# Send SMS with .NET

Source: https://jointextkit.com/docs/send-with-dotnet
Index: https://jointextkit.com/llms.txt

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:

```text
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](/signup) 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:

```bash
export TEXTKIT_API_KEY=ssms_sk_test_...
```

## 1. Create a project

```bash
dotnet new console -o SendSms && cd SendSms
```

## 2. Send an SMS

Replace `Program.cs`:

```csharp
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

```bash
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](/docs/sandbox).

## 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.

| Status | Code | What to do |
| --- | --- | --- |
| 400 | `invalid_request` | A field is missing or malformed; `param` names it. |
| 401 | `invalid_api_key` | The key is missing, revoked or mistyped. |
| 402 | `insufficient_credits` | Add credits; nothing was sent or charged. |
| 403 | `recipient_opted_out` | They texted STOP. Do not retry. |
| 429 | `rate_limited` | Wait `Retry-After` seconds, then retry. |

Every code is in [Errors](/docs/errors).

## Go live

1. **Add a number** in the [console](/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](/docs/compliance).
3. **Swap in a live key** (`ssms_sk_live_...`). The code stays the same.

## Next steps

- [Webhooks](/docs/webhooks): delivery receipts and replies, pushed to your server.
- [Verify](/docs/verify): one-time codes in two calls, billed only when verified.
- [Send an SMS](/docs/api/messages/send): every field the endpoint takes.
- [All quickstarts](/docs/quickstarts): the same guide for other languages and frameworks.