C# SDK

The Anthropic C# SDK provides convenient access to the Anthropic REST API from applications written in C#.

Installation

Install the package from NuGet:

dotnet add package Anthropic

Requirements

This library requires .NET Standard 2.0 or later.

Usage

using System;
using Anthropic;
using Anthropic.Models.Messages;

AnthropicClient client = new();

MessageCreateParams parameters = new()
{
    MaxTokens = 1024,
    Messages =
    [
        new()
        {
            Role = Role.User,
            Content = "Hello, Claude",
        },
    ],
    Model = "claude-opus-4-6",
};

var message = await client.Messages.Create(parameters);

Console.WriteLine(message);

Client configuration

Configure the client using environment variables:

using Anthropic;

// Configured using the ANTHROPIC_API_KEY, ANTHROPIC_AUTH_TOKEN and ANTHROPIC_BASE_URL environment variables
AnthropicClient client = new();

Or manually:

using Anthropic;

AnthropicClient client = new() { ApiKey = "my-anthropic-api-key" };

Or using a combination of the two approaches.

See this table for the available options:

Modifying configuration

To temporarily use a modified client configuration, while reusing the same connection and thread pools, call WithOptions on any client or service:

using System;

var message = await client
    .WithOptions(options =>
        options with
        {
            BaseUrl = "https://example.com",
            Timeout = TimeSpan.FromSeconds(42),
        }
    )
    .Messages.Create(parameters);

Console.WriteLine(message);

Using a with expression makes it easy to construct the modified options.

The WithOptions method does not affect the original client or service.

Streaming

The SDK defines methods that return response "chunk" streams, where each chunk can be individually processed as soon as it arrives instead of waiting on the full response. Streaming methods return IAsyncEnumerable:

using System;
using Anthropic.Models.Messages;

MessageCreateParams parameters = new()
{
    MaxTokens = 1024,
    Messages =
    [
        new()
        {
            Role = Role.User,
            Content = "Hello, Claude",
        },
    ],
    Model = "claude-opus-4-6",
};

await foreach (var message in client.Messages.CreateStreaming(parameters))
{
    Console.WriteLine(message);
}

Error handling

The SDK throws custom unchecked exception types:

• AnthropicApiException: Base class for API errors. See this table for which exception subclass is thrown for each HTTP status code:

Additionally, all 4xx errors inherit from Anthropic4xxException.

• AnthropicSseException: thrown for errors encountered during SSE streaming after a successful initial HTTP response.

• AnthropicIOException: I/O networking errors.

• AnthropicInvalidDataException: Failure to interpret successfully parsed data. For example, when accessing a property that's supposed to be required, but the API unexpectedly omitted it from the response.

• AnthropicException: Base class for all exceptions.

Retries

The SDK automatically retries 2 times by default, with a short exponential backoff between requests.

Only the following error types are retried:

• Connection errors (for example, due to a network connectivity problem)
• 408 Request Timeout
• 409 Conflict
• 429 Rate Limit
• 5xx Internal

The API may also explicitly instruct the SDK to retry or not retry a request.

To set a custom number of retries, configure the client using the MaxRetries property:

using Anthropic;

AnthropicClient client = new() { MaxRetries = 3 };

Or configure a single method call using WithOptions:

using System;

var message = await client
    .WithOptions(options =>
        options with { MaxRetries = 3 }
    )
    .Messages.Create(parameters);

Console.WriteLine(message);

Timeouts

Requests time out after 10 minutes by default.

To set a custom timeout, configure the client using the Timeout option:

using System;
using Anthropic;

AnthropicClient client = new() { Timeout = TimeSpan.FromSeconds(42) };

Or configure a single method call using WithOptions:

using System;

var message = await client
    .WithOptions(options =>
        options with { Timeout = TimeSpan.FromSeconds(42) }
    )
    .Messages.Create(parameters);

Console.WriteLine(message);

Pagination

Auto-pagination

To iterate through all results across all pages, use the Paginate method, which automatically fetches more pages as needed. The method returns an IAsyncEnumerable:

using System;

var page = await client.Beta.Messages.Batches.List(parameters);
await foreach (var item in page.Paginate())
{
    Console.WriteLine(item);
}

Manual pagination

To access individual page items and manually request the next page, use the Items property, and HasNext and Next methods:

using System;

var page = await client.Beta.Messages.Batches.List();
while (true)
{
    foreach (var item in page.Items)
    {
        Console.WriteLine(item);
    }
    if (!page.HasNext())
    {
        break;
    }
    page = await page.Next();
}

Response validation

In rare cases, the API may return a response that doesn't match the expected type. By default, the SDK will not throw an exception in this case. It will throw AnthropicInvalidDataException only if you directly access the property.

