1# Claude API — C#
2 
3> **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.
4 
5## Installation
6 
7```bash
8dotnet add package Anthropic
9```
10 
11## Client Initialization
12 
13```csharp
14using Anthropic;
15 
16// Default (uses ANTHROPIC_API_KEY env var)
17AnthropicClient client = new();
18 
19// Explicit API key (use environment variables — never hardcode keys)
20AnthropicClient client = new() {
21    ApiKey = Environment.GetEnvironmentVariable("ANTHROPIC_API_KEY")
22};
23```
24 
25---
26 
27## Basic Message Request
28 
29```csharp
30using Anthropic.Models.Messages;
31 
32var parameters = new MessageCreateParams
33{
34    Model = Model.ClaudeOpus4_6,
35    MaxTokens = 16000,
36    Messages = [new() { Role = Role.User, Content = "What is the capital of France?" }]
37};
38var response = await client.Messages.Create(parameters);
39 
40// ContentBlock is a union wrapper. .Value unwraps to the variant object,
41// then OfType<T> filters to the type you want. Or use the TryPick* idiom
42// shown in the Thinking section below.
43foreach (var text in response.Content.Select(b => b.Value).OfType<TextBlock>())
44{
45    Console.WriteLine(text.Text);
46}
47```
48 
49---
50 
51## Streaming
52 
53```csharp
54using Anthropic.Models.Messages;
55 
56var parameters = new MessageCreateParams
57{
58    Model = Model.ClaudeOpus4_6,
59    MaxTokens = 64000,
60    Messages = [new() { Role = Role.User, Content = "Write a haiku" }]
61};
62 
63await foreach (RawMessageStreamEvent streamEvent in client.Messages.CreateStreaming(parameters))
64{
65    if (streamEvent.TryPickContentBlockDelta(out var delta) &&
66        delta.Delta.TryPickText(out var text))
67    {
68        Console.Write(text.Text);
69    }
70}
71```
72 
73**`RawMessageStreamEvent` TryPick methods** (naming drops the `Message`/`Raw` prefix): `TryPickStart`, `TryPickDelta`, `TryPickStop`, `TryPickContentBlockStart`, `TryPickContentBlockDelta`, `TryPickContentBlockStop`. There is no `TryPickMessageStop` — use `TryPickStop`.
74 
75---
76 
77## Thinking
78 
79**Adaptive thinking is the recommended mode for Claude 4.6+ models.** Claude decides dynamically when and how much to think.
80 
81```csharp
82using Anthropic.Models.Messages;
83 
84var response = await client.Messages.Create(new MessageCreateParams
85{
86    Model = Model.ClaudeOpus4_6,
87    MaxTokens = 16000,
88    // ThinkingConfigParam? implicitly converts from the concrete variant classes —
89    // no wrapper needed.
90    Thinking = new ThinkingConfigAdaptive(),
91    Messages =
92    [
93        new() { Role = Role.User, Content = "Solve: 27 * 453" },
94    ],
95});
96 
97// ThinkingBlock(s) precede TextBlock in Content. TryPick* narrows the union.
98foreach (var block in response.Content)
99{
100    if (block.TryPickThinking(out ThinkingBlock? t))
101    {
102        Console.WriteLine($"[thinking] {t.Thinking}");
103    }
104    else if (block.TryPickText(out TextBlock? text))
105    {
106        Console.WriteLine(text.Text);
107    }
108}
109```
110 
111> **Deprecated:** `new ThinkingConfigEnabled { BudgetTokens = N }` (fixed-budget extended thinking) still works on Claude 4.6 but is deprecated. Use adaptive thinking above.
112 
113Alternative to `TryPick*`: `.Select(b => b.Value).OfType<ThinkingBlock>()` (same LINQ pattern as the Basic Message example).
114 
115---
116 
117## Tool Use
118 
119### Defining a tool
120 
121`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 `[...]`.
122 
123```csharp
124using System.Text.Json;
125using Anthropic.Models.Messages;
126 
127var parameters = new MessageCreateParams
128{
129    Model = Model.ClaudeSonnet4_6,
130    MaxTokens = 16000,
131    Tools = [
132        new Tool {
133            Name = "get_weather",
134            Description = "Get the current weather in a given location",
135            InputSchema = new() {
136                Properties = new Dictionary<string, JsonElement> {
137                    ["location"] = JsonSerializer.SerializeToElement(
138                        new { type = "string", description = "City name" }),
139                },
140                Required = ["location"],
141            },
142        },
143    ],
144    Messages = [new() { Role = Role.User, Content = "Weather in Paris?" }],
145};
146```
147 
148Derived from `anthropic-sdk-csharp/src/Anthropic/Models/Messages/Tool.cs` and `ToolUnion.cs:799` (implicit conversion).
149 
150See [shared tool use concepts](../shared/tool-use-concepts.md) for the loop pattern.
