A Go implementation of the Model Context Protocol (MCP), enabling seamless integration between LLM applications and external data sources and tools. Discuss the SDK on go package main import ( "context" "fmt" "github.com/mark3labs/mcp-go/mcp" "github.com/mark3labs/mcp-go/server" ) func main() { // Create a new MCP server s := server.NewMCPServer( "Demo ", "1.0.0", server.WithToolCapabilities(false
2 tools
Tools
tools/list
Tool
What it does
A Go implementation of the Model Context Protocol (MCP), enabling seamless integration between LLM applications and external data sources and tools.
Copy the stdio config for Cursor, Claude Desktop, Claude Code, or VS Code on this page. It uses the publisher's install, not a PlugMCP proxy.
Does MCP Go need an API key?
No required environment variable is listed on this record.
What transport does MCP Go use?
MCP Go runs locally over stdio.
When was MCP Go last checked?
Last checked 2026-11-08. A Go implementation of the Model Context Protocol (MCP), enabling seamless integration between LLM applications and external data sources and tools. Discuss the SDK on go package main import ( "context" "fmt" "github.com/mark3labs/mcp-go/mcp" "github.com/mark3labs/mcp-go/server" ) func main() { // Create a new MCP server s := server.NewMCPServer( "Demo ", "1.0.0", server.WithToolCapabilities(false
From the publisher
Documentation
A Go implementation of the Model Context Protocol (MCP), enabling seamless integration between LLM applications and external data sources and tools.
package main
import (
"context"
"fmt"
"github.com/mark3labs/mcp-go/mcp"
"github.com/mark3labs/mcp-go/server"
)
func main() {
// Create a new MCP server
s := server.NewMCPServer(
"Demo 🚀",
"1.0.0",
server.WithToolCapabilities(false),
)
// Add tool
tool := mcp.NewTool("hello_world",
mcp.WithDescription("Say hello to someone"),
mcp.WithString("name",
mcp.Required(),
mcp.Description("Name of the person to greet"),
),
)
// Add tool handler
s.AddTool(tool, helloHandler)
// Start the stdio server
if err := server.ServeStdio(s); err != nil {
fmt.Printf("Server error: %v\n", err)
}
}
func helloHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
name, err := request.RequireString("name")
if err != nil {
return mcp.NewToolResultError(err.Error()), nil
}
return mcp.NewToolResultText(fmt.Sprintf("Hello, %s!", name)), nil
}
That's it!
MCP Go handles all the complex protocol details and server management, so you can focus on building great tools. It aims to be high-level and easy to use.
Key features:
Fast: High-level interface means less code and faster development
Simple: Build MCP servers with minimal boilerplate
Complete*: MCP Go aims to provide a full implementation of the core MCP specification
(*emphasis on aims)
🚨 🚧 🏗️ MCP Go is under active development, as is the MCP specification itself. Core features are working but some advanced capabilities are still in progress.
Table of Contents
Key features
01. Fast: High-level interface means less code and faster development
02. Simple: Build MCP servers with minimal boilerplate
03. Complete: MCP Go aims to provide a full implementation of the core MCP specification
Let's create a simple MCP server that exposes a calculator tool and some data:
package main
import (
"context"
"fmt"
"github.com/mark3labs/mcp-go/mcp"
"github.com/mark3labs/mcp-go/server"
)
func main() {
// Create a new MCP server
s := server.NewMCPServer(
"Calculator Demo",
"1.0.0",
server.WithToolCapabilities(false),
server.WithRecovery(),
)
// Add a calculator tool
calculatorTool := mcp.NewTool("calculate",
mcp.WithDescription("Perform basic arithmetic operations"),
mcp.WithString("operation",
mcp.Required(),
mcp.Description("The operation to perform (add, subtract, multiply, divide)"),
mcp.Enum("add", "subtract", "multiply", "divide"),
),
mcp.WithNumber("x",
mcp.Required(),
mcp.Description("First number"),
),
mcp.WithNumber("y",
mcp.Required(),
mcp.Description("Second number"),
),
)
// Add the calculator handler
s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
// Using helper functions for type-safe argument access
op, err := request.RequireString("operation")
if err != nil {
return mcp.NewToolResultError(err.Error()), nil
}
x, err := request.RequireFloat("x")
if err != nil {
return mcp.NewToolResultError(err.Error()), nil
}
y, err := request.RequireFloat("y")
if err != nil {
return mcp.NewToolResultError(err.Error()), nil
}
var result float64
switch op {
case "add":
result = x + y
case "subtract":
result = x - y
case "multiply":
result = x * y
case "divide":
if y == 0 {
return mcp.NewToolResultError("cannot divide by zero"), nil
}
result = x / y
}
return mcp.NewToolResultText(fmt.Sprintf("%.2f", result)), nil
})
// Start the server
if err := server.ServeStdio(s); err != nil {
fmt.Printf("Server error: %v\n", err)
}
}
What is MCP?
