Cost Watchdog

Cost Watchdog

When to run this skill

• Before introducing any third-party service (LLM provider, DB, queue, monitoring, search, image, email).
• Before suggesting a paid tier of any service the user is currently on free tier of.
• When proposing model selection (Claude Opus vs Sonnet vs Haiku, GPT-4 vs 4o-mini, etc.).
• When implementing patterns that scale poorly: N+1 queries, polling loops, full-text scans, unbounded retries.
• When the user says "está caro" / "está cobrando demais" / "fiquei sem créditos".

Cost flags to watch

LLM costs

• Default to Claude Haiku 4.5 or Sonnet 4.6 for routine work. Use Opus only for tasks that justify it (deep reasoning, business strategy, architecture).
• Always cache prompts that have a stable system prefix (saves up to 90% of input tokens).
• Loops are dangerous. If a feature calls the LLM in a loop, calculate worst case before shipping.
• Streaming + early termination when user might stop reading.

Hosting

• Vercel Hobby (free) is fine until ~100 users. Pro ($20/mo) only when you hit limits.
• Supabase Free tier: 500 MB DB, 1 GB storage, 50K monthly active users. Plenty for prototypes.
• Don't enable database point-in-time recovery before paying users exist.

Database patterns

• Unbounded SELECT without pagination → flag.
• Realtime subscriptions on noisy tables → flag.
• Full-table scans in serverless functions → flag (cold-start tax + bandwidth).

Third-party services

• Stripe: percentage-based, no flag below revenue.
• Resend: 3K emails/month free. Flag transactional volume design.
• PostHog free tier: 1M events/mo. Flag if event firing in loops.
• Sentry free: 5K errors/mo. Flag noisy retry catch blocks.

Vibe coder explanation

> Cloud services are like a cab meter. Some run flat-rate (you pay even idle), some run per-trip (per request), some are free up to a limit then suddenly expensive. Before you ship, I'll show you which meter is running and what the worst-case bill looks like if your idea goes viral.

Output

💰 cost-watchdog flags

[HIGH]  Using Claude Opus inside a loop that may run 50+ times per user action.
        Estimated cost: $0.45 per user action. At 1000 users/day = $13,500/month.
        Suggest: switch to Sonnet 4.6 ($0.06/action) or Haiku 4.5 ($0.01/action).

[MED]   Realtime Supabase subscription on `messages` table without filter.
        Will fire on every insert across all users.
        Suggest: filter by chat_id.

[OK]    Stripe checkout: percentage-based, no fixed cost.
[OK]    Vercel deploy: still on Hobby free tier.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: lglucas
• Source: lglucas/ai-dev-operating-system
• License: MIT
• Homepage: https://github.com/lglucas/ai-dev-operating-system

Install and usage instructions live in the source repository linked above.