Comparisons
COMPARISON
Locke vs Prompt Engineering and System Prompts
System prompts instruct a model how to behave — 'don't repeat personal information,' 'refuse to process medical records,' and so on. These are behavioral controls: they shape outputs, not what leaves your device. By the time a model reads its system prompt, the full prompt — sensitive fields included — has already crossed the network. A behavioral instruction and a data control are different things, working at different layers.
When Prompt engineering and system prompts fits
- You need to shape model tone, format, scope, and refusal behavior — system prompts are exactly the right tool for that.
- You are deploying an AI product and need to define the model's persona and constrain its response patterns.
- You want to prevent the model from reproducing sensitive data in its outputs — system prompts can help with that.
When Locke fits
- Your concern is data-in-transit — sensitive fields leaving the device in the first place.
- You need a control that is resilient to prompt injection, which can override system-prompt instructions.
- You need auditable evidence of what was detected and how it was handled, not just a behavioral guideline.
Side-by-side
| Dimension | Locke | System prompts |
|---|---|---|
| When it acts | Before the prompt is sent — data is masked at composition time on the device. | After the prompt is sent — the instruction is part of the transmitted payload. |
| Does sensitive data leave the device? | No — masked content is substituted before transmission. | Yes — the full prompt including sensitive fields is transmitted. |
| Resilience to prompt injection | Not relevant — data is stripped before reaching the model. | Vulnerable — injected instructions can override system-prompt constraints. |
| Auditable evidence | Local detection log of what was flagged and how it was handled. | None — behavioral, not evidential. |
| Best for | Preventing specific data from reaching any model context. | Shaping model behavior once the model has received input. |
Bottom line
System prompts and Locke solve different problems and belong in the same stack. A system prompt can tell a model not to repeat an SSN — but the SSN already left your device in the prompt. Locke removes it before transmission, so it never enters the model's context at all. Use both.
