Why does a system prompt cost so much?

A system prompt is re-sent as input tokens on every single call. A 2,000-token prompt across a million calls a month is two billion input tokens — that adds up even at low per-token prices.

How does prompt caching reduce this?

Providers offer cached input pricing — typically a large discount for repeated, unchanged prefix content like a system prompt. The tool estimates savings using a configurable cache discount applied to the system-prompt portion.

How accurate is the token estimate?

It approximates tokens from character count using a typical ratio. For exact counts use your provider's tokenizer, but the relative figure is reliable for budgeting and for comparing a trimmed prompt against the original.

Is my system prompt sent anywhere?

No. The estimate runs entirely in your browser. Your prompt is never uploaded, stored, or logged.

Does this include output cost?

No. This tool isolates the cost of the always-on system prompt, which is pure input. Use a full API cost calculator to add per-call output costs on top.

Hidden System Prompt Token Cost Estimator

What does your hidden system prompt really cost?

Most production LLM apps prepend a large system prompt the user never sees — instructions, tone, guardrails, tool definitions. Because it is re-sent as input on every call, its cost scales with your traffic and quietly becomes one of your biggest line items. This tool isolates that cost.

How it works

The tool estimates the token size of your system prompt, then multiplies by your monthly call volume and your model’s input price:

monthly_cost = system_prompt_tokens
             × calls_per_month
             / 1,000,000
             × input_price_per_million

It also shows the cached figure — providers discount repeated, unchanged prefix content heavily, and a stable system prompt is the ideal candidate.

Tips to cut the cost

Trim ruthlessly. Every token here is paid on every call. Remove filler, redundant examples, and decorative formatting.
Enable prompt caching. A static system prompt is the textbook caching win — often a large discount on the prefix portion.
Move rarely-needed instructions out. Conditional rules that apply to 5% of requests can be injected only when relevant rather than living in the always-on prompt.