Finance · Module
Tax Engine
One tax determination, returns data and notices service for every app that buys or sells
Tax Engine is a planned shared service that determines GST for Billing, POS, Online Store, Procurement and Expenses from versioned, sourced rules. It also assembles returns data, runs the GSP connection and tracks tax notices.
One answer to "what tax applies?"
Without a shared service, every app that sells or buys ends up with its own copy of GST logic, and they drift apart. Tax Engine is planned as a Core shared service in W3 that the rest of One3 calls instead of reimplementing. In Year 1, tax rules live inside Billing & Invoicing; from W3 Billing delegates to the engine behind a feature flag, and POS receipts, online orders, purchase orders and vendor bills call the same API from W4.
The determination API takes the supplier registration, counterparty profile, item, amounts, supply type, dates and ship-from and ship-to, and returns tax lines, codes and rates. Every determination is logged per document line with an inputs hash and the rule version used, so any past result can be replayed for audit. A "why this tax?" panel shows the rules applied and their sources on any line.
Rules as data, with a change desk
Jurisdictions cover India (states and UTs with GST state codes), SEZ and export, in a model that can extend to other countries. Tax codes cover the GST slabs in force, exempt, nil, non-GST, cess, composition and reverse charge, all effective-dated. Place-of-supply, SEZ, export, reverse-charge and exemption rules are versioned and linked to a Fact Register row.
Rule changes go through a statutory change desk: proposed, sourced in the Fact Register, reviewed by a CA, tested, scheduled and then announced to tenants. A simulation mode runs new rule versions against draft documents and produces a diff before activation. Every rule is verify until confirmed; these are pointers, not legal advice.
GSP, returns and notices
One GSP adapter service handles IRN generation and cancellation, GSTIN validation, GSTR-2B and IMS retrieval and e-way bills, with per-GSTIN credentials and health checks. Returns data for GSTR-1 and GSTR-3B is assembled from Accounts with validation and variance explanations. TDS and TCS rule tables live here too. Tax notices are registered with source, period, due date and amount, assigned, and answered with an AI draft that a CA reviews; the system never submits a reply.
Features
What Tax Engine includes
Jurisdictions
India country, states and UTs with GST state codes, SEZ and export, in a model extensible to other countries.
Effective-dated tax codes
GST slabs as in force, exempt, nil, non-GST, cess, composition and reverse charge.
HSN/SAC master
Codes with descriptions and default rates, effective-dated, with tenant overrides that need a reason and are audited.
Versioned rule tables
Place-of-supply, SEZ/export, reverse-charge and exemption rules, each linked to a Fact Register row.
Determination API and log
Returns tax lines, codes and rates and logs inputs hash and rule version per line for audit replay.
Simulation and what-if
Run determination on drafts or new rule versions before activation and review a diff report.
Statutory change desk
Propose, source, CA review, test, schedule activation and notify tenants for every rule change.
GSP adapter service
One connection for IRN, GSTIN validation, 2B/IMS retrieval and e-way bills, with per-GSTIN credentials and health checks.
E-invoice service
Payload build, local schema validation, submission and response storage for Billing, POS and self-invoices.
E-way bill
Generate from invoice or challan, update vehicle, cancel or extend validity (verify rules and API access).
Returns data assembly
GSTR-1 and GSTR-3B data sets from Accounts with validation and variance explanations.
TDS/TCS rule tables
Withholding sections, thresholds, rates and PAN-unavailable rules, effective-dated (verify).
Tax notices
Register, assign and track notices with deadline alerts and an AI-drafted reply for CA review.
Explainability panel
"Why this tax?" on any line, showing the rules applied and their sources.
Workflow
Activating a rule change
- 1
A change is proposed from a new notification or circular.
- 2
The source is recorded in the Fact Register.
- 3
A CA reviews the rule and the test suite runs.
- 4
Simulation shows the diff on draft documents.
- 5
Activation is scheduled and tenants are notified.
Workflow
Answering a tax notice
- 1
The notice is registered with source, reference, period, due date and amount.
- 2
It is assigned to an owner with deadline alerts.
- 3
The assistant drafts a reply citing ledger evidence and rule versions.
- 4
A CA approves and submits it outside One3.
AI
AI skills and Digital Employees
E-invoice pre-check narrative
Explains which fields will fail validation and why; the validation itself is deterministic.
Rule-change impact summary
Summarises simulation diffs for tenants; a human publishes.
Notice reply drafts
Drafts replies citing evidence and rule versions for CA approval; never submitted by the system.
Blocked-credit suggestion
Suggests ITC blocked-credit classification at bill entry with a reason; a human can override.
E-invoice rejection explainer
Maps IRP, GSP and e-way bill error codes to a plain-language cause and fix steps.
Digital Employees that use this module
They act through the same governed Actions as people; risky steps wait for approval.
Frequently asked questions
Do I buy Tax Engine separately?
No. It is planned as a Core shared service used by Billing, POS, Online Store, Procurement and Expenses.
How are rate changes handled?
Through a change desk: each change is sourced, CA-reviewed, tested in simulation and scheduled before it takes effect.
Can I see why a tax was applied?
Yes. Each line is planned to show the rules and sources used, and determinations can be replayed for audit.
Does it cover countries outside India?
International jurisdictions are planned later (W5) through licensed tax content partners (verify licence).
See Tax Engine in your workflow
Tell us about your team and the outcomes you want. We will walk you through the apps, the Digital Employees and the plan for your industry.