debug

name

debug

description

This skill should be used when debugging frontend/UI bugs that need runtime evidence. USE THIS SKILL (instead of adding console.log) when you're about to: "add console.log and ask user to check", "open DevTools and tell me what you see", "reproduce the bug and share the output", "check the browser console". Triggers: "debug this", "fix this bug", "why isn't this working", "investigate this issue", "trace the problem", "figure out why X happens", "UI not updating", "state is wrong", "value is null/undefined", "click doesn't work", "modal not showing". Automates log collection server-side - you read logs directly, no user copy-paste needed.

Debug Mode

Fix bugs with runtime evidence, not guesses.

Don't guess → Hypothesize → Instrument → Reproduce → Analyze → Fix → Verify

When to Use

Trigger signals (if you're about to do any of these, use this skill instead):

• "Open DevTools Console and check for..."
• "Reproduce the bug and tell me what you see"
• "Add console.log and let me know the output"
• "Click X, open Y, check if Z appears in console"

Example scenario that should trigger this skill:

❌ Without skill (manual, slow):
"I added debug logging. Please:
1. Open the app in browser
2. Open DevTools Console (F12)
3. Open the defect modal and select a defect
4. Check console for [DEBUG] logs
5. Tell me what you see"

✅ With skill (automated):
Logs are captured server-side → you read them directly → no user copy-paste needed

Use when debugging:

• State/value issues (null, undefined, wrong type)
• Conditional logic (which branch was taken)
• Async timing (race conditions, load order)
• User interaction flows (modals, forms, clicks)

Arguments

/debug /path/to/project

If no path provided, use current working directory.

Workflow

Phase 1: Start Log Server

Step 1: Ensure server is running (starts if needed, no-op if already running):

node skills/debug/scripts/debug_server.js /path/to/project &

Server outputs JSON:

• {"status":"started",...} - new server started
• {"status":"already_running",...} - server was already running (this is fine!)

Step 2: Create session (server generates unique ID from your description):

curl -s -X POST http://localhost:8787/session -d '{"name":"fix-null-userid"}'

Response:

{"session_id":"fix-null-userid-a1b2c3","log_file":"/path/to/project/.debug/debug-fix-null-userid-a1b2c3.log"}

Save the session_id from the response - use it in all subsequent steps.

Server endpoints:

• POST /session with {"name": "description"} → creates session, returns {session_id, log_file}
• POST /log with {"sessionId": "...", "msg": "..."} → writes to log file
• GET / → returns status and log directory

If port 8787 busy: lsof -ti :8787 | xargs kill -9 then restart

──────────

Phase 2: Generate Hypotheses

Before instrumenting, generate 3-5 specific hypotheses:

Hypothesis H1: userId is null when passed to calculateScore()
  Expected: number (e.g., 5)
  Actual: null
  Test: Log userId at function entry

Hypothesis H2: score is string instead of number
  Expected: 85 (number)
  Actual: "85" (string)
  Test: Log typeof score

Each hypothesis must be:

• Specific (not "something is wrong")
• Testable (can confirm/reject with logs)
• Cover different subsystems (don't cluster)

──────────

Phase 3: Instrument Code

Add logging calls to test all hypotheses.

JavaScript/TypeScript:

// #region debug
const SESSION_ID = 'REPLACE_WITH_SESSION_ID'; // e.g. 'fix-null-userid-a1b2c3'
const DEBUG_LOG_URL = 'http://localhost:8787/log';

const debugLog = (msg, data = {}, hypothesisId = null) => {
  const payload = JSON.stringify({
    sessionId: SESSION_ID,
    msg,
    data,
    hypothesisId,
    loc: new Error().stack?.split('\n')[2],
  });

  if (navigator.sendBeacon?.(DEBUG_LOG_URL, payload)) return;
  fetch(DEBUG_LOG_URL, { method: 'POST', body: payload }).catch(() => {});
};
// #endregion

