# 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.

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## 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       |
                                    +------------------------+
```

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## 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.

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## 3. Benefits

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