What is a Policy Engine?
A policy engine evaluates requests against predefined rules and returns allow/deny decisions. In PolicyLayer, the policy engine evaluates every MCP tool call against YAML-defined policies — enforcing tool-level governance in real-time, outside the LLM's reasoning loop.
WHY IT MATTERS
Policy engines are proven in infrastructure — firewalls, IAM systems, API gateways all use them. Define rules declaratively; the engine enforces consistently on every request. The pattern applies directly to AI agent tool calls.
PolicyLayer's policy engine evaluates each tool call against YAML policy files. For every call, it checks the tool name against allow/deny lists, validates arguments against defined constraints, enforces rate limits, and returns an allow or deny decision — all in single-digit milliseconds.
The power lies in composability: "allow read_file for any path, allow write_file only to /tmp/, deny execute_command entirely, rate-limit all tools to 60 calls per minute." These rules compose into a comprehensive policy that governs all tool access.
HOW POLICYLAYER USES THIS
PolicyLayer is a YAML policy engine purpose-built for MCP tool calls. Policies are defined declaratively in YAML files — no code, no complex DSL. The engine evaluates every tool call before it reaches the MCP server, providing deterministic enforcement that cannot be bypassed by the LLM. Open source, Apache 2.0, written in Go.