Numeral vs TaxJar

Numeral

Docs are written for agents, with a public OpenAPI spec for 32 operations, Markdown pages, llms.txt and a remote MCP server whose 14 tools are read-only by default. No status page, numeric rate limit or SLA was found, the API ignores idempotency keys, and API access is sold on a custom plan after a demo.

TaxJar

A 21-operation REST API with a no-card 30-day trial, caller-chosen transaction IDs that make order pushes safe to repeat, and no status-page incident since March 2026. No OpenAPI spec or llms.txt was found, the API changelog stops at January 2022, tokens carry no scopes, and calculation is US-only for new accounts.

Numeral

1. Send X-API-Version: 2026-03-01 on every call to https://api.numeralhq.com. Merchant, platform and certificate endpoints reject requests without it
2. Commit with a stable reference_order_id. On REFERENCE_ORDER_ID_ALREADY_EXISTS look up the existing transaction, because Idempotency-Key is ignored
3. Commit before the calculation's expires_at. If it has expired, calculate again and commit the new calculation_id with the original reference_order_id
4. Send amount as the line total in minor units to /tax/calculations and as the per-unit price to /tax/platform/calculations
5. Start with an sk_test_ key. Certificate endpoints and MCP certificate tools are live only and answer TESTMODE_NOT_SUPPORTED or live_mode_required

TaxJar

1. Send x-api-version: 2022-01-24 on every call. Accounts opened before 3 February 2022 default to an older version with weaker validation
2. Pass nexus_addresses[] or keep nexus states in the account. Without nexus the API returns zero tax and reports no error
3. Create orders with your own transaction_id. On 422 the order already exists, so send a PUT to update it
4. Post JSON with Content-Type: application/json. Form-encoded arrays such as line_items[] can return 406
5. Cache calculations and skip states without nexus. Each calculation or rate lookup counts as a tenth of an order, errors included