Register nexus states via POST /v2/nexus/addresses with the state, zip, city, country, and business type fields; these tell TaxJar where your business has tax obligations
At checkout, call POST /v2/taxes with from_country, from_state, from_zip, to_country, to_state, to_zip, amount (subtotal), shipping, and a line_items array including product_tax_code per item
Map your products to TaxJar product tax codes (PTCs) — e.g., 20010 for clothing, 81100 for software; items without a PTC are taxed at the standard rate which may be incorrect for exempt categories
Parse the response: tax.amount_to_collect is the total tax; breakdown.line_items gives per-line tax for display purposes
For order recording (for AutoFile or audit purposes), POST /v2/transactions/orders after a completed purchase with the same line item detail to persist the transaction in TaxJar
Known gotchas
TaxJar nexus addresses must match the actual business presence; adding nexus for states where you do not have economic nexus will cause TaxJar to collect tax unnecessarily, creating over-collection liability
The SmartCalc API endpoint is rate-limited by plan tier; high-volume stores should cache tax responses for identical from/to/amount/product combinations within the same checkout session rather than calling on every keystroke
TaxJar uses its own product tax code taxonomy — do not map to Avalara PTCs or generic category IDs; incorrect code mapping is the most common cause of incorrect tax calculations for digital goods and clothing
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp