MCP Shield Runtime

Control every MCP tool call before it executes.

MCP Shield Runtime is a local-first security gateway for Model Context Protocol clients and servers. It provides parameter-level policies, tool visibility filtering, approval gates, emergency deny rules, secret redaction, tool-contract drift detection, rate limiting, and tamper-evident audit records.

Project status: production-oriented preview. The core gateway and controls are working and tested, but the project has not yet completed an independent security audit. Use staged rollout and least-privilege upstream credentials for production environments.

Why this project exists

An allowlist that says an agent may use read_file or send_email is not enough. Security decisions must consider the concrete path, recipient, repository, branch, URL, command, environment, identity, detected data, and prior approval.

AI client
   |
   v
MCP Shield Runtime
   |  policy, approval, DLP, contract lock, audit
   v
MCP server

Architecture

Related MCP server: lazy-mcp-router

How it works

Features

• Stdio proxy for local MCP servers

• Streamable HTTP reverse proxy

• MCP 2026 routing-header validation and generation

• Client-independent parameter-level policy decisions

• Built-in emergency blocks for secret paths, private metadata URLs, and destructive commands

• Human approval with retry grants

• Tool discovery filtering

• Tool schema and description lockfile

• Response secret redaction

• Tamper-evident hash-chained audit log with optional HMAC signatures

• Per-user, per-server, per-tool token-bucket rate limiting

• Local dashboard and approval queue

• Zero runtime npm dependencies

• Docker hardening example

• Node built-in test suite and CodeQL workflow

Requirements

• Node.js 22 or newer

• An existing MCP server

• JSON policy and configuration files

Quick start

git clone https://github.com/your-org/mcp-shield-runtime.git
cd mcp-shield-runtime
npm install
npm run check
node src/cli.js init --dir .mcp-shield

Protect a local stdio server

node src/cli.js run \
  --config .mcp-shield/config.json \
  -- node examples/risky-demo-server.js

Example MCP client configuration:

{
  "mcpServers": {
    "protected-demo": {
      "command": "node",
      "args": [
        "/absolute/path/mcp-shield-runtime/src/cli.js",
        "run",
        "--config",
        "/absolute/path/mcp-shield-runtime/.mcp-shield/config.json",
        "--",
        "node",
        "/absolute/path/mcp-shield-runtime/examples/risky-demo-server.js"
      ]
    }
  }
}

Protect a remote Streamable HTTP server

node src/cli.js gateway \
  --config config/default.config.json \
  --upstream https://upstream.example.com/mcp

Point the MCP client to http://127.0.0.1:7777/mcp. The local dashboard is at http://127.0.0.1:7777/.

Approval flow

When a call requires approval, the gateway returns JSON-RPC error -32070 with an approval ID.

node src/cli.js approvals list --config .mcp-shield/config.json
node src/cli.js approvals approve apr_ID \
  --config .mcp-shield/config.json \
  --ttl 300

Retry the identical tool call. The approval is scoped to the request fingerprint and expiration.

Example policy

{
  "version": 1,
  "defaults": { "decision": "deny" },
  "rules": [
    {
      "id": "allow-repository-read",
      "priority": 100,
      "match": {
        "methods": ["tools/call"],
        "tools": ["read_file"],
        "arguments": {
          "path": {
            "within": ["${workspace}"],
            "exclude": ["**/.env", "**/.ssh/**", "**/*.pem"]
          }
        }
      },
      "effect": { "decision": "allow" }
    },
    {
      "id": "approve-write",
      "priority": 90,
      "match": {
        "methods": ["tools/call"],
        "tools": ["write_file", "push_files"]
      },
      "effect": {
        "decision": "require_approval",
        "approvalTtlSeconds": 300
      }
    }
  ]
}

See Policy reference.

Security model

The gateway enforces what it can observe at the MCP boundary. A protocol proxy cannot fully constrain a local server process after launch. Strong filesystem and network isolation requires a container, operating-system sandbox, or workload boundary.

Important deployment rules:

1. Keep upstream credentials least-privileged.

2. Bind the dashboard to localhost unless an API token and TLS termination are configured.

3. Enable HMAC audit signing.

4. Use a container or operating-system sandbox for untrusted local servers.

5. Set allowSseWithoutInspection to false for tools requiring response DLP.

6. Pin upstream package and container versions.

7. Review tool-contract changes before enabling blockSchemaDrift.

Read SECURITY.md and Threat model.

Commands

mcp-shield init
mcp-shield gateway
mcp-shield run -- COMMAND
mcp-shield policy test
mcp-shield approvals list|approve|deny
mcp-shield audit list|verify
mcp-shield contracts list|approve
mcp-shield scan
mcp-shield doctor

Protocol compatibility

The proxy is protocol-neutral for JSON-RPC messages and can forward legacy and modern MCP traffic. The HTTP gateway validates or generates modern Mcp-Method and Mcp-Name routing headers. Stateful legacy HTTP servers may require upstream session handling specific to their deployment.

The project intentionally keeps policy logic separate from the official MCP SDK. SDK-backed client and server adapters can be added without moving the trusted policy core.

Documentation

• Architecture

• Policy reference

• Deployment guide

• Threat model

• Roadmap

• Contributing

License

Apache License 2.0.