Hagfish API

Hagfish brings professional invoicing into your products and automations. Create a client, create an invoice, and ask Hagfish to deliver it without reimplementing invoice math, billing entitlement, PDF generation, or email.

Available API

The client, onboarding, invoice, template, delivery, usage and webhook API is available at the production base URL below. Use disposable accounts when testing. See client onboarding to create links, review sends and inspect durable delivery outcomes.

Base URL

https://hagfish.app/api/v1

The OpenAPI 3.1 document is the machine-readable reference for clients and tools.

Core workflow

1. Create a scoped API key in Hagfish under Settings → Developers.
2. POST /clients to save the client.
3. POST /invoices to create a draft with Hagfish-calculated totals.
4. GET /invoices/{id}/preview to review the revision, totals, and applicable credits.
5. Generate a PDF or explicitly request a delivery with the reviewed revision and credit ceiling.
6. Subscribe to signed webhooks such as invoice.sent.

Every POST or PATCH uses Idempotency-Key, so a network retry cannot create a second resource or billing reservation.

Conventions

• Requests and responses use JSON with snake_case fields.
• Resource IDs are opaque strings. Store them; do not parse them.
• Monetary amounts in responses are decimal strings in the invoice's major currency unit.
• Lists use cursor pagination with limit and after.
• Errors use application/problem+json and include a request_id.
• Invoice deliveries return 202 Accepted because sending is asynchronous.

Continue to Getting started for a complete first invoice.