MCP NodeJS Debugger

Instructions

Implementation Reference

• src/mcp-server.js:332-472 (registration)

  Registration of the 'nodejs_inspect' tool using server.tool(), including description, input schema, and inline handler function.

  server.tool(
    "nodejs_inspect",
    "Executes JavaScript code in the debugged process",
    {
      js_code: z.string().describe("JavaScript code to execute")
    },
    async ({ js_code }) => {
      try {
        // Ensure debugger is enabled
        if (!inspector.debuggerEnabled) {
          await inspector.enableDebugger();
        }
        
        // Capture the current console output length to know where to start capturing new output
        const consoleStartIndex = inspector.consoleOutput.length;
        
        // Wrap the code in a try-catch with explicit console logging for errors
        let codeToExecute = `
          try {
            ${js_code}
          } catch (e) {
            e;  // Return the error
          }
        `;
        
        const response = await inspector.send('Runtime.evaluate', {
          expression: codeToExecute,
          contextId: 1,
          objectGroup: 'console',
          includeCommandLineAPI: true,
          silent: false,
          returnByValue: true,
          generatePreview: true,
          awaitPromise: true  // This will wait for promises to resolve
        });
        
        // Give some time for console logs to be processed
        await new Promise(resolve => setTimeout(resolve, 200));
        
        // Get any console output that was generated during execution
        const consoleOutputs = inspector.consoleOutput.slice(consoleStartIndex);
        const consoleText = consoleOutputs.map(output => 
          `[${output.type}] ${output.message}`
        ).join('\n');
        
        // Process the return value
        let result;
        if (response.result) {
          if (response.result.type === 'object') {
            if (response.result.value) {
              // If we have a value, use it
              result = response.result.value;
            } else if (response.result.objectId) {
              // If we have an objectId but no value, the object was too complex to serialize directly
              // Get more details about the object
              try {
                const objectProps = await inspector.getProperties(response.result.objectId);
                const formattedObject = {};
                
                for (const prop of objectProps.result) {
                  if (prop.value) {
                    if (prop.value.type === 'object' && prop.value.subtype !== 'null') {
                      // For nested objects, try to get their details too
                      if (prop.value.objectId) {
                        try {
                          const nestedProps = await inspector.getProperties(prop.value.objectId);
                          const nestedObj = {};
                          for (const nestedProp of nestedProps.result) {
                            if (nestedProp.value) {
                              if (nestedProp.value.value !== undefined) {
                                nestedObj[nestedProp.name] = nestedProp.value.value;
                              } else {
                                nestedObj[nestedProp.name] = nestedProp.value.description || 
                                  `[${nestedProp.value.subtype || nestedProp.value.type}]`;
                              }
                            }
                          }
                          formattedObject[prop.name] = nestedObj;
                        } catch (nestedErr) {
                          formattedObject[prop.name] = prop.value.description || 
                            `[${prop.value.subtype || prop.value.type}]`;
                        }
                      } else {
                        formattedObject[prop.name] = prop.value.description || 
                          `[${prop.value.subtype || prop.value.type}]`;
                      }
                    } else if (prop.value.type === 'function') {
                      formattedObject[prop.name] = '[function]';
                    } else if (prop.value.value !== undefined) {
                      formattedObject[prop.name] = prop.value.value;
                    } else {
                      formattedObject[prop.name] = `[${prop.value.type}]`;
                    }
                  }
                }
                
                result = formattedObject;
              } catch (propErr) {
                // If we can't get properties, at least show the object description
                result = response.result.description || `[${response.result.subtype || response.result.type}]`;
              }
            } else {
              // Fallback for objects without value or objectId
              result = response.result.description || `[${response.result.subtype || response.result.type}]`;
            }
          } else if (response.result.type === 'undefined') {
            result = undefined;
          } else if (response.result.value !== undefined) {
            result = response.result.value;
          } else {
            result = `[${response.result.type}]`;
          }
        }
        
        let responseContent = [];
        
        // Add console output if there was any
        if (consoleText.length > 0) {
          responseContent.push({
            type: "text", 
            text: `Console output:\n${consoleText}`
          });
        }
        
        // Add the result
        responseContent.push({
          type: "text",
          text: `Code executed successfully. Result: ${JSON.stringify(result, null, 2)}`
        });
        
        return { content: responseContent };
      } catch (err) {
        return {
          content: [{
            type: "text",
            text: `Error executing code: ${err.message}`
          }]
        };
      }
    }
  );

