovered, not to a polished product.
• Assuming one leak did it. The August data dump had the art; the code and maps came later from a different source.