Claude API Cost Tracking

Overview

Track and report Claude API usage costs per session with exact pricing, aggregation, and frontend display.

Core principle: Track every call. Use exact pricing. Display to users. Provide /cost command.

Announce at start: "I'm using the claude-mobile-cost-tracking skill for cost tracking implementation."

When to Use

• Implementing cost tracking service (Task 3.11)
• Integrating cost display in Settings screen
• Implementing /cost slash command
• Calculating API costs
• Displaying cost metrics to users

Exact Pricing (Claude Sonnet 4)

const PRICING = {
  input: 0.003,   // $0.003 per 1k input tokens
  output: 0.015,  // $0.015 per 1k output tokens
};

Source: Anthropic API pricing (verified 2025-10-30)

Implementation Patterns

1. Per-Message Cost Calculation

interface MessageCost {
  inputTokens: number;
  outputTokens: number;
  inputCost: number;
  outputCost: number;
  totalCost: number;
  timestamp: string;
}

function calculateCost(usage: {
  input_tokens: number;
  output_tokens: number;
}): MessageCost {
  const inputCost = (usage.input_tokens / 1000) * PRICING.input;
  const outputCost = (usage.output_tokens / 1000) * PRICING.output;
  
  return {
    inputTokens: usage.input_tokens,
    outputTokens: usage.output_tokens,
    inputCost,
    outputCost,
    totalCost: inputCost + outputCost,
    timestamp: new Date().toISOString()
  };
}

2. Session Aggregation

interface SessionCosts {
  sessionId: string;
  messages: MessageCost[];
  totalInputTokens: number;
  totalOutputTokens: number;
  totalCost: number;
}

function aggregateSessionCosts(messages: MessageCost[]): SessionCosts {
  return {
    messages,
    totalInputTokens: messages.reduce((sum, m) => sum + m.inputTokens, 0),
    totalOutputTokens: messages.reduce((sum, m) => sum + m.outputTokens, 0),
    totalCost: messages.reduce((sum, m) => sum + m.totalCost, 0)
  };
}

3. Storage with Session Data

// In session JSON file
interface Session {
  id: string;
  projectPath: string;
  messages: Message[];
  costs: SessionCosts; // Add this
  createdAt: string;
}

4. Frontend Display (Settings Screen)

// Settings screen cost section
<View testID="cost-section" style={styles.costSection}>
  <Text style={styles.sectionTitle}>API Usage</Text>
  <Text testID="message-count">Messages: {session.messages.length}</Text>
  <Text testID="input-tokens">Input Tokens: {costs.totalInputTokens.toLocaleString()}</Text>
  <Text testID="output-tokens">Output Tokens: {costs.totalOutputTokens.toLocaleString()}</Text>
  <Text testID="total-cost" style={styles.cost}>
    Total Cost: ${costs.totalCost.toFixed(4)}
  </Text>
</View>

5. /cost Slash Command

// In command.service.ts
if (message.startsWith('/cost')) {
  const sessionCosts = getSessionCosts(sessionId);
  return {
    type: 'slash_command_response',
    command: 'cost',
    data: sessionCosts
  };
}

Backend Service (Task 3.11)

// cost.service.ts
export class CostService {
  calculateMessageCost(usage: {input_tokens: number; output_tokens: number}): MessageCost {
    // Implementation from pattern #1
  }
  
  aggregateSessionCosts(sessionId: string): SessionCosts {
    // Implementation from pattern #2
  }
  
  getAllSessionsCosts(): SessionCosts[] {
    // Return costs for all sessions
  }
  
  exportCostsCSV(): string {
    // Export as CSV for analysis
  }
}

Common Mistakes

Mistake	Reality
"Cost tracking is optional"	WRONG. Users need visibility. Required feature.
"Approximate costs are fine"	WRONG. Use exact: $0.003/$0.015 per 1k.
"Track totals only"	WRONG. Per-session tracking enables analysis.
"Backend only is enough"	WRONG. Frontend display is user-facing requirement.

❌ WRONG: No cost tracking

const stream = await client.messages.stream({...});
// No cost tracking

✅ CORRECT: Track every call

const stream = await client.messages.stream({...});
stream.on('message', (msg) => {
  if (msg.usage) {
    const cost = calculateCost(msg.usage);
    saveSessionCost(sessionId, cost);
  }
});

Red Flags

• "Cost tracking is overhead" → WRONG. Required feature.
• "Users check API dashboard" → WRONG. In-app display required.
• "Approximate is fine" → WRONG. Use exact formulas.

Integration

• Use WITH:

  @anthropic-streaming-patterns

  (streams provide usage data)
• Use FOR: Task 3.11 (cost.service.ts), Settings screen display