How to build a prompt firewall

Building an effective prompt firewall requires combining multiple detection techniques with a robust policy engine. This guide covers architecture patterns for enterprise-grade implementation.

Architecture overview

A production prompt firewall consists of four major subsystems: the interception layer (proxy), the detection pipeline, the policy engine, and the logging infrastructure. Each must be designed for low latency, high throughput, and multi-tenant isolation.

Detection pipeline design

The detection pipeline should run multiple engines in parallel: regex-based pattern matching for known attacks, ML classification for intent analysis, entity recognition for PII detection, and embedding similarity for document leak detection. Parallel execution keeps total latency under 100ms.

Proxy deployment patterns

Three main deployment patterns exist: forward proxy (CLI and API traffic), browser extension (web AI tools), and ICAP integration (network gateway). Each intercepts traffic at a different point but feeds into the same detection and policy infrastructure.

Policy engine requirements

The policy engine must support configurable thresholds, multi-signal evaluation, tenant-level customization, and deterministic enforcement. Actions include allow, mask, flag, and block.

Build vs buy analysis

Building in-house requires significant investment in ML training data, detection engine development, and multi-surface proxy infrastructure. Most enterprises find purpose-built platforms like PromptWall provide faster time-to-value. See our build vs buy comparison.