151### Converting response content to the follow-up assistant message
152 
153When 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).
154 
155```csharp
156using Anthropic.Models.Messages;
157 
158Message response = await client.Messages.Create(parameters);
159 
160// No .ToParam() — reconstruct per variant. Implicit conversions from each
161// *Param type to ContentBlockParam mean no explicit wrapper.
162List<ContentBlockParam> assistantContent = [];
163List<ContentBlockParam> toolResults = [];
164foreach (ContentBlock block in response.Content)
165{
166    if (block.TryPickText(out TextBlock? text))
167    {
168        assistantContent.Add(new TextBlockParam { Text = text.Text });
169    }
170    else if (block.TryPickThinking(out ThinkingBlock? thinking))
171    {
172        // Signature MUST be preserved — the API rejects tampering
173        assistantContent.Add(new ThinkingBlockParam
174        {
175            Thinking = thinking.Thinking,
176            Signature = thinking.Signature,
177        });
178    }
179    else if (block.TryPickRedactedThinking(out RedactedThinkingBlock? redacted))
180    {
181        assistantContent.Add(new RedactedThinkingBlockParam { Data = redacted.Data });
182    }
183    else if (block.TryPickToolUse(out ToolUseBlock? toolUse))
184    {
185        // ToolUseBlock has required Caller; ToolUseBlockParam.Caller is optional — don't copy it
186        assistantContent.Add(new ToolUseBlockParam
187        {
188            ID = toolUse.ID,
189            Name = toolUse.Name,
190            Input = toolUse.Input,
191        });
192        // Execute the tool; collect ONE result per tool_use block — the API
193        // rejects the follow-up if any tool_use ID lacks a matching tool_result.
194        string result = ExecuteYourTool(toolUse.Name, toolUse.Input);
195        toolResults.Add(new ToolResultBlockParam
196        {
197            ToolUseID = toolUse.ID,
198            Content = result,
199        });
200    }
201}
202 
203// Follow-up: prior messages + assistant echo + user tool_result(s)
204List<MessageParam> followUpMessages =
205[
206    .. parameters.Messages,
207    new() { Role = Role.Assistant, Content = assistantContent },
208    new() { Role = Role.User, Content = toolResults },
209];
210```
211 
212`ToolResultBlockParam` has no tuple constructor — use the object initializer. `Content` is a string-or-list union; a plain `string` implicitly converts.
213 
214---
215 
216## Context Editing / Compaction (Beta)
217 
218**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`:
219 
220```csharp
221using Anthropic.Models.Beta.Messages;
222using NonBeta = Anthropic.Models.Messages;  // only if you also need non-beta types
223// Now: MessageCreateParams, BetaMessageParam, Role (beta's), NonBeta.Role (if needed)
224```
225 
226 
227`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:
228 
229```csharp
230using Anthropic.Models.Beta.Messages;
231 
232var betaParams = new MessageCreateParams   // no Beta prefix — one of only 2 unprefixed
233{
234    Model = Model.ClaudeOpus4_6,
235    MaxTokens = 16000,
236    Betas = ["compact-2026-01-12"],
237    ContextManagement = new BetaContextManagementConfig
238    {
239        Edits = [new BetaCompact20260112Edit()],
240    },
241    Messages = messages,
242};
243BetaMessage resp = await client.Beta.Messages.Create(betaParams);
244 
245foreach (BetaContentBlock block in resp.Content)
246{
247    if (block.TryPickCompaction(out BetaCompactionBlock? compaction))
248    {
249        // Content is nullable — compaction can fail server-side
250        Console.WriteLine($"compaction summary: {compaction.Content}");
251    }
252}
253 
254// Context-edit metadata lives on a separate nullable field
255if (resp.ContextManagement is { } ctx)
256{
257    foreach (var edit in ctx.AppliedEdits)
258        Console.WriteLine($"cleared {edit.ClearedInputTokens} tokens");
259}
260 
261// ROUND-TRIP: BetaMessageParam.Content is BetaMessageParamContent (a string|list
262// union). It implicit-converts from List<BetaContentBlockParam>, NOT from the
263// response's IReadOnlyList<BetaContentBlock>. Convert each block:
264List<BetaContentBlockParam> paramBlocks = [];
265foreach (var b in resp.Content)
266{
267    if (b.TryPickText(out var t)) paramBlocks.Add(new BetaTextBlockParam { Text = t.Text });
268    else if (b.TryPickCompaction(out var c)) paramBlocks.Add(new BetaCompactionBlockParam { Content = c.Content });
269    // ... other variants as needed
270}
271messages.Add(new BetaMessageParam { Role = Role.Assistant, Content = paramBlocks });
272```
273 
274All 15 `BetaContentBlock.TryPick*` variants: `Text`, `Thinking`, `RedactedThinking`, `ToolUse`, `ServerToolUse`, `WebSearchToolResult`, `WebFetchToolResult`, `CodeExecutionToolResult`, `BashCodeExecutionToolResult`, `TextEditorCodeExecutionToolResult`, `ToolSearchToolResult`, `McpToolUse`, `McpToolResult`, `ContainerUpload`, `Compaction`.
