debug-webapp

name	debug-webapp
description	Debug the trading webapp frontend using Chrome DevTools MCP. Use when frontend is not behaving as expected, WebSocket connections failing, charts not rendering, or API calls returning errors.

Debug Webapp Skill

Systematic debugging workflow for the trading-webapp frontend using Chrome DevTools MCP.

---

Prerequisites

• Chrome browser open with the webapp loaded
• Chrome DevTools MCP server running

---

Step 1: Get Current State

Take Page Snapshot

mcp__chrome-devtools__take_snapshot

This shows the current page structure and identifies interactive elements.

List Open Pages

mcp__chrome-devtools__list_pages

Identify which page/tab to debug. Use select_page if needed.

---

Step 2: Check for Errors

Console Errors

mcp__chrome-devtools__list_console_messages
- Filter by types: ["error", "warn"]

Look for:

• JavaScript errors
• React errors (component stack traces)
• WebSocket connection failures
• API response errors logged by the app

Get Details on Specific Error

mcp__chrome-devtools__get_console_message
- msgid: <from list above>

---

Step 3: Network Analysis

List Network Requests

mcp__chrome-devtools__list_network_requests
- resourceTypes: ["fetch", "xhr", "websocket"]

Look for:

• Failed requests (4xx, 5xx status)
• Slow requests
• WebSocket disconnections
• Missing CORS headers

Inspect Specific Request

mcp__chrome-devtools__get_network_request
- reqid: <from list above>

Check:

• Request headers (auth tokens present?)
• Response body (error messages?)
• Timing (slow backend?)

---

Step 4: Interactive Debugging

Evaluate JavaScript

mcp__chrome-devtools__evaluate_script
- function: "() => { return window.__APP_STATE__ }"

Useful checks:

• () => localStorage.getItem('auth_token')
• () => document.querySelectorAll('[data-error]').length
• () => window.__WEBSOCKET_STATE__

Take Screenshot

mcp__chrome-devtools__take_screenshot
- fullPage: true (for full page)
- uid: <element> (for specific component)

---

Step 5: Performance (if needed)

Start Performance Trace

mcp__chrome-devtools__performance_start_trace
- reload: true
- autoStop: true

Analyze Performance Insights

mcp__chrome-devtools__performance_analyze_insight
- insightSetId: <from trace>
- insightName: "LCPBreakdown" or "DocumentLatency"

---

Common Issues & Checks

WebSocket Not Connecting

1. Check console for WebSocket errors
2. Check network for WS connection attempts
3. Verify backend is running: curl http://localhost:8000/health
4. Check CORS configuration

Chart Not Rendering

1. Take screenshot of chart container
2. Check console for TradingView errors
3. Verify data is being received (network tab)
4. Check element visibility in snapshot

API Calls Failing

1. List network requests filtered to fetch/xhr
2. Check request headers (Authorization present?)
3. Check response status and body
4. Verify backend endpoint exists

State Not Updating

1. Evaluate script to check React state/context
2. Check for console errors during action
3. Verify WebSocket messages arriving (network tab)

---

Quick Debug Sequence

For most issues, run this sequence:

1. list_pages → identify the page
2. take_snapshot → see current UI state
3. list_console_messages (errors only) → find JS errors
4. list_network_requests (fetch/xhr/websocket) → find API issues
5. take_screenshot → visual confirmation

Then dig deeper based on what you find.

---

Webapp-Specific Locations

Component	What to Check
Scanner table	WebSocket messages, /api/candidates
TradingView chart	tradingview console logs, symbol data feed
Watchlist	/api/watchlist endpoint, localStorage
Trade panel	/api/orders, form validation errors
Journal	/api/journal, DecisionSnapshot format