<!--
name: 'Data: Claude API reference — Go'
description: Go SDK reference
ccVersion: 2.1.78
-->
# Claude API — Go

> **Note:** The Go SDK supports the Claude API and beta tool use with \`BetaToolRunner\`. Agent SDK is not yet available for Go.

## Installation

\`\`\`bash
go get github.com/anthropics/anthropic-sdk-go
\`\`\`

## Client Initialization

\`\`\`go
import (
    "github.com/anthropics/anthropic-sdk-go"
    "github.com/anthropics/anthropic-sdk-go/option"
)

// Default (uses ANTHROPIC_API_KEY env var)
client := anthropic.NewClient()

// Explicit API key
client := anthropic.NewClient(
    option.WithAPIKey("your-api-key"),
)
\`\`\`

---

## Basic Message Request

\`\`\`go
response, err := client.Messages.New(context.Background(), anthropic.MessageNewParams{
    Model:     anthropic.ModelClaudeOpus4_6,
    MaxTokens: 16000,
    Messages: []anthropic.MessageParam{
        anthropic.NewUserMessage(anthropic.NewTextBlock("What is the capital of France?")),
    },
})
if err != nil {
    log.Fatal(err)
}
for _, block := range response.Content {
    switch variant := block.AsAny().(type) {
    case anthropic.TextBlock:
        fmt.Println(variant.Text)
    }
}
\`\`\`

---

## Streaming

\`\`\`go
stream := client.Messages.NewStreaming(context.Background(), anthropic.MessageNewParams{
    Model:     anthropic.ModelClaudeOpus4_6,
    MaxTokens: 64000,
    Messages: []anthropic.MessageParam{
        anthropic.NewUserMessage(anthropic.NewTextBlock("Write a haiku")),
    },
})

for stream.Next() {
    event := stream.Current()
    switch eventVariant := event.AsAny().(type) {
    case anthropic.ContentBlockDeltaEvent:
        switch deltaVariant := eventVariant.Delta.AsAny().(type) {
        case anthropic.TextDelta:
            fmt.Print(deltaVariant.Text)
        }
    }
}
if err := stream.Err(); err != nil {
    log.Fatal(err)
}
\`\`\`

**Accumulating the final message** (there is no \`GetFinalMessage()\` on the stream):

\`\`\`go
stream := client.Messages.NewStreaming(ctx, params)
message := anthropic.Message{}
for stream.Next() {
    message.Accumulate(stream.Current())
}
if err := stream.Err(); err != nil { log.Fatal(err) }
// message.Content now has the complete response
\`\`\`


---

## Tool Use

### Tool Runner (Beta — Recommended)

**Beta:** The Go SDK provides \`BetaToolRunner\` for automatic tool use loops via the \`toolrunner\` package.

\`\`\`go
import (
    "context"
    "fmt"
    "log"

    "github.com/anthropics/anthropic-sdk-go"
    "github.com/anthropics/anthropic-sdk-go/toolrunner"
)

// Define tool input with jsonschema tags for automatic schema generation
type GetWeatherInput struct {
    City string \`json:"city" jsonschema:"required,description=The city name"\`
}

// Create a tool with automatic schema generation from struct tags
weatherTool, err := toolrunner.NewBetaToolFromJSONSchema(
    "get_weather",
    "Get current weather for a city",
    func(ctx context.Context, input GetWeatherInput) (anthropic.BetaToolResultBlockParamContentUnion, error) {
        return anthropic.BetaToolResultBlockParamContentUnion{
            OfText: &anthropic.BetaTextBlockParam{
                Text: fmt.Sprintf("The weather in %s is sunny, 72°F", input.City),
            },
        }, nil
    },
)
if err != nil {
    log.Fatal(err)
}

// Create a tool runner that handles the conversation loop automatically
runner := client.Beta.Messages.NewToolRunner(
    []anthropic.BetaTool{weatherTool},
    anthropic.BetaToolRunnerParams{
        BetaMessageNewParams: anthropic.BetaMessageNewParams{
            Model:     anthropic.ModelClaudeOpus4_6,
            MaxTokens: 16000,
            Messages: []anthropic.BetaMessageParam{
                anthropic.NewBetaUserMessage(anthropic.NewBetaTextBlock("What's the weather in Paris?")),
            },
        },
        MaxIterations: 5,
    },
)

// Run until Claude produces a final response
message, err := runner.RunToCompletion(context.Background())
if err != nil {
    log.Fatal(err)
}

// RunToCompletion returns *BetaMessage; content is []BetaContentBlockUnion.
// Narrow via AsAny() switch — note the Beta-namespace types (BetaTextBlock,
// not TextBlock):
for _, block := range message.Content {
    switch block := block.AsAny().(type) {
    case anthropic.BetaTextBlock:
        fmt.Println(block.Text)
    }
}
\`\`\`

**Key features of the Go tool runner:**

- Automatic schema generation from Go structs via \`jsonschema\` tags
- \`RunToCompletion()\` for simple one-shot usage
- \`All()\` iterator for processing each message in the conversation
- \`NextMessage()\` for step-by-step iteration
- Streaming variant via \`NewToolRunnerStreaming()\` with \`AllStreaming()\`

### Manual Loop

For fine-grained control over the agentic loop, define tools with \`ToolParam\`, check \`StopReason\`, execute tools yourself, and feed \`tool_result\` blocks back. This is the pattern when you need to intercept, validate, or log tool calls.

