History

Discovery

Inventory Slot Transfer (IST) was discovered in June 2022 by zxrobbin ( Video 1, Video 2 ). It initially manifested as an innocent glitch to quickly duplicate items in Breath of the Wild. Little did we know that IST will become the most complicated glitch to ever exist in BOTW.

Around June 17, 2022, as the news spread, glitch hunters from the BOTW speedrunning community started to investigate and quickly found some patterns to the glitch.

Hundo Duplication Simulator

Meanwhile, speedrunners of the 100% category started to incorporate it into the route. However, even the simplest form of this glitch required tracking the exact state of the player’s inventory for every action executed, such as picking up or dropping an item. The rules and patterns of this glitch then needs to be applied at each of these steps. It was a painstaking process to manually track all the information.

Luckily, tasks that are difficult for humans are often easy for computers. On June 18, 2022, the Hundo Duplication Simulator was born. Users could type actions such as get 1 core into the simulator. The simulator app will display the inventory state and allow the user to quickly navigate between the steps. This was a massive help for optimizing setups for speedruns because people can change one step and the app will re-calculate the state for every step afterwards. This model is still used by the simulator today.

V2 - IST Simulator

As more applications of IST are found, the glitch has found its use in categories outside of the 100% category, for example in All Shrines and All Dungeons. One of such application is Direct Inventory Corruption. However, the simulator was built in one day and did not support any cases other than the very basic rules that were initially discovered.

Now that this glitch is not specific to 100-percent, the project was renamed to IST Simulator. V2 of the simulator added GameData Inventory and more items to simulate the inventory state closer to how the game handles it. Direct Inventory Corruption support was added. People were happy.

V3 - IST + Weapon Modifier Corruption

…Until more applications of IST are found again, and the simulator being a simulator, did not support these new exploits because I couldn’t predict the future.

Every time a new discovery is made, it almost certainly invalidates assumptions previously made when coding the simulator, and the core has to be re-designed to incorporate the new knowledge. In November 2022, I started re-designing the core by referencing the BOTW decompilation project. This massively improved the accuracy of the simulator and supported new applications such as Weapon Modifier Corruption. The UI was also revamped, adding syntax highlighting to commands and displaying modifier information for items.

V4 - Skybook

After the V3 update, a few small fixes were submitted, but no big re-architecture was made for 2 years.

While V3 was mostly accurate for setups viable for speedruns, it was still far from being perfect:

It didn’t handle special cases for champion abilities or Travel Medallion.
It didn’t handle cases where tabs aren’t discovered
It couldn’t reliably detect and warn user when an action is not possible in game, such as interacting with the inventory when mCount is 0.
…

It was clear that we can not chase these edge cases forever. Something needs to be remade from the ground up, again. Sometime in 2022-2023, I have theorized to build a mini-emulator, to run the simulator from a small part of the game itself. I submitted this idea as a proposal for senior project at my college, and got a student team who was interested in it. I lead the team as the advisor to build a prototype that would later become BlueFlame, the new core for V4.

At the same time as BlueFlame was being developed by the students, I worked on everything else - a new command system, new simulated systems like Overworld and Screens… And finally, in July 2025, Skybook was born.