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.
dotnet add package Anthropicusing 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")
};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);
}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.
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 (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.
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.
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 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.
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.
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.
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.
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" },
},
}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 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().