quickstart-resources/mcp-client-go/main.go

package main

import (
	"bufio"
	"context"
	"encoding/json"
	"fmt"
	"log"
	"os"
	"os/exec"
	"strings"

	"github.com/anthropics/anthropic-sdk-go"
	"github.com/anthropics/anthropic-sdk-go/option"
	"github.com/google/jsonschema-go/jsonschema"
	"github.com/joho/godotenv"
	"github.com/modelcontextprotocol/go-sdk/mcp"
)

var model anthropic.Model = "claude-sonnet-5"

// Sonnet 5 thinks adaptively unless told otherwise, and max_tokens caps thinking
// plus the reply, so leave room for both.
const maxTokens = 10000
const maxToolTurns = 10

type MCPClient struct {
	anthropic *anthropic.Client
	session   *mcp.ClientSession
	tools     []anthropic.ToolUnionParam
	// Compiled outputSchema per tool name, for validating results.
	outputSchemas map[string]*jsonschema.Resolved
}

func NewMCPClient() (*MCPClient, error) {
	// .env is optional; the key may come from the environment instead.
	_ = godotenv.Load()

	client := anthropic.NewClient(option.WithAPIKey(os.Getenv("ANTHROPIC_API_KEY")))

	return &MCPClient{
		anthropic: &client,
	}, nil
}

func (c *MCPClient) ConnectToServer(ctx context.Context, serverArgs []string) error {
	if len(serverArgs) == 0 {
		return fmt.Errorf("no server command provided")
	}

	// Create command to spawn server process
	cmd := exec.CommandContext(ctx, serverArgs[0], serverArgs[1:]...)

	// Create MCP client
	client := mcp.NewClient(
		&mcp.Implementation{
			Name:    "mcp-client-go",
			Version: "0.1.0",
		},
		nil,
	)

	// Connect using CommandTransport
	transport := &mcp.CommandTransport{
		Command: cmd,
	}

	session, err := client.Connect(ctx, transport, nil)
	if err != nil {
		return fmt.Errorf("failed to connect to server: %w", err)
	}

	c.session = session

	// List available tools
	toolsResult, err := session.ListTools(ctx, &mcp.ListToolsParams{})
	if err != nil {
		return fmt.Errorf("failed to list tools: %w", err)
	}

	var toolNames []string

	// Convert MCP tools to Anthropic tool format
	c.outputSchemas = make(map[string]*jsonschema.Resolved)
	for _, tool := range toolsResult.Tools {
		toolNames = append(toolNames, tool.Name)
		anthropicTool, err := mcpToolToAnthropicTool(tool)
		if err != nil {
			return fmt.Errorf("failed to convert mcp tool to anthropic tool: %w", err)
		}
		c.tools = append(c.tools, anthropicTool)

		resolved, err := resolveOutputSchema(tool)
		if err != nil {
			return fmt.Errorf("failed to compile output schema of tool %s: %w", tool.Name, err)
		}
		if resolved != nil {
			c.outputSchemas[tool.Name] = resolved
		}
	}

	fmt.Printf("Connected to server with tools: %v\n", toolNames)
	return nil
}

func mcpToolToAnthropicTool(tool *mcp.Tool) (anthropic.ToolUnionParam, error) {
	var zeroTool anthropic.ToolUnionParam
	schemaJSON, err := json.Marshal(tool.InputSchema)
	if err != nil {
		return zeroTool, fmt.Errorf("failed to marshal input schema of mcp tool: %w", err)
	}
	var schema anthropic.ToolInputSchemaParam
	err = json.Unmarshal(schemaJSON, &schema)
	if err != nil {
		return zeroTool, fmt.Errorf("failed to unmarshal to anthropic input schema: %w", err)
	}

	toolParam := anthropic.ToolParam{
		Name:        tool.Name,
		Description: anthropic.String(tool.Description),
		InputSchema: schema,
	}

	return anthropic.ToolUnionParam{
		OfTool: &toolParam,
	}, nil
}

// resolveOutputSchema compiles a tool's declared outputSchema, or returns
// (nil, nil) when it declares none. OutputSchema is `any` because the root may
// be any JSON Schema, so round-trip through JSON to accept every form.
func resolveOutputSchema(tool *mcp.Tool) (*jsonschema.Resolved, error) {
	if tool.OutputSchema == nil {
		return nil, nil
	}
	raw, err := json.Marshal(tool.OutputSchema)
	if err != nil {
		return nil, fmt.Errorf("failed to marshal output schema: %w", err)
	}
	var schema jsonschema.Schema
	if err := json.Unmarshal(raw, &schema); err != nil {
		return nil, fmt.Errorf("failed to unmarshal output schema: %w", err)
	}
	return schema.Resolve(nil)
}

// validateToolOutput checks a result against its tool's declared outputSchema.
// Error results are exempt: they carry a message, not data.
func (c *MCPClient) validateToolOutput(name string, result *mcp.CallToolResult) error {
	resolved, ok := c.outputSchemas[name]
	if !ok || result.IsError {
		return nil
	}
	if result.StructuredContent == nil {
		return fmt.Errorf("tool %s declares an output schema but returned no structured content", name)
	}
	if err := resolved.Validate(result.StructuredContent); err != nil {
		return fmt.Errorf("structured content from tool %s does not match its output schema: %w", name, err)
	}
	return nil
}

func (c *MCPClient) ProcessQuery(ctx context.Context, query string) (string, error) {
	if c.session == nil {
		return "", fmt.Errorf("client is not connected to any server")
	}

	messages := []anthropic.MessageParam{
		anthropic.NewUserMessage(anthropic.NewTextBlock(query)),
	}

	var finalText []string

	// Initial Claude API call with tools
	response, err := c.createMessage(ctx, messages)
	if err != nil {
		return "", err
	}