Derived from \`anthropic-sdk-go/examples/tools/main.go\`.

\`\`\`go
package main

import (
    "context"
    "encoding/json"
    "fmt"
    "log"

    "github.com/anthropics/anthropic-sdk-go"
)

func main() {
    client := anthropic.NewClient()

    // 1. Define tools. ToolParam.InputSchema uses a map, no struct tags needed.
    addTool := anthropic.ToolParam{
        Name:        "add",
        Description: anthropic.String("Add two integers"),
        InputSchema: anthropic.ToolInputSchemaParam{
            Properties: map[string]any{
                "a": map[string]any{"type": "integer"},
                "b": map[string]any{"type": "integer"},
            },
        },
    }
    // ToolParam must be wrapped in ToolUnionParam for the Tools slice
    tools := []anthropic.ToolUnionParam{{OfTool: &addTool}}

    messages := []anthropic.MessageParam{
        anthropic.NewUserMessage(anthropic.NewTextBlock("What is 2 + 3?")),
    }

    for {
        resp, err := client.Messages.New(context.Background(), anthropic.MessageNewParams{
            Model:     anthropic.ModelClaudeSonnet4_6,
            MaxTokens: 16000,
            Messages:  messages,
            Tools:     tools,
        })
        if err != nil {
            log.Fatal(err)
        }

        // 2. Append the assistant response to history BEFORE processing tool calls.
        //    resp.ToParam() converts Message → MessageParam in one call.
        messages = append(messages, resp.ToParam())

        // 3. Walk content blocks. ContentBlockUnion is a flattened struct;
        //    use block.AsAny().(type) to switch on the actual variant.
        toolResults := []anthropic.ContentBlockParamUnion{}
        for _, block := range resp.Content {
            switch variant := block.AsAny().(type) {
            case anthropic.TextBlock:
                fmt.Println(variant.Text)
            case anthropic.ToolUseBlock:
                // 4. Parse the tool input. Use variant.JSON.Input.Raw() to get the
                //    raw JSON — block.Input is json.RawMessage, not the parsed value.
                var in struct {
                    A int \`json:"a"\`
                    B int \`json:"b"\`
                }
                if err := json.Unmarshal([]byte(variant.JSON.Input.Raw()), &in); err != nil {
                    log.Fatal(err)
                }
                result := fmt.Sprintf("%d", in.A+in.B)
                // 5. NewToolResultBlock(toolUseID, content, isError) builds the
                //    ContentBlockParamUnion for you. block.ID is the tool_use_id.
                toolResults = append(toolResults,
                    anthropic.NewToolResultBlock(block.ID, result, false))
            }
        }

        // 6. Exit when Claude stops asking for tools
        if resp.StopReason != anthropic.StopReasonToolUse {
            break
        }

        // 7. Tool results go in a user message (variadic: all results in one turn)
        messages = append(messages, anthropic.NewUserMessage(toolResults...))
    }
}
\`\`\`

**Key API surface:**

| Symbol | Purpose |
|---|---|
| \`resp.ToParam()\` | Convert \`Message\` response → \`MessageParam\` for history |
| \`block.AsAny().(type)\` | Type-switch on \`ContentBlockUnion\` variants |
| \`variant.JSON.Input.Raw()\` | Raw JSON string of tool input (for \`json.Unmarshal\`) |
| \`anthropic.NewToolResultBlock(id, content, isError)\` | Build \`tool_result\` block |
| \`anthropic.NewUserMessage(blocks...)\` | Wrap tool results as a user turn |
| \`anthropic.StopReasonToolUse\` | \`StopReason\` constant to check loop termination |
| \`anthropic.ToolUnionParam{OfTool: &t}\` | Wrap \`ToolParam\` in the union for \`Tools:\` |

---

## Thinking

Enable Claude's internal reasoning by setting \`Thinking\` in \`MessageNewParams\`. The response will contain \`ThinkingBlock\` content before the final \`TextBlock\`.

**Adaptive thinking is the recommended mode for Claude 4.6+ models.** Claude decides dynamically when and how much to think. Combine with the \`effort\` parameter for cost-quality control.

Derived from \`anthropic-sdk-go/message.go\` (\`ThinkingConfigParamUnion\`, \`NewThinkingConfigAdaptiveParam\`).

