Claude API — C#

Note: The C# SDK is the official Anthropic SDK for C#. Tool use is supported via the Messages API. A class-annotation-based tool runner is not available; use raw tool definitions with JSON schema. The SDK also supports Microsoft.Extensions.AI IChatClient integration with function invocation.

Installation

```bash dotnet add package Anthropic ```

Client Initialization

```csharp using Anthropic;

// Default (uses ANTHROPIC_API_KEY env var) AnthropicClient client = new();

// Explicit API key (use environment variables — never hardcode keys) AnthropicClient client = new() { ApiKey = Environment.GetEnvironmentVariable("ANTHROPIC_API_KEY") }; ```

Basic Message Request

```csharp using Anthropic.Models.Messages;

var parameters = new MessageCreateParams { Model = Model.ClaudeOpus4_6, MaxTokens = 1024, Messages = [new() { Role = Role.User, Content = "What is the capital of France?" }] }; var message = await client.Messages.Create(parameters); Console.WriteLine(message); ```

Streaming

```csharp using Anthropic.Models.Messages;

var parameters = new MessageCreateParams { Model = Model.ClaudeOpus4_6, MaxTokens = 1024, Messages = [new() { Role = Role.User, Content = "Write a haiku" }] };

await foreach (RawMessageStreamEvent streamEvent in client.Messages.CreateStreaming(parameters)) { if (streamEvent.TryPickContentBlockDelta(out var delta) && delta.Delta.TryPickText(out var text)) { Console.Write(text.Text); } } ```

Tool Use (Manual Loop)

The C# SDK supports raw tool definitions via JSON schema. See the shared tool use concepts for the tool definition format and agentic loop pattern.

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