	// Keep calling tools until Claude answers without one, up to a cap.
	for turn := 0; turn < maxToolTurns; turn++ {
		var toolUseBlocks []anthropic.ToolUseBlock
		for _, block := range response.Content {
			switch b := block.AsAny().(type) {
			case anthropic.TextBlock:
				finalText = append(finalText, b.Text)
			case anthropic.ToolUseBlock:
				toolUseBlocks = append(toolUseBlocks, b)
			}
		}

		if len(toolUseBlocks) == 0 {
			return strings.Join(finalText, "\n"), nil
		}

		// Execute every tool call in this response and collect the results
		var anthropicToolResults []anthropic.ContentBlockParamUnion
		for _, toolUseBlock := range toolUseBlocks {
			// Add information about the tool call to final text
			finalText = append(finalText, fmt.Sprintf("[Calling tool %s with args %s]", toolUseBlock.Name, string(toolUseBlock.Input)))

			// Call the MCP server tool
			mcpToolResult, err := c.session.CallTool(ctx, &mcp.CallToolParams{
				Name:      toolUseBlock.Name,
				Arguments: toolUseBlock.Input,
			})
			if err != nil {
				return "", fmt.Errorf("tool call %s failed: %w", toolUseBlock.Name, err)
			}

			if err := c.validateToolOutput(toolUseBlock.Name, mcpToolResult); err != nil {
				return "", err
			}

			// StructuredContent is data the application can use directly.
			if items, ok := mcpToolResult.StructuredContent.([]any); ok {
				finalText = append(finalText, fmt.Sprintf("[%s returned %d items]", toolUseBlock.Name, len(items)))
			}

			// Content is a list of block types; forward only the text ones.
			var texts []string
			for _, block := range mcpToolResult.Content {
				if text, ok := block.(*mcp.TextContent); ok {
					texts = append(texts, text.Text)
				}
			}

			anthropicToolResults = append(anthropicToolResults, anthropic.NewToolResultBlock(
				toolUseBlock.ID,
				strings.Join(texts, "\n"),
				mcpToolResult.IsError,
			))
		}

		// Append the assistant's response and all tool results, in one user
		// message, to the history. Every tool_use needs a matching tool_result.
		messages = append(messages, response.ToParam())
		messages = append(messages, anthropic.NewUserMessage(anthropicToolResults...))

		response, err = c.createMessage(ctx, messages)
		if err != nil {
			return "", err
		}
	}

	// The turn cap was hit. Keep the last response's text. If it asked for
	// more tools, say they were not run.
	wantsTools := false
	for _, block := range response.Content {
		switch b := block.AsAny().(type) {
		case anthropic.TextBlock:
			finalText = append(finalText, b.Text)
		case anthropic.ToolUseBlock:
			wantsTools = true
		}
	}
	if wantsTools {
		finalText = append(finalText, fmt.Sprintf("[Stopped after %d tool-use turns]", maxToolTurns))
	}

	return strings.Join(finalText, "\n"), nil
}

// createMessage calls Claude with the current history. Tools are passed on
// every call so Claude can keep calling them across turns.
func (c *MCPClient) createMessage(ctx context.Context, messages []anthropic.MessageParam) (*anthropic.Message, error) {
	response, err := c.anthropic.Messages.New(ctx, anthropic.MessageNewParams{
		Model:     model,
		MaxTokens: maxTokens,
		Messages:  messages,
		Tools:     c.tools,
	})
	if err != nil {
		return nil, fmt.Errorf("anthropic API request failed: %w", err)
	}
	return response, nil
}

func (c *MCPClient) ChatLoop(ctx context.Context) error {
	fmt.Println("\nMCP Client Started!")
	fmt.Println("Type your queries or 'quit' to exit.")

	scanner := bufio.NewScanner(os.Stdin)

	for {
		fmt.Print("\nQuery: ")
		if !scanner.Scan() {
			break // EOF
		}

		query := strings.TrimSpace(scanner.Text())
		if strings.EqualFold(query, "quit") {
			break
		}
		if query == "" {
			continue
		}

		response, err := c.ProcessQuery(ctx, query)
		if err != nil {
			fmt.Printf("\nError: %v\n", err)
			continue
		}

		fmt.Printf("\n%s\n", response)
	}

	return scanner.Err()
}

func (c *MCPClient) Cleanup() error {
	if c.session != nil {
		if err := c.session.Close(); err != nil {
			return fmt.Errorf("failed to close session: %w", err)
		}
		c.session = nil
	}
	return nil
}

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintln(os.Stderr, "Usage: go run main.go <server_script_or_binary> [args...]")
		os.Exit(1)
	}

	serverArgs := os.Args[1:]

	client, err := NewMCPClient()
	if err != nil {
		log.Fatalf("Failed to create MCP client: %v", err)
	}

	ctx := context.Background()

	if err := client.ConnectToServer(ctx, serverArgs); err != nil {
		log.Fatalf("Failed to connect to MCP server: %v", err)
	}

	// Connecting and listing tools needs no credentials; querying them does.
	// Matching the Python and TypeScript clients, report and exit rather than
	// failing, so the connection itself can be exercised without a key.
	if os.Getenv("ANTHROPIC_API_KEY") == "" {
		fmt.Println("\nNo ANTHROPIC_API_KEY found. To query these tools with Claude, set your API key:")
		fmt.Println("  export ANTHROPIC_API_KEY=your-api-key-here")
		if err := client.Cleanup(); err != nil {
			log.Printf("Cleanup error: %v", err)
		}
		return
	}

	if err := client.ChatLoop(ctx); err != nil {
		log.Printf("ChatLoop error: %v", err)
	}

	if err := client.Cleanup(); err != nil {
		log.Printf("Cleanup error: %v", err)
	}
}