io.github.cleburn/aegis-mcp

Official

by cleburn · TypeScript

Runtime governance enforcement for AI agents. Zero token overhead.

Enforce runtime governance policies on AI agent behavior without token overhead.

aegis-mcp-server

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MCP enforcement layer for the Aegis agent governance specification.

Policy at the root. Enforcement at runtime. Accountability on every action.

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

bash

# 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:

code

Call aegis_policy_summary now. This is your governance contract — it defines your
role, your boundaries, and which tools to use. Do not take any action until you have
called this tool and received confirmation from the user to proceed.

For initial builds, the Aegis CLI generates a custom handoff prompt tailored to your project — use that instead.

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 — including the built-in construction role and all specialist 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.

Construction Mode

The construction role is always available for initial builds and major restructuring. When selected:

The agent has full repository access (all paths writable and readable)
The .agentpolicy/ files serve as the blueprint — the agent reads constitution, governance, and role files to understand the project's architecture, conventions, and quality standards
File operations run through native tools rather than governed tools, for speed
The MCP logs the construction session start to state/overrides.jsonl with a timestamp and human_confirmed: true
When the build is complete, the agent calls aegis_complete_task to run quality gates and close construction mode — the closing timestamp is logged alongside the opening entry
All future sessions after construction should select a specialist role for governed operations

Construction mode is not a bypass — the agent still follows the governance files as its blueprint. It's a speed optimization for greenfield builds where enforcing write restrictions on every file would be counterproductive.

Fixed Mode

If you know which role to assign at startup:

json

{
  "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

Tool	What it does	Token cost
`aegis_policy_summary`	Role boundaries and governance summary (or available roles in universal mode)	~200 tokens
`aegis_select_role`	Select a role (including construction) in universal mode	Tiny
`aegis_check_permissions`	Pre-check if an operation is allowed	Tiny
`aegis_write_file`	Governed write with path + content validation	Same as a normal write
`aegis_read_file`	Governed read with path validation	Same as a normal read
`aegis_delete_file`	Governed delete with path validation	Tiny
`aegis_execute`	Governed command execution	Command output only
`aegis_complete_task`	Run quality gates and close construction mode if active	Gate results only
`aegis_request_override`	Execute a blocked action after human confirmation	Tiny

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 regex patterns (content only, not path-based)
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
Construction session logging — start and end timestamps for initial builds

Override Protocol

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

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

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

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 user permission to route write operations through Aegis tools. The user's authority is the enforcement mechanism.

Native tools for reading, searching, and exploring the codebase work fine without governance gating. Only write, delete, and execute operations benefit from routing through Aegis.

Architecture