/claude-api

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

dotnet add package Anthropic

Client Initialization

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

using Anthropic.Models.Messages;

var parameters = new MessageCreateParams
{
    Model = Model.ClaudeOpus4_6,
    MaxTokens = 16000,
    Messages = [new() { Role = Role.User, Content = "What is the capital of France?" }]
};
var response = await client.Messages.Create(parameters);

// ContentBlock is a union wrapper. .Value unwraps to the variant object,
// then OfType<T> filters to the type you want. Or use the TryPick* idiom
// shown in the Thinking section below.
foreach (var text in response.Content.Select(b => b.Value).OfType<TextBlock>())
{
    Console.WriteLine(text.Text);
}

Streaming

using Anthropic.Models.Messages;

var parameters = new MessageCreateParams
{
    Model = Model.ClaudeOpus4_6,
    MaxTokens = 64000,
    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);
    }
}

RawMessageStreamEvent TryPick methods (naming drops the Message/Raw prefix): TryPickStart, TryPickDelta, TryPickStop, TryPickContentBlockStart, TryPickContentBlockDelta, TryPickContentBlockStop. There is no TryPickMessageStop — use TryPickStop.

Thinking

Adaptive thinking is the recommended mode for Claude 4.6+ models. Claude decides dynamically when and how much to think.

using Anthropic.Models.Messages;

var response = await client.Messages.Create(new MessageCreateParams
{
    Model = Model.ClaudeOpus4_6,
    MaxTokens = 16000,
    // ThinkingConfigParam? implicitly converts from the concrete variant classes —
    // no wrapper needed.
    Thinking = new ThinkingConfigAdaptive(),
    Messages =
    [
        new() { Role = Role.User, Content = "Solve: 27 * 453" },
    ],
});

// ThinkingBlock(s) precede TextBlock in Content. TryPick* narrows the union.
foreach (var block in response.Content)
{
    if (block.TryPickThinking(out ThinkingBlock? t))
    {
        Console.WriteLine($"[thinking] {t.Thinking}");
    }
    else if (block.TryPickText(out TextBlock? text))
    {
        Console.WriteLine(text.Text);
    }
}

Deprecated: new ThinkingConfigEnabled { BudgetTokens = N } (fixed-budget extended thinking) still works on Claude 4.6 but is deprecated. Use adaptive thinking above.

Alternative to TryPick*: .Select(b => b.Value).OfType<ThinkingBlock>() (same LINQ pattern as the Basic Message example).

Tool Use

Defining a tool

Tool (NOT ToolParam) with an InputSchema record. InputSchema.Type is auto-set to "object" by the constructor — don't set it. ToolUnion has an implicit conversion from Tool, triggered by the collection expression [...].

using System.Text.Json;
using Anthropic.Models.Messages;

var parameters = new MessageCreateParams
{
    Model = Model.ClaudeSonnet4_6,
    MaxTokens = 16000,
    Tools = [
        new Tool {
            Name = "get_weather",
            Description = "Get the current weather in a given location",
            InputSchema = new() {
                Properties = new Dictionary<string, JsonElement> {
                    ["location"] = JsonSerializer.SerializeToElement(
                        new { type = "string", description = "City name" }),
                },
                Required = ["location"],
            },
        },
    ],
    Messages = [new() { Role = Role.User, Content = "Weather in Paris?" }],
};

Derived from anthropic-sdk-csharp/src/Anthropic/Models/Messages/Tool.cs and ToolUnion.cs:799 (implicit conversion).

See shared tool use concepts for the loop pattern.

Converting response content to the follow-up assistant message

When echoing Claude's response back in the assistant turn, there is no .ToParam() helper — manually reconstruct each ContentBlock variant as its *Param counterpart. Do NOT use new ContentBlockParam(block.Json): it compiles and serializes, but .Value stays null so TryPick*/Validate() fail (degraded JSON pass-through, not the typed path).

using Anthropic.Models.Messages;

Message response = await client.Messages.Create(parameters);

