Real-Time Calculation

Taxbit's inventory engine is built for real-time, per-user tax calculations at retail and institutional scale. These APIs form the core integration surface for Austria Tax Withholding.

Transactions API

The Transactions API is the entry point for all activity data. Every acquisition, disposal, income event, and transfer must be submitted here. Taxbit uses transaction data to update inventory lots and trigger gain/loss and withholding calculation in near real-time.

• Key endpoint: POST /transactions/{external_id}. Each transaction is identified by a customer-supplied external_id to ensure idempotency.
• The payload includes asset type, quantity, timestamp, transaction type, FMV (in EUR for Austria), and relevant account identifiers.
• Transactions are processed asynchronously — a 200 response confirms successful ingestion; inventory, gains, and withholding are calculated in the background.

Inventory API

The Inventory API provides a real-time view of a user's undisposed asset holdings organised into cost basis lots. Each lot carries metadata including acquisition date, quantity, cost basis, and pool classification.

• GET /inventory — Returns all undisposed lots for a given asset and account. Supports unrealised gain/loss display.
• GET /inventory/summaries — Returns a per-asset summary of total quantity and total cost basis for an account.

Austria requires the moving average cost method at the wallet level for all New Stock. Each new acquisition updates the average cost of the entire holdings pool for that asset. When a disposal occurs, the average cost at the moment of disposal determines the cost basis of the disposed units.

Inventory is organised into four distinct pools that are disposed of in the following priority order. This order is enforced server-side by Taxbit's inventory engine and is not currently configurable per account:

A disposal that exhausts one pool continues into the next pool in priority order, and a single disposal can therefore draw from several pools at once. Because each pool carries its own loss-compensation treatment, the pool a disposal draws from determines how it contributes to the withholding calculation — see Withholding API below.

Gains API

The Gains API surfaces calculated gain and loss data at multiple levels of granularity.

• GET /gains — Full cost basis, proceeds, and gain/loss per disposal event.
• GET /gains/summary — Total gains/losses for a specified period.

For Austria, GET /gains returns one of five pool-specific gain_type values on each row:

A single disposition can emit multiple gain rows if it spans multiple pools (e.g. a large sale that exhausts Old Stock and dips into the Known Cost Basis pool). Old Stock rows are still returned — and still show a real gain or loss — for record-keeping purposes even though Old Stock contributes zero to the withholding calculation.

Withholding API

The Withholding API is the authoritative source for KESt withholding amounts. It is a dedicated, Austria-specific read endpoint — the Gains API returns gain and loss data only and does not return a withholding amount.

Key endpoints:

GET /v1/inventory/withholding/austria?account_id={account_id}&transaction_id={transaction_id}
GET /v1/inventory/withholding/austria?account_id={account_id}&tax_year={YYYY}

Request

Response

{
  "summary": {
    "year_to_date_balance": "1247.835",
    "latest_transaction_datetime": "2026-04-12T14:32:18Z"
  },
  "transaction": {
    "change_in_balance": "42.675",
    "transaction_datetime": "2026-04-12T14:32:18Z",
    "transaction_id": "client-tx-9182"
  },
  "tax_year": 2026,
  "fiat_asset": {
    "uuid": "550e8400-e29b-41d4-a716-446655440000",
    "name": "Euro",
    "code": "EUR",
    "type": "Fiat"
  },
  "calculation_status": "complete"
}

Amounts are returned in full precision as decimal strings (never scientific notation). The CASP applies its own rounding (typically to EUR cents) when reconciling against its actual withheld balance.

Regime formulas

Taxbit resolves each account to one of two formulas: a compensated formula for individual accounts (losses net against gains, clamped at zero), and a non-compensated, gross-only formula for joint, business, and trust accounts. Both amounts below are returned with the 27.5% rate already applied.

Where each disposition's contribution to compensable / non_compensable / gross is determined by pool:

Performance Characteristics

Inventory, gain/loss, and withholding calculations are triggered automatically whenever a transaction is processed. The pipeline is asynchronous — Taxbit's P95 SLA for end-to-end inventory calculation is under 1 second for standard append transactions. This latency window must be factored into the customer's execution flow when determining the timing of fund diversion (see Important Considerations).