Start: User clicks submit button
End: Error shown to user

Midpoint 1: Form validation
→ Data looks correct here? Continue to later code
→ Data already wrong? Focus on earlier code

Midpoint 2: API request construction
→ Request payload correct? Focus on server side
→ Payload already malformed? Focus on form handling

...Continue until exact line is found

# Start bisect
git bisect start

# Mark current (broken) state as bad
git bisect bad

# Mark last known good state
git bisect good v2.3.0

# Git checks out middle commit, test and mark
git bisect good  # or git bisect bad

# Repeat until found
# Git will report: "abc123 is the first bad commit"

# Clean up
git bisect reset

git bisect start HEAD v2.3.0
git bisect run npm test -- --grep "failing test"

function processOrder(order) {
  console.log('[TRACE] processOrder input:', JSON.stringify(order));

  const validated = validateOrder(order);
  console.log('[TRACE] after validation:', JSON.stringify(validated));

  const priced = calculatePrice(validated);
  console.log('[TRACE] after pricing:', JSON.stringify(priced));

  const result = submitOrder(priced);
  console.log('[TRACE] final result:', JSON.stringify(result));

  return result;
}

Input: { userId: "123", items: [...] }
    ↓
validateUser() → { userId: "123", verified: true }
    ↓
enrichItems() → { userId: "123", verified: true, items: [...enriched] }
    ↓
calculateTotals() → { ..., subtotal: 100, tax: 8, total: 108 }
    ↓
Output: { orderId: "456", total: 108 }

function handleRequest(req) {
  console.log('[TRACE] handleRequest entered');

  if (req.authenticated) {
    console.log('[TRACE] authenticated path');
    if (req.isAdmin) {
      console.log('[TRACE] admin path');
      return handleAdminRequest(req);
    } else {
      console.log('[TRACE] user path');
      return handleUserRequest(req);
    }
  } else {
    console.log('[TRACE] unauthenticated path');
    return handlePublicRequest(req);
  }
}

Full System: Frontend → API → Database
              ↓
Test 1: Frontend → Mock API
        → Works? Problem is in API or Database

Test 2: Real API → Mock Database
        → Works? Problem is in Database

Test 3: API with minimal data
        → Works? Problem is data-dependent

function complexFunction(input) {
  // BREAKPOINT 1: Entry - check input
  const step1 = transform(input);
  // BREAKPOINT 2: After first transformation

  for (const item of step1.items) {
    // BREAKPOINT 3: Inside loop - conditional on item
    process(item);
  }

  // BREAKPOINT 4: Exit - check output
  return finalize(step1);
}

-- Create isolated test data
BEGIN TRANSACTION;
-- Insert test data
-- Run test queries
-- Verify results
ROLLBACK;

// Intercept and log all network requests
const originalFetch = window.fetch;
window.fetch = async (...args) => {
  console.log('[TRACE] fetch:', args[0]);
  const response = await originalFetch(...args);
  console.log('[TRACE] response:', response.status);
  return response;
};

// Control time for debugging
const realNow = Date.now;
Date.now = () => {
  const time = realNow();
  console.log('[TRACE] Date.now():', new Date(time).toISOString());
  return time;
};