Understanding Navmesh, NPC Pathing, and Navcutting
The function of Navmesh is to define the path where an NPC (called an Actor) can walk, run, attack, or conduct its primary sandboxing functions, such as sleeping, eating, and interacting with objects in a pre-determined radius around that NPC. Navmesh is a 3D object with the primary purpose of providing AI with navigational details within the 3D space. The word “Navmesh” is a portmanteau of navigation and mesh (meaning the 3D object).
Dynamic navmesh is new to Fallout 4 and occurs only in cells that are flagged in the Creation Kit as Workshop. This allows player-placed objects (settlement objects) to interact with the navmesh and cut holes so that NPCs won’t attempt to travel through that settlement object but will instead walk around it. The word “Navcut” is a portmanteau of navigation and cut or cutting the hole. Navcutting is not unique to Fallout 4 and is present in other BGS games. Only now, in Workshops, it’s supposed to be dynamic and player-placed.
Supposed to be. There appears to be a bug discovered by kinggath, the mod developer of Sim Settlements, that affects the navmesh around all settlement objects, not just within interior workshops such as Home Plate and Vault 88, but in all interior cells at the same 3D spatial coordinates as the settlement object in another interior cell. So if one item is dropped in an interior workshop like the Mechanist's Lair, that hole at that same x,y,z coordinate is present in all interiors impeding NPCs from walking through that particular spot.
How the bug happens and How it can affect gameplay
When a settlement object is placed, it does so with a bounding box in the basic shape of the object to cut the hole in the navmesh. This is supposed to be unique to locations flagged by a mod or by the base game as Workshops, and it is supposed to be unique to each interior location. The unfortunate matter is that there is a missing code check at the engine level and that missing check causes all player-placed navcuts to persist across all interior cells, including mods, Creation Club, and the vanilla game and DLCs.
If an NPC cannot travel correctly to a marker or trigger to advance a quest, it could be detrimental to the save, leaving the player unable to complete quests, especially if the cell does not dump its data upon quitting the game and reloading the save.
So let's say you're sitting on your sofa at home. That sofa takes up space and your dog has to walk around it to get to the other side. There's no way of going through. It takes up a sofa-sized space that people have to walk around. But because of this bug, the space where your sofa is located is now duplicated in everyone's house. Where you have your big toe could trip the guy living in the house down the street.
How the bug affects mod choices for load orders
Interior Settlement mods have to have a unique set of coordinates for each of those interiors per load order to avoid interference with other interior mods (regardless of Workshop flagging) and the base game interiors. That’s right. Per Load Order.
So, just as with Addon Nodes, HumanRace, and Precombines, we can only have one set of interior workshop coordinates that do not share the same spatial coordinates of another interior workshop cell.
Now here is another problem on top of that: cell dimensional limitations. Interiors are limited to the coordinate range of -32767 to 32768. That sounds like a lot until you add several very large interiors with the base game, the DLCs, and all of the Creation Club Creations. Even if there were a community effort to coordinate between all the coordinates, you’ve got a finite amount of space everything can take up.
Prevention and How To Fix
There currently is no engine-level fix for Xbox or PlayStation. Players can, however, take preventative measures by observing good downloading and save management practices. Because there is a lot of panic in the modding community due to this bug, a mod author may attempt to move their cell coordinates so their mod remains unique. Cell data updates usually require new games in order to see any changes to that cell. Only objects that are not persistent will move and the navcut that will be affecting all cells is considered persistent.
While it will not help Vault 88 or Home Plate – those should be fixed officially by Bethesda – prevention is quite simple. Only choose one interior cell mod if you’re going to use interior mods and limit what you build in Home Plate and Vault 88. The current method of playing is to choose between a building load order and a questing load order. It makes things much simpler and load order management is much easier.
Players on PC can attempt to use Interior Navcut Fix to see if it will resolve the problem. This is an engine-level fix that requires F4SE. New games will be required due to the persistence issue described above if currently playing. Other than that, taking preventative measures by observing good downloading and save management practices is key to good load order management. Make note of all workshop mod spatial coordinates in xEdit prior to enabling on an ongoing save or even a new one. You can attempt to dump the data similar to consoles by saving, quitting to desktop, and reloading the save.
Navcut Bug Best Practices
Observe good save management practices. Before entering any interior, make a hard save. This is in case you have to revert. It not only will help you take preventative measures for this but will save you steps in the event of some other event failing.
Observe good download practices. Read the descriptions and use the Framework to make a determination of load order. You only need one interior mod on your framework if you’re going to play with interiors.
Determine your playthrough before downloading. Build your Framework. That is what this guide is about. Determine beforehand if you are playing questing, building, or exploring and build your load order around your pre-determined playstyle. Choose only one interior mod and expect navmesh issues as a result of building within Vault 88 and Home Plate.
Do not update a mod with persistence in the middle of your game. Most cell updates require new games in order to prevent other bugs, other navmesh issues, and problematic persistent objects. If save-quit-reload does not unload the cell properly and quests cannot be completed due to either building an object in the same spot as a quest object or overbuilding your interior settlement, consider restarting on a fresh character.
Reload a previous save. If you find a cell that had loaded data and your companion is not pathing correctly, you can try to reload the cell by dumping all the data. It is a similar process to clearing the system cache. Saving, quitting the game completely, and reloading the save will unload cell data, but do note that it may require a cache clearance to completely finish the process. Power cycle your Xbox and follow cache clearance guidelines from Sony for PlayStation.
Avoid Quick Resume. Quick Resume can break the unloading process on Xbox. If PlayStation has a similar feature, make sure it is not interfering with the unloading process.
Help mod authors. Not everyone hangs out on Reddit or other forums. If you’d like to participate in active discussions on the matter with mod authors to assist the modding community, join us on the Parent Server or join Collective Modding where the discussions are actively taking place. More information is needed from PlayStation users, but Xbox users should join as well.
Mod Authors and Porters
Until Bethesda provides a fix, mod authors of interior cells and workshops who port their work to Bethesda.net are encouraged to provide the cell co-ordinates in the description of their ports so that players without access to tools such as xEdit or the Creation Kit will have the basic information to make a load order determination.
How to test for the bug on your game save
- Start a new test character, or use one that hasn't built anything but has Home Plate available as a player home.
- Grab a companion, any will do, and travel to Vault 111. Proceed to the original room where your player was trapped in the Cryo Pod. At the back of that room is a raised platform. This corresponds with the same spatial coordinates as the red door in Home Plate. Direct the companion to walk on the platform. They should be able to.
- Now go add a bunch of objects around the red door in Home Plate. Cover as much of the area as you can.
- Return to Vault 111 with your companion and direct them to walk on the platform. Note that your companion is now having pathing issues, and probably will not be able to travel correctly or as cleanly as they did before you crammed a bunch of junk around the Home Plate door.
While this bug is not the end of the world, it does require cognizant awareness of how many interiors and interior settlements you choose to add and build in when playing. By observing good downloading practices and using preventative measures you can Survive Load Order Disorder and have a great game.
If you’d like to participate in active discussions on the matter with mod authors to assist the modding community, join us on the Parent Server or join Collective Modding where the discussions are actively taking place. More information is needed from PlayStation users, but Xbox users should join as well.