Your Billing Software Probably Gets Tractor Parts Wrong (Here's the Fix)

If you write or maintain billing software for Indian retailers, here's a bug class that quietly over- or under-charges GST: rates that depend on a tag like "for tractor".

Since 22 September 2025, most vehicle parts, tyres, tubes, batteries, filters and engine oil are 18%. But Schedule I of Notification 9/2025-Central Tax (Rate) puts a named list of tractor parts at 5% (clutch assembly, gear box, brake assembly, radiator, silencer, steering wheel, hydraulics, fuel tank and a few more, S. No. 443 to 454), plus tractor tyres and tubes.

The trap: a tractor battery, bearing, filter or starter is not on that list. Those sit under their own headings at 18% whatever vehicle they go into. So "is_tractor_part = true" cannot decide the rate. The 5% is a whitelist of named items.

A small way to model it:

```python FIVE_PERCENT_TRACTOR = { "clutch assembly", "gear box", "brake assembly", "radiator assembly", "silencer assembly", "steering wheel", "hydraulics", "fuel tank", "tractor tyre", "tractor tube", } # extend from Schedule I, S. No. 443-454, 426, 269-273

def gst_rate(item: str, fits_tractor: bool) -> int: if fits_tractor and item in FIVE_PERCENT_TRACTOR: return 5 return 18

assert gst_rate("clutch assembly", True) == 5 assert gst_rate("battery", True) == 18 # not on the list assert gst_rate("bearing", True) == 18 assert gst_rate("clutch assembly", False) == 18 ```

The consequence of getting this wrong is real money at the counter. On a Rs 4,000 clutch assembly, 5% is Rs 200 and 18% is Rs 720, so habit-billing at 18% overcharges the farmer by Rs 520. Billing a tractor battery at 5% under-collects, and the shop owns the shortfall.

Two practical notes for your item master: keep the HSN and the rate as separate, versioned fields (the rate changed on a date, and old invoices must keep their old rate), and let the shop owner add items to the 5% set explicitly rather than by category.

The full rate table and the warranty-replacement rules from CBIC circulars are in this GST guide for auto spare parts and tyre shops.