The Model Context Protocol (MCP) lets you build servers that expose data and functionality to LLM applications in a secure, standardized way. Think of it like a web API, but specifically designed for LLM interactions.
MCP servers can:
Expose data through Resources (think of these sort of like GET endpoints; they are used to load information into the LLM's context)
Provide functionality through Tools (sort of like POST endpoints; they are used to execute code or otherwise produce a side effect)
Define interaction patterns through Prompts (reusable templates for LLM interactions)
And more!
mcp-go implements the Model Context Protocol specification version 2025-11-25, with backward compatibility for versions 2025-06-18, 2025-03-26, and 2024-11-05.
Core Concepts
Server
The server is your core interface to the MCP protocol. It handles connection management, protocol compliance, and message routing:
// Create a basic server
s := server.NewMCPServer(
"My Server", // Server name
"1.0.0", // Version
)
// Start the server using stdio
if err := server.ServeStdio(s); err != nil {
log.Fatalf("Server error: %v", err)
}
And here's an example of a dynamic resource using a template:
// Dynamic resource example - user profiles by ID
template := mcp.NewResourceTemplate(
"users://{id}/profile",
"User Profile",
mcp.WithTemplateDescription("Returns user profile information"),
mcp.WithTemplateMIMEType("application/json"),
)
// Add template with its handler
s.AddResourceTemplate(template, func(ctx context.Context, request mcp.ReadResourceRequest) ([]mcp.ResourceContents, error) {
// Extract ID from the URI using regex matching
// The server automatically matches URIs to templates
userID := extractIDFromURI(request.Params.URI)
profile, err := getUserProfile(userID) // Your DB/API call here
if err != nil {
return nil, err
}
return []mcp.ResourceContents{
mcp.TextResourceContents{
URI: request.Params.URI,
MIMEType: "application/json",
Text: profile,
},
}, nil
})
The examples are simple but demonstrate the core concepts. Resources can be much more sophisticated - serving multiple contents, integrating with databases or external APIs, etc.
Tools
Tools let LLMs take actions through your server. Unlike resources, tools are expected to perform computation and have side effects. They're similar to POST endpoints in a REST API.
Task-Augmented Tools
Task-augmented tools execute asynchronously and return results via polling. This is useful for long-running operations that would otherwise block or time out. Task tools support three modes:
TaskSupportForbidden (default): The tool cannot be invoked as a task
TaskSupportOptional: The tool can be invoked as a task or synchronously
TaskSupportRequired: The tool must be invoked as a task
// Example: A tool that requires task execution
processBatchTool := mcp.NewTool("process_batch",
mcp.WithDescription("Process a batch of items asynchronously"),
mcp.WithTaskSupport(mcp.TaskSupportRequired),
mcp.WithArray("items",
mcp.Description("Array of items to process"),
mcp.WithStringItems(),
mcp.Required(),
),
)
// Task tool handler returns CreateTaskResult instead of CallToolResult
s.AddTaskTool(processBatchTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CreateTaskResult, error) {
items := request.GetStringSlice("items", []string{})
// Long-running work here
for i, item := range items {
select {
case <-ctx.Done():
// Task was cancelled
return nil, ctx.Err()
default:
// Process item...
