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.

Related MCP server: mcp-trust-guard

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

If arguments do not match an authorized tool's original schema, Slim Guard returns a normal MCP tool error with structuredContent that identifies the schema failure and confirms that the upstream tool was not invoked. Correct the arguments and retry; submitted argument values are not echoed in that error.

Result recovery

When a response needs recoverable delivery, the first response includes a result_ref. Use read_result with query to locate up to three bounded local fragments, or omit query and follow next_cursor to recover the exact snapshot. Both paths use the same immutable capture and never invoke the upstream tool again. Do not combine query with cursor. Native and Compact use the standard delivery boundary. Extreme uses a smaller boundary only when the initial delivered response is at least half the size of the exact response; otherwise it passes the exact response through unchanged.

Evidence and checks

The repository includes a 24-task mode comparison, result-recovery fixtures, and a 100-tool / 8,000-row stress fixture. Run the current artifacts with:

npm run build
npm run bench:task
npm run bench:compression
npm run bench:stress
npm run bench:compression:verify

Their reports are mode-specific and are generated locally; do not treat older alpha benchmark figures as claims for this product line.

Development

npm run build
npm run typecheck
npm run lint
npm test
npm run verify:package-boundary
npm run smoke:package

This project is an alpha. Review authorization patterns and upstream settings before using it with privileged tools.