Arithmetic
Two figures must be equal. No notification can change this and no date applies to it. Most of what fynxIQ does is this, which is the point.
How we know
Every check, and what makes it true.
Any GST tool can tell you a rate. The question a chartered accountant asks next is on what basis, and a product that cannot answer it is asking to be trusted rather than earning it. So here is every check fynxIQ runs, why it is right, and the notification behind it where one applies — along with the honest half: what it does not check at all.
The distinction
Three different reasons a check can be right.
They do not age the same way, and mixing them up is how a tool ends up confidently wrong. This is the whole idea behind the tables further down.
Published format
A structural fact about how an identifier is built, such as the GSTIN check digit.
Statute
A rule of law, with a date it took effect. When the law changes, this check is wrong until we correct it, so the instrument and its date are named beside it.
Portal behaviour
How the GST portal itself treats the return, or a GSTN advisory that governs it.
Your own record
True only for you, checked against your own filing history. Nothing external validates it, and it catches what nothing else can.
The proportions matter more than the total. Most of what fynxIQ does is arithmetic, which is why it does not go stale when the law moves — and why the handful of checks that do depend on a notification are named individually, with the date each took effect.
On the finished return
18 checks before anything downloads.
These run on every plan, including the free one. A blocker keeps the download closed; a warning explains something unusual but legal.
| What is checked | Why it is true | Instrument |
|---|---|---|
| The HSN summary covers the same supplies as the liability tablesArithmetic | Both halves describe one month of the same ledger, so their totals are the same integer number of paise. Compared with exact equality, not a tolerance. | None. Arithmetic needs no authority. |
| An invoice total equals the sum of its own item linesArithmetic | Addition. A document that disagrees with itself is wrong whatever the law says. | None. Arithmetic needs no authority. |
| Tax is the declared rate applied to the declared baseArithmetic | Multiplication, in paise. This is what catches a rate entered as 0.03 where 3 was meant: every figure scales by a hundred and the return still balances internally. | None. Arithmetic needs no authority. |
| Every GSTIN carries its own correct check digitPublished format | The fifteenth character is computed from the first fourteen, so a typo is arithmetic to detect. It proves the number is well formed. It does not prove the registration exists. |
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| Inter-state supplies carry integrated tax, intra-state splits central and stateArithmetic | Decided by comparing two state codes already present in the return. No lookup and no judgement is involved. | None. Arithmetic needs no authority. |
| The HSN summary carries the digit depth your turnover obligesStatute | Six digits above ₹5 crore turnover, four at or below. When turnover has not been entered in settings, fynxIQ says nothing rather than guessing which threshold you are on. |
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| Every HSN code in Table 12 is a code that existsPublished format | The nomenclature itself. A code is four, six or eight digits; chapter 00 does not exist and chapter 77 is reserved with no headings under it. Where fynxIQ lists a heading's subdivisions in full — 7113 to 7118 and 9506 today — a subheading outside that list does not exist either, and 71170990 is the case that proves it: heading 7117 subdivides only into 711711, 711719 and 711790. The portal validates against its own master and rejects the WHOLE of Table 12 when one row fails, so this blocks rather than warns. Outside the listed headings fynxIQ says nothing rather than refusing a code it cannot check. | None. Arithmetic needs no authority. |
| No document number is reported twiceArithmetic | Counting. Two rows carrying one invoice number is a fact about the file. | None. Arithmetic needs no authority. |
| Every rate existed as a GST rate in this filing periodStatute | Rates have commencement dates, so this check is period-aware. A superseded rate is a warning and never a blocker: a credit note against an older invoice legitimately carries the older rate. |
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| A concessional rate belongs to goods that are entitled to itStatute | 3% is the precious-metals rate. On a plastic bottle it is not a discount, it is short tax. |
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| These goods may legitimately carry this rateStatute | Checked against the rate master below, which covers a deliberately small number of codes. Advisory, never blocking, and silent on any code the master does not cover. |
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| A zero-rated line carries no tax, and a taxed line is not zero-ratedArithmetic | Zero times anything is zero. | None. Arithmetic needs no authority. |
| Nil-rated and exempt supplies are not reported in the taxable tablesPortal behaviour | The portal keeps them in Table 8, and a return that puts them elsewhere overstates supply. | None. Arithmetic needs no authority. |
| A buyer GSTIN that failed its check digit is named, not silently droppedPublished format | Dropping it turns a B2B supply into B2C without saying so. The buyer loses input credit and nobody finds out. This check exists because that failure was found in our own code. |
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| Every document reported falls inside a declared range in the document summaryPortal behaviour | Required from the May 2025 period. Every number missing from a range is named, so it can be checked against your own register rather than assumed cancelled. |
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| Supplies through an operator are attributed to that operatorArithmetic | A sum over rows that already carry the operator, reconciled against the total. | None. Arithmetic needs no authority. |
| A rate and state that nets to a negative value is explained, not hiddenArithmetic | Returns exceeding sales in one period is ordinary and the portal accepts it. It is reported so the figure is never a surprise. | None. Arithmetic needs no authority. |
| Every document falls inside the period being filedArithmetic | Date comparison against the period on the return. | None. Arithmetic needs no authority. |
On a photographed invoice
11 checks on every page read.
