Gin-MCP: Zero-Config Gin to MCP Bridge

Enable MCP features for any Gin API with a line of code. Gin-MCP is an opinionated, zero-configuration library that automatically exposes your existing Gin endpoints as Model Context Protocol (MCP) tools, making them instantly usable by MCP-compatible clients like Cursor. Our philosophy is simple: minimal setup, maximum productivity. Just plug Gin-MCP into your Gin application, and it handles the rest.

Why Gin-MCP?

• Effortless Integration: Connect your Gin API to MCP clients without writing tedious boilerplate code.
• Zero Configuration (by default): Get started instantly. Gin-MCP automatically discovers routes and infers schemas.
• Developer Productivity: Spend less time configuring tools and more time building features.
• Flexibility: While zero-config is the default, customize schemas and endpoint exposure when needed.
• Existing API: Works with your existing Gin API - no need to change any code.

Demo

Features

• Automatic Discovery: Intelligently finds all registered Gin routes.
• Schema Inference: Automatically generates MCP tool schemas from route parameters and request/response types (where possible).
• Direct Gin Integration: Mounts the MCP server directly onto your existing gin.Engine.
• Parameter Preservation: Accurately reflects your Gin route parameters (path, query) in the generated MCP tools.
• Customizable Schemas: Manually register schemas for specific routes using RegisterSchema for fine-grained control.
• Selective Exposure: Filter which endpoints are exposed using operation IDs or tags.
• Flexible Deployment: Mount the MCP server within the same Gin app or deploy it separately.

Installation

go get github.com/ckanthony/gin-mcp

Basic Usage: Instant MCP Server

Get your MCP server running in minutes with minimal code:

package main

import (
	"net/http"

	server "github.com/ckanthony/gin-mcp/"
	"github.com/gin-gonic/gin"
)

func main() {
	// 1. Create your Gin engine
	r := gin.Default()

	// 2. Define your API routes (Gin-MCP will discover these)
	r.GET("/ping", func(c *gin.Context) {
		c.JSON(http.StatusOK, gin.H{"message": "pong"})
	})

	r.GET("/users/:id", func(c *gin.Context) {
		// Example handler...
		userID := c.Param("id")
		c.JSON(http.StatusOK, gin.H{"user_id": userID, "status": "fetched"})
	})

	// 3. Create and configure the MCP server
	//    Provide essential details for the MCP client.
	mcp := server.New(r, &server.Config{
		Name:        "My Simple API",
		Description: "An example API automatically exposed via MCP.",
		// BaseURL is crucial! It tells MCP clients where to send requests.
		BaseURL: "http://localhost:8080",
	})

	// 4. Mount the MCP server endpoint
	mcp.Mount("/mcp") // MCP clients will connect here

	// 5. Run your Gin server
	r.Run(":8080") // Gin server runs as usual
}

That's it! Your MCP tools are now available at http://localhost:8080/mcp. Gin-MCP automatically created tools for /ping and /users/:id.

Note on BaseURL: Always provide an explicit BaseURL. This tells the MCP server the correct address to forward API requests to when a tool is executed by the client. Without it, automatic detection might fail, especially in environments with proxies or different internal/external URLs.

Advanced Usage

While Gin-MCP strives for zero configuration, you can customize its behavior.

Fine-Grained Schema Control with RegisterSchema

Sometimes, automatic schema inference isn't enough. RegisterSchema allows you to explicitly define schemas for query parameters or request bodies for specific routes. This is useful when:

• You use complex structs for query parameters (ShouldBindQuery).
• You want to define distinct schemas for request bodies (e.g., for POST/PUT).
• Automatic inference doesn't capture specific constraints (enums, descriptions, etc.) that you want exposed in the MCP tool definition.

package main

import (
	// ... other imports
	"github.com/ckanthony/gin-mcp/pkg/server"
	"github.com/gin-gonic/gin"
)

