Quick Start

What You’ll Build

This guide walks you through building your first API with Contract. By the end, you’ll have a working todo list API accessible via REST and JSON-RPC. You’ll learn the three-step pattern that every Contract service follows:

Define your interface - The contract that describes your API
Implement the interface - Your business logic
Register and serve - Make it available via HTTP

We’ll also show you the recommended project structure using Go packages, where your service lives in a todo package with types named API and Service (the package name provides the “todo” context).

Prerequisites

Before starting, make sure you have:

Go 1.22 or later installed (download Go)
A terminal (Command Prompt, Terminal, or any shell)
A text editor (VS Code, GoLand, or your favorite)
curl for testing (usually pre-installed on Mac/Linux)

If you’re not sure which Go version you have, run:

go version

Step 1: Create Your Project

Create a new directory and initialize a Go module. A Go module is a collection of Go packages with a go.mod file that tracks dependencies:

# Create a new directory for your project
mkdir todo-api
cd todo-api

# Initialize a Go module
# This creates a go.mod file that tracks your dependencies
go mod init todo-api

What this does: Creates a new Go project. The go.mod file tells Go where to find dependencies and what version of Go you’re using. Now create the directory structure for your todo package:

# Create the todo package directory
mkdir todo

Your project structure will look like this:

todo-api/
├── go.mod       # Go module file (created by go mod init)
├── main.go      # We'll create this - wires everything together
└── todo/
    ├── api.go       # We'll create this - the interface (contract)
    ├── service.go   # We'll create this - the implementation
    └── types.go     # We'll create this - input/output types

Step 2: Install Dependencies

Add the required packages. These commands download the mizu web framework and Contract v2:

go get github.com/go-mizu/mizu
go get github.com/go-mizu/mizu/contract/v2

What this does:

github.com/go-mizu/mizu is the mizu web framework that handles HTTP routing
github.com/go-mizu/mizu/contract/v2 is Contract v2, which provides the interface-first API pattern

After running these commands, your go.mod file will list these as dependencies.

Step 3: Create Your Types

Create todo/types.go. This file defines the data structures your API works with. These are called “DTOs” (Data Transfer Objects) because they define what data moves between clients and your server:

// todo/types.go
package todo

// Todo represents a single todo item in our system.
// This is the main data type that clients work with.
//
// The `json` tags tell Go how to convert this struct to/from JSON.
// For example, `json:"id"` means the ID field becomes "id" in JSON.
type Todo struct {
    ID        string `json:"id"`        // Unique identifier for this todo
    Title     string `json:"title"`     // What the todo is about
    Completed bool   `json:"completed"` // Whether it's done
}

// CreateInput is what clients send when creating a new todo.
// We only need the title - the ID is generated automatically,
// and Completed defaults to false for new todos.
type CreateInput struct {
    Title string `json:"title"` // Required: the todo's title
}

// GetInput is what clients send when fetching a specific todo.
// The ID tells us which todo to retrieve.
type GetInput struct {
    ID string `json:"id"` // The ID of the todo to get
}

// DeleteInput is what clients send when deleting a todo.
type DeleteInput struct {
    ID string `json:"id"` // The ID of the todo to delete
}

// ListOutput is what we return when listing all todos.
// It wraps the items in an object so we can add metadata (like count)
// without breaking clients if we add more fields later.
type ListOutput struct {
    Items []*Todo `json:"items"` // The list of todos
    Count int     `json:"count"` // How many todos total
}

Why separate input/output types? You might wonder why we have separate types like CreateInput and GetInput instead of just using Todo everywhere. There are good reasons:

Input types describe what clients send to you. For Create, we only need a title - we generate the ID.
Output types describe what you send back. We return the full Todo with the generated ID.
Flexibility: You can add fields to outputs without requiring clients to send them, and vice versa.
Validation: Input types only contain the fields you actually accept.

Step 4: Define Your Interface (The Contract)

Create todo/api.go. This is the heart of Contract - your interface defines what your API can do. Think of it as a “menu” that lists all available operations:

// todo/api.go
package todo

import "context"

// API defines what operations our todo service supports.
// This interface IS our API contract - it's the single source of truth
// for what methods exist and what they accept/return.
//
// The name is simply "API" because the package name "todo" already
// provides context. When used outside this package, it's "todo.API"
// which reads naturally: "the todo API".
//
// Contract rules for methods:
// - First parameter must be context.Context (for timeouts, cancellation)
// - Input parameters must be pointer to struct (*CreateInput)
// - Output must be pointer to struct (*Todo) or just error
// - Last return value must be error
type API interface {
    // Create adds a new todo item and returns the created todo with its ID.
    // The client sends a CreateInput with the title, and we return the
    // complete Todo including the generated ID.
    Create(ctx context.Context, in *CreateInput) (*Todo, error)

