How to verify a GST number in C# / .NET

1. The call, with HttpClient

There is no C# SDK, and you do not need one — the API is a single GET request with one header.

using System.Net.Http.Json;

using var client = new HttpClient();
client.DefaultRequestHeaders.Add("x-api-key", "gak_your_key_here");

var result = await client.GetFromJsonAsync<GstinResult>(
    "https://gstinapi.in/v1/gstin/33AAACC1206D1ZN");

Console.WriteLine($"{result!.LegalName} — {result.Status}");

public record GstinResult(
    [property: JsonPropertyName("gstin")]             string Gstin,
    [property: JsonPropertyName("legal_name")]        string LegalName,
    [property: JsonPropertyName("trade_name")]        string? TradeName,
    [property: JsonPropertyName("status")]            string Status,
    [property: JsonPropertyName("taxpayer_type")]     string TaxpayerType,
    [property: JsonPropertyName("registration_date")] string? RegistrationDate,
    [property: JsonPropertyName("address")]           string? Address);

2. Validate the GSTIN offline first

A malformed GSTIN returns 400 and costs no credit, but the round trip is still wasted. Validate the check digit locally and typos never reach the network.

private const string Codes = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

public static bool IsValidGstin(string? gstin)
{
    var value = (gstin ?? string.Empty).Trim().ToUpperInvariant();
    if (value.Length != 15) return false;

    var total = 0;
    for (var i = 0; i < 14; i++)
    {
        var index = Codes.IndexOf(value[i]);
        if (index < 0) return false;
        var product = index * (i % 2 == 1 ? 2 : 1);
        total += product / 36 + product % 36;
    }

    var expected = Codes[(36 - total % 36) % 36];
    return value[14] == expected;
}

IsValidGstin("33AAACC1206D1ZN"); // true
IsValidGstin("33AAACC1206D1ZZ"); // false

This proves the GSTIN is well-formed. It cannot tell you whether the registration exists or is still active — only a live lookup does that, because registrations get cancelled.

3. Handle every status code

private static readonly HashSet<int> Retryable = new() { 429, 502 };

public static async Task<GstinResult?> VerifyAsync(
    HttpClient client, string gstin, int attempts = 3)
{
    for (var attempt = 0; attempt < attempts; attempt++)
    {
        var res = await client.GetAsync($"https://gstinapi.in/v1/gstin/{gstin}");

        if (res.IsSuccessStatusCode)
            return await res.Content.ReadFromJsonAsync<GstinResult>();

        var status = (int)res.StatusCode;
        if (status == 404) return null;                 // not registered
        if (status == 402) throw new InvalidOperationException("Out of credits");

        if (!Retryable.Contains(status))
            throw new HttpRequestException($"Lookup failed with HTTP {status}");

        if (attempt < attempts - 1)
            await Task.Delay(TimeSpan.FromSeconds(Math.Pow(2, attempt)));
    }

    throw new HttpRequestException("GST provider unavailable after retries");
}

4. Wiring it into ASP.NET Core

Register a named HttpClient in DI rather than newing one up per call — a fresh HttpClient per request exhausts sockets under load. Read the key from configuration, not from source.

builder.Services.AddHttpClient("gstin", client =>
{
    client.BaseAddress = new Uri("https://gstinapi.in/");
    client.DefaultRequestHeaders.Add(
        "x-api-key", builder.Configuration["Gstin:ApiKey"]);
    client.Timeout = TimeSpan.FromSeconds(10);
});

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