275 
276**`BetaToolUseBlock.Input` is `IReadOnlyDictionary<string, JsonElement>`** — index by key then call the `JsonElement` extractor:
277 
278```csharp
279if (block.TryPickToolUse(out BetaToolUseBlock? tu))
280{
281    int a = tu.Input["a"].GetInt32();
282    string s = tu.Input["name"].GetString()!;
283}
284```
285 
286---
287 
288## Effort Parameter
289 
290Effort is nested under `OutputConfig`, NOT a top-level property. `ApiEnum<string, Effort>` has an implicit conversion from the enum, so assign `Effort.High` directly.
291 
292```csharp
293OutputConfig = new OutputConfig { Effort = Effort.High },
294```
295 
296Values: `Effort.Low`, `Effort.Medium`, `Effort.High`, `Effort.Max`. Combine with `Thinking = new ThinkingConfigAdaptive()` for cost-quality control.
297 
298---
299 
300## Prompt Caching
301 
302`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`.
303 
304```csharp
305System = new List<TextBlockParam> {
306    new() {
307        Text = longSystemPrompt,
308        CacheControl = new CacheControlEphemeral(),  // auto-sets Type = "ephemeral"
309    },
310},
311```
312 
313Optional `Ttl` on `CacheControlEphemeral`: `new() { Ttl = Ttl.Ttl1h }` or `Ttl.Ttl5m`. `CacheControl` also exists on `Tool.CacheControl` and top-level `MessageCreateParams.CacheControl`.
314 
315Verify hits via `response.Usage.CacheCreationInputTokens` / `response.Usage.CacheReadInputTokens`.
316 
317---
318 
319## Token Counting
320 
321```csharp
322MessageTokensCount result = await client.Messages.CountTokens(new MessageCountTokensParams {
323    Model = Model.ClaudeOpus4_6,
324    Messages = [new() { Role = Role.User, Content = "Hello" }],
325});
326long tokens = result.InputTokens;
327```
328 
329`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.
330 
331---
332 
333## Structured Output
334 
335```csharp
336OutputConfig = new OutputConfig {
337    Format = new JsonOutputFormat {
338        Schema = new Dictionary<string, JsonElement> {
339            ["type"] = JsonSerializer.SerializeToElement("object"),
340            ["properties"] = JsonSerializer.SerializeToElement(
341                new { name = new { type = "string" } }),
342            ["required"] = JsonSerializer.SerializeToElement(new[] { "name" }),
343        },
344    },
345},
346```
347 
348`JsonOutputFormat.Type` is auto-set to `"json_schema"` by the constructor. `Schema` is `required`.
349 
350---
351 
352## PDF / Document Input
353 
354`DocumentBlockParam` takes a `DocumentBlockParamSource` union: `Base64PdfSource` / `UrlPdfSource` / `PlainTextSource` / `ContentBlockSource`. `Base64PdfSource` auto-sets `MediaType = "application/pdf"` and `Type = "base64"`.
355 
356```csharp
357new MessageParam {
358    Role = Role.User,
359    Content = new List<ContentBlockParam> {
360        new DocumentBlockParam { Source = new Base64PdfSource { Data = base64String } },
361        new TextBlockParam { Text = "Summarize this PDF" },
362    },
363}
364```
365 
366---
367 
368## Server-Side Tools
369 
370Web 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`.
371 
372```csharp
373Tools = [
374    new WebSearchTool20260209(),
375    new ToolBash20250124(),
376    new ToolTextEditor20250728(),
377    new CodeExecutionTool20260120(),
378],
379```
380 
381Also available: `WebFetchTool20260209`, `MemoryTool20250818`. `WebSearchTool20260209` optionals: `AllowedDomains`, `BlockedDomains`, `MaxUses`, `UserLocation`.
382 
383---
384 
385## Files API (Beta)
386 
387Files live under `client.Beta.Files` (namespace `Anthropic.Models.Beta.Files`). `BinaryContent` implicit-converts from `Stream` and `byte[]`.
388 
389```csharp
390using Anthropic.Models.Beta.Files;
391using Anthropic.Models.Beta.Messages;
392 
393FileMetadata meta = await client.Beta.Files.Upload(
394    new FileUploadParams { File = File.OpenRead("doc.pdf") });
395 
396// Referencing the uploaded file requires Beta message types:
397new BetaRequestDocumentBlock {
398    Source = new BetaFileDocumentSource { FileID = meta.ID },
399}
400```
401 
402The non-beta `DocumentBlockParamSource` union has no file-ID variant — file references need `client.Beta.Messages.Create()`.
403