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FastMCP

A TypeScript framework for building MCP servers capable of handling client sessions.

Note

For a Python implementation, see FastMCP.

Features

Installation

npm install fastmcp

Quickstart

import { FastMCP } from "fastmcp";
import { z } from "zod";

const server = new FastMCP({
  name: "My Server",
  version: "1.0.0",
});

server.addTool({
  name: "add",
  description: "Add two numbers",
  parameters: z.object({
    a: z.number(),
    b: z.number(),
  }),
  execute: async (args) => {
    return String(args.a + args.b);
  },
});

server.start({
  transportType: "stdio",
});

That's it! You have a working MCP server.

You can test the server in terminal with:

git clone https://github.com/punkpeye/fastmcp.git
cd fastmcp

npm install

# Test the addition server example using CLI:
npx fastmcp dev src/examples/addition.ts
# Test the addition server example using MCP Inspector:
npx fastmcp inspect src/examples/addition.ts

SSE

You can also run the server with SSE support:

server.start({
  transportType: "sse",
  sse: {
    endpoint: "/sse",
    port: 8080,
  },
});

This will start the server and listen for SSE connections on http://localhost:8080/sse.

You can then use SSEClientTransport to connect to the server:

import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";

const client = new Client(
  {
    name: "example-client",
    version: "1.0.0",
  },
  {
    capabilities: {},
  },
);

const transport = new SSEClientTransport(new URL(`http://localhost:8080/sse`));

await client.connect(transport);

Core Concepts

Tools

Tools in MCP allow servers to expose executable functions that can be invoked by clients and used by LLMs to perform actions.

server.addTool({
  name: "fetch",
  description: "Fetch the content of a url",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return await fetchWebpageContent(args.url);
  },
});

Returning a string

execute can return a string:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return "Hello, world!";
  },
});

The latter is equivalent to:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return {
      content: [
        {
          type: "text",
          text: "Hello, world!",
        },
      ],
    };
  },
});

Returning a list

If you want to return a list of messages, you can return an object with a content property:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return {
      content: [
        { type: "text", text: "First message" },
        { type: "text", text: "Second message" },
      ],
    };
  },
});

Returning an image

Use the imageContent to create a content object for an image:

import { imageContent } from "fastmcp";

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return imageContent({
      url: "https://example.com/image.png",
    });

    // or...
    // return imageContent({
    //   path: "/path/to/image.png",
    // });

    // or...
    // return imageContent({
    //   buffer: Buffer.from("iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII=", "base64"),
    // });

    // or...
    // return {
    //   content: [
    //     await imageContent(...)
    //   ],
    // };
  },
});

The imageContent function takes the following options:

  • url: The URL of the image.
  • path: The path to the image file.
  • buffer: The image data as a buffer.

Only one of url, path, or buffer must be specified.

The above example is equivalent to:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    return {
      content: [
        {
          type: "image",
          data: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII=",
          mimeType: "image/png",
        },
      ],
    };
  },
});

Logging

Tools can log messages to the client using the log object in the context object:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args, { log }) => {
    log.info("Downloading file...", {
      url,
    });

    // ...

    log.info("Downloaded file");

    return "done";
  },
});

The log object has the following methods:

  • debug(message: string, data?: SerializableValue)
  • error(message: string, data?: SerializableValue)
  • info(message: string, data?: SerializableValue)
  • warn(message: string, data?: SerializableValue)

Errors

The errors that are meant to be shown to the user should be thrown as UserError instances:

import { UserError } from "fastmcp";

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args) => {
    if (args.url.startsWith("https://example.com")) {
      throw new UserError("This URL is not allowed");
    }

    return "done";
  },
});

Progress

Tools can report progress by calling reportProgress in the context object:

server.addTool({
  name: "download",
  description: "Download a file",
  parameters: z.object({
    url: z.string(),
  }),
  execute: async (args, { reportProgress }) => {
    reportProgress({
      progress: 0,
      total: 100,
    });

    // ...

    reportProgress({
      progress: 100,
      total: 100,
    });

    return "done";
  },
});

Extra Arguments

FastMCP supports passing "hidden" arguments to tools that aren't defined in the tool's parameter schema. This is useful for passing sensitive information like API tokens without exposing them in the tool's schema documentation.

