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Claude API — Java

Note: The Java SDK supports the Claude API and beta tool use with annotated classes. Agent SDK is not yet available for Java.

Installation

Maven:

<dependency>
    <groupId>com.anthropic</groupId>
    <artifactId>anthropic-java</artifactId>
    <version>2.17.0</version>
</dependency>

Gradle:

implementation("com.anthropic:anthropic-java:2.17.0")

Client Initialization

import com.anthropic.client.AnthropicClient;
import com.anthropic.client.okhttp.AnthropicOkHttpClient;

// Default (reads ANTHROPIC_API_KEY from environment)
AnthropicClient client = AnthropicOkHttpClient.fromEnv();

// Explicit API key
AnthropicClient client = AnthropicOkHttpClient.builder()
    .apiKey("your-api-key")
    .build();

Basic Message Request

import com.anthropic.models.messages.MessageCreateParams;
import com.anthropic.models.messages.Message;
import com.anthropic.models.messages.Model;

MessageCreateParams params = MessageCreateParams.builder()
    .model(Model.CLAUDE_OPUS_4_6)
    .maxTokens(16000L)
    .addUserMessage("What is the capital of France?")
    .build();

Message response = client.messages().create(params);
response.content().stream()
    .flatMap(block -> block.text().stream())
    .forEach(textBlock -> System.out.println(textBlock.text()));

Streaming

import com.anthropic.core.http.StreamResponse;
import com.anthropic.models.messages.RawMessageStreamEvent;

MessageCreateParams params = MessageCreateParams.builder()
    .model(Model.CLAUDE_OPUS_4_6)
    .maxTokens(64000L)
    .addUserMessage("Write a haiku")
    .build();

try (StreamResponse<RawMessageStreamEvent> streamResponse = client.messages().createStreaming(params)) {
    streamResponse.stream()
        .flatMap(event -> event.contentBlockDelta().stream())
        .flatMap(deltaEvent -> deltaEvent.delta().text().stream())
        .forEach(textDelta -> System.out.print(textDelta.text()));
}

Thinking

Adaptive thinking is the recommended mode for Claude 4.6+ models. Claude decides dynamically when and how much to think. The builder has a direct .thinking(ThinkingConfigAdaptive) overload — no manual union wrapping.

import com.anthropic.models.messages.ContentBlock;
import com.anthropic.models.messages.MessageCreateParams;
import com.anthropic.models.messages.Model;
import com.anthropic.models.messages.ThinkingConfigAdaptive;

MessageCreateParams params = MessageCreateParams.builder()
    .model(Model.CLAUDE_SONNET_4_6)
    .maxTokens(16000L)
    .thinking(ThinkingConfigAdaptive.builder().build())
    .addUserMessage("Solve this step by step: 27 * 453")
    .build();

for (ContentBlock block : client.messages().create(params).content()) {
    block.thinking().ifPresent(t -> System.out.println("[thinking] " + t.thinking()));
    block.text().ifPresent(t -> System.out.println(t.text()));
}

Deprecated: ThinkingConfigEnabled.builder().budgetTokens(N) (and the .enabledThinking(N) shortcut) still works on Claude 4.6 but is deprecated. Use adaptive thinking above.

ContentBlock narrowing: .thinking() / .text() return Optional<T> — use .ifPresent(...) or .stream().flatMap(...). Alternative: isThinking() / asThinking() boolean+unwrap pairs (throws on wrong variant).

Tool Use (Beta)

The Java SDK supports beta tool use with annotated classes. Tool classes implement Supplier<String> for automatic execution via BetaToolRunner.

Tool Runner (automatic loop)

import com.anthropic.models.beta.messages.MessageCreateParams;
import com.anthropic.models.beta.messages.BetaMessage;
import com.anthropic.helpers.BetaToolRunner;
import com.fasterxml.jackson.annotation.JsonClassDescription;
import com.fasterxml.jackson.annotation.JsonPropertyDescription;
import java.util.function.Supplier;

@JsonClassDescription("Get the weather in a given location")
static class GetWeather implements Supplier<String> {
    @JsonPropertyDescription("The city and state, e.g. San Francisco, CA")
    public String location;

    @Override
    public String get() {
        return "The weather in " + location + " is sunny and 72°F";
    }
}

BetaToolRunner toolRunner = client.beta().messages().toolRunner(
    MessageCreateParams.builder()
        .model("{{OPUS_ID}}")
        .maxTokens(16000L)
        .putAdditionalHeader("anthropic-beta", "structured-outputs-2025-11-13")
        .addTool(GetWeather.class)
        .addUserMessage("What's the weather in San Francisco?")
        .build());

for (BetaMessage message : toolRunner) {
    System.out.println(message);
}

Memory Tool

The Java SDK provides BetaMemoryToolHandler for implementing the memory tool backend. You supply a handler that manages file storage, and the BetaToolRunner handles memory tool calls automatically.

