Overview

When you build an API with Contract, you write your business logic once. But different clients need to talk to your API in different ways. That’s what transports are for - they let the same code speak multiple languages.

What Is a Transport?

Think of a transport like a translator at the United Nations. The speaker (your service) gives the same message, but each translator converts it into a language their audience understands. In Contract terms:

Your service is the speaker - it has the business logic
Transports are the translators - they convert requests/responses
Clients are the audience - browsers, apps, AI assistants, etc.

┌──────────────────────────────────────────────────────────────────┐
│                        Your Service                               │
│                   (Written once, in Go)                           │
└──────────────────────────────────────────────────────────────────┘
                              │
        ┌─────────────────────┼─────────────────────┐
        ▼                     ▼                     ▼
┌──────────────┐      ┌──────────────┐      ┌──────────────┐
│     REST     │      │   JSON-RPC   │      │     MCP      │
│  (Familiar)  │      │  (Powerful)  │      │ (AI-Ready)   │
└──────────────┘      └──────────────┘      └──────────────┘
        │                     │                     │
        ▼                     ▼                     ▼
   Web Browsers          Backend Services        AI Assistants
   Mobile Apps           Microservices           Claude Desktop

Available Transports

Contract supports three transports plus documentation generation:

Transport	What It Is	Best For
REST	Traditional HTTP API	Web apps, mobile apps, curl
JSON-RPC	Remote procedure calls over HTTP	Batch operations, microservices
MCP	Model Context Protocol	AI assistants like Claude
OpenAPI	API documentation standard	Docs, code generation

Decision Guide: Which Transport Should I Use?

Use this flowchart to pick the right transport for your use case:

Start Here
    │
    ▼
Who is calling your API?
    │
    ├─► Web browsers, mobile apps, or curl
    │   → Use REST (most familiar)
    │
    ├─► Other backend services
    │   → Use JSON-RPC (supports batching)
    │
    ├─► AI assistants (Claude, etc.)
    │   → Use MCP (designed for AI)
    │
    └─► Need API documentation
        → Add OpenAPI (generates docs)

Pro tip: You can (and should!) use multiple transports at once. They don’t conflict.

Detailed Comparison

Feature Comparison

Feature	REST	JSON-RPC	MCP
HTTP Methods	GET/POST/PUT/DELETE	POST only	POST only
Batch Requests	No	Yes	No
Cacheable	Yes (GET requests)	No	No
Self-Documenting	Via OpenAPI	No	Via tools/list
Error Format	HTTP status codes	JSON-RPC errors	isError flag

REST - The Classic Choice

REST is the most widely known API style. It uses HTTP methods to represent actions:

# Create a todo (POST = create new resource)
curl -X POST http://localhost:8080/todos \
  -H "Content-Type: application/json" \
  -d '{"title": "Buy milk"}'

# List all todos (GET = retrieve data)
curl http://localhost:8080/todos

# Get one todo (GET with ID)
curl http://localhost:8080/todos/123

# Update a todo (PUT = replace resource)
curl -X PUT http://localhost:8080/todos/123 \
  -d '{"title": "Buy milk", "completed": true}'

# Delete a todo (DELETE = remove resource)
curl -X DELETE http://localhost:8080/todos/123

When to use REST:

Your clients are web browsers or mobile apps
You want caching (browsers cache GET requests)
Your team is familiar with REST APIs
You’re building a public API

Pros: Familiar, cacheable, works everywhere Cons: No batching, multiple round trips for complex operations

JSON-RPC - The Power User’s Choice

JSON-RPC is a simple protocol where you explicitly name the method to call. All requests use POST.

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

# Response
{"jsonrpc": "2.0", "id": 1, "result": {"id": "1", "title": "Buy milk"}}

The killer feature is batching - send multiple operations in one request:

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

# All three operations execute, returns array of results

When to use JSON-RPC:

Service-to-service communication
You need to batch multiple operations
Network latency is a concern
Your operations don’t map cleanly to REST verbs

Pros: Batching, explicit method names, standard protocol Cons: POST only, less familiar to web developers

MCP - The AI-Native Choice

MCP (Model Context Protocol) is designed for AI assistants. It lets AI models discover and use your API as “tools”.

# Step 1: Initialize the connection
curl -X POST http://localhost:8080/mcp \
  -d '{"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"protocolVersion": "2025-06-18"}}'

# Step 2: Discover available tools
curl -X POST http://localhost:8080/mcp \
  -d '{"jsonrpc": "2.0", "id": 2, "method": "tools/list"}'

# Response shows what the AI can do:
{
  "result": {
    "tools": [
      {
        "name": "todos.create",
        "description": "Create a new todo",
        "inputSchema": {...}
      }
    ]
  }
}

# Step 3: Call a tool
curl -X POST http://localhost:8080/mcp \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "todos.create",
      "arguments": {"title": "Buy milk"}
    }
  }'

When to use MCP:

You want Claude or other AI assistants to use your API
You’re building AI-powered applications
You want automatic tool discovery

Pros: AI assistants understand it natively, self-documenting Cons: Specialized use case, more complex protocol

OpenAPI - The Documentation Choice

OpenAPI isn’t really a transport - it generates documentation from your service. Use it alongside other transports.

