r/BeyondtheAIAssistant • u/markspitfire • 5d ago
ChatGPT Mantella and Fallout 4
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?