If you would prefer to check that the response is completely well-typed upfront, then either call Validate:

var message = await client.Messages.Create(parameters);
message.Validate();

Or configure the client using the ResponseValidation option:

using Anthropic;

AnthropicClient client = new() { ResponseValidation = true };

Or configure a single method call using WithOptions:

using System;

var message = await client
    .WithOptions(options =>
        options with { ResponseValidation = true }
    )
    .Messages.Create(parameters);

Console.WriteLine(message);

IChatClient integration

The SDK provides an implementation of the IChatClient interface from the Microsoft.Extensions.AI.Abstractions library. This enables AnthropicClient to be used with other libraries that integrate with these core abstractions. For example, tools in the MCP C# SDK (ModelContextProtocol) library can be used directly with an AnthropicClient exposed via IChatClient.

using Anthropic;
using Microsoft.Extensions.AI;
using ModelContextProtocol.Client;

// Configured using the ANTHROPIC_API_KEY, ANTHROPIC_AUTH_TOKEN and ANTHROPIC_BASE_URL environment variables
IChatClient chatClient = client.AsIChatClient("claude-opus-4-6")
    .AsBuilder()
    .UseFunctionInvocation()
    .Build();

// Using McpClient from the MCP C# SDK
McpClient learningServer = await McpClient.CreateAsync(
    new HttpClientTransport(new() { Endpoint = new("https://learn.microsoft.com/api/mcp") }));

ChatOptions options = new() { Tools = [.. await learningServer.ListToolsAsync()] };

Console.WriteLine(await chatClient.GetResponseAsync("Tell me about IChatClient", options));

HTTP client customization

To send a request to the Claude API, build an instance of some Params class and pass it to the corresponding client method. When the response is received, it will be deserialized into an instance of a C# class.

For example, client.Messages.Create should be called with an instance of MessageCreateParams, and it will return an instance of Task<Message>.

Advanced usage

Binary responses

The SDK defines methods that return binary responses, which are used for API responses that shouldn't necessarily be parsed, like non-JSON data.

These methods return HttpResponse:

using System;
using Anthropic.Models.Beta.Files;

FileDownloadParams parameters = new() { FileID = "file_id" };

var response = await client.Beta.Files.Download(parameters);

Console.WriteLine(response);

To save the response content to a file:

using System.IO;

using var response = await client.Beta.Files.Download(parameters);
using var contentStream = await response.ReadAsStream();
using var fileStream = File.Open(path, FileMode.OpenOrCreate);
await contentStream.CopyToAsync(fileStream);

Raw responses

The SDK defines methods that deserialize responses into instances of C# classes. To access response headers, status code, or the raw response body, prefix any HTTP method call on a client or service with WithRawResponse:

var response = await client.WithRawResponse.Messages.Create(parameters);
var statusCode = response.StatusCode;
var headers = response.Headers;

The raw HttpResponseMessage can also be accessed through the RawMessage property.

For non-streaming responses, you can deserialize the response into an instance of a C# class if needed:

using System;
using Anthropic.Models.Messages;

var response = await client.WithRawResponse.Messages.Create(parameters);
Message deserialized = await response.Deserialize();
Console.WriteLine(deserialized);

For streaming responses, you can deserialize the response to an IAsyncEnumerable if needed:

using System;

var response = await client.WithRawResponse.Messages.CreateStreaming(parameters);
await foreach (var item in response.Enumerate())
{
    Console.WriteLine(item);
}

Logging

Enable debug logging via environment variable:

export ANTHROPIC_LOG=debug

Undocumented API functionality

The SDK is typed for convenient usage of the documented API. However, it also supports working with undocumented or not yet supported parts of the API.

Beta features

Additional resources

• GitHub repository
• NuGet package
• API reference
• Streaming guide