# Get the OpenAPI specification
curl http://localhost:8080/openapi.json

The spec can be used with:

Swagger UI: Interactive API documentation
Code generators: Generate client SDKs in any language
API testing tools: Import into Postman, Insomnia, etc.

Using All Transports Together

Here’s how to serve your API via all protocols at once:

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

    "yourapp/todo"
)

func main() {
    // Create your service implementation
    impl := todo.NewService()

    // Register your service
    svc := contract.Register[todo.API](impl,
        contract.WithDefaultResource("todos"),
    )

    app := mizu.New()

    // REST: Traditional HTTP API
    // Endpoints: POST/GET /todos, GET/PUT/DELETE /todos/{id}
    rest.Mount(app.Router, svc)

    // JSON-RPC: RPC-style with batching
    // Endpoint: POST /rpc
    jsonrpc.Mount(app.Router, "/rpc", svc)

    // MCP: AI assistant integration
    // Endpoint: POST /mcp
    mcp.Mount(app.Router, "/mcp", svc)

    // OpenAPI: Serve API documentation
    spec, _ := rest.OpenAPI(svc.Descriptor())
    app.Get("/openapi.json", func(c *mizu.Ctx) error {
        return c.JSON(200, spec)
    })

    fmt.Println("Server running on http://localhost:8080")
    app.Listen(":8080")
}

Now the same service is accessible via:

http://localhost:8080/todos (REST)
http://localhost:8080/rpc (JSON-RPC)
http://localhost:8080/mcp (MCP)
http://localhost:8080/openapi.json (OpenAPI spec)

How Errors Work Across Transports

One of Contract’s best features is consistent error handling. When you return an error:

return nil, contract.ErrNotFound("todo not found")

Each transport formats it appropriately:

Transport	Error Response
REST	HTTP 404 status, body: `{"error": {"code": "NOT_FOUND", "message": "todo not found"}}`
JSON-RPC	`{"error": {"code": -32603, "message": "todo not found", "data": {"code": "NOT_FOUND"}}}`
MCP	`{"content": [{"type": "text", "text": "todo not found"}], "isError": true}`

You don’t need to handle each protocol separately.

Complete Example

Here’s a complete example with all transports. We organize the code with the todo service in its own package:

// todo/types.go
package todo

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

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

type GetInput struct {
    ID string `json:"id" path:"id"`
}

type ListOutput struct {
    Items []*Todo `json:"items"`
}

// todo/api.go
package todo

import "context"

// API defines the contract for todo operations
type API interface {
    Create(ctx context.Context, in *CreateInput) (*Todo, error)
    List(ctx context.Context) (*ListOutput, error)
    Get(ctx context.Context, in *GetInput) (*Todo, error)
}

// todo/service.go
package todo

import (
    "context"
    "fmt"
    "sync"

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

// Service implements todo.API
type Service struct {
    mu     sync.RWMutex
    todos  map[string]*Todo
    nextID int
}

// Compile-time check
var _ API = (*Service)(nil)

func NewService() *Service {
    return &Service{todos: make(map[string]*Todo)}
}

func (s *Service) Create(ctx context.Context, in *CreateInput) (*Todo, error) {
    if in.Title == "" {
        return nil, contract.ErrInvalidArgument("title is required")
    }

    s.mu.Lock()
    defer s.mu.Unlock()

    s.nextID++
    todo := &Todo{
        ID:    fmt.Sprintf("%d", s.nextID),
        Title: in.Title,
    }
    s.todos[todo.ID] = todo
    return todo, nil
}

func (s *Service) List(ctx context.Context) (*ListOutput, error) {
    s.mu.RLock()
    defer s.mu.RUnlock()

    items := make([]*Todo, 0, len(s.todos))
    for _, t := range s.todos {
        items = append(items, t)
    }
    return &ListOutput{Items: items}, nil
}

func (s *Service) Get(ctx context.Context, in *GetInput) (*Todo, error) {
    s.mu.RLock()
    defer s.mu.RUnlock()

    todo, ok := s.todos[in.ID]
    if !ok {
        return nil, contract.ErrNotFound("todo not found")
    }
    return todo, nil
}

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

    "yourapp/todo"
)

func main() {
    impl := todo.NewService()

    svc := contract.Register[todo.API](impl,
        contract.WithName("Todo"),
        contract.WithDefaultResource("todos"),
    )

    app := mizu.New()

    // Mount all transports
    rest.Mount(app.Router, svc)
    jsonrpc.Mount(app.Router, "/rpc", svc)
    mcp.Mount(app.Router, "/mcp", svc,
        mcp.WithInstructions("Use these tools to manage a todo list."),
    )

