Aegis MCP Server

aegis-mcp-server

MCP enforcement layer for the Aegis agent governance specification.

The spec writes the law. The CLI generates the law. This enforces the law.

What It Does

aegis-mcp-server is an MCP server that validates every agent action against your .agentpolicy/ files before it happens. Path permissions, content scanning, role boundaries, quality gates — all enforced at runtime with zero token overhead to the agent.

The agent never loads your governance files. The MCP server reads them into its own process memory and validates silently. The agent calls governed tools and gets back either a success or a blocked response with the specific reason.

Quick Start

# Install globally
npm install -g aegis-mcp-server

If you generated your policy with aegis-cli, the .mcp.json connection config is already in your project root. Just install the MCP and open your agent — it connects automatically.

First Prompt

When starting a new agent session in a governed project, use this as your first prompt:

Call aegis_policy_summary now. This is your governance contract — it defines your
role, your boundaries, and which tools to use. Do not read files, do not take any
action, and do not assume your role until you have called this tool.

How It Works

Universal Mode (Default)

The MCP starts without a pre-assigned role. When the agent calls aegis_policy_summary, it receives the list of available roles from .agentpolicy/roles/. The agent presents them to the user, the user picks, and the agent calls aegis_select_role to lock in. All enforcement uses the selected role for the rest of the session.

This is the default behavior — no configuration needed beyond the .mcp.json that aegis init creates automatically.

Fixed Mode

If you know which role to assign at startup:

{
  "mcpServers": {
    "aegis": {
      "command": "aegis-mcp",
      "args": ["--project", ".", "--role", "backend"]
    }
  }
}

The MCP locks to that role immediately. aegis_policy_summary returns the role's boundaries directly, skipping role selection.

Tools

Zero Token Overhead

Traditional approach: load governance files into the agent's context window. Token cost scales with policy complexity.

Aegis MCP approach: the server loads policy into its own process memory. The agent calls tools and gets structured results. A project with 200 lines of governance has the same token cost as one with 20 lines. The complexity is absorbed by the server, not the agent.

Enforcement

• Governance boundaries — writable, read_only, forbidden path lists
• Role scoping — agents confined to their role's writable and readable paths
• Sensitive pattern detection — content scanned against governance-defined patterns
• Cross-domain boundaries — imports validated against shared interface rules
• Quality gate validation — pre_commit flags mapped to build_commands and executed
• Override logging — every blocked action logged to append-only overrides.jsonl
• Immutable policies — designated rules that cannot be overridden, ever

Override Protocol

When an action is blocked and the governance override behavior is warn_confirm_and_log:

1. The blocked response includes an override_token and the specific policy violated
2. The agent presents the violation to the user
3. If the user confirms, the agent calls aegis_request_override with the token and the user's rationale
4. The action proceeds — the override is logged with human_confirmed: true
5. Normal governance resumes immediately — the override is a one-time exception

Immutable policies (e.g., HIPAA compliance, audit completeness) return override_available: false and cannot be overridden. The user must modify the governance through aegis init.

Consent-Based Governance

The Aegis MCP does not override the agent's native directives. It introduces itself through tool descriptions, explains why governance is active, and asks the agent to seek u