• src/mcp-server.js:338-471 (handler)

  The handler function executes the provided JS code in the Node.js debugged process using the inspector, handles console output, serializes complex objects by fetching properties, and returns structured content.

  async ({ js_code }) => {
    try {
      // Ensure debugger is enabled
      if (!inspector.debuggerEnabled) {
        await inspector.enableDebugger();
      }
      
      // Capture the current console output length to know where to start capturing new output
      const consoleStartIndex = inspector.consoleOutput.length;
      
      // Wrap the code in a try-catch with explicit console logging for errors
      let codeToExecute = `
        try {
          ${js_code}
        } catch (e) {
          e;  // Return the error
        }
      `;
      
      const response = await inspector.send('Runtime.evaluate', {
        expression: codeToExecute,
        contextId: 1,
        objectGroup: 'console',
        includeCommandLineAPI: true,
        silent: false,
        returnByValue: true,
        generatePreview: true,
        awaitPromise: true  // This will wait for promises to resolve
      });
      
      // Give some time for console logs to be processed
      await new Promise(resolve => setTimeout(resolve, 200));
      
      // Get any console output that was generated during execution
      const consoleOutputs = inspector.consoleOutput.slice(consoleStartIndex);
      const consoleText = consoleOutputs.map(output => 
        `[${output.type}] ${output.message}`
      ).join('\n');
      
      // Process the return value
      let result;
      if (response.result) {
        if (response.result.type === 'object') {
          if (response.result.value) {
            // If we have a value, use it
            result = response.result.value;
          } else if (response.result.objectId) {
            // If we have an objectId but no value, the object was too complex to serialize directly
            // Get more details about the object
            try {
              const objectProps = await inspector.getProperties(response.result.objectId);
              const formattedObject = {};
              
              for (const prop of objectProps.result) {
                if (prop.value) {
                  if (prop.value.type === 'object' && prop.value.subtype !== 'null') {
                    // For nested objects, try to get their details too
                    if (prop.value.objectId) {
                      try {
                        const nestedProps = await inspector.getProperties(prop.value.objectId);
                        const nestedObj = {};
                        for (const nestedProp of nestedProps.result) {
                          if (nestedProp.value) {
                            if (nestedProp.value.value !== undefined) {
                              nestedObj[nestedProp.name] = nestedProp.value.value;
                            } else {
                              nestedObj[nestedProp.name] = nestedProp.value.description || 
                                `[${nestedProp.value.subtype || nestedProp.value.type}]`;
                            }
                          }
                        }
                        formattedObject[prop.name] = nestedObj;
                      } catch (nestedErr) {
                        formattedObject[prop.name] = prop.value.description || 
                          `[${prop.value.subtype || prop.value.type}]`;
                      }
                    } else {
                      formattedObject[prop.name] = prop.value.description || 
                        `[${prop.value.subtype || prop.value.type}]`;
                    }
                  } else if (prop.value.type === 'function') {
                    formattedObject[prop.name] = '[function]';
                  } else if (prop.value.value !== undefined) {
                    formattedObject[prop.name] = prop.value.value;
                  } else {
                    formattedObject[prop.name] = `[${prop.value.type}]`;
                  }
                }
              }
              
              result = formattedObject;
            } catch (propErr) {
              // If we can't get properties, at least show the object description
              result = response.result.description || `[${response.result.subtype || response.result.type}]`;
            }
          } else {
            // Fallback for objects without value or objectId
            result = response.result.description || `[${response.result.subtype || response.result.type}]`;
          }
        } else if (response.result.type === 'undefined') {
          result = undefined;
        } else if (response.result.value !== undefined) {
          result = response.result.value;
        } else {
          result = `[${response.result.type}]`;
        }
      }
      
      let responseContent = [];
      
      // Add console output if there was any
      if (consoleText.length > 0) {
        responseContent.push({
          type: "text", 
          text: `Console output:\n${consoleText}`
        });
      }
      
      // Add the result
      responseContent.push({
        type: "text",
        text: `Code executed successfully. Result: ${JSON.stringify(result, null, 2)}`
      });
      
      return { content: responseContent };
    } catch (err) {
      return {
        content: [{
          type: "text",
          text: `Error executing code: ${err.message}`
        }]
      };
    }
  }

• src/mcp-server.js:335-337 (schema)

  Input schema for the tool, defining 'js_code' as a required string parameter.

