Slim Guard MCP server

MCP Slim Guard

MCP Slim Guard gives a host one deliberate way to reach authorized upstream tools. It keeps input schemas intact, validates calls before the upstream is invoked, and can recover an oversized result exactly without repeating that invocation.

Choose a mode

Compact is the default for start. Native is the default installation plan for Codex. Compact is the default for Claude Code; Claude Code supports Compact and Extreme installation plans, not Native.

All modes apply the same allow/deny authorization. Compact and Extreme return at most three discovery matches, each with the original required fields, enums, and nested schema. A call is validated against that original schema before the upstream server is contacted.

find_tool rejects unknown fields and queries longer than 256 Unicode code points locally. It searches only the current authorized catalog and never falls back to an unauthorized entry.

Quick start

0.1.1-alpha.2 is a source candidate and is not published to npm yet. From a reviewed checkout of this candidate:

npm ci
npm run build
npm link
cd /path/to/project-with-mcp-config
mcp-slim-guard init
mcp-slim-guard validate

# Run the default Compact mode.
mcp-slim-guard start

init writes mcp-slim-guard.yml with version 2. The file contains upstream and security settings only. Select a mode in the host command, not in YAML.

# Inspect a host-specific plan without changing host files.
mcp-slim-guard plan --host codex
mcp-slim-guard plan --host claude-code --mode extreme

# Verify the configured runtime without changing Host configuration or calling a business Tool.
mcp-slim-guard verify --host codex

# Apply a reviewed plan.
mcp-slim-guard install --host codex --mode native

Host entries

For Codex, use Native when you want the normal authorized MCP catalog:

[mcp_servers.slim_guard]
command = "mcp-slim-guard"
args = ["start", "--mode", "native"]
cwd = "/absolute/path/to/project"

For Claude Code, use Compact by default:

{
  "mcpServers": {
    "slim-guard": {
      "command": "mcp-slim-guard",
      "args": ["start", "--mode", "compact"],
      "cwd": "/absolute/path/to/project"
    }
  }
}

See host setup for the full configuration and verification steps.

Configuration

version: 2
tools:
  allow:
    - "upstream_*"
  deny:
    - "*_delete_*"
ssrf:
  mode: block
  block_private_ips: true
  allow_domains: []
  block_domains: []
rate_limit:
  default: "60/min"
injection_detection:
  enabled: true
  sensitivity: medium
  mode: block
audit:
  output: file
  filePath: mcp-slim-guard-audit.log
servers:
  upstream:
    command: npx
    args: ["-y", "@your/mcp-server"]

Version 1 configuration is intentionally unsupported. The removed compressor section is rejected rather than migrated automatically.

Repair an invalid call

Shortened. The full README is on GitHub.