Dev

Build MCP servers with an Express-like API.

npm install @mctx-ai/mcp-server

import { createServer, T } from "@mctx-ai/mcp-server";

const app = createServer({
  instructions: "A greeting server. Use the greet tool to say hello.",
});

function greet({ name }) {
  return `Hello, ${name}!`;
}
greet.description = "Greet someone by name";
greet.input = {
  name: T.string({ required: true, description: "Name to greet" }),
};
app.tool("greet", greet);

export default { fetch: app.fetch };

That's a working MCP server. The framework handles protocol negotiation, input validation, error sanitization, CORS, and capability detection. You write the business logic.

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Tools

Tools are functions that AI can call -- like API endpoints. Define a function, attach .description and .input, and register it.

function add({ a, b }) {
  return a + b;
}
add.description = "Add two numbers";
add.input = {
  a: T.number({ required: true, description: "First number" }),
  b: T.number({ required: true, description: "Second number" }),
};
app.tool("add", add);

Return a string and it becomes the tool's text response. Return an object and it gets JSON-serialized automatically.

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Resources

Resources are read-only data that AI can pull for context. They use URI schemes you define -- docs://, db://, anything.

// Static resource
function readme() {
  return "# My Project\nWelcome to the docs.";
}
readme.mimeType = "text/markdown";
app.resource("docs://readme", readme);

// Dynamic template
function getUser({ userId }) {
  return JSON.stringify(db.findUser(userId));
}
getUser.description = "Fetch a user by ID";
getUser.mimeType = "application/json";
app.resource("user://{userId}", getUser);

Static URIs show up in resources/list. Templates with {param} placeholders show up in resources/templates/list and receive extracted params as the first argument.

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Prompts

Prompts are reusable message templates for AI interactions. Return a string for simple cases, or use conversation() for multi-message flows.

function codeReview({ code, language }) {
  return `Review this ${language} code for bugs and style issues:\n\n${code}`;
}
codeReview.description = "Review code for issues";
codeReview.input = {
  code: T.string({ required: true, description: "Code to review" }),
  language: T.string({ description: "Programming language" }),
};
app.prompt("code-review", codeReview);

For multi-message prompts with images or embedded resources:

import { conversation } from "@mctx-ai/mcp-server";

function debug({ error, screenshot }) {
  return conversation(({ user, ai }) => [
    user.say("I hit this error:"),
    user.say(error),
    user.attach(screenshot, "image/png"),
    ai.say("I'll analyze the error and screenshot together."),
  ]);
}
debug.description = "Debug with error + screenshot";
debug.input = {
  error: T.string({ required: true }),
  screenshot: T.string({ required: true, description: "Base64 image data" }),
};
app.prompt("debug", debug);

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Type System

The T object builds JSON Schema definitions for tool and prompt inputs.

Type	Example	Key Options
T.string()	T.string({ required: true })	enum, minLength, maxLength, pattern, format
T.number()	T.number({ min: 0, max: 100 })	min, max, enum
T.boolean()	T.boolean({ default: false })	default
T.array()	T.array({ items: T.string() })	items
T.object()	T.object({ properties: { ... } })	properties, additionalProperties

All types accept required, description, and default.

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Advanced Features

Progress Reporting

Use generator functions and createProgress() for long-running tools.

import { createProgress } from "@mctx-ai/mcp-server";

function* migrate({ tables }) {
  const step = createProgress(tables.length);
  for (const table of tables) {
    yield step();
    copyTable(table);
  }
  return `Migrated ${tables.length} tables`;
}
migrate.description = "Migrate database tables";
migrate.input = {
  tables: T.array({ required: true, items: T.string() }),
};
app.tool("migrate", migrate);

Structured Logging

import { log } from "@mctx-ai/mcp-server";

log.info("Server started");
log.warning("Rate limit approaching");
log.error("Connection failed");

Levels follow RFC 5424: debug, info, notice, warning, error, critical, alert, emergency.

Sampling (ask)

Tools receive an optional ask function as their second argument for LLM-in-the-loop patterns.

async function summarize({ url }, ask) {
  const content = await fetchPage(url);
  if (!ask) return content;
  return await ask(`Summarize this page:\n\n${content}`);
}

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Development

Scaffold a new project:

npm create mctx-server my-server
cd my-server
npm install
npm run dev

npm run dev starts mctx-dev with hot reload for local testing.

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Deploy

Push to GitHub and connect your repo at mctx.ai. Set your price per request and start earning.

Full deployment guide at docs.mctx.ai.

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Links

• Documentation
• Example Server
• GitHub Issues

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mctx is a trademark of mctx, Inc.
Licensed under MIT