The fnvdecomp community has finished reconstructing Fallout: New Vegas's code in C++: on 2 November 2026, the project website closed submissions, and the result was published on GitHub. According to the team, all 70,611 functions within the project's scope compile into machine code that matches the original byte for byte. Other mod and engine developers are already using the work.
What was reconstructed, and how it helps modders
The team reconstructed functions from Steam's 1.4.0.525 version of FalloutNV.exe. Where researchers previously had to decipher machine instructions, they now have a C++ representation of the game's logic to compare against the original. The report lists 11,537,067 matching bytes; code generated by the compiler and linker is excluded from the count.
This is reconstructed code, not source code released by Obsidian. The authors specifically warn that many function names are still provisional and that AI-written comments may contain mistakes. Matching machine code verifies the compilation result; it doesn't automatically make every explanation of that code correct.
- According to the team's report, 70,611 out of 70,611 functions within the project's scope reproduce the original bytes.
- The repository is open for research, modding and game preservation; improvements to names and comments can be submitted through pull requests.
- Changes undergo automated code-matching checks; the documentation also describes comparing the main branch against the original EXE on the project's machine.
- Working with it requires your own Steam copy of Fallout: New Vegas 1.4.0.525. Game assets and executables are not distributed.
In practice, the decompilation has already become a reference for several different projects:
- Gargoyle's New Vegas Co-op: co-op for the original game with one to three friends. Its README credits fnvdecomp, and the version 0.1.9 changelog lists nine fixes resulting from checking the entire mod against the decompiled game. These include fixes for container contents when reconnecting and object desynchronisation after loading an older save.
- slaterain's nv-rs: a new engine written in Rust. In a decision dated 2 November, the team allowed fnv_decomp to be used as a reference for names, types, data structures and logic being translated into Rust.
- cranebeaker's FNV Co-Op: a separate story co-op project built on nv-rs. Its README labels it a developer preview; shared quest progress, dialogue, karma and reputation are among the project's stated features.
🔥 For players, the main benefits are currently coming through individual mods and engines. fnv_decomp itself is not a playable port: building it produces object files for comparison, rather than a runnable game. Removing the memory limit and adding new co-op features require further work in the relevant projects; publishing the C++ code doesn't enable those features by itself.
Reconstructing the code helps researchers check how the game actually handles data, rather than relying on guesses about its engine. I explored a similar interest in older RPGs in my article on the Gothic 3 world viewer and Genome engine. In New Vegas's case, the team has reached a public repository with a documented procedure for verifying code matches.
The next project: Fallout 3
Work on Fallout 3 is already open at f3decomp.com. The website tracks the reconstruction of functions from version 1.7.0.3 and says the result will be published once every function within the project's scope matches and has been integrated. No completion date has been announced.
I'd pay closest attention to the specific fixes that emerge from this reference: their value will be easier to judge in the game itself. I collect updates like these in the modding news section.
