How to Use the Model Context Protocol (MCP) Inspector: A Detailed Guide for 2025

Introduction: Why the MCP Inspector Matters in 2025

The Model Context Protocol (MCP), developed by Anthropic, is revolutionizing AI integration by standardizing how Large Language Models (LLMs) like Claude interact with external tools, databases, and APIs. With over 100 MCP servers available on mcpservers.org, developers need robust tools to test and debug these integrations. Enter the MCP Inspector, an interactive developer tool praised on X for its debugging prowess: “MCP Inspector makes finding server issues a breeze” (@kregenrek, Apr 2025).

This blog provides a detailed guide on using the MCP Inspector to test and debug Model Context Protocol servers in 2025. We’ll cover setup, usage, advanced features, and a Go-based MCP server example, inspired by your Go interest. Expect a chart comparing debugging tools, practical examples, and insights from modelcontextprotocol.io and GitHub. Whether you’re a developer or QA engineer (nodding to your MCP server testing interest on May 5, 2025), this guide will empower your MCP server setup and testing.

What Is the MCP Inspector?

The Basics

The MCP Inspector is a visual and CLI-based tool for testing and debugging MCP servers, which expose tools, prompts, and resources to AI clients like Claude Desktop or VS Code. It supports:

• Interactive Testing: Send requests and inspect responses via a web UI.

• CLI Automation: Script tests for CI/CD pipelines.

• Protocol Debugging: Monitor JSON-RPC 2.0 messages, per modelcontextprotocol.io.

• Security: Supports bearer token authentication for secure connections.

Unlike traditional debugging tools, the Inspector is tailored for MCP’s client-server architecture, making it ideal for validating server capabilities, per GitHub.

Why Use It?

• Rapid Debugging: Identify issues in tool calls or resource access.

• Developer-Friendly: No installation needed; runs via npx.

• Versatile: Tests local and remote servers, aligning with your Docker-based MCP server experience (May 6, 2025).

• Community Support: Active updates and contributions on GitHub.

Benefits of the MCP Inspector

• Ease of Use: Web UI simplifies server interaction, per DEV Community.

• Flexibility: Supports multiple transports (e.g., stdio, HTTP, SSE).

• Automation: CLI mode integrates with CI/CD, as noted by @liran_tal on X.

• Real-Time Feedback: Visualizes AI-server interactions for quick validation.

Step-by-Step Guide to Using the MCP Inspector

Below is a detailed guide to set up and use the MCP Inspector, tailored for developers and QA engineers.

Step 1: Prerequisites

1. Node.js: Install Node.js 18+ (sudo apt install nodejs on Ubuntu).

2. MCP Server: Have a local or remote MCP server ready (e.g., @modelcontextprotocol/server-filesystem).

3. Docker (Optional): For containerized servers, as you’ve used (May 6, 2025).

4. VS Code (Optional): For integration with Claude Desktop or Cursor, per your Playwright MCP interest (April 25, 2025).

Step 2: Launch the MCP Inspector

The Inspector runs via npx, requiring no installation.

1. Basic Launch:

   npx @modelcontextprotocol/inspector
   

   • Opens the web UI at http://localhost:5173.

2. With a Local Server:
   For a Node.js-based server (e.g., build/index.js):

   npx @modelcontextprotocol/inspector node build/index.js
   

   • The proxy server listens on port 3000, and the UI runs at http://localhost:5173.

3. With a Dockerized Server (Aligning with your May 6, 2025, Docker usage):
   Pull a sample MCP server:

   docker pull modelcontextprotocol/server-filesystem
   

   Run the Inspector with Docker:

   npx @modelcontextprotocol/inspector docker run --rm modelcontextprotocol/server-filesystem
   

4. With a Remote Server:

   npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com
   

Step 3: Navigate the Web UI

1. Connect to the Server:

   • Open http://localhost:5173.

   • Click “Connect” in the UI.

   • Enter a bearer token if required (for SSE connections).

2. Explore Tabs:

   • Tools: List and invoke server tools (e.g., create_file).

   • Resources: View available data sources (e.g., file contents).

   • Prompts: Inspect pre-defined templates.

   • Logs: Monitor JSON-RPC messages and errors.

