Why Your AI Agent's Token Bill Exploded
Nothing failed. The loop just paid for its own history, twelve times per task.
What you'll learn
- Why an agent loop’s input tokens grow with the square of its step count
- What prompt caching actually charges — reads at 0.1×, writes at 1.25×
- The one line that silently stops the cache being read, with no error
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Understand it one step at a time
The short runs these in order in about 51 seconds. Here they are written out — pick any step to jump the short straight to it.
The agent cost $4,260 last month
One customer-support triage agent. Four thousand completed tasks in a month. The invoice landed at six times the original estimate.
Frequently asked questions
- Why does an AI agent's token bill grow faster than the number of steps it takes?
- Most agent loops resend the entire conversation history with every step — step 10 re-sends everything from steps 1 through 9. That makes total input tokens grow roughly with the square of the step count, not linearly with it.
- How does prompt caching change the cost of that resent history?
- A cached read is billed at roughly 0.1× the normal input rate, while writing something new into the cache costs about 1.25×. The saving only materializes if the prefix you're resending is byte-identical each time — one line that changes something early in the prompt can silently break the cache and put you back to paying full price.
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