cost-verification-auditor

Cost Verification Auditor

Verify that token cost estimates are within ±20% of actual Claude API usage.

When to Use

✅ Use for:

• Validating token estimation systems after implementation
• Pre-deployment cost accuracy checks
• Debugging unexpected API bills
• Periodic estimation drift detection

❌ NOT for:

• Looking up model pricing (use pricing docs)
• Budget planning or forecasting
• Cost optimization strategies
• Comparing models by price

Core Audit Process

Decision Tree

Has estimator? ──No──→ Build estimator first (see Calibration Guidelines)
      │
     Yes
      ↓
Define 3+ test cases (simple/medium/complex)
      ↓
Estimate BEFORE execution (no peeking!)
      ↓
Execute against real API
      ↓
Calculate variance: (actual - estimated) / estimated
      ↓
Variance ≤ ±20%? ──Yes──→ PASS ✓
      │
     No
      ↓
Apply fixes from Anti-Patterns section
      ↓
Re-run verification

Variance Formula

const inputVariance = (actual.inputTokens - estimate.inputTokens) / estimate.inputTokens;
const outputVariance = (actual.outputTokens - estimate.outputTokens) / estimate.outputTokens;
const costVariance = (actual.totalCost - estimate.totalCost) / estimate.totalCost;

// PASS if both input AND output within ±20%
const passed = Math.abs(inputVariance) <= 0.20 && Math.abs(outputVariance) <= 0.20;

Common Anti-Patterns

Anti-Pattern: The 500-Token Overhead Myth

Novice thinking: "Claude Code adds ~500 tokens overhead, so add that to every estimate."

Reality: Direct API calls have ~10 token overhead. The 500+ overhead is ONLY when using Claude Code's full context (system prompts, tools, conversation history).

Timeline:

• Pre-2025: Many tutorials used 500+ token estimates
• 2025+: Direct API overhead is minimal (~10 tokens)

What to use instead:

Context	Overhead
Direct API call	~10 tokens
With system prompt	50-200 tokens
With tools/functions	100-500 tokens
Claude Code full context	500-2000 tokens