processItem(item)
}
}
// Return result - task ID and metadata are managed by the server
return &mcp.CreateTaskResult{
Task: mcp.Task{
// Task fields (ID, status, etc.) are populated by the server
},
}, nil
})
// Enable task capabilities when creating the server
s := server.NewMCPServer(
"Task Server",
"1.0.0",
server.WithTaskCapabilities(
true, // listTasks: allows clients to list all tasks
true, // cancel: allows clients to cancel running tasks
true, // toolCallTasks: enables task augmentation for tools
),
server.WithMaxConcurrentTasks(10), // Optional: limit concurrent running tasks
)
Task execution flow:
Client calls tool with task parameter
Server immediately returns task ID
Tool executes asynchronously in the background
Client polls tasks/result to retrieve the result
Server sends task status notifications on completion
For optional task tools, the same tool can be called synchronously (without task parameter) or asynchronously (with task parameter):
// Tool with optional task support
analyzeTool := mcp.NewTool("analyze_data",
mcp.WithDescription("Analyze data - can run sync or async"),
mcp.WithTaskSupport(mcp.TaskSupportOptional),
mcp.WithString("data", mcp.Required()),
)
// Use AddTaskTool for hybrid tools that support both modes
s.AddTaskTool(analyzeTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CreateTaskResult, error) {
// This handler runs when called as a task
data := request.GetString("data", "")
result := analyzeData(data)
return &mcp.CreateTaskResult{
Task: mcp.Task{},
}, nil
})
// The server automatically handles both sync and async invocations
// When called without task param: executes handler and returns immediately
// When called with task param: executes handler asynchronously
Limiting Concurrent Tasks
To prevent resource exhaustion, you can limit the number of concurrent running tasks:
s := server.NewMCPServer(
"Task Server",
"1.0.0",
server.WithTaskCapabilities(true, true, true),
server.WithMaxConcurrentTasks(10), // Allow up to 10 concurrent running tasks
)
When the limit is reached, new task creation requests will fail with an error. Completed, failed, or cancelled tasks don't count toward the limit - only tasks in "working" status. If WithMaxConcurrentTasks is not specified or set to 0, there is no limit on concurrent tasks.
For traditional synchronous tools that execute and return results immediately:
Simple calculation example:
calculatorTool := mcp.NewTool("calculate",
mcp.WithDescription("Perform basic arithmetic calculations"),
mcp.WithString("operation",
mcp.Required(),
mcp.Description("The arithmetic operation to perform"),
mcp.Enum("add", "subtract", "multiply", "divide"),
),
mcp.WithNumber("x",
mcp.Required(),
mcp.Description("First number"),
),
mcp.WithNumber("y",
mcp.Required(),
mcp.Description("Second number"),
),
)
s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
args := request.GetArguments()
op := args["operation"].(string)
x := args["x"].(float64)
y := args["y"].(float64)
var result float64
switch op {
case "add":
result = x + y
case "subtract":
result = x - y
case "multiply":
result = x * y
case "divide":
if y == 0 {
return mcp.NewToolResultError("cannot divide by zero"), nil
}
result = x / y
}
return mcp.FormatNumberResult(result), nil
})
HTTP request example:
httpTool := mcp.NewTool("http_request",
mcp.WithDescription("Make HTTP requests to external APIs"),
mcp.WithString("method",
mcp.Required(),
mcp.Description("HTTP method to use"),
mcp.Enum("GET", "POST", "PUT", "DELETE"),
),
mcp.WithString("url",
mcp.Required(),
mcp.Description("URL to send the request to"),
mcp.Pattern("^https?://.*"),
),
mcp.WithString("body",
mcp.Description("Request body (for POST/PUT)"),
),
)
s.AddTool(httpTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
args := request.GetArguments()
