MCP Toolbox Core SDK

This SDK allows you to seamlessly integrate the functionalities of Toolbox allowing you to load and use tools defined in the service as standard Go structs within your GenAI applications.

This simplifies integrating external functionalities (like APIs, databases, or custom logic) managed by the Toolbox into your workflows, especially those involving Large Language Models (LLMs).

Installation
Quickstart
Usage
Loading Tools
- Load a toolset
- Load a single tool
Invoking Tools
Synchronous Usage
Client to Server Authentication
Authenticating Tools
Binding Parameter Values
Using with Orchestration Frameworks
Contributing
License
Support

Installation

go get github.com/googleapis/mcp-toolbox-sdk-go

This SDK is supported on Go version 1.24.2 and higher.

Note

While the SDK itself is synchronous, you can execute its functions within goroutines to achieve asynchronous behavior.

Quickstart

Here's a minimal example to get you started. Ensure your Toolbox service is running and accessible.

package main

import (
	"context"
	"fmt"
	"github.com/googleapis/mcp-toolbox-sdk-go/core"
)

func quickstart() string {
	ctx := context.Background()
	inputs := map[string]any{"location": "London"}
	client, err := core.NewToolboxClient("http://localhost:5000")
	if err != nil {
		return fmt.Sprintln("Could not start Toolbox Client", err)
	}
	tool, err := client.LoadTool("get_weather", ctx)
	if err != nil {
		return fmt.Sprintln("Could not load Toolbox Tool", err)
	}
	result, err := tool.Invoke(ctx, inputs)
	if err != nil {
		return fmt.Sprintln("Could not invoke tool", err)
	}
	return fmt.Sprintln(result)
}

func main() {
	fmt.Println(quickstart())
}

Usage

Import and initialize a Toolbox client, pointing it to the URL of your running Toolbox service.

import "github.com/googleapis/mcp-toolbox-sdk-go/core"

client, err := core.NewToolboxClient("http://localhost:5000")

All interactions for loading and invoking tools happen through this client.

Note

For advanced use cases, you can provide an external custom http.Client during initialization (e.g., core.NewToolboxClient(URL, core.WithHTTPClient(myClient)). If you provide your own session, you are responsible for managing its lifecycle; ToolboxClient will not close it.

Important

Closing the ToolboxClient also closes the underlying network session shared by all tools loaded from that client. As a result, any tool instances you have loaded will cease to function and will raise an error if you attempt to invoke them after the client is closed.

Loading Tools

You can load tools individually or in groups (toolsets) as defined in your Toolbox service configuration. Loading a toolset is convenient when working with multiple related functions, while loading a single tool offers more granular control.

Load a toolset

A toolset is a collection of related tools. You can load all tools in a toolset or a specific one:

// Load default toolset by providing an empty string as the name
tools, err := client.LoadToolset("", ctx)

// Load a specific toolset
tools, err := client.LoadToolset("my-toolset", ctx)

Load a single tool

Loads a specific tool by its unique name. This provides fine-grained control.

tool, err = client.LoadTool("my-tool", ctx)

Invoking Tools

Once loaded, tools behave like Go structs. You invoke them using Invoke method by passing arguments corresponding to the parameters defined in the tool's configuration within the Toolbox service.

tool, err = client.LoadTool("my-tool", ctx)
inputs := map[string]any{"location": "London"}
result, err := tool.Invoke(ctx, inputs)

Tip

For a more comprehensive guide on setting up the Toolbox service itself, which you'll need running to use this SDK, please refer to the Toolbox Quickstart Guide.

Client to Server Authentication

This section describes how to authenticate the ToolboxClient itself when connecting to a Toolbox server instance that requires authentication. This is crucial for securing your Toolbox server endpoint, especially when deployed on platforms like Cloud Run, GKE, or any environment where unauthenticated access is restricted.

This client-to-server authentication ensures that the Toolbox server can verify the identity of the client making the request before any tool is loaded or called. It is different from Authenticating Tools, which deals with providing credentials for specific tools within an already connected Toolbox session.

When is Client-to-Server Authentication Needed?

You'll need this type of authentication if your Toolbox server is configured to deny unauthenticated requests. For example:

Your Toolbox server is deployed on Cloud Run and configured to "Require authentication."
Your server is behind an Identity-Aware Proxy (IAP) or a similar authentication layer.
You have custom authentication middleware on your self-hosted Toolbox server.

Without proper client authentication in these scenarios, attempts to connect or make calls (like LoadTool) will likely fail with Unauthorized errors.

How it works

The ToolboxClient allows you to specify TokenSources that dynamically generate HTTP headers for every request sent to the Toolbox server. The most common use case is to add an Authorization header with a bearer token (e.g., a Google ID token).