    // List returns all todos in the system.
    // This method has no input (just context) because listing doesn't
    // require any parameters. We return a ListOutput containing the items.
    List(ctx context.Context) (*ListOutput, error)

    // Get retrieves a single todo by its ID.
    // If the todo doesn't exist, we return an error.
    Get(ctx context.Context, in *GetInput) (*Todo, error)

    // Delete removes a todo by its ID.
    // This method only returns error (no output) because there's nothing
    // meaningful to return after deletion.
    Delete(ctx context.Context, in *DeleteInput) error
}

Key points about the interface:

Package naming: The interface is named API (not TodoAPI) because todo.API reads naturally when imported.
Context first: Every method starts with ctx context.Context. This is a Go pattern for passing request-scoped data like timeouts and cancellation signals.
Pointer inputs: Input types are pointers (*CreateInput) so they can be nil for methods that don’t need input.
Error handling: Every method returns error as the last return value. This is how you communicate failures to clients.
Method naming: Names like Create, List, Get, Delete automatically map to HTTP verbs when using REST.

Step 5: Implement Your Interface

Create todo/service.go. This is where your actual code lives - the “kitchen” that prepares the dishes from your “menu” (the interface):

// todo/service.go
package todo

import (
    "context"
    "errors"
    "fmt"
    "sync"
)

// Service implements the todo.API interface.
// This is where your actual business logic lives.
//
// The name is simply "Service" because the package name "todo" already
// provides context. When used outside this package, it's "todo.Service".
//
// In a real application, this struct would hold dependencies like:
// - Database connections (*sql.DB, *gorm.DB)
// - Cache clients (*redis.Client)
// - External service clients
//
// For this example, we use in-memory storage to keep things simple.
type Service struct {
    mu     sync.RWMutex     // Protects concurrent access to the map
    todos  map[string]*Todo // In-memory storage (ID -> Todo)
    nextID int              // Simple counter for generating IDs
}

// NewService creates a new Service instance ready to use.
// This is called a "constructor function" in Go.
func NewService() *Service {
    return &Service{
        todos: make(map[string]*Todo),
    }
}

// Create adds a new todo to our storage.
// This implements the API.Create method defined in our interface.
func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error) {
    // Step 1: Validate the input
    // Always validate user input! Never trust data from clients.
    if in.Title == "" {
        return nil, errors.New("title is required")
    }

    // Step 2: Lock for thread-safe access
    // Multiple HTTP requests might call Create at the same time,
    // so we need to protect our shared map.
    s.mu.Lock()
    defer s.mu.Unlock() // Unlock when this function returns

    // Step 3: Generate a unique ID
    s.nextID++
    id := fmt.Sprintf("todo_%d", s.nextID)

    // Step 4: Create and store the todo
    todo := &Todo{
        ID:        id,
        Title:     in.Title,
        Completed: false, // New todos start as not completed
    }
    s.todos[id] = todo

    // Step 5: Return the created todo
    // The client gets back the full todo including the generated ID
    return todo, nil
}

// List returns all todos in storage.
// This implements the API.List method defined in our interface.
func (s *Service) List(ctx context.Context) (*ListOutput, error) {
    // Use RLock (read lock) since we're only reading, not writing.
    // Multiple readers can hold RLock simultaneously.
    s.mu.RLock()
    defer s.mu.RUnlock()

    // Convert map values to a slice
    items := make([]*Todo, 0, len(s.todos))
    for _, todo := range s.todos {
        items = append(items, todo)
    }

    // Return wrapped in ListOutput
    return &ListOutput{
        Items: items,
        Count: len(items),
    }, nil
}

// Get retrieves a single todo by ID.
// This implements the API.Get method defined in our interface.
func (s *Service) Get(ctx context.Context, in *GetInput) (*Todo, error) {
    s.mu.RLock()
    defer s.mu.RUnlock()

    // Look up the todo in our map
    todo, exists := s.todos[in.ID]
    if !exists {
        // Todo not found - return an error
        // In production, use contract.ErrNotFound for proper HTTP 404
        return nil, errors.New("todo not found")
    }

    return todo, nil
}

// Delete removes a todo from storage.
// This implements the API.Delete method defined in our interface.
func (s *Service) Delete(ctx context.Context, in *DeleteInput) error {
    s.mu.Lock()
    defer s.mu.Unlock()

    // Check if the todo exists before deleting
    if _, exists := s.todos[in.ID]; !exists {
        return errors.New("todo not found")
    }

    // Remove from map
    delete(s.todos, in.ID)

    // Return nil to indicate success (no error)
    return nil
}

Important concepts:

Package naming: The struct is named Service (not TodoService) because todo.Service reads naturally.
Constructor function: NewService() is a common Go pattern for creating initialized instances.
Thread safety: We use sync.RWMutex because HTTP servers handle multiple requests concurrently.
Error handling: Return nil, error to indicate failure. Contract translates this to the appropriate protocol response.

