Originally published at claudeguide.io/claude-api-csharp-dotnet

Claude API C# .NET Tutorial: Complete Integration Guide (2026)

You can call the Claude API from C# .NET using the official Anthropic NuGet package — install with dotnet add package Anthropic.SDK, set your ANTHROPIC_API_KEY, and send messages in under 15 lines of code. The SDK supports .NET 6+, streaming, tool use, prompt caching, and multi-turn conversations. This tutorial walks through setup, authentication, every major feature, and production patterns with tested code examples.

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Installation and Setup

Create a new project or add the SDK to an existing one:

dotnet new console -n ClaudeDemo
cd ClaudeDemo
dotnet add package Anthropic.SDK

Set your API key as an environment variable:

# Linux/macOS
export ANTHROPIC_API_KEY="sk-ant-..."

# Windows PowerShell
$env:ANTHROPIC_API_KEY = "sk-ant-..."

Or store it in appsettings.json for ASP.NET Core projects:

{
  "Anthropic": {
    "ApiKey": "sk-ant-..."
  }
}

Security note: Never hardcode API keys in source files. Use environment variables, user secrets (dotnet user-secrets), or Azure Key Vault in production.

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Your First API Call

using Anthropic.SDK;
using Anthropic.SDK.Messaging;

var client = new AnthropicClient();

var message = await client.Messages.CreateAsync(new MessageParameters
{
    Model = "claude-sonnet-4-5",
    MaxTokens = 1024,
    Messages = new List<Message

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## Multi-Turn Conversations

Build conversation history by tracking messages:

csharp
var conversation = new List<Message