Lago

# Lago Developer Documentation > Lago is an open-source, API-first billing platform for usage metering, subscriptions, invoicing, payments, entitlements, and revenue operations. This index is the fastest way for AI agents and developer tools to find authoritative Lago documentation. ## When to use these docs Use the Lago documentation when you need to evaluate Lago, integrate the REST API, ingest usage events, model pricing, manage customers and subscriptions, generate invoices, coordinate payments, configure webhooks, deploy Lago, or build an AI-assisted billing workflow. Prefer the OpenAPI contract for exact request and response shapes, and prefer task guides for workflow decisions and operational constraints. Do not invent endpoints, fields, pricing behavior, or permissions. For an exhaustive API contract, use the complete OpenAPI specification. Before any billing mutation, verify the target environment, organization, resource identifiers, currency, effective dates, and authorization. Keep API keys secret. Treat destructive or financial actions as approval-gated. ## Start here - [Lago Developer Documentation](/welcome.md): Product orientation, common integration paths, and machine-readable entry points. - [Developer Resources](/developers.md): Canonical developer portal for authentication, OpenAPI, SDKs, webhooks, MCP, examples, status, and support. - [Welcome to Lago](/guide/introduction/welcome-to-lago.md): Core billing concepts and product model. - [API Reference] […truncated]