Step 6: Wire Everything Together

Create main.go in your project root. This file imports your todo package and wires everything together:

// main.go
package main

import (
    "fmt"

    "github.com/go-mizu/mizu"
    contract "github.com/go-mizu/mizu/contract/v2"
    "github.com/go-mizu/mizu/contract/v2/transport/rest"
    "github.com/go-mizu/mizu/contract/v2/transport/jsonrpc"

    // Import your local todo package
    "todo-api/todo"
)

func main() {
    // Step 1: Create an instance of your service
    // This is the struct that contains your business logic
    impl := todo.NewService()

    // Step 2: Register it with Contract
    //
    // The generic parameter [todo.API] tells Contract which interface to use.
    // Contract uses Go's reflection to inspect this interface and discover:
    // - All the methods (Create, List, Get, Delete)
    // - The input/output types for each method
    // - JSON schemas for those types
    //
    // This is the "magic" step where your plain Go code becomes an API!
    svc := contract.Register[todo.API](impl,
        // WithDefaultResource groups all methods under "todos"
        // This affects:
        // - REST paths: /todos, /todos/{id}
        // - JSON-RPC methods: todos.create, todos.list, etc.
        contract.WithDefaultResource("todos"),
    )

    // Step 3: Create a mizu app
    // Mizu is a lightweight web framework that handles HTTP routing
    app := mizu.New()

    // Step 4: Mount the REST transport
    // This creates HTTP endpoints based on your method names:
    //   Create -> POST   /todos      (create a new resource)
    //   List   -> GET    /todos      (list all resources)
    //   Get    -> GET    /todos/{id} (get one resource)
    //   Delete -> DELETE /todos/{id} (delete a resource)
    rest.Mount(app.Router, svc)

    // Step 5: Mount the JSON-RPC transport
    // All methods are available at /rpc with names like:
    //   todos.create, todos.list, todos.get, todos.delete
    jsonrpc.Mount(app.Router, "/rpc", svc)

    // Step 6: Print helpful info and start the server
    fmt.Println("═══════════════════════════════════════════════════════════")
    fmt.Println("  Todo API is running!")
    fmt.Println("═══════════════════════════════════════════════════════════")
    fmt.Println()
    fmt.Println("REST endpoints:")
    fmt.Println("  POST   http://localhost:8080/todos      - Create a todo")
    fmt.Println("  GET    http://localhost:8080/todos      - List all todos")
    fmt.Println("  GET    http://localhost:8080/todos/{id} - Get a todo")
    fmt.Println("  DELETE http://localhost:8080/todos/{id} - Delete a todo")
    fmt.Println()
    fmt.Println("JSON-RPC endpoint:")
    fmt.Println("  POST   http://localhost:8080/rpc")
    fmt.Println("  Methods: todos.create, todos.list, todos.get, todos.delete")
    fmt.Println()

    // Start the HTTP server on port 8080
    // This blocks until the server is shut down
    app.Listen(":8080")
}

Complete Project Structure

Your project should now look like this:

todo-api/
├── go.mod
├── go.sum          # Created automatically when you run go mod tidy
├── main.go         # Wires everything together
└── todo/
    ├── api.go      # Interface definition (todo.API)
    ├── service.go  # Implementation (todo.Service)
    └── types.go    # Input/output types

Why organize code this way?

Clear separation: Each file has a single responsibility
Package-based naming: todo.API and todo.Service are clearer than TodoAPI and todoService
Testability: You can easily mock todo.API for testing
Scalability: Add more services by creating new packages (user/, order/, etc.)

Step 7: Run Your Server

Before running, ensure all dependencies are properly resolved:

# Download dependencies and update go.sum
go mod tidy

Then start your API server:

go run main.go

You should see:

═══════════════════════════════════════════════════════════
  Todo API is running!
═══════════════════════════════════════════════════════════

REST endpoints:
  POST   http://localhost:8080/todos      - Create a todo
  GET    http://localhost:8080/todos      - List all todos
  GET    http://localhost:8080/todos/{id} - Get a todo
  DELETE http://localhost:8080/todos/{id} - Delete a todo

JSON-RPC endpoint:
  POST   http://localhost:8080/rpc
  Methods: todos.create, todos.list, todos.get, todos.delete

Leave this terminal running and open a new terminal for testing.