// Usage
debugLog('Function entry', { userId, score, typeScore: typeof score }, 'H1,H2');

Python:

# #region debug
import requests, traceback
SESSION_ID = 'REPLACE_WITH_SESSION_ID'  # e.g. 'fix-null-userid-a1b2c3'
def debug_log(msg, data=None, hypothesis_id=None):
    try:
        requests.post('http://localhost:8787/log', json={
            'sessionId': SESSION_ID, 'msg': msg, 'data': data,
            'hypothesisId': hypothesis_id, 'loc': traceback.format_stack()[-2].strip()
        }, timeout=0.5)
    except: pass
# #endregion

# Usage
debug_log('Function entry', {'user_id': user_id, 'type': type(user_id)}, 'H1')

Guidelines:

• 3-8 instrumentation points
• Cover: entry/exit, before/after critical ops, branch paths
• Tag each log with hypothesisId
• Wrap in // #region debug ... // #endregion
• High-frequency events (mousemove, scroll): log only on state change
• Log both intent and result

──────────

Phase 4: Clear and Reproduce

1. Clear logs:

   : > /path/to/project/.debug/debug-$SESSION_ID.log
   

2. Provide reproduction steps:

   <reproduction_steps>
   1. Start app: yarn dev
   2. Navigate to /users
   3. Click "Calculate Score"
   4. Observe NaN displayed
   </reproduction_steps>
   

3. User reproduces bug

──────────

Phase 5: Analyze Logs

Read and evaluate:

cat /path/to/project/.debug/debug-$SESSION_ID.log

For each hypothesis:

Hypothesis H1: userId is null
  Status: CONFIRMED
  Evidence: {"msg":"Function entry","data":{"userId":null}}

Hypothesis H2: score is string
  Status: REJECTED
  Evidence: {"data":{"typeScore":"number"}}

Status options:

• CONFIRMED: Logs prove it
• REJECTED: Logs disprove it
• INCONCLUSIVE: Need more instrumentation

If all INCONCLUSIVE/REJECTED: Generate new hypotheses, add more logs, iterate.

──────────

Phase 6: Fix

Only fix when logs confirm root cause.

Keep instrumentation active (don't remove yet).

Tag verification logs with runId: "post-fix":

debugLog('Function entry', { userId, runId: 'post-fix' }, 'H1');

──────────

Phase 7: Verify

1. Clear logs
2. User reproduces (bug should be gone)
3. Compare before/after:

   Before: {"data":{"userId":null},"runId":"run1"}
   After:  {"data":{"userId":5},"runId":"post-fix"}
   

4. Confirm with log evidence

If still broken: New hypotheses, more logs, iterate.

──────────

Phase 8: Five Whys (Optional)

When to run: Recurring bug, prod incident, security issue, or "this keeps happening".

After fixing, ask "Why did this bug exist?" to find systemic causes:

Bug: API returns NaN

Why 1: userId was null → Code fix: null check
Why 2: No input validation → Add validation
Why 3: No test for null case → Add test
Why 4: Review didn't catch → (one-off, acceptable)

Categories:

Type	Action
CODE	Fix immediately
TEST	Add test
PROCESS	Update checklist/review
SYSTEMIC	Document patterns

Skip if: Simple one-off bug, low impact, not recurring.

──────────

Phase 9: Clean Up

Remove instrumentation only after:

• Post-fix logs prove success
• User confirms resolved

Search for #region debug and remove all debug code.

