The sales tax API for calculation, VAT, & filing

What is a sales tax API?

What the Kintsugi API does

• Tax calculation

  Quote tax at checkout or on an invoice. Returns tax, taxable amount, and rate per line item, broken out by the jurisdictions that levy it, with the reason for any exemption. Nothing is stored, so call it as often as the cart changes.

  • POST /v1/tax/estimate

• Transactions and refunds

  Record each completed sale and refund. Synced transactions are what Kintsugi uses to track nexus and prepare your returns.

  • POST /v1/transactions
  • POST /v1/transactions/{original_transaction_id}/credit_notes

• Address validation

  Standardise a customer address before you calculate. The response fills in fields such as county, which local rates depend on.

  • POST /v1/address_validation/search
  • POST /v1/address_validation/suggestions

• Products and taxability

  Build your catalogue once and classify each product into a Kintsugi category and subcategory, which decides how it is taxed in each state.

  • POST /v1/products
  • GET /v1/products/categories

• Customers and exemptions

  Store customers and their exemption certificates, and Kintsugi applies the exemption on every estimate that references the customer.

  • POST /v1/customers
  • POST /v1/exemptions
  • POST /v1/exemptions/{exemption_id}/attachments

• Nexus

  Read where you have nexus, and record physical nexus such as offices, employees, or inventory so it counts alongside economic nexus.

  • GET /v1/nexus
  • POST /v1/nexus/physical_nexus

• Registrations

  Start and track sales tax, VAT, and GST registrations from your own product, including the jurisdiction-specific fields each one needs.

  • POST /v1/registrations
  • GET /v1/registrations/jurisdictions

• Filings

  List the returns Kintsugi prepares for you, see the transactions in each one, and approve a filing through the API.

  • GET /v1/filings
  • PUT /v1/filings/{filing_id}/approve

See every endpoint in the API reference.

Calculate tax with one request

Authenticate with your API key and organisation ID, send the sale, and read the tax from the response. Amounts come back as decimal strings, so parse them with a decimal-safe type.

curl -X POST https://api.trykintsugi.com/v1/tax/estimate \
  -H "x-api-key: $KINTSUGI_API_KEY" \
  -H "x-organization-id: $KINTSUGI_ORG_ID" \
  -H "Content-Type: application/json" \
  -d '{
    "date": "2026-01-15T10:00:00Z",
    "external_id": "EST-2026-001",
    "currency": "USD",
    "transaction_items": [
      {
        "external_id": "ITEM-001",
        "date": "2026-01-15T10:00:00Z",
        "external_product_id": "PROD-12345",
        "quantity": 1,
        "amount": 100.00
      }
    ],
    "addresses": [
      {
        "type": "SHIP_TO",
        "street_1": "123 Main St",
        "city": "Seattle",
        "state": "WA",
        "postal_code": "98101",
        "country": "US"
      }
    ]
  }'

From the sales tax calculation guide, which covers required fields, exemptions, and the response in full.

Official SDKs

• Pythonpip install kintsugi-tax-platform-sdk
• TypeScriptnpm add @kintsugi-tax/tax-platform-sdk
• Javacom.trykintsugi:kintsugi-tax-java-sdk
• PHPcomposer require kintsugi-tax/tax-platform-sdk
• Rubygem install kintsugi_sdk

Developer resources

• Getting started

  Connect your data and make your first authenticated call.

• API reference

  Every endpoint, with request and response schemas.

• Sales tax calculation guide

  When to call the estimate endpoint and how to shape the request.

• SDKs

  Python, TypeScript, Java, PHP, and Ruby clients.

• API Lab

  Run each workflow interactively in a simulated sandbox.

• Kintsugi MCP

  Give your AI coding assistant the live API spec and docs.

• Migrating from Avalara or TaxJar

  Map your existing calls to Kintsugi endpoints and cut over cleanly.