
Verify Indian GST numbers (GSTIN): legal name, status, address and return filing history; free tier.
gstinapi is a free Business API that requires apiKey authentication, is served over HTTPS and does not support CORS, so it should be called from a server.
GSTIN verification REST API docs: endpoint, authentication, response schema, status codes and code samples in curl, Python, JavaScript, PHP and C#.
Details from gstinapi.in.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Parse and validate received e-invoices: XRechnung, ZUGFeRD, Peppol, FatturaPA, KSeF
The gstinapi is provided by gstinapi.in (gstinapi.in). GSTIN verification REST API docs: endpoint, authentication, response schema, status codes and code samples in curl, Python, JavaScript, PHP and C#.
Yes. The gstinapi is listed in the Business category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The gstinapi uses apiKey authentication, so you need to register for credentials before making requests.
Not directly. The gstinapi does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The gstinapi is served over HTTPS, so it's safe to use in production.