// No .ToParam() — reconstruct per variant. Implicit conversions from each
// *Param type to ContentBlockParam mean no explicit wrapper.
List<ContentBlockParam> assistantContent = [];
List<ContentBlockParam> toolResults = [];
foreach (ContentBlock block in response.Content)
{
    if (block.TryPickText(out TextBlock? text))
    {
        assistantContent.Add(new TextBlockParam { Text = text.Text });
    }
    else if (block.TryPickThinking(out ThinkingBlock? thinking))
    {
        // Signature MUST be preserved — the API rejects tampering
        assistantContent.Add(new ThinkingBlockParam
        {
            Thinking = thinking.Thinking,
            Signature = thinking.Signature,
        });
    }
    else if (block.TryPickRedactedThinking(out RedactedThinkingBlock? redacted))
    {
        assistantContent.Add(new RedactedThinkingBlockParam { Data = redacted.Data });
    }
    else if (block.TryPickToolUse(out ToolUseBlock? toolUse))
    {
        // ToolUseBlock has required Caller; ToolUseBlockParam.Caller is optional — don't copy it
        assistantContent.Add(new ToolUseBlockParam
        {
            ID = toolUse.ID,
            Name = toolUse.Name,
            Input = toolUse.Input,
        });
        // Execute the tool; collect ONE result per tool_use block — the API
        // rejects the follow-up if any tool_use ID lacks a matching tool_result.
        string result = ExecuteYourTool(toolUse.Name, toolUse.Input);
        toolResults.Add(new ToolResultBlockParam
        {
            ToolUseID = toolUse.ID,
            Content = result,
        });
    }
}

// Follow-up: prior messages + assistant echo + user tool_result(s)
List<MessageParam> followUpMessages =
[
    .. parameters.Messages,
    new() { Role = Role.Assistant, Content = assistantContent },
    new() { Role = Role.User, Content = toolResults },
];

ToolResultBlockParam has no tuple constructor — use the object initializer. Content is a string-or-list union; a plain string implicitly converts.

Context Editing / Compaction (Beta)

Beta-namespace prefix is inconsistent (source-verified against src/Anthropic/Models/Beta/Messages/*.cs @ 12.9.0). No prefix: MessageCreateParams, MessageCountTokensParams, Role. Everything else has the Beta prefix: BetaMessageParam, BetaMessage, BetaContentBlock, BetaToolUseBlock, all block param types. The unprefixed Role WILL collide with Anthropic.Models.Messages.Role if you import both namespaces (CS0104). Safest: import only Beta; if mixing, alias the beta Role:

using Anthropic.Models.Beta.Messages;
using NonBeta = Anthropic.Models.Messages;  // only if you also need non-beta types
// Now: MessageCreateParams, BetaMessageParam, Role (beta's), NonBeta.Role (if needed)

BetaMessage.Content is IReadOnlyList<BetaContentBlock> — a 15-variant discriminated union. Narrow with TryPick*. Response BetaContentBlock is NOT assignable to param BetaContentBlockParam — there's no .ToParam() in C#. Round-trip by converting each block:

using Anthropic.Models.Beta.Messages;

var betaParams = new MessageCreateParams   // no Beta prefix — one of only 2 unprefixed
{
    Model = Model.ClaudeOpus4_6,
    MaxTokens = 16000,
    Betas = ["compact-2026-01-12"],
    ContextManagement = new BetaContextManagementConfig
    {
        Edits = [new BetaCompact20260112Edit()],
    },
    Messages = messages,
};
BetaMessage resp = await client.Beta.Messages.Create(betaParams);

foreach (BetaContentBlock block in resp.Content)
{
    if (block.TryPickCompaction(out BetaCompactionBlock? compaction))
    {
        // Content is nullable — compaction can fail server-side
        Console.WriteLine($"compaction summary: {compaction.Content}");
    }
}

// Context-edit metadata lives on a separate nullable field
if (resp.ContextManagement is { } ctx)
{
    foreach (var edit in ctx.AppliedEdits)
        Console.WriteLine($"cleared {edit.ClearedInputTokens} tokens");
}

// ROUND-TRIP: BetaMessageParam.Content is BetaMessageParamContent (a string|list
// union). It implicit-converts from List<BetaContentBlockParam>, NOT from the
// response's IReadOnlyList<BetaContentBlock>. Convert each block:
List<BetaContentBlockParam> paramBlocks = [];
foreach (var b in resp.Content)
{
    if (b.TryPickText(out var t)) paramBlocks.Add(new BetaTextBlockParam { Text = t.Text });
    else if (b.TryPickCompaction(out var c)) paramBlocks.Add(new BetaCompactionBlockParam { Content = c.Content });
    // ... other variants as needed
}
messages.Add(new BetaMessageParam { Role = Role.Assistant, Content = paramBlocks });

All 15 BetaContentBlock.TryPick* variants: Text, Thinking, RedactedThinking, ToolUse, ServerToolUse, WebSearchToolResult, WebFetchToolResult, CodeExecutionToolResult, BashCodeExecutionToolResult, TextEditorCodeExecutionToolResult, ToolSearchToolResult, McpToolUse, McpToolResult, ContainerUpload, Compaction.

