C# SDK

The Anthropic C# SDK provides convenient access to the Claude 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 = Model.ClaudeOpus5,
};

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

foreach (var block in message.Content)
{
    if (block.TryPickText(out var textBlock))
    {
        Console.WriteLine(textBlock.Text);
    }
}

For authentication options including Workload Identity Federation, see Authentication. If your API key is a personal or service account key with access to multiple workspaces, set the workspace ID in the anthropic-workspace-id request header; Select a workspace shows the per-request option for this SDK.

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 generally correspond to SSE or JSONL responses.

A streaming method always has a Streaming suffix in its name, even if it doesn't have a non-streaming variant.

These streaming methods return IAsyncEnumerable:

using System;
using Anthropic.Models.Messages;

MessageCreateParams parameters = new()
{
    MaxTokens = 1024,
    Messages =
    [
        new()
        {
            Role = Role.User,
            Content = "Hello, Claude",
        },
    ],
    Model = Model.ClaudeOpus5,
};

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, because of 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

The SDK defines methods that return paginated lists of results. It provides convenient ways to access the results either one page at a time or item-by-item across all pages.

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.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:

var page = await client.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 does not throw an exception in this case. It throws 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 (and Anthropic.Services.IBetaService) 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 through 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
AnthropicClient client = new();

IChatClient chatClient = client.AsIChatClient("claude-opus-5")
    .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));

Requests and responses

To send a request to the Claude API, build an instance of a Params class and pass it to the corresponding client method. When the response is received, it's 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.Files;

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

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

Console.WriteLine(response);

To save the response content to a file, or any Stream, use the CopyToAsync method:

using System.IO;

using var response = await client.Files.Download(parameters);
using var contentStream = await response.ReadAsStream();
using var fileStream = File.Open(path, FileMode.OpenOrCreate);
await contentStream.CopyToAsync(fileStream); // Or any other Stream

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 by setting an 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.

Platform integrations

The C# SDK supports the following platforms through separate NuGet packages:

• Agent Platform: Anthropic.Vertex. See Claude on Google Cloud for client setup.
• Bedrock: Anthropic.Bedrock. Use AnthropicBedrockMantleClient for the Messages-API Bedrock endpoint, or AnthropicBedrockClient (bedrock-runtime path). AnthropicBedrockMantleClient takes an optional MantleAwsClientOptions config object; AnthropicBedrockClient accepts AnthropicBedrockCredentialsHelper.FromEnv() or explicit credentials.
• Claude Platform on AWS: Anthropic.Aws. Use AnthropicAwsClient; set WorkspaceId on the client or the ANTHROPIC_AWS_WORKSPACE_ID environment variable (see Workspaces). Available in beta.
• Foundry: Anthropic.Foundry. Use AnthropicFoundryClient with DefaultAnthropicFoundryCredentials.FromEnv() or explicit credentials.

Use AnthropicBedrockMantleClient for new projects; AnthropicBedrockClient remains for existing applications using the Bedrock InvokeModel API.

Semantic versioning

This package generally follows SemVer conventions, though certain backward-incompatible changes may be released as minor versions:

1. Changes to library internals that are technically public but not intended or documented for external use.
2. Changes that aren't expected to impact the vast majority of users in practice.

Backward-compatibility is taken seriously to ensure you can rely on a smooth upgrade experience.

Additional resources

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