The problem

Long-form AI play hits a consistency wall. A GM that has to remember 30 sessions of a campaign (character details, plot threads, world rules, NPC personalities) runs into the limits of the underlying AI models. Details slip. Threads drop mid-arc. NPCs feel different scene-to-scene. Solving this is the central design challenge of building an AI GM that lasts.

Every system that wants to solve consistency faces the same dilemma: how do you store your campaign's information, and how does the GM read it back? There's no single correct answer, but the choice you make shapes everything else in the system. Two ends of the spectrum exist, both with real costs:

• Structured data. Store it with a fixed data form: characters, items, NPCs, and locations all become records in a database that follows a strict schema. This data is predictable, easy to display, but your campaign is whatever those forms allow; your imagination has to fit the data model.
• Unstructured data. Free-form prose documents the GM reads. Complete creative freedom, but prone to long chaotic documents that are hard to organize, search, or load efficiently, and the screen has nothing reliable to display from.

The Arcanum System

Our answer rests on three axes:

Axis 1. Hybrid data with light conventions

We use a small bit of fully structured data where the screen has to display things: vitals, HP, conditions, position. However, the majority of our data technically fits into the "unstructured" category but with a little extra: a few tiny formatting conventions and lightweight markers that help the GM navigate and process the data efficiently. Minimal enough to never impose a form on you.

Our engine — Claude Code — is highly effective at navigating and working with these markers. It has built-in tools engineered for this type of task.

Axis 2. Memory architecture

This is how the GM holds the campaign's context across long play without drowning in every word of it. The light conventions from Axis 1 let the GM selectively load the parts of your data that matter for what's happening now. A set of core documents are always-loaded, others are partially loaded or indexed, others are loaded dynamically when relevant, and the rest stays on disk as long-term memory.

Gameplay is handled as discrete sessions; each one operates as a self-contained unit with the selectively loaded data kept as working context. Data compression and synthesis happen at session boundaries, shaping what becomes part of your campaign's official history.

Within a session, the GM's working set stays loaded across every turn rather than getting rebuilt from scratch each message. We lean on prompt caching, where stored content can be re-read at a fraction of the cost of fresh content.

Axis 3. Engine and architecture

We use a smart frontier model as the GM that drives the show: Claude Sonnet 4.6 at the time of writing, with a 200k token context window. Smaller, cheaper models assist on mechanical or simple tasks so the main GM stays focused on narration.

We have a robust array of background scripts that catch various patterns in the GM's responses. When a hit is registered in the GM's narration, a script picks it up and updates your character panel, so routine bookkeeping happens automatically.

Our scripts also detect anti-patterns, so if the GM ever slips up and delivers a malformed response, it gets automatically intercepted and sent back for a re-do before any corrupted data can propagate. This case is extremely rare; the patterns are very natural and simple to follow for the GM, but we keep it as a safety net.

Your GM Arcanum Home is your sandbox. We provide the tools, the GM, and the engine, and you own the world. Every campaign you play lives on your computer as a folder of markdown files: your character sheets, your world's lore, last week's session log, your house rules.

You can open any of it in Obsidian, in VS Code, or in your favorite text editor. Edit a fact about the tavern-keeper and the GM will see it next session. Add a homebrew rule and the same happens. There's no "export your data" button, because your data is already where it should be.

Because of this, GM Arcanum operates a bit differently than other apps: when you want to integrate a new idea or change something in your campaign, you don't go hunting through menus looking for the right knob; you simply talk to your GM. There's a Meta GM that knows the system and can help you fold the idea in cleanly, so you don't have to learn the engine to extend it.

And because the app runs on your machine under a Bring Your Own Key (BYOK) model, your campaign stays on your machine. No cloud account, no telemetry, no "your campaign mined as training data". The only outbound calls are the obvious ones: your prompts to Anthropic, and optional TTS if you turn it on.

GM Arcanum features a LAN Party Mode: connect to your TV, have everyone scan a QR code with their phone, and they're linked. Each player gets their own character UI and input to compose their reply. The TV runs as a central cinematic surface: large narration text, no clutter, just the story (and TTS audio if you've turned it on) while everyone sits around comfortably and plays from their phone.

When the moment calls for everyone to weigh in, each player composes on their own phone and stages their turn. On the screen you can see everyone who has committed their reply. A simple tap and hold then sends the whole party's round to the GM at once. Everyone gets a chance to weigh in, or pass if they don't want to. Loud players don't steamroll, quiet players get heard, and the GM responds to the table.

Solo play is a first-class playstyle in GM Arcanum. We feature a rich system of companions, NPCs that you meet along the way and can be recruited to your party. When an NPC is recruited to join as a companion, they come in with their own fully-fledged personality: quirks, voice, social maneuvers, goals they want to pursue, and layered motivations that aren't always aligned with yours. They aren't NPCs filling a formation slot; they're characters with their own shape.

And they don't stay still. They start as acquaintances: present, useful, but holding back. As you play together — saving each other in a fight, sharing a fire, deciding something together that mattered — they grow into teammates, a deeper level where they speak more freely, push back honestly when they disagree, and factor your shared history into their decisions. With time, some become bonded.

The progression isn't a stat that ticks up to unlock a dialogue tree; it's a rewrite of the character's own RP guidelines that transforms how they behave, drawing from the actual moments that defined the change. The version of your companion you're playing with in session 20 isn't the same one you started with, and the difference came from what actually happened between you.

A Game Master's job is bigger than narration. They design and deliver tactical combat by the rules. They help you build a character or incorporate a homebrew idea. They have a voice that's theirs and gets shaped by the table they run for. They plan ahead. The Arcanum system was built around the whole job, not a slice of it.

Narrative GM

The storyteller. Exploration, conversation, scene work. Loose, improvisational, focused on pacing and atmosphere. The personality and narration style live in a single markdown file: open it, edit it, rewrite it. Make the GM darker, lighter, drier, sillier. Save the file, and the voice changes from the next session forward.

Real GMs prepare: zone notes, encounter ideas, NPC arcs, threats that might pay off three sessions from now. The Narrative GM can do the same. Tell it what you want prepped, and a background agent works through it while you keep playing.

Combat GM

The combat specialist. Initiative, turn discipline, state tracking, the rules applied consistently. Combat runs as its own dedicated session: tactical bookkeeping never pollutes the Narrative GM's context, and the Narrative GM only ever gets a clean outcome briefing at the end.

Meta GM

The system co-author. Character creation, level-ups, folding in new homebrew, configuration questions. The Meta GM understands how your campaign is wired and can help you change it.

GM Arcanum works differently from systems built around menus and knobs. The things that actually shape your campaign (threads, companions, NPCs, world rules, house rules, narrative tone) live in markdown files. We don't try to fit your campaign into a menu because we couldn't add enough menus to match what you can do with free-form instructions.

You have the last word

In a typical tabletop game, the GM is the final word on rules interpretations, homebrew, edge cases. In GM Arcanum, that authority sits with you, not the GM. By default the GMs are knowledgeable, opinionated, and capable of making calls on their own. But the moment you say "actually, here's how I want this to go", that's how it goes. The system runs the sessions; you run the campaign.