Lago

Lago is an open-source metered billing platform designed for usage-based SaaS pricing. Core capabilities: ingest usage events (API calls, compute minutes, messages sent), define billable metrics that aggregate raw events, attach metrics to subscription plans with flexible pricing models (flat, per-unit, tiered, graduated, percentage, package), and generate invoices automatically. For agents, the usage ingestion API is the primary integration point: POST events as users consume your product, and Lago handles aggregation, billing calculation, and invoice generation. Self-hostable or Lago Cloud. Genuinely developer-first API design. Active open-source project with strong community. Confidence is docs-derived.

REST API at api.getlago.com/api/v1 (Cloud) or self-hosted base URL. Resources: events, customers, subscriptions, plans, billable_metrics, invoices, credit_notes, wallets. POST /api/v1/events ingests a usage event (customer_id, transaction_id, code, timestamp, properties). GET /api/v1/customers/{id}/current_usage returns real-time usage for the current billing period. POST /api/v1/subscriptions assigns a customer to a plan. GET /api/v1/invoices lists generated invoices. Response payloads are JSON with consistent object shapes. Batch event ingestion via POST /api/v1/events/batch.

Bearer token auth via Authorization: Bearer header using API key from Lago organization settings. Keys are org-scoped with full access. No granular scoping. Key management via Lago dashboard or admin API. Self-hosted instances manage their own key generation. HTTPS enforced on Lago Cloud; operator-configured for self-hosted. Webhook signing (HMAC-SHA256) for outbound billing events (invoice finalized, payment received, etc.).

Structured JSON error responses with code (string) and error_details (field-level validation) fields. 401 for invalid API key, 404 for missing resource, 422 for validation errors (e.g., duplicate transaction_id — Lago idempotency key for events). Event ingestion is idempotent on transaction_id. Webhook retry for billing events on delivery failure. Rate limits documented. Lago Cloud uptime at status.getlago.com. Self-hosted reliability depends on deployment (Docker/Kubernetes with Redis and PostgreSQL).

getlago.com/docs provides API reference, usage-based billing concepts (billable metrics, aggregation types), plan/pricing model configuration guides, subscription lifecycle docs, invoice management, and self-hosting guide (Docker + docker-compose). Documentation is thorough and well-maintained. Getting started requires a Lago Cloud account (free tier) or self-hosted Docker setup. Active open-source community on GitHub and Slack. SDKs for Ruby, Python, Node.js, Go, Java, PHP.