Pricing & GST

The Paise Convention

All prices are stored as integers in paise (₹1 = 100 paise). This avoids float rounding errors — a well-known problem in financial software.

User Says	Stored As	Display Formula
"₹14"	1400	1400 / 100 = 14.00
"₹99.50"	9950	9950 / 100 = 99.50
"₹250"	25000	25000 / 100 = 250.00

The system prompt tells the LLM to convert:

Prices are stored in paise (₹1 = 100 paise) — convert user amounts to paise when storing.
Prices quoted by sellers include GST — the stored sell_price is GST-inclusive (MRP).

GST-Inclusive Pricing

In India, retail prices (MRP) include GST. The sell_price field stores the GST-inclusive price. When displaying a bill, the system:

1. Derives the taxable value from the inclusive price
2. Splits the tax into CGST (Central) and SGST (State) at half the slab rate each

The split_gst Algorithm

def split_gst(inclusive_paise: int, gst_slab: int) -> tuple[int, int]:
    if gst_slab == 0:
        return (0, 0)
    taxable = round(inclusive_paise * 100 / (100 + gst_slab))
    total_tax = inclusive_paise - taxable
    cgst = round(total_tax / 2)
    sgst = total_tax - cgst
    return (cgst, sgst)

Examples

Sell Price	GST Slab	Taxable	CGST	SGST	Total Tax	Grand Total
₹100 (10000 paise)	0%	10000	0	0	0	10000
₹100 (10000 paise)	5%	9524	238	238	476	10000
₹100 (10000 paise)	12%	8929	536	535	1071	10000
₹100 (10000 paise)	18%	8475	762	763	1525	10000
₹37 (3700 paise)	5%	3524	88	88	176	3700

Note: cgst and sgst may differ by 1 paise due to rounding (total tax is always correct).

Per-Item GST Computation

GST is computed per line item, not per bill. Each inventory item has its own gst_slab:

class Inventory(SQLModel, table=True):
    gst_slab: int = 0           # 0, 5, 12, 18, 28
    sell_price: int             # GST-inclusive MRP in paise
    hsn_code: str | None        # Harmonized System code (8 chars)

For a bill with multiple items at different slabs:

for item in bill_items:
    inv = session.get(Inventory, item.inventory_id)
    line_total = inv.sell_price * item.quantity
    cgst, sgst = split_gst(line_total, inv.gst_slab)
    subtotal += line_total - (cgst + sgst)
    cgst_total += cgst
    sgst_total += sgst
    grand_total = subtotal + cgst_total + sgst_total

HSN Code Auto-Lookup

When adding a new product, the discover_product_info tool uses the LLM (via a prompt) to suggest the correct HSN code and GST slab. This is the only place the model's "knowledge" is trusted — for product classification, not pricing.

Invoice Display

The bill display shows the full GST breakup:

Bill #42 — Finalized
  Subtotal:        ₹952.38
  CGST (2.5%):     ₹23.81
  SGST (2.5%):     ₹23.81
  ─────────────────────
  Grand Total:   ₹1,000.00
  ─────────────────────
  Payment: UPI (ref: 12345)

This is rendered by billing/formatter.py using the BillDisplay dataclass.

Guardrails

if sell_price < cost_price:
    raise ValueError(
        f"Sell price (₹{sell_price/100:.2f}) must be >= "
        f"cost price (₹{cost_price/100:.2f})"
    )

The system prevents below-cost sales. The LLM sees this error and can explain it to the user.