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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.