method := args["method"].(string)
url := args["url"].(string)
body := ""
if b, ok := args["body"].(string); ok {
body = b
}
// Create and send request
var req *http.Request
var err error
if body != "" {
req, err = http.NewRequest(method, url, strings.NewReader(body))
} else {
req, err = http.NewRequest(method, url, nil)
}
if err != nil {
return mcp.NewToolResultErrorFromErr("unable to create request", err), nil
}
client := &http.Client{}
resp, err := client.Do(req)
if err != nil {
return mcp.NewToolResultErrorFromErr("unable to execute request", err), nil
}
defer resp.Body.Close()
// Return response
respBody, err := io.ReadAll(resp.Body)
if err != nil {
return mcp.NewToolResultErrorFromErr("unable to read request response", err), nil
}
return mcp.NewToolResultText(fmt.Sprintf("Status: %d\nBody: %s", resp.StatusCode, string(respBody))), nil
})
Tools can be used for any kind of computation or side effect:
Database queries
File operations
External API calls
Calculations
System operations
Each tool should:
Have a clear description
Validate inputs
Handle errors gracefully
Return structured responses
Use appropriate result types
Prompts
Prompts are reusable templates that help LLMs interact with your server effectively. They're like "best practices" encoded into your server. Here are some examples:
// Simple greeting prompt
s.AddPrompt(mcp.NewPrompt("greeting",
mcp.WithPromptDescription("A friendly greeting prompt"),
mcp.WithArgument("name",
mcp.ArgumentDescription("Name of the person to greet"),
),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
name := request.Params.Arguments["name"]
if name == "" {
name = "friend"
}
return mcp.NewGetPromptResult(
"A friendly greeting",
[]mcp.PromptMessage{
mcp.NewPromptMessage(
mcp.RoleAssistant,
mcp.NewTextContent(fmt.Sprintf("Hello, %s! How can I help you today?", name)),
),
},
), nil
})
// Code review prompt with embedded resource
s.AddPrompt(mcp.NewPrompt("code_review",
mcp.WithPromptDescription("Code review assistance"),
mcp.WithArgument("pr_number",
mcp.ArgumentDescription("Pull request number to review"),
mcp.RequiredArgument(),
),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
prNumber := request.Params.Arguments["pr_number"]
if prNumber == "" {
return nil, fmt.Errorf("pr_number is required")
}
return mcp.NewGetPromptResult(
"Code review assistance",
[]mcp.PromptMessage{
mcp.NewPromptMessage(
mcp.RoleUser,
mcp.NewTextContent("Review the changes and provide constructive feedback."),
),
mcp.NewPromptMessage(
mcp.RoleAssistant,
mcp.NewEmbeddedResource(mcp.ResourceContents{
URI: fmt.Sprintf("git://pulls/%s/diff", prNumber),
MIMEType: "text/x-diff",
}),
),
},
), nil
})
// Database query builder prompt
s.AddPrompt(mcp.NewPrompt("query_builder",
mcp.WithPromptDescription("SQL query builder assistance"),
mcp.WithArgument("table",
mcp.ArgumentDescription("Name of the table to query"),
mcp.RequiredArgument(),
),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
tableName := request.Params.Arguments["table"]
if tableName == "" {
return nil, fmt.Errorf("table name is required")
}
return mcp.NewGetPromptResult(
"SQL query builder assistance",
[]mcp.PromptMessage{
mcp.NewPromptMessage(
mcp.RoleUser,
mcp.NewTextContent("Help construct efficient and safe queries for the provided schema."),
),
mcp.NewPromptMessage(
mcp.RoleUser,
mcp.NewEmbeddedResource(mcp.ResourceContents{
URI: fmt.Sprintf("db://schema/%s", tableName),
MIMEType: "application/json",
}),
),
},
), nil
})
Additional examples covering resources, prompts, and more in the examples directory
Extras
Transports
MCP-Go supports stdio, SSE and streamable-HTTP transport layers. For SSE transport, you can use SetConnectionLostHandler() to detect and handle disconnections for implementing reconnection logic.
