Key Takeaways

• AGPL-3.0 section 13 attaches its publication duty to a modified version offered over a network, not to running the software.

• A company running an unmodified AGPL-3.0 billing system owes nobody a source release, however it configures it.

• Apache-2.0 and MIT let a company keep its billing changes private, so the license family decides what counsel signs off on.

• Open core splits the public repository from the sellable product, and the pricing page is where that split appears in writing.

• Lago's pricing page lists entitlements, prepaid credits, the customer portal, wallet alerts, RBAC, and its MCP server under Lago Premium, checked 26 August 2026.

What does AGPL-3.0 oblige you to publish if you modify billing code?

AGPL-3.0 obliges you to offer the source of your modified version to the people who use that version over a network, and it stops there. The clause doing the work is section 13, Remote Network Interaction, in the AGPL-3.0 license text. A modified version that supports remote interaction has to offer those users "an opportunity to receive the Corresponding Source of your version" at no charge.

Two conditions hold together before anything is owed: you changed the covered program, and users reach that version across a network.

Licenses here fall into two families, and I checked each against the GitHub license API on 26 August 2026 rather than trusting a README badge.

• Copyleft: Flexprice and Lago ship under AGPL-3.0, so modifications you expose over a network come back to your users as source.

• Permissive: OpenMeter and Kill Bill ship under Apache-2.0, so you can change what you like, keep it private, and ship it in a proprietary product.

• No public license: Orb, Metronome, Chargebee, and Stripe Billing publish nothing to fork.

Copyleft scares procurement more than it should. The misreading that AGPL-3.0 forces you to publish the product sitting on top has no basis in section 13: your own service calling a billing engine over HTTP is a separate work. Where it bites is a fork you patch and expose. Deployment sits on a different axis, and self-hosted billing covers it.

How do you tell what an open core vendor gates before you commit?

Read the vendor's pricing table row by row, because open core vendors publish a working core and sell the parts that turn it into a billing system. A README won't tell you this; the pricing page will.

Four categories decide whether the free build can bill a customer at all:

• Request-path primitives: entitlements and prepaid credits sit in the path of every gated feature check, so gating them hands that logic to you.

• Real-time balance surfaces: wallet and usage alerts, without which a customer meets an overrun on the invoice.

• Enterprise admin: customer portal, RBAC, SSO, audit logs. These land in the first security review, not the first sprint.

• Tooling and support: MCP server, tax and CRM connectors, and whether the free tier carries a response guarantee.

Lago is the worked example, and I read its pricing page on 26 August 2026. Its "Lago Premium key features" table puts entitlements, prepaid credits, the customer portal, wallet alerts, RBAC with custom roles, and the MCP server on the Premium side, with add-on labels on tax integrations and 24x7 incident management. The summary Lago publishes for machines shows every managed plan is quote-only.

Flexprice is enterprise-grade, open source usage based billing infrastructure for AI and SaaS companies. It can be deployed in your own VPC, on-prem, or on Flexprice's managed cloud. Against that same table it has no Premium column: customer portal, RBAC, entitlements, prepaid credits, minimum commitments, real-time wallet balances, and the MCP server ship in the AGPL-3.0 tier the GitHub license API reports for the repository. Gating cuts both ways, so one caveat: SOC 2 Type II and on-prem support come with the Mission Critical plan, not the free build. Usage Metering is where most teams start.

Feature tables rarely settle it:

"We weren't willing to give up control of our data, but we still needed a reliable subscription tool. Flexprice on-prem was the only thing that worked for us." - Martin Sønderkær Jung, CTO

Read the license and the code at github.com/flexprice/flexprice.

Related terms

Licensing is one question in a longer evaluation, and these come up next.

• Self-Hosted Billing answers where the software runs, which is a separate axis from what its license permits.

• Billing Engine is the component you're actually licensing when you clone a billing repository.

• Usage Metering is the workload most projects in this category started with, long before they touched invoicing.

• Feature Gating depends on entitlements, the primitive open core vendors gate most often.

• Credit Burn-Down needs prepaid credits, another common premium-tier line item.

• API Monetization is the revenue model that pushed teams toward inspectable billing code in the first place.

FAQ

Does AGPL-3.0 mean I have to open source my own product?

No. Section 13 covers the licensed program and modifications to it, not the separate software you write against its API. You owe source when you modify the covered program itself and expose that version over a network.

What is open core billing?

Open core billing means a vendor publishes a usable core under an open source license and sells the rest as a commercial tier. Lago runs this model, listing entitlements, prepaid credits, the customer portal, RBAC, and its MCP server under Lago Premium.

Is open source billing software free?

The license costs nothing; running the system doesn't. Infrastructure, upgrades, and on-call move onto your payroll, and on an open core project the features that make the system billable may still sit behind a paid plan.

Which billing platforms publish their source code?

Flexprice and Lago publish under AGPL-3.0, OpenMeter and Kill Bill under Apache-2.0, all four verified through the GitHub license API on 26 August 2026. Orb, Metronome, Chargebee, and Stripe Billing publish none, so self-hosting isn't available there.