// Example struct for query parameters
type ListProductsParams struct {
	Page  int    `form:"page,default=1" json:"page,omitempty" jsonschema:"description=Page number,minimum=1"`
	Limit int    `form:"limit,default=10" json:"limit,omitempty" jsonschema:"description=Items per page,maximum=100"`
	Tag   string `form:"tag" json:"tag,omitempty" jsonschema:"description=Filter by tag"`
}

// Example struct for POST request body
type CreateProductRequest struct {
	Name  string  `json:"name" jsonschema:"required,description=Product name"`
	Price float64 `json:"price" jsonschema:"required,minimum=0,description=Product price"`
}

func main() {
	r := gin.Default()

	// --- Define Routes ---
	r.GET("/products", func(c *gin.Context) { /* ... handler ... */ })
	r.POST("/products", func(c *gin.Context) { /* ... handler ... */ })
	r.PUT("/products/:id", func(c *gin.Context) { /* ... handler ... */ })


	// --- Configure MCP Server ---
	mcp := server.New(r, &server.Config{
		Name:        "Product API",
		Description: "API for managing products.",
		BaseURL:     "http://localhost:8080",
	})

	// --- Register Schemas ---
	// Register ListProductsParams as the query schema for GET /products
	mcp.RegisterSchema("GET", "/products", ListProductsParams{}, nil)

	// Register CreateProductRequest as the request body schema for POST /products
	mcp.RegisterSchema("POST", "/products", nil, CreateProductRequest{})

	// You can register schemas for other methods/routes as needed
	// e.g., mcp.RegisterSchema("PUT", "/products/:id", nil, UpdateProductRequest{})

	mcp.Mount("/mcp")
	r.Run(":8080")
}

Explanation:

• mcp.RegisterSchema(method, path, querySchema, bodySchema)
• method: HTTP method (e.g., "GET", "POST").
• path: Gin route path (e.g., "/products", "/products/:id").
• querySchema: An instance of the struct used for query parameters (or nil if none). Gin-MCP uses reflection and jsonschema tags to generate the schema.
• bodySchema: An instance of the struct used for the request body (or nil if none).

Filtering Exposed Endpoints

Control which Gin endpoints become MCP tools using operation IDs or tags (if your routes define them).

// Only include specific operations by their Operation ID (if defined in your routes)
mcp := server.New(r, &server.Config{
    // ... other config ...
    IncludeOperations: []string{"getUser", "listUsers"},
})

// Exclude specific operations
mcp := server.New(r, &server.Config{
    // ... other config ...
    ExcludeOperations: []string{"deleteUser"}, // Don't expose deleteUser tool
})

// Only include operations tagged with "public" or "users"
mcp := server.New(r, &server.Config{
    // ... other config ...
    IncludeTags: []string{"public", "users"},
})

// Exclude operations tagged with "admin" or "internal"
mcp := server.New(r, &server.Config{
    // ... other config ...
    ExcludeTags: []string{"admin", "internal"},
})

Filtering Rules:

• You can only use one inclusion filter (IncludeOperations OR IncludeTags).
• You can only use one exclusion filter (ExcludeOperations OR ExcludeTags).
• You can combine an inclusion filter with an exclusion filter (e.g., include tag "public" but exclude operation "legacyPublicOp"). Exclusion takes precedence.

Customizing Schema Descriptions (Less Common)

For advanced control over how response schemas are described in the generated tools (often not needed):

mcp := server.New(r, &server.Config{
    // ... other config ...
    DescribeAllResponses:    true, // Include *all* possible response schemas (e.g., 200, 404) in tool descriptions
    DescribeFullResponseSchema: true, // Include the full JSON schema object instead of just a reference
})

Examples

See the examples directory for complete, runnable examples demonstrating various features.

Connecting an MCP Client (e.g., Cursor)

Once your Gin application with Gin-MCP is running:

1. Start your application.
2. In your MCP client (like Cursor Settings -> MCP), provide the URL where you mounted the MCP server (e.g., http://localhost:8080/mcp) as the SSE endpoint.
3. The client will connect and automatically discover the available API tools.

Contributing

Contributions are welcome! Please feel free to submit issues or Pull Requests.