Lago vs Metronome

Lago

Lago's open-source edition runs usage metering, plans, wallets and invoicing on the owner's servers at no charge, with an OpenAPI 3.1 file of 217 operations and releases about every two weeks. The free edition has one unscoped API key and no audit logs, and Lago's llms.txt understates what its MCP server can change.

Metronome

Metronome's API carries an Idempotency-Key on every POST, a public OpenAPI description of 142 operations, and agentic tokens whose writes wait for a person's approval. Prices for the Startup plan are public. Errors return only a message string, token scopes are set through support, and the status feed lists 13 incidents since 22 July 2026.

Lago

1. Call <your Lago API origin>/api/v1 with Authorization: Bearer <key>. On Lago Cloud the hosts are api.getlago.com and api.eu.getlago.com
2. Send every usage event with a deterministic transaction_id and an explicit timestamp, and resend the same payload on retry. On the ClickHouse event store a changed timestamp is billed again
3. Expect 422 value_already_exist for a repeated event on the Postgres event store. Treat it as success for a retry
4. Page with page and per_page and read meta.next_page and meta.total_count. The default page size is 100 in most cases
5. Treat the free edition's API key as full access. It can void invoices and terminate subscriptions, so keep it out of prompts and ask a person before financial changes
6. Before giving an assistant the MCP server, note that it includes delete_invoice, void_invoice, delete_plan and create_payment and sets no read-only annotations

Metronome

1. Send an Idempotency-Key header on every POST and reuse it on retry. A cached error is returned for the same key, so use a new key after fixing the cause
2. Give each usage event a stable transaction_id. Metronome ignores repeats for 34 days and accepts at most 100 events per /v1/ingest call
3. On 429 read X-Metronome-Rate-Limit-Type (client or customer) and back off. Unlisted endpoints default to 8 requests a second
4. With an agentic token, expect 403 AgenticMutationBlocked on writes. Post the original method, path and body to /v1/approval_requests and show the person the approval_url
5. Use a sandbox token for testing. The token alone decides whether a call reaches sandbox or production data