Your MCP Tool Schemas Are Eating 86% of Your Context — Fix It with One Proxy
mcp-slim-guard is a free open-source proxy that compresses MCP tool schemas by up to 86%, saving thousands of tokens per call — while adding SSRF protection, injection detection, rate limiting, and audit logging.
Your MCP Tool Schemas Are Eating 86% of Your Context — Fix It with One Proxy
When you connect 10-15 MCP tools to your AI agent, you're silently burning $40-50/month on tool schema tokens alone. Here's a one-command fix.
The Silent Context Killer
If you use Claude Code, Cursor, Codex, or any AI coding agent with MCP servers, you've probably noticed something: the more tools you add, the dumber your agent gets.
That's not your imagination. Every MCP server exposes a tools/list response with JSON schemas for each tool — parameter names, types, descriptions, required fields. With 10-15 tools, that's 1,700+ tokens of pure schema metadata, loaded on every conversation turn.
At DeepSeek V4 Flash pricing ($0.15/M tokens), 10,000 tool calls/month without compression:
| Setup | Tokens/call | Monthly cost |
|---|---|---|
| 14 tools, full schema | 1,736 | ~$52 |
| With mcp-slim-guard (light) | 300 | ~$9 |
That's an 83% savings — $43/month — just from schema compression.
But compression alone isn't enough. You also need security.
Introducing mcp-slim-guard
mcp-slim-guard is a free open-source MCP proxy that sits between your AI agent and your MCP servers, adding two superpowers in one CLI:
🗜️ Schema Compression — 5 Levels
| Level | Strategy | Savings | Best for |
|---|---|---|---|
light | 3 wrapper tools, on-demand schema | -83% | ⭐ Default. Best balance |
normal | 2 wrapper tools | -86% | 30+ tools, strong LLMs |
extreme | Strip descriptions in-place | -22% | Few tools with complex schemas |
maximum | Signature only | -25% | Very large individual schemas |
lazy | Budget preload + on-demand | -5% | 30+ tools, rarely used |
The key insight: light/normal replace all tools with 2-3 wrapper tools. The LLM fetches the actual schema on demand only when it needs to call a specific tool. Most LLMs never use all tools in a single conversation, so this saves context on every turn.
🛡️ Security Pipeline — Defense in Depth
Every tool call runs through a serial pipeline:
Agent Request → 1. Allow/Deny → 2. SSRF Shield → 3. Injection Detection → 4. Rate Limit → 5. Audit Log → Upstream
- Allow/Deny: Glob-based pattern matching. Fail-closed by default.
- SSRF Shield: Blocks 10., 192.168., 169.254.* — prevents internal network attacks
- Injection Detection: 17 heuristic patterns for Shell/SQL/NoSQL/Prompt injection
- Rate Limiting: Token bucket, per-tool configurable (default 60 req/min)
- Audit Logging: Structured JSON with rotation + gzip compression
Bonus Features
| Feature | Why it matters |
|---|---|
| Request cache | TTL+LRU for read-only tools — 0.01ms cache hits |
| Hot reload | kill -HUP <pid> — zero-downtime config changes |
| Multi-server routing | One proxy, multiple upstream servers |
| STDIO + HTTP | Works with any MCP client |
Quick Start
$ pnpm add -g mcp-slim-guard
# Auto-discover your MCP servers
cd your-project/
mcp-slim-guard init
# Dry-run to check for false positives
mcp-slim-guard validate
# Start the proxy (STDIO mode)
mcp-slim-guard start
Then update your AI agent's config to point to mcp-slim-guard instead of individual servers. That's it — you're now saving 83% on tool schema tokens with full security coverage.
Compare for Yourself
| Feature | mcp-slim-guard | slim-mcp | mcp-compressor | mcp-guardian |
|---|---|---|---|---|
| Schema compression | ✅ 5 levels, -86% | ✅ 5 levels, -77% | ✅ | ❌ |
| Accuracy validated | ✅ 180 API calls | ✅ 120 | ❌ | — |
| Request cache | ✅ TTL+LRU | ❌ | ❌ | ❌ |
| Tool allow/deny | ✅ Glob-based | ❌ | ❌ | ✅ |
| SSRF protection | ✅ IP + domain | ❌ | ❌ | ✅ |
| Injection detection | ✅ 17 patterns | ❌ | ❌ | ✅ |
| Rate limiting | ✅ Token bucket | ❌ | ❌ | ✅ |
| Audit log | ✅ JSON, rotation | ❌ | ❌ | ✅ |
| Hot reload | ✅ SIGHUP | ❌ | ❌ | ❌ |
| Multi-server routing | ✅ Prefix auto-route | ❌ | ❌ | ❌ |
| HTTP transport | ✅ Streamable HTTP | ❌ | ✅ | ❌ |
| Compression + Security | ✅ One proxy | ❌ Compression only | ❌ Compression only | ❌ Security only |
The Bottom Line
mcp-slim-guard is the only MCP proxy that combines schema compression with security. You don't need to choose between saving tokens and staying safe — you get both in one npm install.
- GitHub: lennney/mcp-slim-guard
- npm:
npm install -g mcp-slim-guard - 402 tests, 5 dependencies, MIT licensed