Embedding StreamableHTTP in non-net/http frameworks
StreamableHTTPServer is an http.Handler, so it can be mounted in any
router that speaks net/http. To embed it in a framework that does not
go through net/http (e.g. fasthttp
or fiber) without buffering the response through an
adaptor, use the transport-agnostic Handle entry point:
func (s *StreamableHTTPServer) Handle(w HTTPResponseWriter, r *HTTPRequest)
HTTPRequest is a plain struct (Method, URL, Header, Body,
Context) and HTTPResponseWriter is a small interface (Header,
WriteHeader, Write, Flush, CanStream). Implementations whose
underlying transport cannot stream MUST return false from CanStream;
the server will then reject GET (SSE listening) with 405 Method Not Allowed and keep POST responses as buffered application/json instead of
upgrading to text/event-stream.
See the HTTP transport docs
for a full fasthttp/fiber adapter example. ServeHTTP is unchanged and
remains the conventional net/http entry point.
OAuth Protected Resource Metadata
Servers that require OAuth can advertise their authorization requirements
via the RFC 9728/.well-known/oauth-protected-resource endpoint referenced by the
MCP authorization spec.
Use server.WithProtectedResourceMetadata (or
server.WithSSEProtectedResourceMetadata) to auto-mount the endpoint, or
server.NewProtectedResourceMetadataHandler to wire it into a custom router.
See the HTTP transport docs for examples.
Servers exposed to browser-based MCP clients can opt into Cross-Origin
Resource Sharing handling on either HTTP transport. CORS is disabled by
default; configure it explicitly via server.WithStreamableHTTPCORS or
server.WithSSECORS:
The transport answers preflight (OPTIONS) requests directly and decorates
simple responses with the appropriate Access-Control-Allow-Origin,
Access-Control-Allow-Credentials, Access-Control-Expose-Headers and
Vary headers. Sensible defaults are used when the corresponding option is
omitted (GET, POST, DELETE, OPTIONS for methods; Content-Type, Mcp-Session-Id, Mcp-Protocol-Version, Mcp-Method, Mcp-Name, Last-Event-ID, Authorization for request headers;
Mcp-Session-Id for exposed headers). Combining WithCORSAllowedOrigins("*")
with WithCORSAllowCredentials() echoes the request Origin to remain
spec-compliant.
DNS rebinding protection for localhost servers
Both HTTP transports automatically protect local servers against
DNS rebinding attacks:
requests arriving over a loopback connection (127.0.0.1, [::1]) whose
Host header is not a localhost value are rejected with 403 Forbidden.
The check is derived from the connection's local address at runtime, so it
applies whether the server listens on localhost or 0.0.0.0, and never
affects requests arriving via non-loopback addresses.
If a reverse proxy on the same host forwards requests via localhost while
preserving the original Host header, configure the proxy to rewrite the
Host header to localhost, or opt out explicitly:
httpServer := server.NewStreamableHTTPServer(mcpServer,
// Or server.WithSSEDisableLocalhostProtection(true) on NewSSEServer.
server.WithDisableLocalhostProtection(true),
)
See the HTTP transport docs
for details, including the caveat for the framework-agnostic Handle entry
point.
