A GST utilities API that validates GSTINs, calculates tax and looks up HSN/SAC codes performs three tasks that India’s official GST systems also handle. The title does not name the provider, though, so this article cannot describe that product’s endpoints, validation method, data sources, update schedule or pricing. What it can do is set out how the official e-invoice and e-Way Bill systems handle the same three tasks, so developers and billing teams know what a utility should match and what to test before relying on it.
What the title can and cannot establish
The title describes three jobs. It does not identify a vendor, a documentation page or a pricing model. The word “Free” is a claim in the title only. No pricing page or free-tier terms could be confirmed for any specific utility, so treat free access, rate limits and data-refresh timing as questions to put to the provider rather than facts to assume.
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The official GST ecosystem is a useful benchmark. The Goods and Services Tax Network (GSTN) publishes an e-invoicing integration model, and the Invoice Registration Portal (IRP), whose underlying system IRIS powers for GSTN, documents core APIs and validation rules. The E-Way Bill API developer portal documents GSTIN and HSN detail functions. Those documents describe related capabilities. They do not show that an unnamed utility uses the same infrastructure, exposes matching endpoints or follows the same rules.
GSTIN validation: format checks versus live status
“Validate GSTIN” can mean two different things, and a utility may do only one of them.
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- Structural (format) check. The utility confirms that the 15-character string follows the GSTIN structure, such as a valid state code and a correctly formed PAN-based segment. This runs offline and cannot tell you whether the taxpayer is active.
- Registration or status lookup. The utility queries official GSTIN detail services for the registered legal name, trade name, registration status and related details. The GSTN E-Way Bill portal and the IRIS IRP core API list “Get GSTIN Details” among their functions, and IRIS also lists a “Sync GSTIN details from the Common Portal” operation. Either route needs valid credentials and returns data only for the taxpayer’s current record.
Ask the provider which of these it performs. A format-only result passing does not mean the business is registered or active, and an invoice-level check in the IRP can still reject a GSTIN for other reasons. The IRIS validation rules document describes checks on GSTIN and other invoice fields as part of e-invoice processing, which is a different scope from a standalone utility.
Tax calculation: intra-state, inter-state and the exception flag
Whether a supply attracts CGST and SGST or IGST is the core logic any tax calculator must get right. According to the IRIS IRP validation rules at https://einvoice6.gst.gov.in/content/validation-rules/:
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- An intra-state transaction, where supplier and recipient are in the same state, generally carries CGST and SGST.
- An inter-state transaction, where they are in different states, carries IGST.
- The documentation describes a flag for exception circumstances in which IGST applies to an intra-state supply. A calculator that hard-codes the state comparison without this exception will produce incorrect totals for those cases.
The same rules describe validation of HSN codes, item-level tax calculations and invoice totals. A utility that calculates tax should be tested against these outcomes: the tax split, the rounding of item-level amounts and the invoice total. Any test cases should include at least one intra-state, one inter-state and one flagged exception scenario.
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HSN codes classify goods and tax rates attach to them through notifications. Both are maintained data. A lookup table that was correct last year can be wrong now, so the update date of any HSN or rate table matters as much as the table itself.
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January 2025 HSN database update
IRIS’s release dated January 28, 2025 states that its HSN code database was updated in alignment with the National Informatics Centre (NIC) following specified notifications. The release is dated, and the change applies to IRIS’s system. It does not establish that any other utility refreshed its data on the same date.
September 2025 addition of a 40% rate
IRIS’s production release dated September 21, 2025 states that a 40% GST rate was added to its production tax-rate master and made available through the API, the web form, the Excel utility and JSON upload. This is a rate that exists in the master, not a rate that applies to every supply. Whether it applies to a given good or service depends on the classification and the notification in force for that transaction and date.
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SAC codes
The title covers SAC codes, which classify services. The sources reviewed here document HSN handling in detail, and the SAC treatment in the IRIS and E-Way Bill functions could not be confirmed from them. Ask any utility to show which master it uses for SAC codes and when that master was last updated.
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GSTN’s e-invoicing page at https://www.gstn.org.in/e-invoicing states that e-invoice API clients authenticate with valid credentials generated by taxpayers and then receive an authorization token for subsequent calls. GSTN gives that token a validity of 360 minutes. The page reviewed does not state the year of that figure, so confirm it against the current documentation before building token refresh logic.
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These are access rules for GSTN’s e-invoice API. A third-party utility may use a different credential model, a different token lifetime or none at all. Do not carry the 360-minute figure into the utility’s design without checking its documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Official API functions compared
The table below lists only functions that the official sources name. A blank entry means the source did not list that function, not that it is missing from the system.
| System | Documented functions named in the source | HSN lookup listed | Source |
|---|---|---|---|
| GSTN e-invoice API | Taxpayer-generated credentials and authorization token (360-minute validity); integration by taxpayers, GSPs and accounting and billing software providers | Not stated on the page reviewed | GSTN, E-Invoice |
| IRIS IRP core APIs | Authentication, Generate IRN, Cancel IRN, Get IRN, retrieval by document details, health status, Get GSTIN Details, Sync GSTIN details from the Common Portal | Not listed among core operations; HSN database changes appear in release notes | IRIS IRP, Core APIs |
| GST E-Way Bill API portal | Get GSTIN Details, GET HSN Details; documentation version v1.03 at the time of review | Yes (GET HSN Details) | GSTN E-Way Bill API Developer Portal |
The IRIS validation rules and the e-invoice integration page at https://einvoice6.gst.gov.in/content/api-integration/ give further dated integration and release information. Read the release notes directly for any change that affects your data.
How to test a GST utility API before relying on it
- Confirm the provider and documentation. Ask for the company name, the documentation URL and the terms governing free use, including any rate limits or volume caps.
- Establish what “validate GSTIN” means. Run a malformed GSTIN, a well-formed GSTIN for a taxpayer you know is cancelled or suspended, and an active GSTIN. If the utility returns the same result for the last two, it is doing a format check only.
- Test the tax split. Run an intra-state invoice, an inter-state invoice and an intra-state invoice under the IGST exception described in the IRIS rules. Compare the CGST, SGST and IGST amounts and the total.
- Check the data date. Ask which HSN and rate masters are used, when they were last updated and which notification each change follows. Test a code whose rate changed in a dated release, such as the 2025 IRIS updates above, and confirm the utility returns the current rate.
- Verify SAC coverage separately. Look up several SAC codes for services, since HSN results do not prove SAC support.
- Review error handling. Check how the utility reports an invalid GSTIN, an unknown code, an expired token or a service outage, and whether it distinguishes those cases.
Where a tax utility fits in a billing workflow
GSTN names accounting and billing software providers among the parties that can integrate their systems with the e-invoice system through its APIs. In practice, a utility is most useful when it runs before the invoice is generated, so that GSTIN, HSN and tax-split errors are caught on the business’s own side before the record is registered with the e-invoice system. Keep the official validation as the final authority, because the registration step applies its own checks.
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