Go SDK

The Anthropic Go library provides convenient access to the Anthropic REST API from applications written in Go.

Installation

import (
	"github.com/anthropics/anthropic-sdk-go" // imported as anthropic
)

Or to pin the version:

go get -u 'github.com/anthropics/[email protected]'

Requirements

This library requires Go 1.23+.

Usage

package main

import (
	"context"
	"fmt"

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

func main() {
	client := anthropic.NewClient(
		option.WithAPIKey("my-anthropic-api-key"), // defaults to os.LookupEnv("ANTHROPIC_API_KEY")
	)
	message, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
		MaxTokens: 1024,
		Messages: []anthropic.MessageParam{
			anthropic.NewUserMessage(anthropic.NewTextBlock("What is a quaternion?")),
		},
		Model: anthropic.ModelClaudeOpus4_6,
	})
	if err != nil {
		panic(err.Error())
	}
	fmt.Printf("%+v\n", message.Content)
}

Request fields

The anthropic library uses the omitzero semantics from the Go 1.24+ encoding/json release for request fields.

Required primitive fields (int64, string, etc.) feature the tag `json:"...,required"`. These fields are always serialized, even their zero values.

Optional primitive types are wrapped in a param.Opt[T]. These fields can be set with the provided constructors, anthropic.String(string), anthropic.Int(int64), etc.

Any param.Opt[T], map, slice, struct or string enum uses the tag `json:"...,omitzero"`. Its zero value is considered omitted.

The param.IsOmitted(any) function can confirm the presence of any omitzero field.

p := anthropic.ExampleParams{
	ID:   "id_xxx",                // required property
	Name: anthropic.String("..."), // optional property

	Point: anthropic.Point{
		X: 0,                // required field will serialize as 0
		Y: anthropic.Int(1), // optional field will serialize as 1
		// ... omitted non-required fields will not be serialized
	},

	Origin: anthropic.Origin{}, // the zero value of [Origin] is considered omitted
}

To send null instead of a param.Opt[T], use param.Null[T](). To send null instead of a struct T, use param.NullStruct[T]().

p.Name = param.Null[string]()       // 'null' instead of string
p.Point = param.NullStruct[Point]() // 'null' instead of struct

param.IsNull(p.Name)  // true
param.IsNull(p.Point) // true

Request structs contain a .SetExtraFields(map[string]any) method which can send non-conforming fields in the request body. Extra fields overwrite any struct fields with a matching key.

To send a custom value instead of a struct, use the generic function param.Override (for example, param.Override[anthropic.FooParams](12)).

// In cases where the API specifies a given type,
// but you want to send something else, use [SetExtraFields]:
p.SetExtraFields(map[string]any{
	"x": 0.01, // send "x" as a float instead of int
})

// Send a number instead of an object
custom := param.Override[anthropic.FooParams](12)

Request unions

Unions are represented as a struct with fields prefixed by "Of" for each of its variants, only one field can be non-zero. The non-zero field will be serialized.

Sub-properties of the union can be accessed via methods on the union struct. These methods return a mutable pointer to the underlying data, if present.

// Only one field can be non-zero, use param.IsOmitted() to check if a field is set
type AnimalUnionParam struct {
	OfCat *Cat `json:",omitzero,inline`
	OfDog *Dog `json:",omitzero,inline`
}

animal := AnimalUnionParam{
	OfCat: &Cat{
		Name: "Whiskers",
		Owner: PersonParam{
			Address: AddressParam{Street: "3333 Coyote Hill Rd", Zip: 0},
		},
	},
}

// Mutating a field
if address := animal.GetOwner().GetAddress(); address != nil {
	address.ZipCode = 94304
}

Deserializing params

Param types (types ending in Param, such as MessageNewParams or ToolUnionParam) are designed for outgoing requests only. They marshal correctly to JSON but do not fully support round-trip deserialization. If you unmarshal raw JSON into a param struct, typed union fields like OfBashTool20250124 will be nil even when the underlying JSON is valid.

If you need to reconstruct params from raw JSON (for example, from a database, middleware, or a previous request), call UnmarshalJSON to populate non-union fields, then use param.SetJSON to attach the raw bytes for correct re-serialization:

// Serialize params (for example, for storage or forwarding)
b, err := json.Marshal(original)
if err != nil {
	panic(err)
}

// Later, reconstruct params from the stored JSON
var params anthropic.MessageNewParams
if err := params.UnmarshalJSON(b); err != nil {
	panic(err)
}
param.SetJSON(b, &params)

// params.Model and other scalar fields are populated by UnmarshalJSON.
// params.Tools[0].OfBashTool20250124 is nil (the union limitation),
// but the raw JSON is preserved. When params is marshaled again
// for the API call, the tools serialize correctly.
b2, _ := json.Marshal(params)
fmt.Println(string(b) == string(b2)) // true

For this use case, param.SetJSON (available since v1.20.0) is preferred over the more general param.Override[T](any) because it doesn't require spelling out the type parameter and makes the round-trip intent explicit.