A handwritten invoice cannot be trusted because a model read it confidently. It can be trusted when the figures on it agree with each other across encodings that share no strokes. That is what these do — and the last one is the only check the page itself cannot make.
| What is checked | Why it is true | Instrument |
|---|---|---|
| Every amount is written unambiguouslyArithmetic | Three digits after a decimal point is not how rupees are written, so the mark was almost certainly a thousands separator read as a point — which is a hundred-fold error. | None. Arithmetic needs no authority. |
| The line items add up to the amount before taxArithmetic | Addition, against a figure written separately on the same page. | None. Arithmetic needs no authority. |
| The tax equals the rate applied to the baseArithmetic | Multiplication, against three figures written separately on the same page. | None. Arithmetic needs no authority. |
| Base plus tax equals the invoice totalArithmetic | Addition, against a fourth figure written separately on the same page. | None. Arithmetic needs no authority. |
| The amount in words equals the amount in figuresArithmetic | The strongest check on a photographed invoice. Words and digits share no strokes, so a misreading of one cannot produce the same misreading of the other. A figure that agrees with its own words was read correctly, whatever the handwriting looked like. | None. Arithmetic needs no authority. |
| The seller GSTIN on the page is the one being filed forPublished format | Compared character by character against the registration on the return. |
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| The buyer GSTIN carries its own correct check digitPublished format | Roughly thirty-five in thirty-six misreadings fail this arithmetic. The one in thirty-six that does not is what the buyer record below exists to catch. |
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| The state in the GSTIN matches the state written beside itPublished format | The first two characters are the state code, and the address is written independently of it. Two encodings of the same fact, cross-checked. |
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| The tax head agrees with the two states on the pageArithmetic | Integrated tax between two states, central and state tax within one. Derived from the GSTINs, not from which boxes the invoice happened to fill in. | None. Arithmetic needs no authority. |
| The invoice date falls inside the period being filedArithmetic | Date comparison. Catches a year misread as well as a page from the wrong month. | None. Arithmetic needs no authority. |
| This buyer was invoiced under this same registration beforeYour own record | The only check an invoice cannot make about itself. A GSTIN misread into a different but valid registration passes the check digit, matches its state, and agrees with every figure on the page. Nothing on that invoice can catch it. Only the memory of last month can, and it is silent about any buyer it has not seen before. | None. Arithmetic needs no authority. |
Arithmetic verifies; a person confirms. They are different claims and fynxIQ badges them differently. A tick you place on a field is recorded as confirmed by you, and it can never overrule a check digit that failed.
Not a check, still a decision
Why a supply landed where it did.
Nothing warns you about these, because nothing went wrong. The engine simply had to choose, and the choice rests on an instrument exactly as a check does.
An inter-state B2C invoice above ₹1,00,000 is reported on its own in Table 5. At or below it, the sale is aggregated by rate and state into Table 7. The threshold was ₹2,50,000 for supplies before 1 August 2024.
- Notification 12/2024-Central Taxfrom 1 August 2024
Six digits above ₹5 crore aggregate annual turnover in the previous financial year, four at or below. Turnover is taken from your settings; when it has not been entered, fynxIQ reports the depth it found without asserting which threshold you are on.
- Notification 78/2020-Central Taxfrom 1 April 2021
Rates are resolved against the period, not against a single current list. September 2025 is treated as transitional because the restructuring took effect on the 22nd, so both structures are accepted inside that one return.
- Notification 1/2017-Central Tax (Rate)from 1 July 2017
- Notification 9/2025-Central Tax (Rate)from 22 September 2025
- Notification 19/2025-Central Tax (Rate)from 1 February 2026
- Notification 14/2025-Central Tax (Rate)from 22 September 2025
Table 13 is built and required from the May 2025 return period onwards. Zero-value replacement documents are counted as issued, not as cancelled — counting them as cancellations understates documents issued, which is a real error we found in a filed return.