These header-generating functions are called just before each request, ensuring that fresh credentials or header values can be used.

Configuration

You can configure these dynamic headers as seen below:

import "github.com/googleapis/mcp-toolbox-sdk-go/core"

tokenProvider := func() string {
	return "header3_value"
}

staticTokenSource := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: "header2_value"})
dynamicTokenSource := core.NewCustomTokenSource(tokenProvider)

client, err := core.NewToolboxClient(
  "toolbox-url",
  core.WithClientHeaderString("header1", "header1_value"),
  core.WithClientHeaderTokenSource("header2", staticTokenSource),
  core.WithClientHeaderTokenSource("header3", dynamicTokenSource),
)

Authenticating with Google Cloud Servers

For Toolbox servers hosted on Google Cloud (e.g., Cloud Run) and requiring Google ID token authentication, the helper module auth_methods provides utility functions.

Step by Step Guide for Cloud Run

Configure Permissions: Grant the roles/run.invoker IAM role on the Cloud Run service to the principal. This could be your user account email or a service account.
Configure Credentials
- Local Development: Set up ADC.
- Google Cloud Environments: When running within Google Cloud (e.g., Compute Engine, GKE, another Cloud Run service, Cloud Functions), ADC is typically configured automatically, using the environment's default service account.

Connect to the Toolbox Server

import "github.com/googleapis/mcp-toolbox-sdk-go/core"
import "context"

ctx := context.Background()

token, err := core.GetGoogleIDToken(ctx, URL)

client, err := core.NewToolboxClient(
  URL,
  core.WithClientHeaderString("Authorization", token),
)

// Now, you can use the client as usual.

Authenticating Tools

Warning

Always use HTTPS to connect your application with the Toolbox service, especially in production environments or whenever the communication involves sensitive data (including scenarios where tools require authentication tokens). Using plain HTTP lacks encryption and exposes your application and data to significant security risks, such as eavesdropping and tampering.

Tools can be configured within the Toolbox service to require authentication, ensuring only authorized users or applications can invoke them, especially when accessing sensitive data.

When is Authentication Needed?

Authentication is configured per-tool within the Toolbox service itself. If a tool you intend to use is marked as requiring authentication in the service, you must configure the SDK client to provide the necessary credentials (currently Oauth2 tokens) when invoking that specific tool.

Supported Authentication Mechanisms

The Toolbox service enables secure tool usage through Authenticated Parameters. For detailed information on how these mechanisms work within the Toolbox service and how to configure them, please refer to Toolbox Service Documentation - Authenticated Parameters.

Step 1: Configure Tools in Toolbox Service

First, ensure the target tool(s) are configured correctly in the Toolbox service to require authentication. Refer to the Toolbox Service Documentation - Authenticated Parameters for instructions.

Step 2: Configure SDK Client

Your application needs a way to obtain the required Oauth2 token for the authenticated user. The SDK requires you to provide a function capable of retrieving this token when the tool is invoked.

Provide an ID Token Retriever Function

You must provide the SDK with a function that returns the necessary token when called. The implementation depends on your application's authentication flow (e.g., retrieving a stored token, initiating an OAuth flow).

Important

The name used when registering the getter function with the SDK (e.g., "my_api_token") must exactly match the name of the corresponding authServices defined in the tool's configuration within the Toolbox service.

func getAuthToken() string {
  // ... Logic to retrieve ID token (e.g., from local storage, OAuth flow)
  // This example just returns a placeholder. Replace with your actual token retrieval.
	return "YOUR_ID_TOKEN" // Placeholder
}

Tip

Your token retriever function is invoked every time an authenticated parameter requires a token for a tool call. Consider implementing caching logic within this function to avoid redundant token fetching or generation, especially for tokens with longer validity periods or if the retrieval process is resource-intensive.

Option A: Add Default Authentication to a Client

You can add default tool level authentication to a client. Every tool / toolset loaded by the client will contain the auth token.

ctx := context.Background()

client, err := core.NewToolboxClient("http://127.0.0.1:5000",
	core.WithDefaultToolOptions(
		core.WithAuthTokenString("my-auth-1", "auth-value"),
	),
)

AuthTool, err := client.LoadTool("my-tool", ctx)

Option B: Add Authentication to a Loaded Tool

You can add the token retriever function to a tool object after it has been loaded. This modifies the specific tool instance.

ctx := context.Background()

client, err := core.NewToolboxClient("http://127.0.0.1:5000")

tool, err := client.LoadTool("my-tool", ctx)

AuthTool, err := tool.ToolFrom(
  core.WithAuthTokenSource("my-auth", headerTokenSource),
  core.WithAuthTokenString("my-auth-1", "value"),
  )

Option C: Add Authentication While Loading Tools

You can provide the token retriever(s) directly during the LoadTool or LoadToolset calls. This applies the authentication configuration only to the tools loaded in that specific call, without modifying the original tool objects if they were loaded previously.