\`\`\`go
// There is no ThinkingConfigParamOfAdaptive helper — construct the union
// struct-literal directly and take the address of the variant.
adaptive := anthropic.NewThinkingConfigAdaptiveParam()
params := anthropic.MessageNewParams{
    Model:     anthropic.ModelClaudeSonnet4_6,
    MaxTokens: 16000,
    Thinking:  anthropic.ThinkingConfigParamUnion{OfAdaptive: &adaptive},
    Messages: []anthropic.MessageParam{
        anthropic.NewUserMessage(anthropic.NewTextBlock("How many r's in strawberry?")),
    },
}

resp, err := client.Messages.New(context.Background(), params)
if err != nil {
    log.Fatal(err)
}

// ThinkingBlock(s) precede TextBlock in content
for _, block := range resp.Content {
    switch b := block.AsAny().(type) {
    case anthropic.ThinkingBlock:
        fmt.Println("[thinking]", b.Thinking)
    case anthropic.TextBlock:
        fmt.Println(b.Text)
    }
}
\`\`\`

> **Deprecated:** \`ThinkingConfigParamOfEnabled(budgetTokens)\` (fixed-budget extended thinking) still works on Claude 4.6 but is deprecated. Use adaptive thinking above.

To disable: \`anthropic.ThinkingConfigParamUnion{OfDisabled: &anthropic.ThinkingConfigDisabledParam{}}\`.

---

## Server-Side Tools

Version-suffixed struct names with \`Param\` suffix. \`Name\`/\`Type\` are \`constant.*\` types — zero value marshals correctly, so \`{}\` works. Wrap in \`ToolUnionParam\` with the matching \`Of*\` field.

\`\`\`go
Tools: []anthropic.ToolUnionParam{
    {OfWebSearchTool20260209: &anthropic.WebSearchTool20260209Param{}},
    {OfBashTool20250124: &anthropic.ToolBash20250124Param{}},
    {OfTextEditor20250728: &anthropic.ToolTextEditor20250728Param{}},
    {OfCodeExecutionTool20260120: &anthropic.CodeExecutionTool20260120Param{}},
},
\`\`\`

Also available: \`WebFetchTool20260209Param\`, \`MemoryTool20250818Param\`, \`ToolSearchToolBm25_20251119Param\`, \`ToolSearchToolRegex20251119Param\`.

---

## PDF / Document Input

\`NewDocumentBlock\` generic helper accepts any source type. \`MediaType\`/\`Type\` are auto-set.

\`\`\`go
b64 := base64.StdEncoding.EncodeToString(pdfBytes)

msg := anthropic.NewUserMessage(
    anthropic.NewDocumentBlock(anthropic.Base64PDFSourceParam{Data: b64}),
    anthropic.NewTextBlock("Summarize this document"),
)
\`\`\`

Other sources: \`URLPDFSourceParam{URL: "https://..."}\`, \`PlainTextSourceParam{Data: "..."}\`.

---

## Files API (Beta)

Under \`client.Beta.Files\`. Method is **\`Upload\`** (NOT \`New\`/\`Create\`), params struct is \`BetaFileUploadParams\`. The \`File\` field takes an \`io.Reader\`; use \`anthropic.File()\` to attach a filename + content-type for the multipart encoding.

\`\`\`go
f, _ := os.Open("./upload_me.txt")
defer f.Close()

meta, err := client.Beta.Files.Upload(ctx, anthropic.BetaFileUploadParams{
    File:  anthropic.File(f, "upload_me.txt", "text/plain"),
    Betas: []anthropic.AnthropicBeta{anthropic.AnthropicBetaFilesAPI2025_04_14},
})
// meta.ID is the file_id to reference in subsequent message requests
\`\`\`

Other \`Beta.Files\` methods: \`List\`, \`Delete\`, \`Download\`, \`GetMetadata\`.

---

## Context Editing / Compaction (Beta)

Use \`Beta.Messages.New\` with \`ContextManagement\` on \`BetaMessageNewParams\`. There is no \`NewBetaAssistantMessage\` — use \`.ToParam()\` for the round-trip.

\`\`\`go
params := anthropic.BetaMessageNewParams{
    Model:     anthropic.ModelClaudeOpus4_6,  // also supported: ModelClaudeSonnet4_6
    MaxTokens: 16000,
    Betas:     []anthropic.AnthropicBeta{"compact-2026-01-12"},
    ContextManagement: anthropic.BetaContextManagementConfigParam{
        Edits: []anthropic.BetaContextManagementConfigEditUnionParam{
            {OfCompact20260112: &anthropic.BetaCompact20260112EditParam{}},
        },
    },
    Messages: []anthropic.BetaMessageParam{ /* ... */ },
}

resp, err := client.Beta.Messages.New(ctx, params)
if err != nil {
    log.Fatal(err)
}

// Round-trip: append response to history via .ToParam()
params.Messages = append(params.Messages, resp.ToParam())

// Read compaction blocks from the response
for _, block := range resp.Content {
    if c, ok := block.AsAny().(anthropic.BetaCompactionBlock); ok {
        fmt.Println("compaction summary:", c.Content)
    }
}
\`\`\`

Other edit types: \`BetaClearToolUses20250919EditParam\`, \`BetaClearThinking20251015EditParam\`.