contentFilter

Creates a builder for content filtering rules using custom strings and regular expressions.

Signature

const builder = contentFilter(patterns, options?);

Parameters

Parameter	Type	Description
patterns	(RegExp | string)[]	Array of detection patterns (required)
options.name	string	Rule name (auto-generated if omitted)
options.label	string	Label for identification in detection results

String patterns are automatically converted to case-insensitive regular expressions.

Methods

Method	Returns	Description
.scope(...tools)	ActionBuilder	Limit to specific tools (string | RegExp)
.block(message?)	Rule	Block on violation (severity: "error")
.warn(message?)	Rule	Warn on violation (severity: "warn")
.log(message?)	Rule	Log violation (severity: "info")

Returns

DetectorBuilder — terminal methods return a Rule.

Examples

Basic

import { contentFilter } from "open-mcp-guardrails";

contentFilter(["confidential", /classified/i]).block();

With label

contentFilter(
  ["internal only", /do not distribute/i],
  { label: "internal_document" }
).warn();

Custom message

contentFilter(["top secret", "classified", /Project\s*X/i])
  .block("Confidential information detected");

Scope to specific tools

contentFilter(["classified"]).scope("filesystem__read_file").block();

JSON Config

Equivalent configurations using guardrails.json:

{
  "type": "content-filter",
  "action": "block",
  "patterns": ["confidential", "/classified/i"]
}

{
  "type": "content-filter",
  "action": "warn",
  "patterns": ["internal only", "/do not distribute/i"],
  "label": "internal_document"
}

{
  "type": "content-filter",
  "action": "block",
  "patterns": ["top secret", "classified", "/Project\\s*X/i"],
  "message": "Confidential information detected"
}

{
  "type": "content-filter",
  "action": "block",
  "patterns": ["classified"],
  "scope": ["filesystem__read_file"]
}

String patterns are matched case-insensitively. Use /pattern/flags syntax for regex patterns.

Related

• pii — PII detection
• secrets — Secret detection
• Custom rules guide — How to create custom rules