I built a niche API for Indian developers because no one else did — here's the whole story

I've been building in public for a while now, and this is the project I'm most excited to share.
The problem
If you've ever worked on Indian invoicing, cheque printing, or accounting software, you've run into this: you need to print a number in words. Not just "one hundred fifty thousand" — but "Rupees One Lakh Fifty Thousand Only."
The Indian number system uses Lakhs and Crores, not Millions and Billions. Every Indian fintech app, every GST invoice generator, every cheque printing system needs this.
I searched RapidAPI. Nothing. Stack Overflow is full of half-working snippets. PyPI has abandoned packages. So I decided to fix that.
What I built
RupeesInWords is a REST API that converts any number to Indian Rupees in words.

GET /api/v1/convert?number=150000

Response:

{
  "number": 150000,
  "words": "Rupees One Lakh Fifty Thousand Only",
  "indian_formatted": "1,50,000",
  "currency_symbol": "Rupees",
  "success": true
}

Features:

Full Lakh/Crore/Paise support
Batch conversion (up to 50 numbers per request)
Indian comma formatting (1,50,000)
Multiple currency symbols (₹, Rs., INR, Rupees)
Clean error handling for edge cases

The open-core model
The core converter logic is open-source on GitHub:
👉 https://github.com/NEXUS-Lord/rupees-in-words
Pure Python. Zero dependencies. MIT license. If you want to self-host or use it as a library, it's right there.
The hosted API — with API keys, rate limiting, batch endpoints, and uptime — is on RapidAPI:
👉 https://rapidapi.com/NEXUSLord/api/rupeesinwords-indian-number-to-words
Free tier available: 100 requests/month, no credit card required.
This is a classic open-core model. The library is free. The service is monetized. The open-source version builds trust and GitHub stars — the hosted API generates revenue.

The tech stack

Python + FastAPI — automatic Swagger UI at /docs out of the box
Render.com — deploys directly from GitHub, free tier to start
RapidAPI — marketplace and billing layer

The entire infrastructure setup took less than an hour. FastAPI's auto-generated docs alone saved me hours of documentation work.
What I learned
The hardest part wasn't building the API — it was validating the idea first. Before writing a single line of code, I searched RapidAPI, GitHub, and PyPI to confirm nothing like this existed for Indian developers. That 20 minutes of research saved me from building something nobody needed.
The Indian developer market is massively underserved by API tooling. Western developers have thousands of niche APIs available. Indian developers are still copy-pasting Stack Overflow snippets for basic financial formatting. That gap is the opportunity.
What's next
I'm targeting 5,000 GitHub stars by July 2026 and building this as a proof-of-concept for the full pipeline:
Identify a niche problem → build a clean solution → open-source the core → monetize the hosted version.
If you're building Indian fintech tools, invoicing software, or anything that needs Indian number formatting — try the free tier and tell me what I'm missing. Edge cases, feature requests, weird number formats — I want to hear all of it.
GitHub: https://github.com/NEXUS-Lord/rupees-in-words
RapidAPI: https://rapidapi.com/NEXUSLord/api/rupeesinwords-indian-number-to-words
Building in public. One niche API at a time.