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; } } }