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The contract template creates a service based on transport-neutral contracts. A “contract” in this context means a formal definition of what your API can do - like a menu at a restaurant. You define the methods once as Go code, and the framework automatically creates multiple ways to call them (REST, JSON-RPC) plus generates documentation (OpenAPI).

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When to Use This Template

Use the contract template when:
  • Multiple protocols needed - REST for web clients, JSON-RPC for internal services
  • API-first design - Define contracts before implementation
  • Auto-generated docs - OpenAPI specs from your code
  • Clean architecture - Business logic separate from transport

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What You Get

A complete service with:
  • Service contracts - Plain Go structs with typed methods
  • REST transport - RESTful HTTP endpoints
  • JSON-RPC transport - JSON-RPC 2.0 endpoint
  • OpenAPI spec - Auto-generated at /openapi.json
  • Discovery endpoint - List available methods

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Quick Start

# Create the project
mizu new myservice --template contract

# Enter the project
cd myservice

# Install dependencies
go mod tidy

# Run the server
mizu dev
Test the different protocols: 
# REST
curl http://localhost:8080/api/todo

# JSON-RPC
curl -X POST http://localhost:8080 \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"todo.List","id":1}'

# OpenAPI spec
curl http://localhost:8080/openapi.json

# Discovery
mizu contract ls

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How Contracts Work

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1. Define a Service

// service/todo/todo.go
type Service struct {
    // dependencies here
}

type Todo struct {
    ID        string `json:"id"`
    Title     string `json:"title"`
    Completed bool   `json:"completed"`
}

type CreateInput struct {
    Title string `json:"title"`
}

func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error) {
    // Implementation
}

func (s *Service) List(ctx context.Context) ([]*Todo, error) {
    // Implementation
}

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2. Register with Contract System

// In app setup
registry := contract.NewRegistry()
registry.Register("todo", &todo.Service{})

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3. Mount Transports

// REST transport
rest := contract.NewREST(registry)
app.Mount("/api", rest.Handler())

// JSON-RPC transport
rpc := contract.NewJSONRPC(registry)
app.Post("/", rpc.Handler())

// OpenAPI
openapi := contract.NewOpenAPI(registry)
app.Get("/openapi.json", openapi.Handler())

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Benefits

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Write Once, Expose Multiple Ways

Your business logic is in plain Go methods: 
func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error) {
    // This same code handles REST and JSON-RPC
}
Both transports call the same method: 
# REST: POST /api/todo
# JSON-RPC: {"method": "todo.Create", ...}

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Automatic Documentation

OpenAPI spec is generated from your Go types: 
type CreateInput struct {
    Title string `json:"title" doc:"The title of the todo"`
}
Generates: 
{
  "CreateInput": {
    "type": "object",
    "properties": {
      "title": {
        "type": "string",
        "description": "The title of the todo"
      }
    }
  }
}

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Type Safety

Everything is strongly typed: 
// Input and output types are checked at compile time
func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error)

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Discoverable

Clients can discover available methods: 
mizu contract ls

todo
  todo.Create    POST   /api/todo       Create a new todo
  todo.List      GET    /api/todo       List all todos
  todo.Get       GET    /api/todo/{id}  Get a todo by ID

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Project Structure

myservice/
├── cmd/
│   └── api/
│       └── main.go           # Entry point
├── app/
│   └── server/
│       ├── config.go         # Configuration
│       └── server.go         # Server setup
├── service/
│   └── todo/
│       └── todo.go           # Todo service contract
├── go.mod
└── .gitignore

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Available Transports

TransportEndpointUse Case
REST/api/*Web clients, mobile apps
JSON-RPC/ (POST)Internal services, RPC clients
OpenAPI/openapi.jsonDocumentation, code generation
OpenRPC/openrpc.jsonJSON-RPC documentation

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CLI Integration

Use mizu contract to interact with your service: 
# List all methods
mizu contract ls

# Show method details
mizu contract show todo.Create

# Call a method
mizu contract call todo.Create '{"title":"Buy milk"}'

# Export OpenAPI spec
mizu contract spec > openapi.json

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When to Use Contract vs API Template

AspectAPI TemplateContract Template
ProtocolsREST onlyREST + JSON-RPC
DocumentationManualAuto-generated
Type safetyAt boundariesThroughout
ComplexityLowerHigher
Best forSimple APIsMulti-protocol services

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Next Steps

Project Structure

Understand every file in detail

Tutorial

Build a contract-based service