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

go get github.com/anthropics/anthropic-sdk-go

Client Initialization

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

response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
    Model:     anthropic.ModelClaudeOpus4_6,
    MaxTokens: 1024,
    Messages: []anthropic.MessageParam{
        anthropic.NewUserMessage(anthropic.NewTextBlock("What is the capital of France?")),
    },
})
if err != nil {
    log.Fatal(err)
}
fmt.Println(response.Content[0].Text)

Streaming

stream := client.Messages.NewStreaming(context.TODO(), anthropic.MessageNewParams{
    Model:     anthropic.ModelClaudeOpus4_6,
    MaxTokens: 1024,
    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)
}

Tool Use

Tool Runner (Beta — Recommended)

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

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: 1024,
            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)
}
fmt.Println(message.Content[0].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, use raw tool definitions via JSON schema. See the shared tool use concepts for the tool definition format and agentic loop pattern.

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