Response objects

All fields in response structs are ordinary value types (not pointers or wrappers). Response structs also include a special JSON field containing metadata about each property.

type Animal struct {
	Name   string `json:"name,nullable"`
	Owners int    `json:"owners"`
	Age    int    `json:"age"`
	JSON   struct {
		Name        respjson.Field
		Owner       respjson.Field
		Age         respjson.Field
		ExtraFields map[string]respjson.Field
	} `json:"-"`
}

To handle optional data, use the .Valid() method on the JSON field. .Valid() returns true if a field is not null, not present, or couldn't be marshaled.

If .Valid() is false, the corresponding field will simply be its zero value.

raw := `{"owners": 1, "name": null}`

var res Animal
json.Unmarshal([]byte(raw), &res)

// Accessing regular fields

res.Owners // 1
res.Name   // ""
res.Age    // 0

// Optional field checks

res.JSON.Owners.Valid() // true
res.JSON.Name.Valid()   // false
res.JSON.Age.Valid()    // false

// Raw JSON values

res.JSON.Owners.Raw()                  // "1"
res.JSON.Name.Raw() == "null"          // true
res.JSON.Name.Raw() == respjson.Null   // true
res.JSON.Age.Raw() == ""               // true
res.JSON.Age.Raw() == respjson.Omitted // true

These .JSON structs also include an ExtraFields map containing any properties in the json response that were not specified in the struct. This can be useful for API features not yet present in the SDK.

body := res.JSON.ExtraFields["my_unexpected_field"].Raw()

Response unions

In responses, unions are represented by a flattened struct containing all possible fields from each of the object variants. To convert it to a variant use the .AsFooVariant() method or the .AsAny() method if present.

If a response value union contains primitive values, primitive fields will be alongside the properties but prefixed with Of and feature the tag json:"...,inline".

type AnimalUnion struct {
	// From variants [Dog], [Cat]
	Owner Person `json:"owner"`
	// From variant [Dog]
	DogBreed string `json:"dog_breed"`
	// From variant [Cat]
	CatBreed string `json:"cat_breed"`
	// ...

	JSON struct {
		Owner respjson.Field
		// ...
	} `json:"-"`
}

// If animal variant
if animal.Owner.Address.ZipCode == "" {
	panic("missing zip code")
}

// Switch on the variant
switch variant := animal.AsAny().(type) {
case Dog:
case Cat:
default:
	panic("unexpected type")
}

Error handling

When the API returns a non-success status code, the SDK returns an error with type *anthropic.Error. This contains the StatusCode, *http.Request, and *http.Response values of the request, as well as the JSON of the error body (much like other response objects in the SDK). The error also includes the RequestID from the response headers, which is useful for troubleshooting with Anthropic support.

To handle errors, use the errors.As pattern:

_, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
	MaxTokens: 1024,
	Messages: []anthropic.MessageParam{{
		Content: []anthropic.ContentBlockParamUnion{{
			OfText: &anthropic.TextBlockParam{
				Text: "What is a quaternion?",
			},
		}},
		Role: anthropic.MessageParamRoleUser,
	}},
	Model: anthropic.ModelClaudeOpus4_6,
})
if err != nil {
	var apierr *anthropic.Error
	if errors.As(err, &apierr) {
		println("Request ID:", apierr.RequestID)
		println(string(apierr.DumpRequest(true)))  // Prints the serialized HTTP request
		println(string(apierr.DumpResponse(true))) // Prints the serialized HTTP response
	}
	panic(err.Error()) // GET "/v1/messages": 400 Bad Request (Request-ID: req_xxx) { ... }
}

When other errors occur, they are returned unwrapped; for example, if HTTP transport fails, you might receive *url.Error wrapping *net.OpError.

Retries

Certain errors will be automatically retried 2 times by default, with a short exponential backoff. The SDK retries by default all connection errors, 408 Request Timeout, 409 Conflict, 429 Rate Limit, and >=500 Internal errors.

