Autumn is a billing layer for AI and SaaS startups that sits between an application and Stripe, the payment processor. It keeps the billing state of each customer: subscription plan, credit balances, feature entitlements (which features a plan allows) and usage limits. Without it, this logic lives in the application's own code, its database and its Stripe webhooks, the messages Stripe sends when a payment event happens.
At the moment of use, the application asks Autumn whether a customer may do something, such as send an AI message or add a seat, and reports the usage so that Autumn deducts it from the balance. Pricing is configured in Autumn: recurring plans, one-off purchases, free plans, credit systems, usage-based and per-unit prices, trials, add-ons, spend limits and automatic top-ups. A price change or a custom contract is then a configuration change, not new code. Checks can also run through an edge cache in the customer's own cloud.
Autumn is a hosted service, and its docs describe it as open source. It has a free plan and monthly paid plans priced by billing volume; Stripe's own fees still apply.