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The fastmcp dev command starts your MCP server in development mode with an interactive command-line interface for testing tools, resources, and prompts.

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Command Syntax

npx fastmcp dev <file> [options]

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Arguments

  • <file> - Required. Path to your server file (TypeScript or JavaScript)

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Options

  • --watch, -w - Watch for file changes and automatically restart the server
  • --verbose, -v - Enable verbose logging for debugging
  • --help - Display help information

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Basic Usage

Run your server in development mode: 
npx fastmcp dev src/server.ts
This launches an interactive CLI powered by @wong2/mcp-cli that allows you to:
  • List and execute tools
  • Browse and read resources
  • Test prompts with different arguments
  • View server capabilities
  • Monitor server logs in real-time

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Watch Mode

Enable automatic server restarts when files change: 
npx fastmcp dev src/server.ts --watch
This is particularly useful during development as it automatically reloads your server whenever you save changes to your code.
Watch mode uses tsx --watch under the hood, which monitors your source files and dependencies for changes.

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Verbose Logging

Enable detailed logging to troubleshoot issues: 
npx fastmcp dev src/server.ts --verbose
Verbose mode displays:
  • The exact command being executed
  • File path being loaded
  • Watch mode status
  • Detailed error stack traces

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Examples

npx fastmcp dev src/examples/addition.ts

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Transport Options

The dev command works with servers using the stdio transport, which is the default for development:
src/server.ts
import { FastMCP } from "fastmcp";

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

// Add your tools, resources, and prompts...

server.start({
  transportType: "stdio", // Default transport for CLI
});
The dev command is designed for stdio transport. For testing HTTP-based transports (httpStream, sse), you should run your server directly and connect using an appropriate client.

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Interactive CLI Features

Once your server starts, the interactive CLI provides:

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Tool Execution

  • List all available tools
  • Execute tools with parameters
  • View tool results in formatted output

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Resource Access

  • Browse resource URIs
  • Read resource contents
  • Test resource templates with different arguments

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Prompt Testing

  • List available prompts
  • Test prompts with various arguments
  • Preview generated prompt content

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Server Information

  • View server name and version
  • Check enabled capabilities
  • Monitor connection status

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Troubleshooting

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Server Won’t Start

If your server fails to start, try running with verbose logging: 
npx fastmcp dev src/server.ts --verbose
Common issues:
  • TypeScript errors: Fix compilation errors in your server file
  • Missing dependencies: Ensure all imports are installed
  • Port conflicts: Check if the required port is already in use (for HTTP transports)

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File Not Found

Ensure the path to your server file is correct: 
# Relative path from current directory
npx fastmcp dev ./src/server.ts

# Absolute path
npx fastmcp dev /path/to/project/src/server.ts

Inspect Command

Visual debugging with MCP Inspector

CLI Overview

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