Step 8: Test Your API

Let’s test your API using curl. Open a new terminal window (keep the server running in the first one).

Create a Todo (REST)

curl -X POST http://localhost:8080/todos \
  -H "Content-Type: application/json" \
  -d '{"title": "Buy groceries"}'

What this does:

POST tells the server we want to create something
/todos is the resource path
-H "Content-Type: application/json" tells the server we’re sending JSON
-d '{"title": "Buy groceries"}' is the JSON body (our CreateInput)

Expected output:

{"id":"todo_1","title":"Buy groceries","completed":false}

The server generated an ID (todo_1) and set completed to false.

Create Another Todo

curl -X POST http://localhost:8080/todos \
  -H "Content-Type: application/json" \
  -d '{"title": "Learn Contract"}'

Expected output:

{"id":"todo_2","title":"Learn Contract","completed":false}

List All Todos (REST)

curl http://localhost:8080/todos

What this does: A simple GET request to /todos calls our List method. Expected output:

{"items":[{"id":"todo_1","title":"Buy groceries","completed":false},{"id":"todo_2","title":"Learn Contract","completed":false}],"count":2}

Get a Specific Todo (REST)

curl http://localhost:8080/todos/todo_1

What this does: GET request to /todos/{id} calls our Get method with id=todo_1. Expected output:

{"id":"todo_1","title":"Buy groceries","completed":false}

Delete a Todo (REST)

curl -X DELETE http://localhost:8080/todos/todo_1

What this does: DELETE request removes the todo with the given ID. Expected output: Empty (HTTP 204 No Content means success with no body) Verify it’s deleted:

curl http://localhost:8080/todos

Now you should only see one todo.

Step 9: Try JSON-RPC

The same service is also available via JSON-RPC. JSON-RPC uses a different format where you specify the method name in the request body. This is useful for:

Batching: Send multiple requests in one HTTP call
RPC-style clients: Some languages prefer explicit method names over HTTP verbs

Create via JSON-RPC

curl -X POST http://localhost:8080/rpc \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "todos.create",
    "params": {"title": "Call mom"}
  }'

What this does:

All JSON-RPC requests go to /rpc as POST
"jsonrpc": "2.0" identifies this as JSON-RPC (required)
"id": 1 helps match requests to responses (you choose the ID)
"method": "todos.create" is {resource}.{method}
"params" is our CreateInput

Expected output:

{"jsonrpc":"2.0","id":1,"result":{"id":"todo_3","title":"Call mom","completed":false}}

List via JSON-RPC

curl -X POST http://localhost:8080/rpc \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "todos.list"
  }'

Batch Requests (JSON-RPC Only)

One of JSON-RPC’s killer features is batching. Send multiple requests in one HTTP call:

curl -X POST http://localhost:8080/rpc \
  -H "Content-Type: application/json" \
  -d '[
    {"jsonrpc":"2.0","id":1,"method":"todos.create","params":{"title":"First task"}},
    {"jsonrpc":"2.0","id":2,"method":"todos.create","params":{"title":"Second task"}},
    {"jsonrpc":"2.0","id":3,"method":"todos.list"}
  ]'

All three operations execute in one HTTP call! You get an array of responses back, matched by their id values.

Understanding What Happened

Let’s recap what Contract did for you behind the scenes:

Inspected your interface: When you called contract.Register[todo.API](), Contract used Go’s reflection to discover all methods in the todo.API interface and their input/output types.
Generated JSON schemas: Your Go structs (Todo, CreateInput, etc.) were automatically converted to JSON schemas. These schemas are used for documentation and AI tool definitions.
Created REST endpoints: Based on your method names, Contract created HTTP endpoints:
- Create → POST /todos (HTTP convention: POST creates resources)
- List → GET /todos (HTTP convention: GET retrieves resources)
- Get → GET /todos/ (path parameter for specific resource)
- Delete → DELETE /todos/ (HTTP convention: DELETE removes resources)
Created JSON-RPC handlers: All methods became available at /rpc:
- todos.create, todos.list, todos.get, todos.delete
Compile-time safety: If your Service didn’t implement all methods in API, Go’s compiler would have caught it before you even ran the program.

Common Questions

Why doesn’t my method appear as an endpoint?

