Agents multiply tokens
A chat answer is one model call. An agent works in a loop: read the task, plan, call a tool, read the result, decide again. Each step resends the instructions and everything gathered so far, so input grows with every call. Ten steps on a long document can cost more than a hundred simple summaries.
Rule of thumbBudget agents at 4 to 8 calls per task until you have measured the real number.