You can use the WithMaxRetries option to configure or disable this:

// Configure the default for all requests:
client := anthropic.NewClient(
	option.WithMaxRetries(0), // default is 2
)

// Override per-request:
// ...
	client.Messages.New(
		context.TODO(),
		anthropic.MessageNewParams{
			MaxTokens: 1024,
			Messages: []anthropic.MessageParam{{
				Content: []anthropic.ContentBlockParamUnion{{
					OfText: &anthropic.TextBlockParam{
						Text: "What is a quaternion?",
					},
				}},
				Role: anthropic.MessageParamRoleUser,
			}},
			Model: anthropic.ModelClaudeOpus4_6,
		},
		option.WithMaxRetries(5),
	)

Timeouts

Requests do not time out by default; use context to configure a timeout for a request lifecycle.

Note that if a request is retried, the context timeout does not start over. To set a per-retry timeout, use option.WithRequestTimeout().

// This sets the timeout for the request, including all the retries.
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
defer cancel()
// ...
	client.Messages.New(
		ctx,
		anthropic.MessageNewParams{
			MaxTokens: 1024,
			Messages: []anthropic.MessageParam{{
				Content: []anthropic.ContentBlockParamUnion{{
					OfText: &anthropic.TextBlockParam{
						Text: "What is a quaternion?",
					},
				}},
				Role: anthropic.MessageParamRoleUser,
			}},
			Model: anthropic.ModelClaudeOpus4_6,
		},
		// This sets the per-retry timeout
		option.WithRequestTimeout(20*time.Second),
	)

Long requests

Avoid setting a large MaxTokens value without using streaming as some networks may drop idle connections after a certain period of time, which can cause the request to fail or timeout without receiving a response from Anthropic.

This SDK will also return an error if a non-streaming request is expected to be above roughly 10 minutes long. Calling .Messages.NewStreaming() or setting a custom timeout disables this error.

File uploads

Request parameters that correspond to file uploads in multipart requests are typed as io.Reader. The contents of the io.Reader will by default be sent as a multipart form part with the file name of "anonymous_file" and content-type of "application/octet-stream", so the recommended approach is to specify a custom content-type with the anthropic.File(reader io.Reader, filename string, contentType string) helper, which easily wraps any io.Reader with the appropriate file name and content type.

// A file from the file system
file, err := os.Open("/path/to/file.json")
anthropic.BetaFileUploadParams{
	File:  anthropic.File(file, "custom-name.json", "application/json"),
	Betas: []anthropic.AnthropicBeta{anthropic.AnthropicBetaFilesAPI2025_04_14},
}

// A file from a string
anthropic.BetaFileUploadParams{
	File:  anthropic.File(strings.NewReader("my file contents"), "custom-name.json", "application/json"),
	Betas: []anthropic.AnthropicBeta{anthropic.AnthropicBetaFilesAPI2025_04_14},
}

The file name and content-type can also be customized by implementing Name() string or ContentType() string on the run-time type of io.Reader. Note that os.File implements Name() string, so a file returned by os.Open will be sent with the file name on disk.

Pagination

This library provides some conveniences for working with paginated list endpoints.

You can use .ListAutoPaging() methods to iterate through items across all pages:

iter := client.Messages.Batches.ListAutoPaging(context.TODO(), anthropic.MessageBatchListParams{
	Limit: anthropic.Int(20),
})
// Automatically fetches more pages as needed.
for iter.Next() {
	messageBatch := iter.Current()
	fmt.Printf("%+v\n", messageBatch)
}
if err := iter.Err(); err != nil {
	panic(err.Error())
}

Or you can use simple .List() methods to fetch a single page and receive a standard response object with additional helper methods like .GetNextPage():

page, err := client.Messages.Batches.List(context.TODO(), anthropic.MessageBatchListParams{
	Limit: anthropic.Int(20),
})
for page != nil {
	for _, batch := range page.Data {
		fmt.Printf("%+v\n", batch)
	}
	page, err = page.GetNextPage()
}
if err != nil {
	panic(err.Error())
}

RequestOptions

This library uses the functional options pattern. Functions defined in the option package return a RequestOption, which is a closure that mutates a RequestConfig. These options can be supplied to the client or at individual requests. For example:

client := anthropic.NewClient(
	// Adds a header to every request made by the client
	option.WithHeader("X-Some-Header", "custom_header_info"),
)

client.Messages.New(context.TODO(), // ...,
	// Override the header
	option.WithHeader("X-Some-Header", "some_other_custom_header_info"),
	// Add an undocumented field to the request body, using sjson syntax
	option.WithJSONSet("some.json.path", map[string]string{"my": "object"}),
)

The request option option.WithDebugLog(nil) may be helpful while debugging.

