mcp-shield

The nginx of MCP — drop-in resilience middleware for any MCP server.

Timeout · Retry · Circuit Breaker · Structured Logging

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Why

MCP servers have zero built-in resilience. A hung GitHub API blocks your agent for 600 seconds. A transient network blip crashes the entire chain. A dead server keeps getting hammered with requests.

mcp-shield is a transparent stdio proxy that sits between your agent and MCP server. One command, zero code changes.

Agent ←stdio→ mcp-shield ←stdio→ MCP Server

Install

npm install -g @daino/mcp-shield

Or run directly with npx:

npx @daino/mcp-shield wrap -- npx @modelcontextprotocol/server-github

Quick Start

# Wrap any MCP server with sensible defaults (30s timeout, 2 retries)
mcp-shield wrap -- npx @modelcontextprotocol/server-github

# Custom timeout and retries
mcp-shield wrap --timeout 60s --retries 5 -- npx server-github

# Using a config file
mcp-shield wrap --config mcp-shield.yaml --server github

Claude Desktop

Add to your claude_desktop_config.json:

{
  "mcpServers": {
    "github": {
      "command": "npx",
      "args": [
        "@daino/mcp-shield", "wrap",
        "--timeout", "30s", "--retries", "3",
        "--",
        "npx", "@modelcontextprotocol/server-github"
      ],
      "env": {
        "GITHUB_TOKEN": "your-token-here"
      }
    }
  }
}

Done. Your GitHub MCP server now has timeout, retry, and circuit breaker protection.

Features

Timeout

Kill hung tool calls. No more 600-second waits.

timeout: 30s  # per-tool override available

Retry

Exponential backoff + jitter. Deterministic errors (invalid params, method not found) are never retried.

retries:
  max: 3
  backoff: exponential  # 1s → 2s → 4s
  jitter: true

Circuit Breaker

After repeated failures, fail fast instead of burning tokens on a dead server.

circuit_breaker:
  threshold: 5      # open after 5 consecutive failures
  reset_after: 60s  # try again after 60 seconds

States: Closed (normal) → Open (rejecting) → Half-Open (testing)

Structured Logging

Every tool call logged as structured JSON to stderr:

{
  "level": "info",
  "msg": "tool_call_end",
  "server": "github",
  "tool": "get_file_contents",
  "duration_ms": 245,
  "status": "success",
  "attempt": 1
}

Config File

For multi-server setups:

# mcp-shield.yaml
defaults:
  timeout: 30s
  retries:
    max: 3
    backoff: exponential
    jitter: true
  circuit_breaker:
    threshold: 5
    reset_after: 60s

servers:
  github:
    command: "npx @modelcontextprotocol/server-github"
    env:
      GITHUB_TOKEN: "${GITHUB_TOKEN}"
    tools:
      get_file_contents:
        timeout: 60s          # slow tool gets more time
      search_repositories:
        retries:
          max: 5              # flaky tool gets more retries

  filesystem:
    command: "npx @modelcontextprotocol/server-filesystem /home/user"
    timeout: 10s
    retries:
      max: 1

Programmatic Usage

import { shield } from '@daino/mcp-shield';

const proxy = shield({
  command: 'npx',
  args: ['@modelcontextprotocol/server-github'],
  timeout: 30_000,
  retries: { max: 3, backoff: 'exponential', jitter: true },
  circuitBreaker: { threshold: 5, resetAfter: 60_000 },
});

proxy.start();

Comparison

	mcp-shield	No protection	General retry libs
MCP-native (JSON-RPC aware)	✅	—	❌
Per-tool config	✅	—	❌
Zero agent code changes	✅	—	❌
Circuit breaker	✅	❌	✅
Structured MCP logging	✅	❌	❌
Drop-in Claude Desktop support	✅	—	❌

Roadmap

• Timeout + Retry + Circuit Breaker + Logging
• Response Validation (schema check)
• Tool Filtering (expose only specific tools)
• Rate Limiting (per-tool call caps)
• Metrics Export (Prometheus-compatible)
• Multi-server Composition
• Hot-reload config
• Dashboard UI

Contributing

Contributions welcome! Please open an issue first to discuss what you'd like to change.

git clone https://github.com/DainoJung/mcp-shield.git
cd mcp-shield
npm install
npm test

License

MIT