Saving & Validating

Check project

Run "Check project" before saving. It walks the whole behaviour and flags problems, including the one that matters most: a state with no generator, which crashes the game on load rather than on entering the state. The crash happens during an unconditional node walk while the graph loads, not when the player reaches that part of the state machine. Affected states are named directly in the results so you know exactly what to fix, and clicking a result jumps you straight to the node on the canvas.

Other checks include unreachable-state warnings (a state nothing transitions into, sometimes legitimate if something external sets it, so this is a warning, not a hard error) and repack comparisons that catch drift between what you changed and what actually got written.

Saving

Save writes your changes back to .hkx and keeps your original file as a .bak alongside it. Most edits, such as changing a value, renaming something, or growing an array, are written directly into the file's own bytes or appended to the end without moving anything that's already there. Only a change that adds or removes objects, or lengthens an array of text, goes through a repack step. Before that step overwrites anything, it reads back what it just produced and counts the objects in it. If anything went missing on the way through, nothing is written, and you're told what was lost instead of ending up with a silently broken file.

Current status

Field-level edits are confirmed working in the game. A garage door whose sequence generator was retargeted and given new timing values, edited entirely through this tool, sat open in-game with no issues. Structural edits, such as adding a state, removing one, or retargeting a transition, are validated against the file format and Havok's own tooling, but have not yet been confirmed loading in Fallout 4 itself. Keep the .bak file, and treat structural changes as beta until you've tested them in your own game.