See the full list of request options.

HTTP client customization

Middleware

The SDK provides option.WithMiddleware, which applies the given middleware to requests.

client := anthropic.NewClient(
	option.WithMiddleware(func(req *http.Request, next option.MiddlewareNext) (res *http.Response, err error) {
		// Before the request
		start := time.Now()
		LogReq(req)

		// Forward the request to the next handler
		res, err = next(req)

		// Handle stuff after the request
		LogRes(res, err, time.Since(start))

		return res, err
	}),
)

When multiple middlewares are provided as variadic arguments, the middlewares are applied left to right. If option.WithMiddleware is given multiple times, for example first in the client then the method, the middleware in the client will run first and the middleware given in the method will run next.

You may also replace the default http.Client with option.WithHTTPClient(client). Only one http client is accepted (this overwrites any previous client) and receives requests after any middleware has been applied.

Platform integrations

The Go SDK supports Amazon Bedrock and Google Vertex AI through subpackages:

• Bedrock: import "github.com/anthropics/anthropic-sdk-go/bedrock". Use bedrock.WithLoadDefaultConfig(ctx) or bedrock.WithConfig(cfg). Importing this package globally registers a decoder for application/vnd.amazon.eventstream for streaming.
• Vertex AI: import "github.com/anthropics/anthropic-sdk-go/vertex". Use vertex.WithGoogleAuth(ctx, region, projectID) or vertex.WithCredentials(ctx, region, projectID, creds).

Advanced usage

Accessing raw response data (for example, response headers)

You can access the raw HTTP response data by using the option.WithResponseInto() request option. This is useful when you need to examine response headers, status codes, or other details.

// Create a variable to store the HTTP response
var response *http.Response
message, err := client.Messages.New(
	context.TODO(),
	anthropic.MessageNewParams{
		MaxTokens: 1024,
		Messages: []anthropic.MessageParam{{
			Content: []anthropic.ContentBlockParamUnion{{
				OfText: &anthropic.TextBlockParam{
					Text: "What is a quaternion?",
				},
			}},
			Role: anthropic.MessageParamRoleUser,
		}},
		Model: anthropic.ModelClaudeOpus4_6,
	},
	option.WithResponseInto(&response),
)
if err != nil {
	// handle error
}
fmt.Printf("%+v\n", message)

fmt.Printf("Status Code: %d\n", response.StatusCode)
fmt.Printf("Headers: %+#v\n", response.Header)

Making custom/undocumented requests

This library is typed for convenient access to the documented API. If you need to access undocumented endpoints, params, or response properties, the library can still be used.

Undocumented endpoints

To make requests to undocumented endpoints, you can use client.Get, client.Post, and other HTTP verbs. RequestOptions on the client, such as retries, will be respected when making these requests.

var (
	// params can be an io.Reader, a []byte, an encoding/json serializable object,
	// or a "...Params" struct defined in this library.
	params map[string]any

	// result can be an []byte, *http.Response, a encoding/json deserializable object,
	// or a model defined in this library.
	result *http.Response
)
err := client.Post(context.Background(), "/unspecified", params, &result)
if err != nil {
	// ...
}

Undocumented request params

To make requests using undocumented parameters, you may use either the option.WithQuerySet() or the option.WithJSONSet() methods.

params := FooNewParams{
	ID: "id_xxxx",
	Data: FooNewParamsData{
		FirstName: anthropic.String("John"),
	},
}
client.Foo.New(context.Background(), params, option.WithJSONSet("data.last_name", "Doe"))

Undocumented response properties

To access undocumented response properties, you may either access the raw JSON of the response as a string with result.JSON.RawJSON(), or get the raw JSON of a particular field on the result with result.JSON.Foo.Raw().

Any fields that are not present on the response struct will be saved and can be accessed by result.JSON.ExtraFields() which returns the extra fields as a map[string]Field.

Semantic versioning

This package generally follows SemVer conventions, though certain backwards-incompatible changes may be released as minor versions:

1. Changes to library internals which are technically public but not intended or documented for external use. (Please open a GitHub issue to let the maintainers know if you're relying on such internals.)
2. Changes that aren't expected to impact the vast majority of users in practice.

Backwards-compatibility is taken seriously to ensure you can rely on a smooth upgrade experience.

Your feedback is welcome; please open an issue with questions, bugs, or suggestions.

Additional resources

• GitHub repository
• Go package documentation
• API reference
• Streaming guide