- GSTN advisory of 2 May 2025 (Phase III, Tables 12 and 13)from May 2025 return period
The rate master
31 entries, and silence everywhere else.
This is the table behind “these goods may not carry that rate”. It is deliberately small. An entry that is not certain is not written, because silence costs you nothing while a confident wrong accusation teaches you to ignore the warnings that matter. Every return reports how much of it your codes were covered by, as a figure.
| HSN | Goods | Rate | In force | Source |
|---|---|---|---|---|
| 3305 | Preparations for use on the hair | 5% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 3305 | Preparations for use on the hair | 18% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 3306 | Oral and dental hygiene preparations | 5% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 3306 | Oral and dental hygiene preparations | 18% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 340111 | Soap in bars for toilet use | 5% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 340111 | Soap in bars for toilet use | 18% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 3924 | Tableware, kitchenware and household articles of plastic | 18% | July 2017 onwards | Schedule III, unchanged by Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 4202 | Trunks, suitcases, handbags and similar containers | 18% | July 2017 onwards | Schedule III, unchanged by Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 61 | Articles of apparel and made-up textile articlesup to ₹2,500 a piece at 5%, above it at 18% | 5% or 18% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 61 | Articles of apparel and made-up textile articlesup to ₹1,000 a piece at 5%, above it at 12% | 5% or 12% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 62 | Articles of apparel and made-up textile articlesup to ₹2,500 a piece at 5%, above it at 18% | 5% or 18% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 62 | Articles of apparel and made-up textile articlesup to ₹1,000 a piece at 5%, above it at 12% | 5% or 12% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 63 | Articles of apparel and made-up textile articlesup to ₹2,500 a piece at 5%, above it at 18% | 5% or 18% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 63 | Articles of apparel and made-up textile articlesup to ₹1,000 a piece at 5%, above it at 12% | 5% or 12% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 64 | Footwearup to ₹2,500 a pair at 5%, above it at 18% | 5% or 18% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 681599 | Bricks, blocks and earthen roofing tiles6% without input tax credit, 12% with it | 6% or 12% | April 2022 onwards | Brick-kiln scheme, continued by Notification 14/2025-Central Tax (Rate) |
| 6901 | Bricks, blocks and earthen roofing tiles6% without input tax credit, 12% with it | 6% or 12% | April 2022 onwards | Brick-kiln scheme, continued by Notification 14/2025-Central Tax (Rate) |
| 6904 | Bricks, blocks and earthen roofing tiles6% without input tax credit, 12% with it | 6% or 12% | April 2022 onwards | Brick-kiln scheme, continued by Notification 14/2025-Central Tax (Rate) |
| 6905 | Bricks, blocks and earthen roofing tiles6% without input tax credit, 12% with it | 6% or 12% | April 2022 onwards | Brick-kiln scheme, continued by Notification 14/2025-Central Tax (Rate) |
| 7106 | Silver, unwrought or semi-manufactured | 3% | July 2017 onwards | Notification 1/2017-Central Tax (Rate) |
| 7108 | Gold, unwrought or semi-manufactured | 3% | July 2017 onwards | Notification 1/2017-Central Tax (Rate) |
| 7113 | Articles of jewellery of precious metal | 3% | July 2017 onwards | Notification 1/2017-Central Tax (Rate) |
| 7114 | Goldsmiths' and silversmiths' wares | 3% | July 2017 onwards | Notification 1/2017-Central Tax (Rate) |
| 7117 | Imitation jewellery | 3% | July 2017 onwards | Notification 1/2017-Central Tax (Rate) |
| 8424 | Sprayers, spray guns and similar appliances | 18% | July 2017 onwards | Schedule III, unchanged by Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 9503 | Toyselectronic and battery-operated toys stay at 18% | 5% or 18% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 9503 | Toyselectronic and battery-operated toys sit at the higher rate | 12% or 18% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 9506 | Sports goods and equipment for physical exercise | 5% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 9506 | Sports goods and equipment for physical exercise | 12% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
| 960321 | Tooth brushes | 5% | September 2025 onwards | Notification 9/2025-Central Tax (Rate), w.e.f. 22 September 2025 |
| 960321 | Tooth brushes | 18% | July 2017 to September 2025 | Notification 1/2017-Central Tax (Rate) |
Where goods can legitimately carry more than one rate — apparel above or below a price per piece, bricks with or without input credit — every permitted rate is listed and the condition says what decides between them. An HSN summary row aggregates many sales, so the engine cannot resolve those from the return alone and does not pretend to. It objects only when the declared rate is none of the permitted ones.
