Product Design: The MCP Constraint Builder (Standalone)

This document outlines the product design and system architecture for a standalone security product: The MCP Constraint Builder. This tool acts as an auditing and policy-enforcement layer for AI agents, allowing users to visually configure execution policies and parameters for third-party MCP servers.

1. Core Concept

The MCP Constraint Builder is a security firewall and configuration manager for local AI agents. It intercepts JSON-RPC requests sent by AI clients (such as Claude, Cursor, or local coding assistants) to local MCP servers.

+------------+       JSON-RPC       +------------------------+       JSON-RPC       +------------------+
| AI Client  |  ----------------->  |   MCP Policy Proxy     |  ----------------->  | Target MCP Server|
| (Claude/etc|                      | (Enforces rules.json)  |                      |   (Proprietary)  |
+------------+                      +-----------+------------+                      +------------------+
                                                |
                                                | Reads policies
                                                v
                                    +------------------------+
                                    |       rules.json       |
                                    +------------------------+

2. Key Features

Visual Constraint Editor

A desktop application that allows users to import any standard MCP server configuration. The tool inspects the server's available tools (list_tools) and dynamically renders a visual configuration interface:

Allow/Deny lists: Explicitly toggle which tools (e.g. run_command, read_file) the agent is allowed to access.
Parameter boundaries: Set regex patterns or value ranges for arguments (e.g. "Only allow read_file if the path starts with C:/Users/Owner/workspace").
Execution budget: Set rate limits (e.g. "maximum 10 tool calls per minute").

Proxy Firewall (JSON-RPC Middleware)

A lightweight background proxy runner. When the AI agent attempts to execute a tool, the proxy checks the arguments against rules.json:

Pass: If within bounds, forwards the request directly to the target server.
Block: If out of bounds, immediately returns an authorization error back to the AI client without executing the command.
Escalate: If marked for manual verification, prompts the user on their host machine for real-time bypass approval.

3. Benefits

Enterprise Compliance: Solves the data protection and code execution trust gap for running local agents.
IP Protection: Allows creators of advanced AI agents to sell compiled/obfuscated MCP binaries while giving users full visibility and boundary control.
Ecosystem Agnostic: Works with any standard MCP client and server protocol without requiring code changes to either.

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