Log Format

Each line is NDJSON:

{"ts":"2024-01-03T12:00:00.000Z","msg":"Button clicked","data":{"id":5},"hypothesisId":"H1","loc":"app.js:42"}

Critical Rules

1. NEVER fix without runtime evidence - Always collect logs first
2. NEVER remove instrumentation before verification - Keep until fix confirmed
3. NEVER guess - If unsure, add more logs
4. If all hypotheses rejected - Generate new ones from different subsystems

Troubleshooting

Issue	Solution
Server won't start	Check port 8787 not in use: lsof -i :8787
Logs empty	Check browser blocks (mixed content/CSP/CORS), firewall
Wrong log file	Verify session ID matches
Too many logs	Filter by hypothesisId, use state-change logging
Can't reproduce	Ask user for exact steps, check environment

CORS / Mixed Content Workarounds

If logs aren't arriving, it’s usually one of:

• Mixed content: HTTPS app → http://localhost:8787 is blocked. Use a dev-server proxy (same origin) or serve the log endpoint over HTTPS.
• CSP: connect-src blocks the log URL. Use a dev-server proxy or update CSP.
• CORS preflight: Content-Type: application/json triggers OPTIONS. Use a “simple” request (text/plain) or sendBeacon.

1. sendBeacon (avoids preflight; fire-and-forget):

const DEBUG_LOG_URL = 'http://localhost:8787/log';
const debugLog = (msg, data = {}, hypothesisId = null) => {
  const payload = JSON.stringify({ sessionId: SESSION_ID, msg, data, hypothesisId });
  if (navigator.sendBeacon?.(DEBUG_LOG_URL, payload)) return;
  fetch(DEBUG_LOG_URL, { method: 'POST', body: payload }).catch(() => {});
};

Note: still blocked by mixed content + CSP.

2. Dev server proxy (Vite example) - same-origin /__log → http://localhost:8787/log:

// vite.config.js
export default {
  server: {
    proxy: {
      '/__log': {
        target: 'http://localhost:8787',
        changeOrigin: true,
        rewrite: (path) => path.replace(/^\/__log/, '/log'),
      },
    },
  },
};

// Then POST to /__log instead of localhost:8787/log

3. Last resort (local only) - allow insecure content / disable mixed-content blocking in browser settings

Chrome Extension Debugging

Content scripts run in an isolated world with strict CSP - they cannot directly fetch to localhost:8787. The solution is to relay logs through the background script (service worker).

Content Script (sender):

// #region debug
const DEBUG_SESSION_ID = 'your-session-id-here';

const debugLog = (msg, data = {}, hypothesisId = null) => {
  chrome.runtime.sendMessage({
    type: 'DEBUG_LOG',
    payload: {
      sessionId: DEBUG_SESSION_ID,
      msg,
      data,
      hypothesisId,
      loc: new Error().stack?.split('\n')[2]?.trim(),
    },
  }).catch(() => {});
};
// #endregion

// Usage
debugLog('handleMouseMove', { target: target.tagName, rect }, 'H1');

Background Script (relay):

// #region debug - relay logs to debug server
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  if (message.type === 'DEBUG_LOG') {
    fetch('http://localhost:8787/log', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(message.payload),
    }).catch(() => {});
    sendResponse({ ok: true });
    return true;
  }
});
// #endregion

Why this works:

• Background scripts (service workers) have relaxed CSP and can fetch to localhost
• chrome.runtime.sendMessage is the bridge between content script and background
• Keep both debug regions tagged for easy cleanup

Injected scripts (MAIN world): If debugging code injected via <script> into the page context, use window.postMessage to relay to content script, which then relays to background:

// In MAIN world (injected script)
window.postMessage({ type: 'DEBUG_LOG_RELAY', payload: { ... } }, '*');

// In content script
window.addEventListener('message', (e) => {
  if (e.data?.type === 'DEBUG_LOG_RELAY') {
    chrome.runtime.sendMessage({ type: 'DEBUG_LOG', payload: e.data.payload });
  }
});

Checklist

• Server running (started or already_running)
• Session created via POST /session - save the returned session_id
• 3-5 hypotheses generated
• 3-8 logs added, tagged with hypothesisId
• Logs cleared before reproduction
• Reproduction steps provided
• Each hypothesis evaluated (CONFIRMED/REJECTED/INCONCLUSIVE)
• Fix based on evidence only
• Before/after comparison done
• Instrumentation removed after confirmation