Does adding a safety prefix actually cost money?

Yes. Every extra instruction token is billed on every request. A 200-token defense prefix on a million monthly requests is 200 million extra input tokens, which can be a meaningful line item even though each individual call looks cheap.

How do I estimate injection risk and incident cost?

Risk is a rough probability that any given request contains a successful injection — for most apps this is well under 1%. Incident cost should capture remediation, data exposure, reputation, and potential regulatory exposure of a single breach. Both are estimates; use conservative ranges and test the sensitivity.

Is a system-prompt prefix enough defense?

No. Instruction prefixes reduce but do not eliminate injection — determined attackers bypass them. Treat the prefix as one cheap layer alongside input validation, output filtering, least-privilege tool access, and a separate guard model. This calculator only prices the prefix layer.

When is the defense clearly worth it?

When the risk-adjusted expected incident cost exceeds the monthly defense cost, the prefix pays for itself in pure expected-value terms — and that ignores the value of avoiding a tail-risk catastrophe, which usually tips the decision further toward defending.

Is anything uploaded?

No. The calculation is local arithmetic in your browser. Nothing you enter is sent to a server.

Prompt Injection Defense Token Cost Calculator

Prompt injection defense token cost

A safety prefix feels free — it is just a few sentences in the system prompt. But you pay for those sentences on every request, forever. This tool turns that into a monthly number and sets it next to the risk-adjusted cost of an injection attack, so the security decision becomes an expected-value comparison instead of a gut call.

How it works

The tool computes two figures over a 30-day month:

Defense cost = defense_tokens × input_price × requests_per_day × 30. This is the unavoidable overhead of carrying the prefix on every call.
Risk-adjusted incident cost = requests_per_day × 30 × injection_probability × incident_cost. This is the expected loss if you ship without the prefix, assuming the prefix is what stops those injections.

If the expected incident cost exceeds the defense cost, the prefix is worth it on average. The tool also shows the break-even injection probability — the risk level at which the two numbers are equal — so you can sanity-check whether your risk estimate is above or below that threshold.

Tips and notes

The break-even probability is usually tiny, which is why almost every production app carries some injection defense — the prefix is cheap relative to a single bad incident.
Defense tokens add to latency and context usage too, not just cost; keep the prefix tight.
This prices only the instruction-prefix layer. A real defense stack also includes a guard model (see the guard-model cost calculator), input sanitization, and constrained tool permissions — budget for those separately.
Tail risk matters: even when the expected-value math is close, a single catastrophic breach can dwarf years of prefix cost, so bias toward defending when incident cost is high.