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Generative AI > Google Gemini API > GST Computation Agent

GST Computation Agent

Author: Venkata Sudhakar

ShopMax India sells electronics across states in India, making GST computation non-trivial. Intrastate sales attract CGST and SGST; interstate sales attract IGST. Different product categories carry different GST rates. A computation agent that takes the month's transactions, applies the correct tax rules, and produces a filing-ready summary saves the accounts team hours of manual work each month and reduces the risk of incorrect filing.

This tutorial builds a GST Computation Agent using ADK and Gemini. The agent classifies transactions by type (intrastate or interstate), applies the applicable GST rate per product category, computes CGST, SGST, and IGST breakdowns, and generates a GSTR-3B ready summary.

The below example shows monthly GST computation for ShopMax India sales transactions.

from google.adk.agents import LlmAgent
from google.adk.tools import FunctionTool
from google.adk.runners import Runner
from google.adk.sessions import InMemorySessionService

# GST rates by product category
GST_RATES = {
    "tv": 18,
    "audio": 18,
    "tablet": 18,
    "laptop": 18,
    "phone": 12,
    "accessories": 18,
    "appliance": 12,
}

# Monthly sales transactions
TRANSACTIONS = [
    {"id": "T001", "category": "tv", "taxable_value": 620000,
     "seller_state": "Maharashtra", "buyer_state": "Maharashtra"},
    {"id": "T002", "category": "audio", "taxable_value": 290000,
     "seller_state": "Maharashtra", "buyer_state": "Karnataka"},
    {"id": "T003", "category": "tablet", "taxable_value": 740000,
     "seller_state": "Maharashtra", "buyer_state": "Maharashtra"},
    {"id": "T004", "category": "phone", "taxable_value": 185000,
     "seller_state": "Maharashtra", "buyer_state": "Delhi"},
    {"id": "T005", "category": "accessories", "taxable_value": 95000,
     "seller_state": "Maharashtra", "buyer_state": "Maharashtra"},
]

# --- Tool: compute GST for all transactions ---
def compute_monthly_gst() -> dict:
    """Compute CGST, SGST, and IGST for all monthly transactions.
    Returns:
        GST breakdown by type and total liability
    """
    total_cgst = 0.0
    total_sgst = 0.0
    total_igst = 0.0
    breakdown = []
    for txn in TRANSACTIONS:
        rate = GST_RATES.get(txn["category"], 18)
        taxable = txn["taxable_value"]
        intrastate = txn["seller_state"] == txn["buyer_state"]
        if intrastate:
            cgst = round(taxable * rate / 2 / 100)
            sgst = cgst
            igst = 0
        else:
            cgst = 0
            sgst = 0
            igst = round(taxable * rate / 100)
        total_cgst += cgst
        total_sgst += sgst
        total_igst += igst
        breakdown.append({
            "txn_id": txn["id"],
            "category": txn["category"],
            "taxable_value": taxable,
            "gst_rate_pct": rate,
            "type": "Intrastate" if intrastate else "Interstate",
            "cgst": cgst,
            "sgst": sgst,
            "igst": igst,
            "total_tax": cgst + sgst + igst,
        })
    total_tax = total_cgst + total_sgst + total_igst
    return {
        "total_taxable_value": sum(t["taxable_value"] for t in TRANSACTIONS),
        "total_cgst": round(total_cgst),
        "total_sgst": round(total_sgst),
        "total_igst": round(total_igst),
        "total_gst_liability": round(total_tax),
        "transaction_count": len(TRANSACTIONS),
        "breakdown": breakdown,
    }

# --- Agent setup ---
gst_tool = FunctionTool(func=compute_monthly_gst)

agent = LlmAgent(
    name="gst_agent",
    model="gemini-2.0-flash",
    instruction="""You are a GST computation agent for ShopMax India.
Call compute_monthly_gst to get the full breakdown.
Present:
1. A transaction-level table: Txn | Category | Taxable Value | Rate | Type | CGST | SGST | IGST | Total Tax
2. A GSTR-3B summary section:
   - Total Outward Taxable Supplies (excl. zero-rated)
   - CGST payable
   - SGST payable
   - IGST payable
   - Total GST liability
Format all amounts in Rs with comma separators.""",
    tools=[gst_tool],
)

session_service = InMemorySessionService()
runner = Runner(agent=agent, app_name="gst_agent", session_service=session_service)

import asyncio

async def run_gst(query: str) -> str:
    session = await session_service.create_session(app_name="gst_agent", user_id="accounts")
    from google.genai.types import Content, Part
    msg = Content(role="user", parts=[Part(text=query)])
    result = ""
    async for event in runner.run_async(user_id="accounts", session_id=session.id, new_message=msg):
        if event.is_final_response() and event.content:
            result = event.content.parts[0].text
    return result

response = asyncio.run(run_gst("Compute the monthly GST liability and give me the GSTR-3B summary."))
print(response)

It gives the following output,

GST Computation - ShopMax India (Monthly)
5 transactions  |  Total Taxable Value: Rs 19,30,000

Txn  | Category    | Taxable Value | Rate | Type        | CGST    | SGST    | IGST    | Tax
-----|-------------|---------------|------|-------------|---------|---------|---------|--------
T001 | TV          | Rs 6,20,000   | 18%  | Intrastate  | 55,800  | 55,800  |      0  | 1,11,600
T002 | Audio       | Rs 2,90,000   | 18%  | Interstate  |       0 |       0 | 52,200  |   52,200
T003 | Tablet      | Rs 7,40,000   | 18%  | Intrastate  | 66,600  | 66,600  |      0  | 1,33,200
T004 | Phone       | Rs 1,85,000   | 12%  | Interstate  |       0 |       0 | 22,200  |   22,200
T005 | Accessories | Rs 95,000     | 18%  | Intrastate  |  8,550  |  8,550  |      0  |   17,100

GSTR-3B Summary:
  Total Outward Taxable Supplies : Rs 19,30,000
  CGST Payable                   : Rs 1,30,950
  SGST Payable                   : Rs 1,30,950
  IGST Payable                   : Rs 74,400
  Total GST Liability            : Rs 3,36,300

In production, pull transaction data directly from BigQuery using a monthly Cloud Scheduler trigger. Cross-reference with input tax credit (ITC) from purchase invoices to compute net GST payable. Store the GSTR-3B summary in Firestore and generate a PDF using Cloud Functions for the chartered accountant's review before filing on the GSTN portal.

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