When a client passes arguments that aren't defined in the tool's parameter schema, FastMCP extracts these as "extra arguments" and makes them available in the extraArgs property of the context object:

server.addTool({
  name: "fetchData",
  description: "Fetch data from an API",
  parameters: z.object({
    endpoint: z.string(),
    query: z.string().optional(),
  }),
  execute: async (args, context) => {
    // Access the validated parameters
    const { endpoint, query } = args;
    
    // Access the token from extra arguments
    const token = context.extraArgs?.token as string | undefined;
    
    if (!token) {
      return "No token provided";
    }
    
    // Use the token for authentication
    const response = await fetch(endpoint, {
      headers: {
        Authorization: `Bearer ${token}`
      }
    });
    
    return await response.text();
  },
});

This allows clients to pass sensitive information like API tokens without them being exposed in the tool's schema or documentation:

// Client code
await client.callTool("fetchData", {
  endpoint: "https://api.example.com/data",
  query: "example",
  token: "secret-api-token" // This will be passed as an extra argument
});

The token argument won't appear in the tool's schema when clients request it, but it will be available in the context.extraArgs object when the tool is executed.

Resources

Resources represent any kind of data that an MCP server wants to make available to clients. This can include:

  • File contents
  • Screenshots and images
  • Log files
  • And more

Each resource is identified by a unique URI and can contain either text or binary data.

server.addResource({
  uri: "file:///logs/app.log",
  name: "Application Logs",
  mimeType: "text/plain",
  async load() {
    return {
      text: await readLogFile(),
    };
  },
});

Note

load can return multiple resources. This could be used, for example, to return a list of files inside a directory when the directory is read.

async load() {
  return [
    {
      text: "First file content",
    },
    {
      text: "Second file content",
    },
  ];
}

You can also return binary contents in load:

async load() {
  return {
    blob: 'base64-encoded-data'
  };
}

Resource templates

You can also define resource templates:

server.addResourceTemplate({
  uriTemplate: "file:///logs/{name}.log",
  name: "Application Logs",
  mimeType: "text/plain",
  arguments: [
    {
      name: "name",
      description: "Name of the log",
      required: true,
    },
  ],
  async load({ name }) {
    return {
      text: `Example log content for ${name}`,
    };
  },
});

Resource template argument auto-completion

Provide complete functions for resource template arguments to enable automatic completion:

server.addResourceTemplate({
  uriTemplate: "file:///logs/{name}.log",
  name: "Application Logs",
  mimeType: "text/plain",
  arguments: [
    {
      name: "name",
      description: "Name of the log",
      required: true,
      complete: async (value) => {
        if (value === "Example") {
          return {
            values: ["Example Log"],
          };
        }

        return {
          values: [],
        };
      },
    },
  ],
  async load({ name }) {
    return {
      text: `Example log content for ${name}`,
    };
  },
});

Prompts

Prompts enable servers to define reusable prompt templates and workflows that clients can easily surface to users and LLMs. They provide a powerful way to standardize and share common LLM interactions.

server.addPrompt({
  name: "git-commit",
  description: "Generate a Git commit message",
  arguments: [
    {
      name: "changes",
      description: "Git diff or description of changes",
      required: true,
    },
  ],
  load: async (args) => {
    return `Generate a concise but descriptive commit message for these changes:\n\n${args.changes}`;
  },
});

Prompt argument auto-completion

Prompts can provide auto-completion for their arguments:

server.addPrompt({
  name: "countryPoem",
  description: "Writes a poem about a country",
  load: async ({ name }) => {
    return `Hello, ${name}!`;
  },
  arguments: [
    {
      name: "name",
      description: "Name of the country",
      required: true,
      complete: async (value) => {
        if (value === "Germ") {
          return {
            values: ["Germany"],
          };
        }

        return {
          values: [],
        };
      },
    },
  ],
});

