Plans
Try it with no account. Pay when it's worth it.
Sigma Domain is usable before you sign up, free once you do, and $5.99 a month when you start relying on it — under a third of what the major AI assistants charge, with unlimited tool calling on the paid plan.
Guest
$0
No account. Nothing to cancel.
- A small allowance, then sign in to continue
- Full conversation and tool access
- Runs on Sonnet and Haiku
- No persistence guarantee — for trying, not keeping
Free
$0
Your work is saved and follows you across devices.
- $3 of AI usage a month, then $5 more on Haiku
- Runs on Sonnet and Haiku
- Private database and file storage
- Group conversations and sharing
- Browse the whole toolkit marketplace
Standard
$5.99 / month
Or $54 a year — a fraction of the $20 a month the major AI assistants charge.
- Opus and Fable — the most capable Claude models, on paid plans only
- Unlimited tool calling — no daily or monthly cap
- $60 of AI usage a month — the first $50 on any model, then it steps down instead of cutting you off
- Add credit to keep going past it, on any model
- Second opinions from ~70 models across ~20 providers
- Usage analytics
- Everything in Free
Charging
How the money actually works
Allowances are monthly and reset on the 1st. Guest and Free accounts simply stop when theirs is used up — nothing is charged and nothing is owed.
On Standard, tool calling is uncapped: run as many tools as you like, as often as you like. What still carries a monthly allowance is AI usage itself, because every answer costs us real money at the model provider.
Standard never cuts you off mid-month. The last part of the allowance is reserved for the cheaper models, so it always buys a lot of conversation. The first $50 runs any model including Fable. From $50 to $55 you're on Opus or below. The final $5 runs Sonnet, which goes roughly eighteen times further per message than Fable — so a heavy week on the best model costs you some depth later, not the rest of your month.
Past the $60 you can add credit and keep going immediately. Added credit carries no model restriction at all, because you're paying the full rate for it — so $15 of credit runs Fable if you want it. We'll suggest Opus or Sonnet for top-ups under $20, simply because $15 of Fable is about seven messages. Overage is billed; it isn't a free extension.
Free accounts get $3 of normal use and then a further $5 on Haiku before they're asked to upgrade. Guests get a small allowance and are then asked to sign in, which is free.
We'd rather not publish an exact token count while the limits are still being tuned, because a number on this page should be one you can hold us to. What you can see at any time is your own usage: your account's billing page shows the tool calls and AI tokens you've used against your current allowance, and the model picker tells you when a model has stepped down.
Tools published by other people may carry their own per-call price. That price is shown before the tool runs, and it's billed through your Sigma Domain account rather than requiring a separate relationship with the publisher. If you're on the other side of that — publishing and monetization.
Roadmap
Coming to paid plans
In development
Priority handling under load. Paid plans already get Opus and Fable; what we haven't built is giving them priority when the platform is busy, so a paid turn and a free one currently queue the same way.
One-click top-ups. Added credit is metered and billed today, but buying it is still a manual step on the billing page. Putting a "add $15 and keep going" button in the chat itself, at the moment you run out, is the next piece we're finishing.