AuthTool, err := client.LoadTool("my-tool", ctx, core.WithAuthTokenString("my-auth-1", "value"))

// or

AuthTools, err := client.LoadToolset(
  "my-toolset",
  ctx,
  core.WithAuthTokenString("my-auth-1", "value"),
)

Note

Adding auth tokens during loading only affect the tools loaded within that call.

Complete Authentication Example

import "github.com/googleapis/mcp-toolbox-sdk-go/core"
import "fmt"

func getAuthToken() string {
  // ... Logic to retrieve ID token (e.g., from local storage, OAuth flow)
  // This example just returns a placeholder. Replace with your actual token retrieval.
	return "YOUR_ID_TOKEN" // Placeholder
}

func main() {
  ctx := context.Background()
  inputs := map[string]any{"input": "some input"}

  dynamicTokenSource := core.NewCustomTokenSource(getAuthToken)

  client, err := core.NewToolboxClient("http://127.0.0.1:5000")
  tool, err := client.LoadTool("my-tool", ctx)
  AuthTool, err := tool.ToolFrom(core.WithAuthTokenSource("my_auth", dynamicTokenSource))

  result, err := AuthTool.Invoke(ctx, inputs)

  fmt.Println(result)
}

Note

An auth token getter for a specific name (e.g., "GOOGLE_ID") will replace any client header with the same name followed by "_token" (e.g., "GOOGLE_ID_token").

Binding Parameter Values

The SDK allows you to pre-set, or "bind", values for specific tool parameters before the tool is invoked or even passed to an LLM. These bound values are fixed and will not be requested or modified by the LLM during tool use.

Why Bind Parameters?

Protecting sensitive information: API keys, secrets, etc.
Enforcing consistency: Ensuring specific values for certain parameters.
Pre-filling known data: Providing defaults or context.

Important

The parameter names used for binding (e.g., "api_key") must exactly match the parameter names defined in the tool's configuration within the Toolbox service.

Note

You do not need to modify the tool's configuration in the Toolbox service to bind parameter values using the SDK.

Option A: Add Default Bound Parameters to a Client

You can add default tool level bound parameters to a client. Every tool / toolset
loaded by the client will have the bound parameter.

ctx := context.Background()

client, err := core.NewToolboxClient("http://127.0.0.1:5000",
	core.WithDefaultToolOptions(
		core.WithBindParamString("param1", "value"),
	),
)

boundTool, err := client.LoadTool("my-tool", ctx)

Option B: Binding Parameters to a Loaded Tool

Bind values to a tool object after it has been loaded. This modifies the specific tool instance.

client, err := core.NewToolboxClient("http://127.0.0.1:5000")

tool, err := client.LoadTool("my-tool", ctx)

boundTool, err := tool.ToolFrom(
  core.WithBindParamString("param1", "value"),
  core.WithBindParamString("param2", "value")
  )

Option C: Binding Parameters While Loading Tools

Specify bound parameters directly when loading tools. This applies the binding only to the tools loaded in that specific call.

boundTool, err := client.LoadTool("my-tool", ctx, core.WithBindParamString("param", "value"))

// OR

boundTool, err := client.LoadToolset("", ctx, core.WithBindParamString("param", "value"))

Note

Bound values during loading only affect the tools loaded in that call.

Binding Dynamic Values

Instead of a static value, you can bind a parameter to a synchronous or asynchronous function. This function will be called each time the tool is invoked to dynamically determine the parameter's value at runtime. Functions with the return type (data_type, error) can be provided.

getDynamicValue := func() (string, error) { return "req-123", nil }

dynamicBoundTool, err := tool.ToolFrom(core.WithBindParamStringFunc("param", getDynamicValue))

Important

You don't need to modify tool configurations to bind parameter values.

Using with Orchestration Frameworks

To see how the MCP Toolbox Go SDK works with orchestration frameworks, check out the end-to-end examples in the /samples/ folder.

Use the tbgenkit package to convert Toolbox Tools into Genkit compatible tools.

Contributing

Contributions are welcome! Please refer to the DEVELOPER.md file for guidelines on how to set up a development environment and run tests.

License

This project is licensed under the Apache License 2.0. See the LICENSE file for details.

Support

If you encounter issues or have questions, check the existing GitHub Issues for the main Toolbox project.

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