You are helping me build the following AI agent workflow.

## Goal
Audit SaaS Subscription Waste: Monthly audit scans Gmail for billing emails, extracts vendor details via GPT-4.1-mini, updates a Google Sheet.

## Specification
- What it does: You automatically scan your Gmail for billing emails to identify subscription costs and renewal dates without manual tracking. The agent extracts vendor details, calculates annualized spending, and updates a Google Sheet while emailing you a summary of total waste to help you cut unnecessary expenses.
- Trigger: On a schedule (Scheduled · monthly)
- Autonomy: Fully hands-off
- Expected setup effort: about an afternoon
- Tools/services involved:
  - Gmail: Search billing emails and send digest report
  - OpenAI Chat Model: Extract subscription details and classify spend
  - Google Sheets: Store vendor data and upsert results
  - Code: Clean JSON and calculate annualized costs

## Known pitfalls, handle each one explicitly in your implementation
1. Currency symbols vary across providers so parse amounts as strings first then convert to a float for accurate summation.

## Reference implementation
https://n8n.io/workflows/15945 (workflow template)
Fetch and inspect this before building. If it matches my stack, adapt it;
if not, rebuild the pattern with my tools.

## Process requirements
1. Before building: ask me which of the listed tools I actually use and
   what my platform is (n8n / Make / code / other). Do not assume.
2. Adapt the pattern to my answers; do not force the reference stack.
3. Address every pitfall above; tell me how you handled each.
4. Provide a test plan I can run before letting this touch real data.
5. Ask before any step that sends messages, modifies data, or spends money.

Source: https://usecasesforagents.com/use-case/audit-saas-subscription-waste/ via usecasesforagents.com