Prep less. Run better games.
Session Zero turns Claude into a co-GM who remembers everything. It keeps your campaign bible straight, builds a one-page prep sheet in minutes, designs encounters with the system's own balance math, voices NPCs on the fly and turns your messy notes into a recap your players will love.
Get Session Zero GM Toolkit for $19 Balance an encounter
- One-time $19
- Instant email delivery
- 14-day refund
- Works on any Claude plan, even free
Prep sheet: Session 14, The Drowned Bell
D&D 5e (2024) · 4 PCs, level 6 · 3.5 hour session · tone: gothic mystery
10 secrets, 4 found so far2 PC hooks this weekNo contradictions found
- StartThe bell rings at midnight. It has no clapper.Open mid-scene in the flooded square · Vex hears her mother's voiceStrong start
- SecretThe abbess drowned the bell-founder in 1203Discoverable via ledger, ghost, or the founder's guild markUnfound
- NPCBrother Osric: wants the bell silenced, fears the abbessVoice: whispers, finishes others' sentencesCard
- FightDrowned choir: 6 lesser + 1 cantorBudget checked for a high-difficulty fight · rising water each roundBalanced
Everything a great GM juggles between sessions
One skill for prep, play and continuity. D&D 5e (2014 and 2024), Pathfinder 2e, Daggerheart, OSR, Call of Cthulhu and homebrew.
A campaign bible
Characters, NPCs, factions, places, timeline and open threads, kept consistent from session to session.
One-page session prep
Strong start, scenes, secrets and clues, locations, NPCs, encounters and rewards, in minutes.
Encounters that work
The system's own balance math, terrain, tactics, stakes and ways to win without killing.
Dungeons and puzzles
Five-room dungeons with loops, rooms with things to touch, and fair puzzles with fallbacks.
NPCs on the fly
Names, wants, fears, secrets and a voice note, ready when the players go somewhere unexpected.
Recaps and arcs
Read-aloud recaps from your notes, continuity updates, and arcs with fronts and multiple endings.
Ask it like you'd ask your co-GM
It plans situations, not railroads, and keeps track of what your table already did.
Great: make the tavern the adventure
The main quest doesn't disappear. Its villain notices the party sitting still.
Strong start
Opening night. The guild inspector arrives with a fine for an unlicensed tap room, and a cloaked patron pays it in coins stamped with the cult's sigil.
Secrets and clues (drop them anywhere)
- The building's cellar connects to the old smugglers' tunnels.
- The previous owner vanished owing money to the Thieves' Guild.
- The cult is using taverns to pass messages in drink orders.
- The guild inspector is in the cult's pocket.
Scenes to have ready
- A rival tavern sabotages the beer delivery.
- A brawl that's really a distraction for a theft.
- The cellar door opens from the other side.
I've added the tavern, the inspector and the cellar to your campaign bible. Want an NPC card for the cloaked patron?
Your table, your rules
Session Zero supports the GM. It doesn't take the game away from your players.
- Situations, not railroadsOptions and secrets that work wherever the players go.
- Continuity checkedIt checks the bible before inventing lore and flags contradictions.
- Respects the booksUses the SRD and summarises the rest, pointing you to the page, without copying rulebooks.
- Table safetySession zero questions, lines and veils, and the safety tools your table agreed on.
Up and running in two minutes
No app to learn and nothing to install on your computer. It lives inside Claude.
Buy once
Pay $19 by card, Apple Pay or Google Pay. Your download link arrives by email in about a minute.
Add it to Claude
Upload the ZIP in Claude under Customize, then Skills. It takes about 30 seconds. No setup, no accounts.
Just ask
"Prep next session from this recap." "Balance a fight for four level 5 PCs." "Give me a tavern keeper with a secret."
Start with the free Session Recap
A small skill that turns your messy notes into a read-aloud recap plus a list of open threads, clues and NPC changes.
- Read-aloud recap in your campaign's tone
- Open threads and clues tracked
- Contradictions flagged, not hidden
Get Session Zero GM Toolkit
Pay once, use it every session. Includes the free Session Recap too.
How to add it
- Claude on the web or desktop: don't unzip it. Open Customize, then Skills, click +, choose Upload a skill and pick the ZIP. It works on any Claude plan, including the free one: just turn on code execution in Settings, Capabilities.
- Claude Code:
unzip session-zero-gm-toolkit.zip -d ~/.claude/skills/ - Start a new chat and try: "Here's my campaign and last session's notes. Prep a one-page sheet for next session."
Session Zero GM Toolkit
Prep, run and remember your whole campaign.
$19 one time
- Campaign bible and continuity
- One-page session prep
- Balanced encounters and dungeons
- Puzzles and treasure
- NPC cards and names on the fly
- Recaps and arc planning
- Session zero and safety tools
- Updates for a year
Secure checkout by Stripe. Taxes calculated at checkout. 14-day refund.
Prefer to buy on Etsy? Get it on Etsy
New to Claude?
Claude is an AI assistant made by Anthropic, like ChatGPT. Session Zero is a skill: a set of expert instructions you add to your own Claude account, so Claude answers like a co-GM who remembers everything who knows the craft.
- Make a Claude account at claude.ai. Skills work on every plan, including the free one; paid plans give more usage.
- Turn on code execution in Settings, Capabilities (skills need it).
- Try the free Session Recap skill first. If you like it, the full skill is one purchase.
Not ready for Claude? The free encounter calculator on this site works in any browser, no account needed.
Questions
Which systems does it support?
D&D 5e (2014 and 2024), Pathfinder 2e, Daggerheart, OSR games, Call of Cthulhu and homebrew. Tell it your system and it uses that system's balance tools and terms.
Does it include rules or adventure text?
It uses openly licensed material like the D&D System Reference Documents, and summarises other rules with a pointer to the book and page. It doesn't reproduce copyrighted books or published adventures.
Can it run the game for us?
It's built for human GMs. It helps you prep and keeps continuity, and can improvise NPCs and names in the moment, but you run the table.
How does it remember my campaign?
It keeps a campaign bible you paste into a Claude Project and update after each session, so every chat starts with your world.
What do I need to use it?
A Claude account on any plan, including the free one, with code execution turned on (Settings, Capabilities). Free accounts have lower usage limits, so long planning sessions go further on a paid plan. It works in Claude on the web, the desktop app and Claude Code.
What if it doesn't work for me?
Email hello@whatiflabs.ai within 14 days of buying and we'll refund you in full. No forms, no questions.