First a bit of background:

(This is with the current version of the Creation Kit that matches the current version of Skyrim)

I have a mod that I created that adds a cabin to the exterior cell "RiverwoodPath". In order to provide NPC access to the part of the cell where the cabin is I had to modify the navmesh. This worked fine for quite a while. Then one day I noticed that my NPC follower could no longer enter the RiverwoodPath cell via the main road from the south. NPCs would just get to the cell border and get completely confused. I had not made any changes to my navmesh.

I worked on my navmesh for quite some time but could not resolve this issue and could not find anything wrong with the navmesh. Finally I disabled all other mods and found that the problem no longer happend. The NPCs were perfectly happy to cross the cell border. I then re-enabled each mod one by one until the problem started happening again.

It turns out that the problem only happens when I enable a particular 3rd party mod. And that mod is the only one I have that edits the cell directly south of the RiverwoodPath cell. I looked at that mod's navmesh but could not see anything unusual or problematic about it.

Now, finally, my question:

What happens if two mods edit navmesh in adjacent exterior cells? I know that when you finalize an external navmesh it creates links to all adjacent cells so that NPCs can traverse the borders. But, presumably these links would be to the vanilla navmesh in any adjacent cells that your mod didn't modify. Now let's say another mod edits the navmesh in an adjacent cell and finalizes that cell. It would seem that that cell's links would be to the vanilla navmesh in your first mod's cell and not to the edited navmesh in your first mod's cell.

Are my assumptions correct and could that be why NPCs are unable to traverse the cell border when I have both of these mods enabled? And if that is true, is there any way around it? Or is this a case where there is no way to resolve the conflict between the two mods and the user just has to decide which one they want to use?

One other unusual thing is that the cell traversal problem seemed to be somewhat limited to the preferred navmesh triangles over the road. If I led my follower off the road I could sometimes get her to follow me into my modded cell. But, after getting her into my cell she seemed to be following the vanilla navmesh instead of my edited navmesh. I could make my modded cell work perfectly just by disabling the other mod. But, as I said before I don't think there were any actual navmesh problems with the other mod (at least I couldn't see any).

If you load the other mod after yours, your changes to the navmesh in your cell are overwritten by the other mod. When they finalized the mesh in their cell, it finalizes the mesh in the surrounding cells as well. I played with that a lot when I put my Bedroom mod right next to my Warehouse mod and thought I could publish them as separate mods. There are other problems too but I ended up putting them together into one mod because they could not coexist as separate mods. Navmesh and landscape are both problems for adjacent cells.

I believe the mod that is loaded last would 'win'. Meaning the navmesh changes in the losing mod would no longer exist. But I'm not certain.

That is exactly what happens AJ. Last mod wins.

Thanks guys! It's not a big deal at the moment because I have not released my mod. But I still try to make it as "correct" as possible in case I ever decide to release it in the future. From what you are saying though, it sounds like the behavior I experienced is somewhat expected and probably unavoidable.

Chok3hold:
How did you merge the two mods?
I might have a similar issue.

Does setting your mod to be an ESM fix this issue at all (at least, the navmesh and world changes, while everything else is done in a seperate ESP)?

No, if you make it an esm, it will not be able to overwrite the vanilla navmesh in Skyrim.esm. As it stands at the moment, only an esp can alter things in Skyrim.esm, using the standard features of the CK.

So I'm assuming the "navmesh bug workaround" of having your navmesh be made in an ESM only applies to totally new worldspaces/interiors?

Shalani, I merged them in the CK by copying the interior cell from the smaller of the two into the larger. Just good old hard work. Currently, the alternatives are not very attractive. TESVSnip corrupts data and the CK Version Control probably takes longer to set up than the merge does. I'll be happy to go through the process for you if you think it will help you. You may already know about it and I don't want to go through it all if you know more about it than I do.

Actually, I am in the middle of another merge right now as I have created an armory that I am adding into the mod.

Correct. The navmesh workaround (which, btw, is no longer needed) only worked because the game didn't lose track of the navmesh data from an ESM during the play session.

An ESM is intended to be new content, with no overwrites of content from any other file. Which is why you can't edit anything in a vanilla cell/worldspace using an ESM or those edits are completely ignored.

