> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/punkpeye/fastmcp/llms.txt
> Use this file to discover all available pages before exploring further.

# fastmcp dev

> Run MCP servers in development mode with interactive CLI testing

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

## Command Syntax

```bash theme={null}
npx fastmcp dev <file> [options]
```

### Arguments

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

### Options

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

## Basic Usage

Run your server in development mode:

```bash theme={null}
npx fastmcp dev src/server.ts
```

This launches an interactive CLI powered by [@wong2/mcp-cli](https://github.com/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

## Watch Mode

Enable automatic server restarts when files change:

```bash theme={null}
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.

<Note>
  Watch mode uses `tsx --watch` under the hood, which monitors your source files and dependencies for changes.
</Note>

## Verbose Logging

Enable detailed logging to troubleshoot issues:

```bash theme={null}
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

## Examples

<CodeGroup>
  ```bash Basic Development theme={null}
  npx fastmcp dev src/examples/addition.ts
  ```

  ```bash Watch Mode theme={null}
  npx fastmcp dev src/examples/addition.ts --watch
  ```

  ```bash Verbose + Watch theme={null}
  npx fastmcp dev src/examples/addition.ts --watch --verbose
  ```

  ```bash Short Flags theme={null}
  npx fastmcp dev src/server.ts -wv
  ```
</CodeGroup>

## Transport Options

The dev command works with servers using the `stdio` transport, which is the default for development:

```typescript src/server.ts theme={null}
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
});
```

<Note>
  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.
</Note>

## Interactive CLI Features

Once your server starts, the interactive CLI provides:

### Tool Execution

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

### Resource Access

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

### Prompt Testing

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

### Server Information

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

## Troubleshooting

### Server Won't Start

If your server fails to start, try running with verbose logging:

```bash theme={null}
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)

### File Not Found

Ensure the path to your server file is correct:

```bash theme={null}
# Relative path from current directory
npx fastmcp dev ./src/server.ts

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

## Related Commands

<CardGroup cols={2}>
  <Card title="Inspect Command" icon="magnifying-glass" href="/cli/inspect">
    Visual debugging with MCP Inspector
  </Card>

  <Card title="CLI Overview" icon="terminal" href="/cli/overview">
    Back to CLI overview
  </Card>
</CardGroup>