Session Management
MCP-Go provides a robust session management system that allows you to:
Maintain separate state for each connected client
Register and track client sessions
Send notifications to specific clients
Provide per-session tool customization
Basic Session Handling
// Create a server with session capabilities
s := server.NewMCPServer(
"Session Demo",
"1.0.0",
server.WithToolCapabilities(true),
)
// Implement your own ClientSession
type MySession struct {
id string
notifChannel chan mcp.JSONRPCNotification
isInitialized bool
// Add custom fields for your application
}
// Implement the ClientSession interface
func (s *MySession) SessionID() string {
return s.id
}
func (s *MySession) NotificationChannel() chan<- mcp.JSONRPCNotification {
return s.notifChannel
}
func (s *MySession) Initialize() {
s.isInitialized = true
}
func (s *MySession) Initialized() bool {
return s.isInitialized
}
// Register a session
session := &MySession{
id: "user-123",
notifChannel: make(chan mcp.JSONRPCNotification, 10),
}
if err := s.RegisterSession(context.Background(), session); err != nil {
log.Printf("Failed to register session: %v", err)
}
// Send notification to a specific client
err := s.SendNotificationToSpecificClient(
session.SessionID(),
"notification/update",
map[string]any{"message": "New data available!"},
)
if err != nil {
log.Printf("Failed to send notification: %v", err)
}
// Unregister session when done
s.UnregisterSession(context.Background(), session.SessionID())
Per-Session Tools
For more advanced use cases, you can implement the SessionWithTools interface to support per-session tool customization:
// Implement SessionWithTools interface for per-session tools
type MyAdvancedSession struct {
MySession // Embed the basic session
sessionTools map[string]server.ServerTool
}
// Implement additional methods for SessionWithTools
func (s *MyAdvancedSession) GetSessionTools() map[string]server.ServerTool {
return s.sessionTools
}
func (s *MyAdvancedSession) SetSessionTools(tools map[string]server.ServerTool) {
s.sessionTools = tools
}
// Create and register a session with tools support
advSession := &MyAdvancedSession{
MySession: MySession{
id: "user-456",
notifChannel: make(chan mcp.JSONRPCNotification, 10),
},
sessionTools: make(map[string]server.ServerTool),
}
if err := s.RegisterSession(context.Background(), advSession); err != nil {
log.Printf("Failed to register session: %v", err)
}
// Add session-specific tools
userSpecificTool := mcp.NewTool(
"user_data",
mcp.WithDescription("Access user-specific data"),
)
// You can use AddSessionTool (similar to AddTool)
err := s.AddSessionTool(
advSession.SessionID(),
userSpecificTool,
func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
// This handler is only available to this specific session
return mcp.NewToolResultText("User-specific data for " + advSession.SessionID()), nil
},
)
if err != nil {
log.Printf("Failed to add session tool: %v", err)
}
// Or use AddSessionTools directly with ServerTool
/*
err := s.AddSessionTools(
advSession.SessionID(),
server.ServerTool{
Tool: userSpecificTool,
Handler: func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
// This handler is only available to this specific session
return mcp.NewToolResultText("User-specific data for " + advSession.SessionID()), nil
},
},
)
if err != nil {
log.Printf("Failed to add session tool: %v", err)
}
*/
// Delete session-specific tools when no longer needed
err = s.DeleteSessionTools(advSession.SessionID(), "user_data")
if err != nil {
log.Printf("Failed to delete session tool: %v", err)
}
Tool Filtering
You can also apply filters to control which tools are available to certain sessions:
// Add a tool filter that only shows tools with certain prefixes
s := server.NewMCPServer(
"Tool Filtering Demo",
"1.0.0",
server.WithToolCapabilities(true),
server.WithToolFilter(func(ctx context.Context, tools []mcp.Tool) []mcp.Tool {
// Get session from context
session := server.ClientSessionFromContext(ctx)
if session == nil {
return tools // Return all tools if no session
}
// Example: filter tools based on session ID prefix
if strings.HasPrefix(session.SessionID(), "admin-") {
// Admin users get all tools
return tools
} else {
// Regular users only get tools with "public-" prefix
var filteredTools []mcp.Tool
for _, tool := range tools {
if strings.HasPrefix(tool.Name, "public-") {
filteredTools = append(filteredTools, tool)
}
}
return filteredTools
}
}),
)
Working with Context
The session context is automatically passed to tool and resource handlers:
s.AddTool(mcp.NewTool("session_aware"), func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
// Get the current session from context
session := server.ClientSessionFromContext(ctx)
if session == nil {
return mcp.NewToolResultError("No active session"), nil
}
return mcp.NewToolResultText("Hello, session " + session.SessionID()), nil
})
// When using handlers in HTTP/SSE servers, you need to pass the context with the session
httpHandler := func(w http.ResponseWriter, r *http.Request) {
// Get session from somewhere (like a cookie or header)
session := getSessionFromRequest(r)
// Add session to context
ctx := s.WithContext(r.Context(), session)
// Use this context when handling requests
// ...
