fastmcp inspect

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Command Syntax

npx fastmcp inspect <file>

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Arguments

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

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Basic Usage

npx fastmcp inspect src/server.ts

1. Starts your MCP server
2. Launches the MCP Inspector web interface
3. Automatically opens your browser to the inspector UI
4. Connects the inspector to your running server

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What is MCP Inspector?

• Interactive Tool Testing - Execute tools with a visual form interface
• Resource Browser - Browse and preview resources with syntax highlighting
• Prompt Testing - Test prompts and view generated content
• Real-time Logs - Monitor server logs and events
• Request/Response Viewer - Inspect MCP protocol messages
• Schema Validation - Verify tool parameters and responses

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Examples

npx fastmcp inspect src/examples/addition.ts

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Inspector Interface

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Tools Tab

• View all available tools and their descriptions
• See tool parameter schemas
• Fill in parameters using a visual form
• Execute tools and view results
• Test error handling

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Resources Tab

• List of all available resources and templates
• Resource URI structure
• Preview resource contents
• Test resource templates with different arguments
• View MIME types and metadata

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Prompts Tab

• Browse available prompts
• Test prompt argument completion
• Preview generated prompt messages
• Inspect prompt metadata

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Logs Tab

• Server startup logs
• Tool execution logs
• Error messages and stack traces
• Protocol-level messages
• Connection events

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Transport Compatibility

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", // Compatible with inspector
});

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Debugging Workflow

1. Start the Inspector
   npx fastmcp inspect src/server.ts
   
2. Verify Server Connection
   • Check that the server appears connected in the UI
   • Review the server name and version
3. Test Tools
   • Navigate to the Tools tab
   • Select a tool to test
   • Fill in required parameters
   • Execute and verify the response
4. Browse Resources
   • Switch to the Resources tab
   • Test resource URIs
   • Verify resource content and MIME types
5. Monitor Logs
   • Keep the Logs tab open
   • Watch for errors or warnings
   • Verify log messages from your tools

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Troubleshooting

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Inspector Won’t Start

# Check if the server file is valid
npx fastmcp validate src/server.ts

# Try running with dev command first
npx fastmcp dev src/server.ts --verbose

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Browser Doesn’t Open

• Check the terminal for the inspector URL
• Manually open the URL in your browser
• Ensure no firewall is blocking the connection

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Connection Issues

• Verify your server is using stdio transport
• Check for errors in the Logs tab
• Restart the inspector command

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Comparison with Dev Command

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Related Commands

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Learn More