BetaToolUseBlock.Input is IReadOnlyDictionary<string, JsonElement> — index by key then call the JsonElement extractor:

if (block.TryPickToolUse(out BetaToolUseBlock? tu))
{
    int a = tu.Input["a"].GetInt32();
    string s = tu.Input["name"].GetString()!;
}

Effort Parameter

Effort is nested under OutputConfig, NOT a top-level property. ApiEnum<string, Effort> has an implicit conversion from the enum, so assign Effort.High directly.

OutputConfig = new OutputConfig { Effort = Effort.High },

Values: Effort.Low, Effort.Medium, Effort.High, Effort.Max. Combine with Thinking = new ThinkingConfigAdaptive() for cost-quality control.

Prompt Caching

System takes MessageCreateParamsSystem? — a union of string or List<TextBlockParam>. There is no SystemTextBlockParam; use plain TextBlockParam. The implicit conversion needs the concrete List<TextBlockParam> type (array literals won't convert). For placement patterns and the silent-invalidator audit checklist, see shared/prompt-caching.md.

System = new List<TextBlockParam> {
    new() {
        Text = longSystemPrompt,
        CacheControl = new CacheControlEphemeral(),  // auto-sets Type = "ephemeral"
    },
},

Optional Ttl on CacheControlEphemeral: new() { Ttl = Ttl.Ttl1h } or Ttl.Ttl5m. CacheControl also exists on Tool.CacheControl and top-level MessageCreateParams.CacheControl.

Verify hits via response.Usage.CacheCreationInputTokens / response.Usage.CacheReadInputTokens.

Token Counting

MessageTokensCount result = await client.Messages.CountTokens(new MessageCountTokensParams {
    Model = Model.ClaudeOpus4_6,
    Messages = [new() { Role = Role.User, Content = "Hello" }],
});
long tokens = result.InputTokens;

MessageCountTokensParams.Tools uses a different union type (MessageCountTokensTool) than MessageCreateParams.Tools (ToolUnion) — if you're passing tools, the compiler will tell you when it matters.

Structured Output

OutputConfig = new OutputConfig {
    Format = new JsonOutputFormat {
        Schema = new Dictionary<string, JsonElement> {
            ["type"] = JsonSerializer.SerializeToElement("object"),
            ["properties"] = JsonSerializer.SerializeToElement(
                new { name = new { type = "string" } }),
            ["required"] = JsonSerializer.SerializeToElement(new[] { "name" }),
        },
    },
},

JsonOutputFormat.Type is auto-set to "json_schema" by the constructor. Schema is required.

PDF / Document Input

DocumentBlockParam takes a DocumentBlockParamSource union: Base64PdfSource / UrlPdfSource / PlainTextSource / ContentBlockSource. Base64PdfSource auto-sets MediaType = "application/pdf" and Type = "base64".

new MessageParam {
    Role = Role.User,
    Content = new List<ContentBlockParam> {
        new DocumentBlockParam { Source = new Base64PdfSource { Data = base64String } },
        new TextBlockParam { Text = "Summarize this PDF" },
    },
}

Server-Side Tools

Web search, bash, text editor, and code execution are built-in server tools. Type names are version-suffixed; constructors auto-set name/type. All implicit-convert to ToolUnion.

Tools = [
    new WebSearchTool20260209(),
    new ToolBash20250124(),
    new ToolTextEditor20250728(),
    new CodeExecutionTool20260120(),
],

Also available: WebFetchTool20260209, MemoryTool20250818. WebSearchTool20260209 optionals: AllowedDomains, BlockedDomains, MaxUses, UserLocation.

Files API (Beta)

Files live under client.Beta.Files (namespace Anthropic.Models.Beta.Files). BinaryContent implicit-converts from Stream and byte[].

using Anthropic.Models.Beta.Files;
using Anthropic.Models.Beta.Messages;

FileMetadata meta = await client.Beta.Files.Upload(
    new FileUploadParams { File = File.OpenRead("doc.pdf") });

// Referencing the uploaded file requires Beta message types:
new BetaRequestDocumentBlock {
    Source = new BetaFileDocumentSource { FileID = meta.ID },
}

The non-beta DocumentBlockParamSource union has no file-ID variant — file references need client.Beta.Messages.Create().