My Claude API bill hit $340 last month.
Not because I was doing anything wrong — I was just building something that scales, and the costs were scaling with it.
I spent a few weeks auditing every single API call. What I found was embarrassingly fixable: about 75% of my costs came from three patterns I kept repeating without realizing it.
The 3 patterns eating my budget
1. Sending the same context on every call
I had a ~2,000 token system prompt going out with every request. No caching. Pure waste.
Fix: prompt caching. Two lines of code. Cut 40% of the bill overnight.
2. Using Sonnet where Haiku was more than enough
Classification tasks. Simple extraction. Routing decisions. All running on Sonnet.
Fix: model routing logic. Haiku costs ~20x less for tasks that don't need reasoning depth.
3. Measuring tokens instead of cost-per-task
I was watching token counts but had no idea what each feature actually cost end-to-end.
Fix: instrument cost-per-task. Immediately obvious where the leaks are.
What I built
I turned this process into an interactive workbook: 5 modules, runs in the browser, no login, no install.
Each module has an actual exercise you run against your own API key — so you see the token diff in real time, not just in theory.
It's free. Pay what you want if you find it useful.
Results
My bill went from $340 → $67/month. Your mileage will vary depending on your use case, but the patterns are consistent across most Claude API setups.
Curious what optimizations others have found — drop them in the comments.
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