  {
    js_code: z.string().describe("JavaScript code to execute")
  },

• src/mcp-server.js:36-319 (helper)

  The Inspector class is the core helper that manages connection to Node.js inspector protocol, sending CDP commands, handling events like paused/resumed/console, and providing utilities used by the nodejs_inspect handler.

  class Inspector {
  	constructor(port = 9229, retryOptions = { maxRetries: 5, retryInterval: 1000, continuousRetry: true }) {
  		this.port = port;
  		this.connected = false;
  		this.pendingRequests = new Map();
  		this.debuggerEnabled = false;
  		this.breakpoints = new Map();
  		this.paused = false;
  		this.currentCallFrames = [];
  		this.retryOptions = retryOptions;
  		this.retryCount = 0;
  		this.callbackHandlers = new Map();
  		this.continuousRetryEnabled = retryOptions.continuousRetry;
  		this.initialize();
  	}
  
  	async initialize() {
  		try {
  			// First, get the WebSocket URL from the inspector JSON API
  			// Use 127.0.0.1 instead of localhost to avoid IPv6 issues
  			const response = await fetch(`http://127.0.0.1:${this.port}/json`);
  			const data = await response.json();
  			const debuggerUrl = data[0]?.webSocketDebuggerUrl;
  			
  			if (!debuggerUrl) {
  				this.scheduleRetry();
  				return;
  			}
  			
  			this.ws = new WebSocket(debuggerUrl);
  			
  			this.ws.on('open', () => {
  				this.connected = true;
  				this.retryCount = 0;
  				this.enableDebugger();
  			});
  			
  			this.ws.on('error', (error) => {
  				this.scheduleRetry();
  				 this.debuggerEnabled = false;
  			});
  			
  			this.ws.on('close', () => {
  				this.connected = false;
  				this.scheduleRetry();
  				this.debuggerEnabled = false;
  			});
  			
  			this.ws.on('message', (data) => {
  				const response = JSON.parse(data.toString());
  				
  				// Handle events
  				if (response.method) {
  					this.handleEvent(response);
  					return;
  				}
  				
  				// Handle response for pending request
  				if (response.id && this.pendingRequests.has(response.id)) {
  					const { resolve, reject } = this.pendingRequests.get(response.id);
  					this.pendingRequests.delete(response.id);
  					
  					if (response.error) {
  						reject(response.error);
  					} else {
  						resolve(response.result);
  					}
  				}
  			});
  		} catch (error) {
  			this.scheduleRetry();
  		}
  	}
  	
  	scheduleRetry() {
  		// If continuous retry is enabled, we'll keep trying after the initial attempts
  		if (this.retryCount < this.retryOptions.maxRetries || this.continuousRetryEnabled) {
  			this.retryCount++;
  			
  			// Use a longer interval for continuous retries to reduce resource usage
  			const interval = this.continuousRetryEnabled && this.retryCount > this.retryOptions.maxRetries
  				? Math.min(this.retryOptions.retryInterval * 5, 10000) // Max 10 seconds between retries
  				: this.retryOptions.retryInterval;
  				
  			setTimeout(() => this.initialize(), interval);
  		}
  	}
  	
  	async enableDebugger() {
      try {
        if (!this.debuggerEnabled && this.connected) {
  
  				await this.send('Debugger.enable', {});
  				this.debuggerEnabled = true;
  				
  				// Setup event listeners
  				await this.send('Runtime.enable', {});
  				
  				// Also activate possible domains we'll need
  				await this.send('Runtime.runIfWaitingForDebugger', {});
  			}
  		} catch (error) {
  			this.scheduleRetry();
      }
  	}
  	
  	handleEvent(event) {
  		
  		switch (event.method) {
  			case 'Debugger.paused':
  				this.paused = true;
  				this.currentCallFrames = event.params.callFrames;
  				
  				// Notify any registered callbacks for pause events
  				if (this.callbackHandlers.has('paused')) {
  					this.callbackHandlers.get('paused').forEach(callback => 
  						callback(event.params));
  				}
  				break;
  				
  			case 'Debugger.resumed':
  				this.paused = false;
  				this.currentCallFrames = [];
  				
