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ChatGPT Custom Actions + GST Maker

Custom GPT Actions

Build autonomous billing agents and custom GPTs equipped with real-time Indian GST APIs

ChatGPT Tool Page

OpenAPI 3.1.0 Schema for Custom GPT Actions

Paste this schema into your ChatGPT Custom GPT builder under Actions

{
  "openapi": "3.1.0",
  "info": {
    "title": "GST Maker Public API",
    "version": "1.0.0"
  },
  "servers": [
    { "url": "https://gstmaker.com" }
  ],
  "paths": {
    "/tools-api/gstin-decode": {
      "get": {
        "summary": "Validate and decode Indian GSTIN",
        "operationId": "decodeGstin",
        "parameters": [
          {
            "name": "gstin",
            "in": "query",
            "required": true,
            "schema": { "type": "string" }
          }
        ]
      }
    },
    "/tools-api/hsn-search": {
      "get": {
        "summary": "Search HSN and SAC tax codes",
        "operationId": "searchHsn",
        "parameters": [
          {
            "name": "q",
            "in": "query",
            "required": true,
            "schema": { "type": "string" }
          }
        ]
      }
    }
  }
}

How to Setup ChatGPT Custom Actions

Connect GPTs with GST Maker endpoints

  1. 1
    Create a Custom GPT Go to Explore GPTs → Create and name your agent (e.g. 'Indian Billing & Tax Bot').
  2. 2
    Add New Action Scroll to the bottom of the Configure tab and click Create new action.
  3. 3
    Paste OpenAPI Schema Paste the OpenAPI 3.1.0 schema above. ChatGPT will auto-detect the available endpoints.
  4. 4
    Test in Playground In the preview chat, say: 'Validate GSTIN 27AAPCS1234A1Z5' and verify the action response!

Pro Tips for Custom GPT Actions

Reliable execution without hallucinatory errors

  • Set Strict System Instructions Add to Instructions: 'Always call the decodeGstin action to verify GST numbers before generating invoice summaries. Never guess or hallucinate GST states.'
  • Privacy Policy URL Requirement When publishing public Custom GPTs with Actions, provide our privacy policy link: https://gstmaker.com/privacy.
  • Handle State-wise IGST Logic Instruct the GPT: 'If customer state does not match supplier state, calculate IGST instead of CGST+SGST.'
  • Use for WhatsApp Support Bots You can connect your Custom GPT to WhatsApp via Twilio or WATI to answer customer billing inquiries autonomously.