What is Multi-Tenant?
A multi-tenant MCP server is one canister that serves many users, where each user’s data is segregated by their principal ID. When to Use:- ✅ Todo/task lists, notes, calendars, settings
- ❌ Token custody, financial operations, per-user contracts
- Pros: Cost-effective, simple deployment, shared resources
- Cons: Shared storage limits, requires careful access control
Example: Multi-Tenant Todo List
Let’s build a simple todo list MCP where each user has their own isolated tasks.1. Define Your Data Model
// src/Types.mo
module {
public type Todo = {
id: Text;
title: Text;
completed: Bool;
createdAt: Int;
};
public type UserData = {
todos: [Todo];
createdAt: Int;
};
};
2. Set Up Principal-Based Storage
// src/main.mo
import Map "mo:map/Map";
import { phash } "mo:map/Map";
import Principal "mo:base/Principal";
import Time "mo:base/Time";
import Array "mo:base/Array";
actor TodoMCP {
// Store each user's data by their principal
stable var userData = Map.new<Principal, Types.UserData>();
// Helper to get or create user data
private func getUserData(caller: Principal) : Types.UserData {
switch (Map.get(userData, phash, caller)) {
case (?data) data;
case (null) {
let newData: Types.UserData = {
todos = [];
createdAt = Time.now();
};
Map.set(userData, phash, caller, newData);
newData
};
};
};
// Helper to update user data
private func setUserData(caller: Principal, data: Types.UserData) {
Map.set(userData, phash, caller, data);
};
}
3. Create MCP Tools
// Tool 1: Add Todo
public func todo_add(title: Text, auth: ?AuthTypes.AuthInfo) : async Result.Result<Text, Text> {
// Get authenticated user's principal
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
// Get user's data (creates if new user)
var data = getUserData(caller);
// Create new todo
let newTodo: Types.Todo = {
id = Principal.toText(caller) # "-" # Int.toText(Time.now());
title = title;
completed = false;
createdAt = Time.now();
};
// Add to user's todos
data := {
data with
todos = Array.append(data.todos, [newTodo]);
};
// Save updated data
setUserData(caller, data);
#ok(newTodo.id)
};
// Tool 2: List Todos
public func todo_list(auth: ?AuthTypes.AuthInfo) : async Result.Result<[Types.Todo], Text> {
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
let data = getUserData(caller);
#ok(data.todos)
};
// Tool 3: Toggle Todo
public func todo_toggle(id: Text, auth: ?AuthTypes.AuthInfo) : async Result.Result<(), Text> {
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
var data = getUserData(caller);
// Find and toggle the todo
let updatedTodos = Array.map<Types.Todo, Types.Todo>(
data.todos,
func(todo) {
if (todo.id == id) {
{ todo with completed = not todo.completed }
} else {
todo
}
}
);
data := { data with todos = updatedTodos };
setUserData(caller, data);
#ok()
};
// Tool 4: Delete Todo
public func todo_delete(id: Text, auth: ?AuthTypes.AuthInfo) : async Result.Result<(), Text> {
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
var data = getUserData(caller);
// Remove the todo
let filteredTodos = Array.filter<Types.Todo>(
data.todos,
func(todo) { todo.id != id }
);
data := { data with todos = filteredTodos };
setUserData(caller, data);
#ok()
};
4. Register with MCP Server
import Mcp "mo:mcp-motoko-sdk/mcp/Mcp";
import McpTypes "mo:mcp-motoko-sdk/mcp/Types";
// Define tool schemas
let todoAddTool: McpTypes.Tool = {
name = "todo_add";
title = ?"Add Todo";
description = ?"Create a new todo item in your personal list";
payment = null;
inputSchema = Json.obj([
("type", Json.str("object")),
("properties", Json.obj([
("title", Json.obj([
("type", Json.str("string")),
("description", Json.str("Todo title"))
]))
])),
("required", Json.arr([Json.str("title")]))
]);
outputSchema = ?Json.obj([
("type", Json.str("object")),
("properties", Json.obj([
("id", Json.obj([("type", Json.str("string"))]))
]))
]);
};
// Register all tools
let mcpConfig: McpTypes.McpConfig = {
self = Principal.fromActor(self);
serverInfo = {
name = "io.example.todo";
title = "Multi-Tenant Todo List";
version = "1.0.0";
};
tools = [todoAddTool, todoListTool, todoToggleTool, todoDeleteTool];
toolImplementations = [
("todo_add", handleTodoAdd),
("todo_list", handleTodoList),
("todo_toggle", handleTodoToggle),
("todo_delete", handleTodoDelete)
];
// ... other MCP config
};
let mcpServer = Mcp.createServer(mcpConfig);
Key Security Principles
1. Always Check the Caller
// ✅ CORRECT - Each user only accesses their own data
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
let data = getUserData(caller); // Automatically scoped to this user
// ❌ WRONG - Never search across all users
for ((principal, data) in userData.entries()) {
// This can leak data from other users!
}
2. No Cross-User Access
// ❌ NEVER allow accessing another user's data
public func get_user_todos(userId: Text) : async [Types.Todo] {
// This is dangerous! Any user could access anyone's data
}
// ✅ ALWAYS use the authenticated principal
public func get_my_todos(auth: ?AuthTypes.AuthInfo) : async [Types.Todo] {
let caller = switch (auth) {
case (?authInfo) authInfo.principal;
case (null) { return #err("Authentication required") };
};
let data = getUserData(caller);
data.todos
}
3. Validate All Inputs
// Check title length
if (title.size() == 0) {
return #err("Title cannot be empty");
};
if (title.size() > 500) {
return #err("Title too long (max 500 characters)");
};
Testing Your Multi-Tenant App
1. Deploy Locally
dfx start --background
dfx deploy
2. Create API Keys for Different Users
# User 1
dfx identity use alice
dfx canister call todo_mcp create_my_api_key '("Test Key", vec {})'
# Save the key: KEY_ALICE="..."
# User 2
dfx identity use bob
dfx canister call todo_mcp create_my_api_key '("Test Key", vec {})'
# Save the key: KEY_BOB="..."