go-micro/examples/mcp/documented

Documented Service Example

This example demonstrates how to document your go-micro service handlers so that AI agents can understand them better. The MCP gateway parses Go comments and struct tags to generate rich tool descriptions.

Documentation Features

1. Go Doc Comments

Standard Go documentation comments are used as the tool description:

// GetUser retrieves a user by ID from the database.
//
// This endpoint fetches a user's complete profile including their name,
// email, and age. If the user doesn't exist, an error is returned.
func (u *Users) GetUser(ctx context.Context, req *GetUserRequest, rsp *GetUserResponse) error {
    // ...
}

2. JSDoc-Style Tags

Use @param, @return, and @example tags for detailed documentation:

// CreateUser creates a new user in the system.
//
// @param name {string} User's full name (required, 1-100 characters)
// @param email {string} User's email address (required, must be valid email format)
// @param age {number} User's age (optional, must be 0-150 if provided)
// @return {User} The newly created user with generated ID
// @example
//   {
//     "name": "Alice Smith",
//     "email": "alice@example.com",
//     "age": 30
//   }
func (u *Users) CreateUser(ctx context.Context, req *CreateUserRequest, rsp *CreateUserResponse) error {
    // ...
}

3. Struct Tags

Add description tags to struct fields for better schema:

type User struct {
    ID    string `json:"id" description:"User's unique identifier (UUID format)"`
    Name  string `json:"name" description:"User's full name"`
    Email string `json:"email" description:"User's email address"`
    Age   int    `json:"age,omitempty" description:"User's age (optional)"`
}

Running the Example

1. Start the Service

cd examples/mcp/documented
go run main.go

Output:

Users service starting...
Service: users
Endpoints:
  - Users.GetUser
  - Users.CreateUser
MCP Gateway: http://localhost:3000

2. Test MCP Tools

List available tools:

curl http://localhost:3000/mcp/tools | jq

You'll see rich descriptions:

{
  "tools": [
    {
      "name": "users.Users.GetUser",
      "description": "GetUser retrieves a user by ID from the database",
      "inputSchema": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string",
            "description": "User ID in UUID format (e.g., \"123e4567-e89b-12d3-a456-426614174000\")"
          }
        },
        "required": ["id"],
        "examples": [
          "{\"id\": \"123e4567-e89b-12d3-a456-426614174000\"}"
        ]
      }
    },
    {
      "name": "users.Users.CreateUser",
      "description": "CreateUser creates a new user in the system",
      "inputSchema": {
        "type": "object",
        "properties": {
          "name": {
            "type": "string",
            "description": "User's full name (required, 1-100 characters)"
          },
          "email": {
            "type": "string",
            "description": "User's email address (required, must be valid email format)"
          },
          "age": {
            "type": "number",
            "description": "User's age (optional, must be 0-150 if provided)"
          }
        },
        "required": ["name", "email"],
        "examples": [
          "{\"name\": \"Alice Smith\", \"email\": \"alice@example.com\", \"age\": 30}"
        ]
      }
    }
  ]
}

3. Call a Tool

Get existing user:

curl -X POST http://localhost:3000/mcp/call \
  -H "Content-Type: application/json" \
  -d '{
    "tool": "users.Users.GetUser",
    "input": {"id": "user-1"}
  }'

Create new user:

curl -X POST http://localhost:3000/mcp/call \
  -H "Content-Type: application/json" \
  -d '{
    "tool": "users.Users.CreateUser",
    "input": {
      "name": "Alice Smith",
      "email": "alice@example.com",
      "age": 30
    }
  }'

4. Use with Claude Code

Add to your claude_desktop_config.json:

{
  "mcpServers": {
    "users-service": {
      "command": "go",
      "args": ["run", "/path/to/examples/mcp/documented/main.go"]
    }
  }
}

Then in Claude Code, ask:

> You: "Show me user-1's profile"

Claude will:
1. See the GetUser tool with rich description
2. Understand it needs an "id" parameter (UUID format)
3. Call users.Users.GetUser with {"id": "user-1"}
4. Return the user profile

Documentation Best Practices

DO: Write Clear Descriptions

// ✅ Good: Clear, explains what and why
// GetUser retrieves a user by ID from the database.
// Returns full profile including email, name, and preferences.

// ❌ Bad: Vague, no context
// Get gets a user

DO: Specify Parameter Constraints

// ✅ Good: Specifies format and constraints
// @param id {string} User ID in UUID format (e.g., "123e4567-e89b-12d3-a456-426614174000")
// @param age {number} User's age (must be 0-150)

// ❌ Bad: No constraints or format
// @param id {string} The ID

DO: Provide Examples

// ✅ Good: Real example agents can use
// @example
//   {
//     "name": "Alice Smith",
//     "email": "alice@example.com",
//     "age": 30
//   }

// ❌ Bad: No example
// (agents have to guess the format)

DO: Use Descriptive Struct Tags

// ✅ Good: Explains what the field is
type User struct {
    ID string `json:"id" description:"User's unique identifier (UUID format)"`
}

// ❌ Bad: No description
type User struct {
    ID string `json:"id"`
}

How It Works

1. Go Doc Parsing

   • The MCP gateway reads your service's Go comments
   • First line becomes the tool description
   • Full comment becomes the detailed description

2. JSDoc Tag Parsing

   • @param tags enhance parameter descriptions
   • @return tags describe what the tool returns
   • @example tags provide usage examples

3. Struct Tag Reading

   • description tags add context to fields
   • json:"field,omitempty" marks optional fields
   • Used to generate JSON Schema for parameters

4. Schema Generation

   • Combines parsed documentation with type information
   • Creates rich JSON Schema for each tool
   • Agents use this to understand how to call your service

Impact on Agent Performance

Without Documentation

Tool: users.Users.GetUser
Description: Call GetUser on users service
Parameters: { "id": "string" }

Agent thinks: "What's an ID? What format? What if I pass the wrong thing?"

With Documentation

Tool: users.Users.GetUser
Description: Retrieves a user by ID from the database. Returns full profile
             including email, name, and preferences.
Parameters:
  - id (string, required): User ID in UUID format
    Example: "123e4567-e89b-12d3-a456-426614174000"
Example:
  {"id": "user-1"}

Agent thinks: "I need a UUID format ID. I can use 'user-1' from the example!"

Result: Agent calls your service correctly on the first try!

Next Steps

• Document all your service handlers with clear descriptions
• Add @param, @return, and @example tags
• Use description tags in struct fields
• Test with Claude Code to see how agents understand your services

License

Apache 2.0