    // Serve OpenAPI spec
    spec, _ := rest.OpenAPI(svc.Descriptor())
    app.Get("/openapi.json", func(c *mizu.Ctx) error {
        return c.JSON(200, spec)
    })

    fmt.Println("Server running on http://localhost:8080")
    fmt.Println()
    fmt.Println("Endpoints:")
    fmt.Println("  REST:     POST/GET /todos, GET /todos/{id}")
    fmt.Println("  JSON-RPC: POST /rpc")
    fmt.Println("  MCP:      POST /mcp")
    fmt.Println("  OpenAPI:  GET /openapi.json")

    app.Listen(":8080")
}

Practical Recommendations

Starting a New Project

Start with REST + OpenAPI:

rest.Mount(app.Router, svc)

spec, _ := rest.OpenAPI(svc.Descriptor())
app.Get("/openapi.json", func(c *mizu.Ctx) error {
    return c.JSON(200, spec)
})

Add more transports as needed.

Building a Web Application

REST is perfect for web applications:

rest.Mount(app.Router, svc)

Building Microservices

JSON-RPC for service-to-service (batching is valuable):

jsonrpc.Mount(app.Router, "/rpc", svc)

Building AI-Powered Apps

MCP for AI assistants, plus REST for human debugging:

mcp.Mount(app.Router, "/mcp", svc)
rest.Mount(app.Router, svc)  // For testing with curl

Building for Everything

Use all of them! There’s no conflict:

rest.Mount(app.Router, svc)
jsonrpc.Mount(app.Router, "/rpc", svc)
mcp.Mount(app.Router, "/mcp", svc)

spec, _ := rest.OpenAPI(svc.Descriptor())
app.Get("/openapi.json", func(c *mizu.Ctx) error {
    return c.JSON(200, spec)
})

Common Questions

Can I use multiple transports at once?

Yes! Each transport uses different paths, so they don’t conflict. This is actually recommended.

Which transport is fastest?

They’re all similar in performance. The overhead is minimal compared to your actual business logic.

Do I need to write different code for each transport?

No! That’s the whole point of Contract. Write your service once, expose it via any transport.

Can I customize how a transport works?

Yes, each transport has options for customization. See the individual transport pages for details.

How are method names formatted for each transport?

All transports use the resource.method pattern:

Interface Method	Transport Method Name
`Create`	`todos.create`
`List`	`todos.list`
`Get`	`todos.get`

For REST, method names are mapped to HTTP verbs and paths automatically.

What’s Next?

Each transport has its own detailed documentation:

REST Transport - HTTP verbs, path patterns, customization
JSON-RPC Transport - Batching, notifications, error codes
MCP Transport - AI integration, Claude Desktop setup
OpenAPI - Documentation, Swagger UI, code generation

Getting Started

Core Concepts

Transports

SDKs

Advanced

Reference

What Is a Transport?

Available Transports

Decision Guide: Which Transport Should I Use?

Detailed Comparison

Feature Comparison

REST - The Classic Choice

JSON-RPC - The Power User’s Choice

MCP - The AI-Native Choice

OpenAPI - The Documentation Choice

Using All Transports Together

How Errors Work Across Transports

Complete Example

Practical Recommendations

Starting a New Project

Building a Web Application

Building Microservices

Building AI-Powered Apps

Building for Everything

Common Questions

Can I use multiple transports at once?

Which transport is fastest?

Do I need to write different code for each transport?

Can I customize how a transport works?

How are method names formatted for each transport?

What’s Next?

Getting Started

Core Concepts

Transports

SDKs

Advanced

Reference

​What Is a Transport?

​Available Transports

​Decision Guide: Which Transport Should I Use?

​Detailed Comparison

​Feature Comparison

​REST - The Classic Choice

​JSON-RPC - The Power User’s Choice

​MCP - The AI-Native Choice

​OpenAPI - The Documentation Choice

​Using All Transports Together

​How Errors Work Across Transports

​Complete Example

​Practical Recommendations

​Starting a New Project

​Building a Web Application

​Building Microservices

​Building AI-Powered Apps

​Building for Everything

​Common Questions

​Can I use multiple transports at once?

​Which transport is fastest?

​Do I need to write different code for each transport?

​Can I customize how a transport works?

​How are method names formatted for each transport?

​What’s Next?

What Is a Transport?

Available Transports

Decision Guide: Which Transport Should I Use?

Detailed Comparison

Feature Comparison

REST - The Classic Choice

JSON-RPC - The Power User’s Choice

MCP - The AI-Native Choice

OpenAPI - The Documentation Choice

Using All Transports Together

How Errors Work Across Transports

Complete Example

Practical Recommendations

Starting a New Project

Building a Web Application

Building Microservices

Building AI-Powered Apps

Building for Everything

Common Questions

Can I use multiple transports at once?

Which transport is fastest?

Do I need to write different code for each transport?

Can I customize how a transport works?

How are method names formatted for each transport?

What’s Next?