Send SMS with .NET

Send your first text from C# with .NET's HttpClient and TextKit's REST API. No NuGet package to add.

Prompt for your AI agent

Paste this into Claude Code, Cursor or any coding agent to have it do the integration:

Add SMS sending to this .NET project with TextKit.

- Docs for this stack: https://jointextkit.com/docs/send-with-dotnet.md
- API: POST https://api.jointextkit.com/v1/messages with `Authorization: Bearer
  <key>` and a JSON body `{"to": "+1...", "body": "..."}`.
- There is no .NET SDK: POST JSON to https://api.jointextkit.com/v1/messages
  with HttpClient (System.Net.Http.Json).
- Read the key from the `TEXTKIT_API_KEY` environment variable. Never hardcode
  it or send it to the browser.
- Phone numbers are E.164 and US or Canada only (`+1` followed by 10 digits).
- A test key (`ssms_sk_test_`) simulates: `+15005550006` always ends `delivered`
  and no phone gets a text.
- Errors are JSON: `{"error": {"code", "message", "param"}}` with a 4xx status.
  On `429`, wait `Retry-After` seconds and retry.
- Send only from server code that runs after a real event (a signup, an order).
  Never expose an endpoint that texts any number a visitor types.

Before you start

• An API key. Create a free account and copy the test key (ssms_sk_test_...) from Home in the console. It works at once, no card.
• .NET 6 or later.
• Put the key in an environment variable:

export TEXTKIT_API_KEY=ssms_sk_test_...

1. Create a project

dotnet new console -o SendSms && cd SendSms

2. Send an SMS

Replace Program.cs:

using System.Net.Http.Headers;
using System.Net.Http.Json;

using var http = new HttpClient();
http.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("TEXTKIT_API_KEY"));

var response = await http.PostAsJsonAsync("https://api.jointextkit.com/v1/messages", new
{
    to = "+15005550006",
    body = "Hello from TextKit",
});

var json = await response.Content.ReadAsStringAsync();
if (!response.IsSuccessStatusCode)
{
    Console.Error.WriteLine($"{(int)response.StatusCode} {json}");
    return 1;
}

Console.WriteLine(json);
return 0;

3. Run it

dotnet run

It prints the message as JSON, with its id (msg_...) and "status": "delivered".

What the test key did

+15005550006 is a sandbox number: the send comes back delivered and no phone gets a text (the response says "simulated": true). Put your own verified phone in to and the test key texts it for real. See Sandbox & test numbers.

Handle errors

The program above already stops on an error and prints the body. A failed request returns a 4xx status and a JSON body {"error": {"code": "...", "message": "...", "param": "..."}}. On 429 rate_limited, wait the Retry-After seconds and try again. Send an Idempotency-Key header (any unique string per message) and a retried request returns the first result instead of sending twice.

Every code is in Errors.

Go live

1. Add a number in the console.
2. Register your use case. We file your 10DLC registration; toll-free numbers use a guided verification form. Until carriers approve it, the number texts only your verified numbers. See Compliance & registration.
3. Swap in a live key (ssms_sk_live_...). The code stays the same.

Next steps

• Webhooks: delivery receipts and replies, pushed to your server.
• Verify: one-time codes in two calls, billed only when verified.
• Send an SMS: every field the endpoint takes.
• All quickstarts: the same guide for other languages and frameworks.