import com.anthropic.helpers.BetaMemoryToolHandler;
import com.anthropic.helpers.BetaToolRunner;
import com.anthropic.models.beta.messages.BetaMemoryTool20250818;
import com.anthropic.models.beta.messages.BetaMessage;
import com.anthropic.models.beta.messages.MessageCreateParams;
import com.anthropic.models.beta.messages.ToolRunnerCreateParams;

// Implement BetaMemoryToolHandler with your storage backend (e.g., filesystem)
BetaMemoryToolHandler memoryHandler = new FileSystemMemoryToolHandler(sandboxRoot);

MessageCreateParams createParams = MessageCreateParams.builder()
    .model("{{OPUS_ID}}")
    .maxTokens(4096L)
    .addTool(BetaMemoryTool20250818.builder().build())
    .addUserMessage("Remember that my favorite color is blue")
    .build();

BetaToolRunner toolRunner = client.beta().messages().toolRunner(
    ToolRunnerCreateParams.builder()
        .betaMemoryToolHandler(memoryHandler)
        .initialMessageParams(createParams)
        .build());

for (BetaMessage message : toolRunner) {
    System.out.println(message);
}

See the shared memory tool concepts for more details on the memory tool.

Non-Beta Tool Declaration (manual JSON schema)

Tool.InputSchema.Properties is a freeform Map<String, JsonValue> wrapper — build property schemas via putAdditionalProperty. type: "object" is the default. The builder has a direct .addTool(Tool) overload that wraps in ToolUnion automatically.

import com.anthropic.core.JsonValue;
import com.anthropic.models.messages.Tool;

Tool tool = Tool.builder()
    .name("get_weather")
    .description("Get the current weather in a given location")
    .inputSchema(Tool.InputSchema.builder()
        .properties(Tool.InputSchema.Properties.builder()
            .putAdditionalProperty("location", JsonValue.from(Map.of("type", "string")))
            .build())
        .required(List.of("location"))
        .build())
    .build();

MessageCreateParams params = MessageCreateParams.builder()
    .model(Model.CLAUDE_SONNET_4_6)
    .maxTokens(16000L)
    .addTool(tool)
    .addUserMessage("Weather in Paris?")
    .build();

For manual tool loops, handle tool_use blocks in the response, send tool_result back, loop until stop_reason is "end_turn". See shared tool use concepts.

Building MessageParam with Content Blocks (Tool Result Round-Trip)

MessageParam.Content is an inner union class (string | list). Use the builder's .contentOfBlockParams(List<ContentBlockParam>) alias — there is NO separate MessageParamContent class with a static ofBlockParams:

import com.anthropic.models.messages.MessageParam;
import com.anthropic.models.messages.ContentBlockParam;
import com.anthropic.models.messages.ToolResultBlockParam;

List<ContentBlockParam> results = List.of(
    ContentBlockParam.ofToolResult(ToolResultBlockParam.builder()
        .toolUseId(toolUseBlock.id())
        .content(yourResultString)
        .build())
);

MessageParam toolResultMsg = MessageParam.builder()
    .role(MessageParam.Role.USER)
    .contentOfBlockParams(results)   // builder alias for Content.ofBlockParams(...)
    .build();

Effort Parameter

Effort is nested inside OutputConfig — there is NO .effort() directly on MessageCreateParams.Builder.

import com.anthropic.models.messages.OutputConfig;

.outputConfig(OutputConfig.builder()
    .effort(OutputConfig.Effort.HIGH)  // or LOW, MEDIUM, MAX
    .build())

Combine with Thinking = ThinkingConfigAdaptive for cost-quality control.

Prompt Caching

System message as a list of TextBlockParam with CacheControlEphemeral. Use .systemOfTextBlockParams(...) — the plain .system(String) overload can't carry cache control. For placement patterns and the silent-invalidator audit checklist, see shared/prompt-caching.md.

import com.anthropic.models.messages.TextBlockParam;
import com.anthropic.models.messages.CacheControlEphemeral;

.systemOfTextBlockParams(List.of(
    TextBlockParam.builder()
        .text(longSystemPrompt)
        .cacheControl(CacheControlEphemeral.builder()
            .ttl(CacheControlEphemeral.Ttl.TTL_1H)  // optional; also TTL_5M
            .build())
        .build()))

There's also a top-level .cacheControl(CacheControlEphemeral) on MessageCreateParams.Builder and on Tool.builder().

Verify hits via response.usage().cacheCreationInputTokens() / response.usage().cacheReadInputTokens().