Methods must follow Contract’s rules:

Must be in the interface: Methods only on the struct (not in the interface) won’t be exposed
First parameter must be context.Context: This is required for proper request handling
Input must be pointer to struct: Use *CreateInput, not CreateInput
Last return must be error: Every method needs to report success/failure

How do I add more transports?

Import the transport package and mount it:

import "github.com/go-mizu/mizu/contract/v2/transport/mcp"

// In main():
mcp.Mount(app.Router, "/mcp", svc)  // For AI assistants like Claude

How do I handle errors properly?

The quick start uses simple errors.New(). For production, use Contract’s typed errors for proper HTTP status codes:

import contract "github.com/go-mizu/mizu/contract/v2"

// Returns HTTP 404 Not Found
return nil, contract.ErrNotFound("todo not found")

// Returns HTTP 400 Bad Request
return nil, contract.ErrInvalidArgument("title is required")

// Returns HTTP 401 Unauthorized
return nil, contract.ErrUnauthenticated("please log in")

See the Error Handling guide for all error types.

How do I add a database?

Replace the in-memory map with your database client. The Service struct holds your dependencies:

type Service struct {
    db *sql.DB  // Or *gorm.DB, *pgx.Pool, etc.
}

func NewService(db *sql.DB) *Service {
    return &Service{db: db}
}

func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error) {
    // Use s.db to insert into your database
    result, err := s.db.ExecContext(ctx,
        "INSERT INTO todos (title, completed) VALUES (?, ?)",
        in.Title, false,
    )
    if err != nil {
        return nil, err
    }

    id, _ := result.LastInsertId()
    return &Todo{
        ID:    fmt.Sprintf("todo_%d", id),
        Title: in.Title,
    }, nil
}

What’s Next?

Now that you have a working API, explore these topics:

Defining Services - Learn all the method signature patterns and interface design
Registration - Understand all registration options
Error Handling - Proper error handling across protocols
REST Transport - Deep dive into REST configuration
JSON-RPC Transport - Batch requests and notifications
MCP Transport - Connect your API to AI assistants
Testing - How to test your services

Getting Started

Core Concepts

Transports

SDKs

Advanced

Reference

What You’ll Build

Prerequisites

Step 1: Create Your Project

Step 2: Install Dependencies

Step 3: Create Your Types

Step 4: Define Your Interface (The Contract)

Step 5: Implement Your Interface

Step 6: Wire Everything Together

Complete Project Structure

Step 7: Run Your Server

Step 8: Test Your API

Create a Todo (REST)

Create Another Todo

List All Todos (REST)

Get a Specific Todo (REST)

Delete a Todo (REST)

Step 9: Try JSON-RPC

Create via JSON-RPC

List via JSON-RPC

Batch Requests (JSON-RPC Only)

Understanding What Happened

Common Questions

Why doesn’t my method appear as an endpoint?

How do I add more transports?

How do I handle errors properly?

How do I add a database?

What’s Next?

Getting Started

Core Concepts

Transports

SDKs

Advanced

Reference

​What You’ll Build

​Prerequisites

​Step 1: Create Your Project

​Step 2: Install Dependencies

​Step 3: Create Your Types

​Step 4: Define Your Interface (The Contract)

​Step 5: Implement Your Interface

​Step 6: Wire Everything Together

​Complete Project Structure

​Step 7: Run Your Server

​Step 8: Test Your API

​Create a Todo (REST)

​Create Another Todo

​List All Todos (REST)

​Get a Specific Todo (REST)

​Delete a Todo (REST)

​Step 9: Try JSON-RPC

​Create via JSON-RPC

​List via JSON-RPC

​Batch Requests (JSON-RPC Only)

​Understanding What Happened

​Common Questions

​Why doesn’t my method appear as an endpoint?

​How do I add more transports?

​How do I handle errors properly?

​How do I add a database?

​What’s Next?

What You’ll Build

Prerequisites

Step 1: Create Your Project

Step 2: Install Dependencies

Step 3: Create Your Types

Step 4: Define Your Interface (The Contract)

Step 5: Implement Your Interface

Step 6: Wire Everything Together

Complete Project Structure

Step 7: Run Your Server

Step 8: Test Your API

Create a Todo (REST)

Create Another Todo

List All Todos (REST)

Get a Specific Todo (REST)

Delete a Todo (REST)

Step 9: Try JSON-RPC

Create via JSON-RPC

List via JSON-RPC

Batch Requests (JSON-RPC Only)

Understanding What Happened

Common Questions

Why doesn’t my method appear as an endpoint?

How do I add more transports?

How do I handle errors properly?

How do I add a database?

What’s Next?