Every instrument, in one place
What we rely on, and from when.
| Instrument | Effective | What it does |
|---|---|---|
| Notification 1/2017-Central Tax (Rate) | 1 July 2017 | The original rate schedules. Every rate that has not since been changed still rests here. |
| Notification 9/2025-Central Tax (Rate) | 22 September 2025 | The rate restructuring. 12% ceased to be a general rate and 40% was introduced. Because it took effect mid-month, both structures are legitimate inside the September 2025 return and fynxIQ accepts either for that one period. |
| Notification 14/2025-Central Tax (Rate) | 22 September 2025 | Continues the brick-kiln scheme, under which fly-ash bricks, building bricks and earthen roofing tiles keep 6% without input credit and 12% with it. Without this entry the engine would wrongly flag 12% on those goods after September 2025. |
| Notification 19/2025-Central Tax (Rate) | 1 February 2026 | 28% ceased to be a GST rate. |
| Notification 78/2020-Central Tax | 1 April 2021 | Above ₹5 crore aggregate annual turnover in the previous financial year, the HSN summary must carry six digits; at or below it, four. Exactly ₹5 crore is not above ₹5 crore, and is treated as four. |
| Notification 12/2024-Central Tax | 1 August 2024 | The inter-state B2C invoice value above which a supply is reported individually in Table 5 rather than aggregated into Table 7 dropped from ₹2,50,000 to ₹1,00,000. |
| GSTN advisory of 2 May 2025 (Phase III, Tables 12 and 13) | May 2025 return period | From this period the portal will not accept a return whose supply tables are populated and whose document summary is blank. |
| GSTIN structure published by GSTN | structural | Fifteen characters: a two-digit state code, the ten-character PAN, an entity number, a fixed letter, and a mod-36 check digit computed over the first fourteen. |
Instruments are cited by number and date rather than deep-linked, because the department’s own URLs for individual notifications move, and a dead link on a page about verification is worse than no link. The number is how it is actually looked up.
The honest half
What fynxIQ does not check.
Published on purpose. A list of checks with no list of gaps beside it invites you to assume the gaps are not there — and assuming everything was checked is exactly how a wrong return gets filed with confidence.
Whether a GSTIN actually exists, or is active
The check digit proves a registration number is well formed, not that it was ever issued or is still live. That needs the department's own lookup, which is not connected. A cancelled buyer registration will pass every check here.
Whether goods are classified under the right HSN code
Classification is genuinely arguable and the same physical object can sit under different codes for defensible reasons. fynxIQ checks whether the rate is one the code may carry. It has no opinion on whether the code was the right one to choose.
Most HSN codes, for rate
The rate master covers a small number of prefixes. On every other code the engine says nothing, which is correct behaviour: silence costs you nothing, while a confident wrong accusation teaches you to ignore the warnings that matter. Coverage is reported on every return as a figure, never implied to be complete.
Whether a photographed invoice was read correctly
Nothing proves that. What the checks above establish is narrower and more useful: that a set of figures agrees with itself across four independent encodings, including one — the amount in words — that shares no strokes with the digits. A misreading that satisfies all of them is not a misreading. Anything not settled that way is shown to you unresolved, and a field you tick is recorded as confirmed by a person, never as verified.
Services
The rate master is a goods schedule. Codes in chapter 99 are reported as outside its scope rather than as carrying 0%.
Whether the underlying sales data is complete
fynxIQ reconciles what you give it. If a marketplace report or an invoice is missing from the upload, every check can pass on a return that is still short. The document summary names gaps in your invoice numbering, which is the closest thing to a defence, and it is not the same thing.
Anything at all about the portal submission
fynxIQ produces a JSON file. It never holds portal credentials, never files on your behalf, and cannot see what happened after you uploaded it.
Check us
Go and read the source yourself.
- CBIC GST portal — Acts, rules, notifications and circulars, published by the department that issues them.
- GST portal — Where the return is actually filed, and where a registration can be looked up.
If you find an entry on this page that is out of date or wrong, tell us and we will correct it and say so. That is a cheaper failure than a return filed on it. Write to us.
Run all of it on your own month.
Every check on this page runs on the free plan. None of them are held back for paying customers, because the checks are the product.
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