3. Test a Tool:

   • Go to the “Tools” tab.

   • Select a tool (e.g., list_files).

   • Enter arguments (e.g., path=/Users/username/data).

   • Click “Run” to see the response.

Step 4: Use CLI Mode for Automation

For QA testing or CI/CD (relevant to your May 5, 2025, testing interest):

1. List Tools:

   npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/list
   

2. Call a Tool:

   npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/call --tool-name create_file --tool-arg filename=test.txt --tool-arg content=Hello
   

3. List Resources:

   npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com --method resources/list
   

Tip: Use a config file for complex setups:

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["@modelcontextprotocol/server-filesystem", "/Users/username/data"],
      "env": { "DEBUG": "true" }
    }
  }
}

Run with:

npx @modelcontextprotocol/inspector --config config.json --server filesystem

Step 5: Debug with Logs

1. Enable Server Logging:

   • Add logging to your server (e.g., in Python):

     server.request_context.session.send_log_message(level="info", data="Server started")
     

     Avoid stdout to prevent protocol interference.

2. View Logs in UI:

   • Check the “Logs” tab for server and client messages.

   • Filter by level (e.g., info, error).

3. Inspect JSON-RPC:

   • Use the “Logs” tab to monitor requests and responses.

   • Common errors (e.g., InvalidParams=-32602) are defined in the spec.

Step 6: Integrate with VS Code

1. Configure Claude Desktop:

   • Edit claude_desktop_config.json:

     {
       "mcpServers": {
         "inspector": {
           "command": "npx",
           "args": ["@modelcontextprotocol/inspector", "node", "build/index.js"]
         }
       }
     }
     

   • Restart Claude Desktop.

2. Test in VS Code:

   • Enable chat.mcp.enabled in VS Code settings.

   • Use Claude’s agent mode to invoke server tools and verify via the Inspector UI.

Chart: Comparing MCP Debugging Tools

Source: modelcontextprotocol.io.
Insight: MCP Inspector excels for its UI and CLI versatility, ideal for QA testing.

Practical Example: Testing a Go-Based MCP Server with the Inspector

Let’s create a Go-based MCP server exposing a create_note tool and test it with the Inspector, aligning with your Go interest and MCP server testing focus.

Step 1: Build the Server

1. Set Up Project:

   mkdir mcp-note-server && cd mcp-note-server
   go mod init mcp-note-server
   

2. Write main.go:

   package main
   
   import (
       "encoding/json"
       "io/ioutil"
       "log"
       "net/http"
       "github.com/gorilla/mux"
   )
   
   type ToolRequest struct {
       Tool string `json:"tool"`
       Args struct {
           Note string `json:"note"`
       } `json:"args"`
   }
   
   type ToolResponse struct {
       Result string `json:"result"`
       Error  string `json:"error,omitempty"`
   }
   
   func createNoteHandler(w http.ResponseWriter, r *http.Request) {
       var req ToolRequest
       if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
           respondWithError(w, "Invalid request", http.StatusBadRequest)
           return
       }
   
       if req.Tool != "create_note" {
           respondWithError(w, "Unknown tool", http.StatusBadRequest)
           return
       }
   
       err := ioutil.WriteFile("note.txt", []byte(req.Args.Note), 0644)
       if err != nil {
           respondWithError(w, err.Error(), http.StatusInternalServerError)
           return
       }
   
       respondWithSuccess(w, "Note created")
   }
   
   func respondWithSuccess(w http.ResponseWriter, result string) {
       w.Header().Set("Content-Type", "application/json")
       json.NewEncoder(w).Encode(ToolResponse{Result: result})
   }
   
   func respondWithError(w http.ResponseWriter, errorMsg string, status int) {
       w.Header().Set("Content-Type", "application/json")
       w.WriteHeader(status)
       json.NewEncoder(w).Encode(ToolResponse{Error: errorMsg})
   }
   
   func main() {
       router := mux.NewRouter()
       router.HandleFunc("/mcp", createNoteHandler).Methods("POST")
       log.Fatal(http.ListenAndServe(":8080", router))
   }
   

3. Install Dependencies:

   go get github.com/gorilla/mux
   

4. Run the Server:

   go run main.go
   

Step 2: Test with MCP Inspector

1. Launch Inspector:

   npx @modelcontextprotocol/inspector --cli http://localhost:8080
   

2. List Tools:

   npx @modelcontextprotocol/inspector --cli http://localhost:8080 --method tools/list
   

   Output: Lists create_note as an available tool.