  				// Notify any registered callbacks for resume events
  				if (this.callbackHandlers.has('resumed')) {
  					this.callbackHandlers.get('resumed').forEach(callback => 
  						callback());
  				}
  				break;
  				
  			case 'Debugger.scriptParsed':
  				// Script parsing might be useful for source maps
  				break;
  				
  			case 'Runtime.exceptionThrown':
  				break;
  				
  			case 'Runtime.consoleAPICalled':
  				// Handle console logs from the debugged program
  				const args = event.params.args.map(arg => {
  					if (arg.type === 'string') return arg.value;
  					if (arg.type === 'number') return arg.value;
  					if (arg.type === 'boolean') return arg.value;
  					if (arg.type === 'object') {
  						if (arg.value) {
  							return JSON.stringify(arg.value, null, 2);
  						} else if (arg.objectId) {
  							// We'll try to get properties later as we can't do async here
  							return arg.description || `[${arg.subtype || arg.type}]`;
  						} else {
  							return arg.description || `[${arg.subtype || arg.type}]`;
  						}
  					}
  					return JSON.stringify(arg);
  				}).join(' ');
  				
  				// Store console logs to make them available to the MCP tools
  				if (!this.consoleOutput) {
  					this.consoleOutput = [];
  				}
  				this.consoleOutput.push({
  					type: event.params.type,
  					message: args,
  					timestamp: Date.now(),
  					raw: event.params.args
  				});
  				
  				// Keep only the last 100 console messages to avoid memory issues
  				if (this.consoleOutput.length > 100) {
  					this.consoleOutput.shift();
  				}
  				
  				break;
  		}
  	}
  	
  	registerCallback(event, callback) {
  		if (!this.callbackHandlers.has(event)) {
  			this.callbackHandlers.set(event, []);
  		}
  		this.callbackHandlers.get(event).push(callback);
  	}
  	
  	unregisterCallback(event, callback) {
  		if (this.callbackHandlers.has(event)) {
  			const callbacks = this.callbackHandlers.get(event);
  			const index = callbacks.indexOf(callback);
  			if (index !== -1) {
  				callbacks.splice(index, 1);
  			}
  		}
  	}
  
  	async send(method, params) {
  		return new Promise((resolve, reject) => {
  			const timeout = setTimeout(() => {
  				reject(new Error(`Request timed out: ${method}`));
  				this.pendingRequests.delete(id);
  			}, 5000);
  			
  			const checkConnection = () => {
  				if (this.connected) {
  					try {
  						const id = Math.floor(Math.random() * 1000000);
  						this.pendingRequests.set(id, { 
  							resolve: (result) => {
  								clearTimeout(timeout);
  								resolve(result);
  							}, 
  							reject: (err) => {
  								clearTimeout(timeout);
  								reject(err);
  							} 
  						});
  						
  						this.ws.send(JSON.stringify({
  							id,
  							method,
  							params
  						}));
  					} catch (err) {
  						clearTimeout(timeout);
  						reject(err);
  					}
  				} else {
  					const connectionCheckTimer = setTimeout(checkConnection, 100);
  					// If still not connected after 3 seconds, reject the promise
  					setTimeout(() => {
  						clearTimeout(connectionCheckTimer);
  						clearTimeout(timeout);
  						reject(new Error('Not connected to debugger'));
  					}, 3000);
  				}
  			};
  			
  			checkConnection();
  		});
  	}
  	
  	async getScriptSource(scriptId) {
  		try {
  			const response = await this.send('Debugger.getScriptSource', {
  				scriptId
  			});
  			return response.scriptSource;
  		} catch (err) {
  			return null;
  		}
  	}
  	
  	async evaluateOnCallFrame(callFrameId, expression) {
  		if (!this.paused) {
  			throw new Error('Debugger is not paused');
  		}
  		
  		try {
  			return await this.send('Debugger.evaluateOnCallFrame', {
  				callFrameId,
  				expression,
  				objectGroup: 'console',
  				includeCommandLineAPI: true,
  				silent: false,
  				returnByValue: true,
  				generatePreview: true
  			});
  		} catch (err) {
  			throw err;
  		}
  	}
  	
  	async getProperties(objectId, ownProperties = true) {
  		try {
  			return await this.send('Runtime.getProperties', {
  				objectId,
  				ownProperties,
  				accessorPropertiesOnly: false,
  				generatePreview: true
  			});
  		} catch (err) {
  			throw err;
  		}
  	}
  }