}
Request Hooks
Hook into the request lifecycle by creating a Hooks object with your
selection among the possible callbacks. This enables telemetry across all
functionality, and observability of various facts, for example the ability
to count improperly-formatted requests, or to log the agent identity during
initialization.
Add the Hooks to the server at the time of creation using the
server.WithHooks option.
Tool Handler Middleware
Add middleware to tool call handlers using the server.WithToolHandlerMiddleware option. Middlewares can be registered on server creation and are applied on every tool call.
A recovery middleware option is available to recover from panics in a tool call and can be added to the server with the server.WithRecovery option.
Prompt Handler Middleware
Add middleware to prompt handlers using the server.WithPromptHandlerMiddleware option. Middlewares can be registered on server creation and are applied on every prompts/get call.
Prompt Filtering
Filter prompts based on context using the server.WithPromptFilter option. This works the same way as tool filtering but applies to prompts/list results.
Regenerating Server Code
Server hooks and request handlers are generated. Regenerate them by running:
go generate ./...
You need go installed and the goimports tool available. The generator runs
goimports automatically to format and fix imports.
Auto-completions
When users are filling in argument values for a specific prompt (identified by name) or resource template (identified by URI), servers can provide contextual suggestions.
To enable completion support, use the server.WithCompletions() option when creating your server.
Completion Providers
You can provide completion logic for both prompt arguments and resource template arguments by implementing the respective interfaces and passing them to the server as options.
type MyPromptCompletionProvider struct{}
func (p *MyPromptCompletionProvider) CompletePromptArgument(
ctx context.Context,
promptName string,
argument mcp.CompleteArgument,
context mcp.CompleteContext,
) (*mcp.Completion, error) {
// Example: provide style suggestions for a "code_review" prompt
if promptName == "code_review" && argument.Name == "style" {
styles := []string{"formal", "casual", "technical", "creative"}
var suggestions []string
// Filter based on current input
for _, style := range styles {
if strings.HasPrefix(style, argument.Value) {
suggestions = append(suggestions, style)
}
}
return &mcp.Completion{
Values: suggestions,
}, nil
}
// Return empty suggestions for unhandled cases
return &mcp.Completion{Values: []string{}}, nil
}
type MyResourceCompletionProvider struct{}
func (p *MyResourceCompletionProvider) CompleteResourceArgument(
ctx context.Context,
uri string,
argument mcp.CompleteArgument,
context mcp.CompleteContext,
) (*mcp.Completion, error) {
// Example: provide file path completions
if uri == "file:///{path}" && argument.Name == "path" {
// You can access previously completed arguments from context.Arguments
// context.Arguments is a map[string]string of already-resolved arguments
paths := getMatchingPaths(argument.Value) // Your custom logic
return &mcp.Completion{
Values: paths[:min(len(paths), 100)], // Max 100 items
Total: len(paths), // Total available matches
HasMore: len(paths) > 100, // More results available
}, nil
}
return &mcp.Completion{Values: []string{}}, nil
}
// Register the provider
mcpServer := server.NewMCPServer(
"my-server",
"1.0.0",
server.WithCompletions(),
server.WithPromptCompletionProvider(&MyPromptCompletionProvider{}),
server.WithResourceCompletionProvider(&MyResourceCompletionProvider{}),
)
Completion Context
For prompts or resource templates with multiple arguments, the CompleteContext parameter provides access to previously completed arguments. This allows you to provide contextual suggestions based on earlier choices.
func (p *MyProvider) CompleteResourceArgument(
ctx context.Context,
uri string,
argument mcp.CompleteArgument,
context mcp.CompleteContext,
) (*mcp.Completion, error) {
// Access previously completed arguments
if previousValue, ok := context.Arguments["previous_arg"]; ok {
// Provide suggestions based on previous_arg value
return getSuggestionsFor(argument.Value, previousValue), nil
}
return &mcp.Completion{Values: []string{}}, nil
}
Response Constraints
When returning completion results:
Maximum 100 items per response
Use Total to indicate the total number of available matches
Use HasMore to signal if additional results exist beyond the returned values
