Traditional: Write test code → Execute → Check results → Maintain
Zero Script: Build log infrastructure → Manual UX test → AI log analysis → Auto issue detection

{
  "timestamp": "2026-01-08T10:30:00.000Z",
  "level": "INFO",
  "service": "api",
  "request_id": "req_abc123",
  "message": "API Request completed",
  "data": {
    "method": "POST",
    "path": "/api/users",
    "status": 200,
    "duration_ms": 45
  }
}

Client → API Gateway → Backend → Database
   ↓         ↓           ↓          ↓
req_abc   req_abc     req_abc    req_abc

Trackable with same Request ID across all layers

// middleware.ts
import { v4 as uuidv4 } from 'uuid';

export function generateRequestId(): string {
  return `req_${uuidv4().slice(0, 8)}`;
}

// Propagate via header
headers['X-Request-ID'] = requestId;

// API client
const requestId = headers['X-Request-ID'] || generateRequestId();

// Include in all logs
logger.info('Processing request', { request_id: requestId });

// Include in header when calling downstream services
await fetch(url, {
  headers: { 'X-Request-ID': requestId }
});

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    start_time = time.time()

    # Request logging
    logger.info(
        f"Request started",
        extra={
            'request_id': request_id,
            'data': {
                'method': request.method,
                'path': request.url.path,
                'query': str(request.query_params)
            }
        }
    )

    response = await call_next(request)

    duration = (time.time() - start_time) * 1000

    # Response logging (including 200 OK!)
    logger.info(
        f"Request completed",
        extra={
            'request_id': request_id,
            'data': {
                'status': response.status_code,
                'duration_ms': round(duration, 2)
            }
        }
    )

    response.headers['X-Request-ID'] = request_id
    return response

undefined

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# Business logic
user = User(**data)
db.add(user)
db.commit()

logger.info("User created", extra={
    'request_id': request_id,
    'data': {'user_id': user.id}
})

return user

---

// lib/logger.ts
type LogLevel = 'DEBUG' | 'INFO' | 'WARNING' | 'ERROR';

interface LogData {
  request_id?: string;
  [key: string]: any;
}

const LOG_LEVELS: Record<LogLevel, number> = {
  DEBUG: 0,
  INFO: 1,
  WARNING: 2,
  ERROR: 3,
};

const MIN_LEVEL = process.env.NODE_ENV === 'production' ? 'INFO' : 'DEBUG';

function log(level: LogLevel, message: string, data?: LogData) {
  if (LOG_LEVELS[level] < LOG_LEVELS[MIN_LEVEL]) return;

  const logEntry = {
    timestamp: new Date().toISOString(),
    level,
    service: 'web',
    request_id: data?.request_id || 'N/A',
    message,
    data: data ? { ...data, request_id: undefined } : undefined,
  };

  console.log(JSON.stringify(logEntry));
}

export const logger = {
  debug: (msg: string, data?: LogData) => log('DEBUG', msg, data),
  info: (msg: string, data?: LogData) => log('INFO', msg, data),
  warning: (msg: string, data?: LogData) => log('WARNING', msg, data),
  error: (msg: string, data?: LogData) => log('ERROR', msg, data),
};

// lib/api-client.ts
import { logger } from './logger';
import { v4 as uuidv4 } from 'uuid';

export async function apiClient<T>(
  endpoint: string,
  options: RequestInit = {}
): Promise<T> {
  const requestId = `req_${uuidv4().slice(0, 8)}`;
  const startTime = Date.now();

  logger.info('API Request started', {
    request_id: requestId,
    method: options.method || 'GET',
    endpoint,
  });

  try {
    const response = await fetch(`/api${endpoint}`, {
      ...options,
      headers: {
        'Content-Type': 'application/json',
        'X-Request-ID': requestId,
        ...options.headers,
      },
    });

    const duration = Date.now() - startTime;
    const data = await response.json();

    // Log 200 OK too!
    logger.info('API Request completed', {
      request_id: requestId,
      status: response.status,
      duration_ms: duration,
    });

    if (!response.ok) {
      logger.error('API Request failed', {
        request_id: requestId,
        status: response.status,
        error: data.error,
      });
      throw new ApiError(data.error);
    }

    return data;
  } catch (error) {
    logger.error('API Request error', {
      request_id: requestId,
      error: error instanceof Error ? error.message : 'Unknown error',
    });
    throw error;
  }
}

http {
    log_format json_combined escape=json '{'
        '"timestamp":"$time_iso8601",'
        '"level":"INFO",'
        '"service":"nginx",'
        '"request_id":"$http_x_request_id",'
        '"message":"HTTP Request",'
        '"data":{'
            '"remote_addr":"$remote_addr",'
            '"method":"$request_method",'
            '"uri":"$request_uri",'
            '"status":$status,'
            '"body_bytes_sent":$body_bytes_sent,'
            '"request_time":$request_time,'
            '"upstream_response_time":"$upstream_response_time",'
            '"http_referer":"$http_referer",'
            '"http_user_agent":"$http_user_agent"'
        '}'
    '}';

    access_log /var/log/nginx/access.log json_combined;
}

version: '3.8'
services:
  api:
    build: ./backend
    environment:
      - LOG_LEVEL=DEBUG
      - LOG_FORMAT=json
    logging:
      driver: json-file
      options:
        max-size: "10m"
        max-file: "3"

  web:
    build: ./frontend
    environment:
      - NODE_ENV=development
    depends_on:
      - api

  nginx:
    image: nginx:alpine
    volumes:
      - ./nginx/nginx.conf:/etc/nginx/nginx.conf
    ports:
      - "80:80"
    depends_on:
      - api
      - web

undefined

---

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User tests actual features in browser:
1. Sign up attempt
2. Login attempt
3. Use core features
4. Test edge cases

Claude Code in real-time:
1. Monitor log stream
2. Detect error patterns
3. Detect abnormal response times
4. Track entire flow via Request ID
5. Auto-document issues

undefined

---

{"level":"ERROR","message":"..."}

{"data":{"duration_ms":3000}}

3+ consecutive failures on same endpoint

{"data":{"status":500}}

┌─────────────────────────────────────────────────────────────┐
│                   Iterative Test Cycle                        │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│  Cycle N:                                                   │
│  1. Run test script (E2E or manual)                         │
│  2. Claude monitors logs in real-time                       │
│  3. Record pass/fail results                                │
│  4. Claude identifies root cause of failures                │
│  5. Fix code immediately (hot reload)                       │
│  6. Document: Cycle N → Bug → Fix                           │
│                                                             │
│  Repeat until acceptable pass rate (>85%)                   │
│                                                             │
└─────────────────────────────────────────────────────────────┘

#!/bin/bash

response=$(curl -s -X POST "$API_URL/api/v1/feature/action" \
    -H "Authorization: Bearer $TOKEN" \
    -H "Content-Type: application/json" \
    -d '{"param": "value"}')

if [[ "$response" == *"expected_result"* ]]; then
    echo -e "${GREEN}✅ PASS${NC}"
    ((PASS_COUNT++))
else
    echo -e "${RED}❌ FAIL${NC}"
    echo "Response: $response"
    ((FAIL_COUNT++))
fi

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---

✅ Request start (method, path, params)
✅ Request complete (status, duration_ms)
✅ Major business logic steps
✅ Detailed info on errors
✅ Before/after external API calls
✅ DB queries (in development)

✅ API call start
✅ API response received (status, duration)
✅ Detailed info on errors
✅ Important user actions