Wait, are you saying that after Morrowind and Oblivion, the next evolution effectively eliminates the ability for the community to mod for the same city?

So if one person wants to add a dock to Seyda Neen, and another a small warehouse next to the C&E office, it would no longer be possible, because the areas are technically the same cell, and "navmesh" would explode?

That seems like an inconceivably bad design decision.

Or the developers simply didn't think about it while designing? They can't think of every single little thing that we may want to do to make it work.

Think about all the things that DO work. How many of those did they have to specifically code for? I'm sure plenty, people just never think about it and take everything that works for granted.

I DO appreciate what works well in Skyrim, I really do!

Still, being able to allow more than one modder to add something to Balmora HAD to have been something they considered. I mean it isn't like a minor oversight, it is what most modders would want to do!

I have to assume this is something they are diligently pursuing resolution to, because it is pretty devastating.

Considering that like 95% of mods are made by one single person, I wouldn't exactly agree with that...

Plus if you merge the files and refinalize the navmesh is no longer broken. It's when multiple esp's alter touching exterior cells. If multiple people want to edit one area, then can team up and combine the mod.

Problem solved, no?

No need to come to Beth's defense, I don't mean to attack them or anything.

But to answer your question, no. Keeping track of everyone that wants to edit something in the vicinity of your mod and merging projects together that might be otherwise unrelated isn't really much of a solution.

I'm sure they'll get it straightened out.

Ahhh, right. I thought you meant multiple people working together on one area.

If they are completely seperate unrelated modders then I do see the issue.

Though I'm not sure what kind of solution could fix this. The game would have to detect when two mods edit bordering navmeshes, then create some kind of a hotfix or something. That sounds pretty complicated.

As long as your cell edits are small, you don't really need to finalize the mesh for everything. As I think Arthmoor pointed out, if your mesh revisions are well away from the cell boundaries , everything continues to work. I have also read some reports that doors will also function without finalizing with just a pure, but I haven't tested that enough to be willing to go out on a limb and say it works all of the time. I still finalize for doors just to be sure it is follower friendly. But then if another modder enters that cell or the next, they could concievably blow up my work, whether it needs to be finalized or not, but finalizing again themselves. Fortunately, that doesn't seem to happen too much with just doors as they continue to work even though the next modder finalizes the mesh again. I have doors all over Skyrim and I am sure that a lot of other mods have also finalized the mesh in the same spots and I have had no complaints about the doors not working. There have been a few possible conflict CTDs that could certainly be based on navmesh conflicts, but very few in my opinion, maybe half a dozen. Of course, I am sure that a lot of people could just figure out the conflicts and move on without saying anything.

It does become a serious problem for more involved edits, though, like a complete rework of the mesh for the cell like I was doing. The landscape becomes another problem as it is difficult to make the edges match up with any amount of modifications unles the two cells are being modified by the same file. The problem is also compounded by the fact that there are so few cells in Skyrim that are empty enough to allow for serious expansion. I wonder if the solution is intended to be in creating custom world spaces but that has it's own set of problems as well.

I agree with you guys that is does seem like a bad design decision if the purpose was to create a game just for modders, but I am sure they had to make compromises that are hard to appreciate without the details. It wasn't a big problem for Bethesda though as they were placing all of their work together into one file.

if you don't alter the actual exterior navmesh itself, you dont need to finalize it. you can drop your load door and teleport marker as long as there are no other vanilla door teleporters in the same cell (but make sure the navmesh is finalized in the interior), and you dont need to finalize the exterior navmesh. however if another mod finalizes that navmesh your door teleporter might not work properly.

you can have 2 mods in the same cell each drop their door markers without finalizing and both will work.

You'd solve the problem of two mods editing the same space the same way we always have: Compatibility patches. There's nothing fundamentally different about taking two mods that edit vanilla navmeshes and making them both play nice together than there was about doing this in Morrowind or Oblivion. Your patch file would simply be the sum product of Mod A and Mod B with a navmesh to make them both work properly, finalization and all.

A navmesh is not really any different from a path grid. You can't just arbitrarily plop stuff down on either one and expect two mods with giant objects to coexist peacefully.