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claude-api/csharp/claude-api.md

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# 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](../shared/tool-use-concepts.md) for the tool definition format and agentic loop pattern.