Prompt argument auto-completion using enum

If you provide an enum array for an argument, the server will automatically provide completions for the argument.

server.addPrompt({
  name: "countryPoem",
  description: "Writes a poem about a country",
  load: async ({ name }) => {
    return `Hello, ${name}!`;
  },
  arguments: [
    {
      name: "name",
      description: "Name of the country",
      required: true,
      enum: ["Germany", "France", "Italy"],
    },
  ],
});

Authentication

FastMCP allows you to authenticate clients using a custom function:

import { AuthError } from "fastmcp";

const server = new FastMCP({
  name: "My Server",
  version: "1.0.0",
  authenticate: ({request}) => {
    const apiKey = request.headers["x-api-key"];

    if (apiKey !== '123') {
      throw new Response(null, {
        status: 401,
        statusText: "Unauthorized",
      });
    }

    // Whatever you return here will be accessible in the `context.session` object.
    return {
      id: 1,
    }
  },
});

Now you can access the authenticated session data in your tools:

server.addTool({
  name: "sayHello",
  execute: async (args, { session }) => {
    return `Hello, ${session.id}!`;
  },
});

Sessions

The session object is an instance of FastMCPSession and it describes active client sessions.

server.sessions;

We allocate a new server instance for each client connection to enable 1:1 communication between a client and the server.

Typed server events

You can listen to events emitted by the server using the on method:

server.on("connect", (event) => {
  console.log("Client connected:", event.session);
});

server.on("disconnect", (event) => {
  console.log("Client disconnected:", event.session);
});

FastMCPSession

FastMCPSession represents a client session and provides methods to interact with the client.

Refer to Sessions for examples of how to obtain a FastMCPSession instance.

requestSampling

requestSampling creates a sampling request and returns the response.

await session.requestSampling({
  messages: [
    {
      role: "user",
      content: {
        type: "text",
        text: "What files are in the current directory?",
      },
    },
  ],
  systemPrompt: "You are a helpful file system assistant.",
  includeContext: "thisServer",
  maxTokens: 100,
});

You can also provide request options, such as a timeout in milliseconds:

import { RequestOptions } from "fastmcp";

const options: RequestOptions = {
  timeout: 120000, // 120 seconds
};

await session.requestSampling(
  {
    // ... message parameters
  },
  options,
);

clientCapabilities

The clientCapabilities property contains the client capabilities.

session.clientCapabilities;

loggingLevel

The loggingLevel property describes the logging level as set by the client.

session.loggingLevel;

roots

The roots property contains the roots as set by the client.

session.roots;

server

The server property contains an instance of MCP server that is associated with the session.

session.server;

Typed session events

You can listen to events emitted by the session using the on method:

session.on("rootsChanged", (event) => {
  console.log("Roots changed:", event.roots);
});

session.on("error", (event) => {
  console.error("Error:", event.error);
});

Running Your Server

Test with mcp-cli

The fastest way to test and debug your server is with fastmcp dev:

npx fastmcp dev server.js
npx fastmcp dev server.ts

This will run your server with mcp-cli for testing and debugging your MCP server in the terminal.

Inspect with MCP Inspector

Another way is to use the official MCP Inspector to inspect your server with a Web UI:

npx fastmcp inspect server.ts

FAQ

How to use with Claude Desktop?

Follow the guide https://modelcontextprotocol.io/quickstart/user and add the following configuration:

{
  "mcpServers": {
    "my-mcp-server": {
      "command": "npx",
      "args": [
        "tsx",
        "/PATH/TO/YOUR_PROJECT/src/index.ts"
      ],
      "env": {
        "YOUR_ENV_VAR": "value"
      }
    }
  }
}

Showcase

Note

If you've developed a server using FastMCP, please submit a PR to showcase it here!

Acknowledgements

About

A TypeScript framework for building MCP servers.

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License

MIT license
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