Token Counting

import com.anthropic.models.messages.MessageCountTokensParams;

long tokens = client.messages().countTokens(
    MessageCountTokensParams.builder()
        .model(Model.CLAUDE_SONNET_4_6)
        .addUserMessage("Hello")
        .build()
).inputTokens();

Structured Output

The class-based overload auto-derives the JSON schema from your POJO and gives you a typed .text() return — no manual schema, no manual parsing.

import com.anthropic.models.messages.StructuredMessageCreateParams;

record Book(String title, String author) {}
record BookList(List<Book> books) {}

StructuredMessageCreateParams<BookList> params = MessageCreateParams.builder()
    .model(Model.CLAUDE_SONNET_4_6)
    .maxTokens(16000L)
    .outputConfig(BookList.class)  // returns a typed builder
    .addUserMessage("List 3 classic novels")
    .build();

client.messages().create(params).content().stream()
    .flatMap(cb -> cb.text().stream())
    .forEach(typed -> {
        // typed.text() returns BookList, not String
        for (Book b : typed.text().books()) System.out.println(b.title());
    });

Supports Jackson annotations: @JsonPropertyDescription, @JsonIgnore, @ArraySchema(minItems=...). Manual schema path: OutputConfig.builder().format(JsonOutputFormat.builder().schema(...).build()).

PDF / Document Input

DocumentBlockParam builder has source shortcuts. Wrap in ContentBlockParam.ofDocument() and pass via .addUserMessageOfBlockParams().

import com.anthropic.models.messages.DocumentBlockParam;
import com.anthropic.models.messages.ContentBlockParam;
import com.anthropic.models.messages.TextBlockParam;

DocumentBlockParam doc = DocumentBlockParam.builder()
    .base64Source(base64String)  // or .urlSource("https://...") or .textSource("...")
    .title("My Document")        // optional
    .build();

.addUserMessageOfBlockParams(List.of(
    ContentBlockParam.ofDocument(doc),
    ContentBlockParam.ofText(TextBlockParam.builder().text("Summarize this").build())))

Server-Side Tools

Version-suffixed types; name/type auto-set by builder. Direct .addTool() overloads exist for every type — no manual ToolUnion wrapping.

import com.anthropic.models.messages.WebSearchTool20260209;
import com.anthropic.models.messages.ToolBash20250124;
import com.anthropic.models.messages.ToolTextEditor20250728;
import com.anthropic.models.messages.CodeExecutionTool20260120;

.addTool(WebSearchTool20260209.builder()
    .maxUses(5L)                              // optional
    .allowedDomains(List.of("example.com"))   // optional
    .build())
.addTool(ToolBash20250124.builder().build())
.addTool(ToolTextEditor20250728.builder().build())
.addTool(CodeExecutionTool20260120.builder().build())

Also available: WebFetchTool20260209, MemoryTool20250818, ToolSearchToolBm25_20251119.

Beta namespace (MCP, compaction)

For beta-only features use com.anthropic.models.beta.messages.* — class names have a Beta prefix AND live in the beta package. The beta MessageCreateParams.Builder has direct .addTool(BetaToolBash20250124) overloads AND .addMcpServer():

import com.anthropic.models.beta.messages.MessageCreateParams;
import com.anthropic.models.beta.messages.BetaToolBash20250124;
import com.anthropic.models.beta.messages.BetaCodeExecutionTool20260120;
import com.anthropic.models.beta.messages.BetaRequestMcpServerUrlDefinition;

MessageCreateParams params = MessageCreateParams.builder()
    .model(Model.CLAUDE_OPUS_4_6)
    .maxTokens(16000L)
    .addBeta("mcp-client-2025-11-20")
    .addTool(BetaToolBash20250124.builder().build())
    .addTool(BetaCodeExecutionTool20260120.builder().build())
    .addMcpServer(BetaRequestMcpServerUrlDefinition.builder()
        .name("my-server")
        .url("https://example.com/mcp")
        .build())
    .addUserMessage("...")
    .build();

client.beta().messages().create(params);

BetaTool* types are NOT interchangeable with non-beta Tool* — pick one namespace per request.

Reading server-tool blocks in the response: ServerToolUseBlock has .id(), .name() (enum), and ._input() returning raw JsonValue — there is NO typed .input(). For code execution results, unwrap two levels:

for (ContentBlock block : response.content()) {
    block.serverToolUse().ifPresent(stu -> {
        System.out.println("tool: " + stu.name() + " input: " + stu._input());
    });
    block.codeExecutionToolResult().ifPresent(r -> {
        r.content().resultBlock().ifPresent(result -> {
            System.out.println("stdout: " + result.stdout());
            System.out.println("stderr: " + result.stderr());
            System.out.println("exit: " + result.returnCode());
        });
    });
}

Files API (Beta)

Under client.beta().files(). File references in messages need the beta message types (non-beta DocumentBlockParam.Source has no file-ID variant).

import com.anthropic.models.beta.files.FileUploadParams;
import com.anthropic.models.beta.files.FileMetadata;
import com.anthropic.models.beta.messages.BetaRequestDocumentBlock;
import java.nio.file.Paths;

FileMetadata meta = client.beta().files().upload(
    FileUploadParams.builder()
        .file(Paths.get("/path/to/doc.pdf"))  // or .file(InputStream) or .file(byte[])
        .build());

// Reference in a beta message:
BetaRequestDocumentBlock doc = BetaRequestDocumentBlock.builder()
    .fileSource(meta.id())
    .build();

Other methods: .list(), .delete(String fileId), .download(String fileId), .retrieveMetadata(String fileId).