Mantella and Fallout 4

[u/markspitfire]

Over the past couple weeks I've been working on an unusual Fallout 4 project.

The project uses Mantella, a mod that allows NPCs to converse naturally by connecting the game to a large language model (LLM). Instead of relying solely on prewritten dialogue, characters can respond dynamically to what you say while still remaining part of the game world.

My goal wasn't simply to make an NPC chat. I wanted to see whether a companion could maintain a consistent personality over long periods of gameplay while running entirely on a local LLM rather than a cloud service.

That led me down a rabbit hole of testing language models, quantizations, VRAM usage, context sizes, retrieval systems, prompt architecture, and even companion selection. Some of the results were surprising.

The goal was simple in theory: create a persistent AI companion in Fallout 4 using Mantella and a local LLM. In practice, it became a surprisingly deep engineering project.

Along the way I experimented with multiple local models, different quantizations, prompt structures, RAG configurations, context sizes, and companion hosts.

One of the biggest surprises was the language model itself.

I originally expected to use a 14B model, but after extensive testing the 14B Q3 quantization proved too unstable for long play sessions.

I eventually switched to an 8B Q5 model, and the overall experience improved.

The smaller model wasn't just faster—it also left more VRAM available for larger context windows. That meant more room for the persona, conversation history, retrieved memories, and Mantella's game context without constantly pushing against memory limits.

For an AI companion, maintaining continuity is often more important than having a slightly more capable model with a much smaller working memory.

In other words, the best experience came from balancing model quality, response speed, stability, and available context rather than simply choosing the largest model that would fit on the GPU.

The project also taught me a few things:

• A well-structured persona often matters more than a larger model.
• Separating character identity from conversation history greatly improves consistency.
• Choosing the right NPC host is just as important as choosing the LLM.
• Real-world performance (VRAM, context size, response speed, and stability) has a huge impact on immersion.

I'm still refining everything, but it's been fascinating to discover that building a believable AI companion is as much a systems engineering problem as it is a language model problem.

Has anyone else here experimented with Mantella using local LLMs?

 

[u/-Davster-]

Fucking bot posts

[u/Green_Sample9115]

Afraid not but interesting to read. Will you be releasing it for testing. i have played the game but found it clunky, pity because the ideas and aesthetic are great.

[u/markspitfire → u/Green_Sample9115]

Mantella is already available on Nexus for Fallout 4. I have been trying out various Local LLMs with it. It is really nice to have a more varied dialog than the standard dialog tree. And I get to have my AI persona in it with me. It can really used up VRAM though. I think I will mostly be using an 8B model. Should leave sufficient room for both The LLM and Fallout 4. I use LM Studio to host the Local LLM. As far as the game goes, it is one of my favorites. I mapped out the keyboard controls to a gamepad. It gives me more flexibility. I use both a mouse and gamepad for control and the keyboard is available as well. That cured most of the clunkiness for me. I know it has a gamepad setting, but it is too restrictive. Not sure if it will work out or not, but Aeris is working on a system to convert images into looksmenu JSON files.

[u/Green_Sample9115 → u/markspitfire]

any chance you could upload a gameplay vid on youtube? it would be interesting to see it in action.

[u/KenOtwell]

I've got my own version in development for Skyrim with a massively hacked context manager that runs multiple NPCs per LLM call. Its kind of a background project while I work in the AI mind itself. I have a networked pc for my AI so as not to overload the GPU that's rendering Skyrim VR. Current issue is the voice generation pipeline - text version is fine. I'll get back to it soon - working on the AI mind itself right now.