Mythras GM · Claude Code
Run Your First Session
Claude as your Gamesmaster, running Mythras Imperative with a real dice engine and every wound, faction and journal entry persisted in a knowledge graph. This page takes you from nothing to an open scene — and tells you how to check each step actually worked.
Before You Start
Three things. Two of them you probably already have.
Native builds cover macOS and Linux on x86_64 or arm64, and Windows on x86_64. Anywhere else,
the engine can run its database under Docker instead — see /mythras-gm:setup.
Install, One Step at a Time
Install the engine explicitly and first. Campaigns declare it as a dependency, but do not rely on that resolving for you — when it silently doesn't, the campaign's own diagnostics go down with it, and you get a plugin that simply isn't there rather than an error.
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Add the marketplace
One source holds the engine and every campaign.
/plugin marketplace add fourth-wall-gaming/mythras-gmConfirmThe marketplace lists three plugins:
mythras-gm,purewaterandveilwrack. If you see fewer, stop here rather than continuing. -
Install the engine
This is the dice tower, the rules graph and the save file. Nothing else works without it.
/plugin install mythras-gm@fourth-wall-gamingConfirmmythras-gm appears in your installed plugins. Do not move on until it does.
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Install a campaign
Pick one. Purewater is a Classic Fantasy port — a canal city, three days before a tournament.The Veilwrack is an original sky realm — winged peoples on floating bone-spires, and the wind is dying. You can install both and keep two saves side by side.
/plugin install purewater@fourth-wall-gamingConfirmBoth mythras-gm and purewater are installed. Two plugins, not one.
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Start a fresh session and read the first line
Both plugins check the game at session start, so quit Claude Code and open it again. The preflight tells you exactly where you stand — it never imports anything and never blocks the session, so whatever it says is safe to act on.
Confirm — one of these…installed but has not been imported into the database yet— expected on a first run. Continue to step 5.the mythras-gm engine is not installed— step 2 didn't take. Run it again.PREFLIGHT FAILED — uv is not installed— install uv, then start another session. -
Open the scenario
This imports the campaign, briefs you on Mythras if you're new to it, walks you through choosing or rolling a character, and opens the first scene. Run it once, ever.
/purewater:startConfirmYou are asked whether you've played Mythras before, and then offered characters. If instead the narration simply begins, something didn't import — see below.
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Every session after that
One command, forever. The preflight will have already told you the clock and session number.
/mythras-gm:playConfirma game is in progress— with your campaign's clock and session number. Never run the start command again; it is only for a campaign that has not begun.
Where Your Save Lives
The files in a campaign plugin are a scenario seed, not a save. The moment you run the start
command, the database becomes the game — and the database is on disk, at
~/.claude/mythras-gm/typedb/data.
That path is the campaign, and it persists for as long as the disk does. Close the laptop, come
back in a month, pick up mid-scene. If you would rather keep it somewhere you actually back up,
set MYTHRAS_TYPEDB_HOME — the server binary, its log and the data directory all
move together, because a data directory that outlives its binary is just another way to lose
the save.
This is a local game. The engine is brought up by a SessionStart hook, and that
hook does not fire in hosted or disposable environments — so nothing creates the database,
and there is no save to come back to.
When Something Is Wrong, It Will Be Quiet About It
The characteristic failure here is not a crash. It's a database that isn't there, or a schema that never loaded, while the game carries on narrating over the top of it. Learn the tells.
- Nothing is printed at session start. Both plugins always say something; silence means neither hook ran.
- The GM narrates straight into a scene without offering you a character.
- A wound, a spent luck point or a journal entry from last session has quietly reverted.
- Dice results appear in the prose but no roll is reported.
One command answers all of them. It checks the install end to end and reports every stage as pass or fail, in order:
/mythras-gm:setupEvery check reads true, ending in Ready. Any check that reads false is named, with the reason attached.
Two Campaigns, and a Word About Spoilers
And Then the Dragons Came: Purewater
A canal city where the Lake Lady's waters meet the sea. Baron Hanzo di Teufel has arrived for the Tourney with a champion nobody is allowed to see, and there are three days left before it all goes wrong.
4 entry points · 42 characters · 15 agendas on clocksThe Veilwrack: The Stilling
No ground — an endless sky above the toxic Undermist. The winged Alar live on the floating bone-spires of dead sky-leviathans, and Lanternfall's windlane began Stilling this season. Nobody has said the word aloud yet.
4 spires · 7 factions · 3 playable AlarDon't browse the campaign repositories. They are the published source of the scenario, so they hold the Gamesmaster's material — the secrets, the timeline and who everyone actually is. Much of it is marked GM-only, but not all of it, and a scenario is easy to spoil by accident. Install the plugin and let the game show you. Both worlds are built so that what you don't witness happens anyway, which only works if you haven't read ahead.
Love the System? Buy the Books
This runs on the Mythras Imperative SRD under the ORC License. The Design Mechanism publishes the full Mythras line — get the real thing and play it at a real table too.