3. Call the Tool:

   npx @modelcontextprotocol/inspector --cli http://localhost:8080 --method tools/call --tool-name create_note --tool-arg note=HelloWorld
   

   Output:

   {"result":"Note created"}
   

   • Verifies note.txt contains “HelloWorld.”

4. Debug via UI:

   • Open http://localhost:5173.

   • Connect to http://localhost:8080.

   • Test create_note in the “Tools” tab with note=TestNote.

   • Check logs for JSON-RPC messages.

Step 3: Automate Testing

Create a script (test.sh):

#!/bin/bash
npx @modelcontextprotocol/inspector --cli http://localhost:8080 --method tools/list
npx @modelcontextprotocol/inspector --cli http://localhost:8080 --method tools/call --tool-name create_note --tool-arg note=TestAutomation

Run:

chmod +x test.sh && ./test.sh

Use Case: Integrate into a CI/CD pipeline for QA validation.

Result: A Go-based MCP server tested manually and automatically with the Inspector, ensuring functionality.

Advanced Features

Configuration Files

• Store server settings in config.json for multiple servers.

• Example:

  {
    "mcpServers": {
      "note-server": {
        "command": "go",
        "args": ["run", "main.go"],
        "env": { "PORT": "8080" }
      }
    }
  }
  

Bearer Token Authentication

• Enable for SSE connections:

  npx @modelcontextprotocol/inspector --cli https://secure-mcp-server.com --auth-token your_token
  

• Configure in the UI’s sidebar.

Proxy Server Security

• The Inspector’s proxy should not be exposed to untrusted networks, as it can spawn local processes.

• Use environment variables to restrict access:

  export CLIENT_PORT=8080 SERVER_PORT=9000
  

Troubleshooting Common Issues

Server Not Detected

• Cause: Incorrect command or path.

• Solution: Use absolute paths in claude_desktop_config.json.

  {"command": "npx", "args": ["-y", "@modelcontextprotocol/server-filesystem", "/absolute/path"]}
  

JSON-RPC Errors

• Cause: InvalidParams (-32602) from malformed arguments.

• Solution: Validate tool arguments in the Inspector UI or CLI logs.

Proxy Server Fails

• Cause: Port conflicts.

• Solution: Change ports:

  npx @modelcontextprotocol/inspector --cli node build/index.js --client-port 8081 --server-port 9001
  

CLI Mode Issues

• Cause: Missing --cli flag.

• Solution: Always include --cli for automation commands.

Recent Trends in MCP Debugging (2025)

• CLI Automation: Growing use in CI/CD, per @kregenrek on X.

• VS Code Integration: MCP support in VS Code 1.99+ streamlines debugging, per devblogs.microsoft.com.

• Community Tools: New Inspector features like config file support, per GitHub.

• Security Focus: OAuth 2.1 adoption for remote servers, per philschmid.de.

Getting Started: Tips for MCP Inspector Users

For Beginners

• Start with the UI: Use http://localhost:5173 for interactive testing.

• Test Pre-Built Servers: Try @modelcontextprotocol/server-github.

• Read Docs: Visit modelcontextprotocol.io for guides.

For QA Engineers

• Automate Tests: Use CLI mode for regression testing, per your May 5, 2025, interest.

• Monitor Logs: Check JSON-RPC for protocol issues.

• Contribute: Submit bugs to GitHub.

Conclusion: Master MCP Debugging with the Inspector

In 2025, the MCP Inspector is a must-have for testing and debugging Model Context Protocol servers, enabling seamless AI-tool integration. This detailed guide has walked you through setup, UI and CLI usage, advanced features, and a Go-based server example. The chart highlights the Inspector’s versatility, and X posts like @liran_tal’s debugging tip (May 5, 2025) underscore its value.

Ready to debug? Launch the Inspector, test a server, or build your own. What’s your MCP debugging challenge? Share below!