Lens

CI npm version npm downloads license: MIT

mcp-lens is an MCP (Model Context Protocol) proxy/router that sits in front of your fleet of downstream MCP servers and exposes only a handful of meta-tools to the host, instead of every downstream tool definition.

The problem

Power users of Claude Code / Claude Desktop / Cursor connect 10+ MCP servers. Each one dumps the full definition (name, description, JSON Schema) of every tool it has — often 100-200 tools total — straight into the model's context window, on every single turn. That's tokens (and money) spent before the model has even done anything.

How mcp-lens fixes it

Instead of connecting the host directly to N servers, you connect it to one mcp-lens process, and mcp-lens connects to the N servers on your behalf:

Host (Claude) ── mcp-lens ──┬── github MCP server
                             ├── slack MCP server
                             ├── filesystem MCP server
                             └── ... (N more)

The host only ever sees 4 small meta-tools:

When the model needs a capability, it calls search_tools to find the right downstream tool, then call_tool to actually run it — the full tool list is discovered on demand, never loaded up front.

Definition-size savings

Measured by scripts/benchmark.ts (npm run benchmark), against 100 fictitious downstream tools with realistic descriptions and JSON schemas, spread across 8 servers:

Run it yourself: npm run benchmark (accepts an optional tool-count argument, e.g. npm run benchmark -- 200).

Honest caveat: mcp-lens's own 4 meta-tool definitions have a small, fixed cost (~1.4 KB). For a single downstream server exposing only 1-2 trivial tools, that fixed cost can outweigh the savings — mcp-lens earns its keep once your fleet grows past roughly a handful of tools, and the benefit compounds from there (98%+ at 100 tools, as measured above). If you only ever connect one or two lightweight MCP servers, connecting them directly may be simpler.

Install for Claude Code / Claude Desktop

1. Create a fleet config (see examples/mcp-lens.config.json):

   {
     "servers": {
       "github": { "command": "npx", "args": ["-y", "@modelcontextprotocol/server-github"] },
       "filesystem": { "command": "npx", "args": ["-y", "@modelcontextprotocol/server-filesystem", "/path/to/dir"] }
     }
   }
   

   The shape matches the mcpServers object you already have in claude_desktop_config.json / .mcp.json — each entry is just { command, args?, env? }.

2. Point your MCP client at mcp-lens instead of at each server individually:

   {
     "mcpServers": {
       "lens": {
         "command": "npx",
         "args": ["-y", "@m8t-jacob/mcp-lens", "/absolute/path/to/mcp-lens.config.json"],
         "env": { "MCP_LENS_CONFIG": "/absolute/path/to/mcp-lens.config.json" }
       }
     }
   }
   

   The config path can be passed either as the first CLI argument or via MCP_LENS_CONFIG (the argument wins if both are set).

3. Move your existing per-server entries out of the host's own MCP config and into the mcp-lens fleet config instead — the host now only launches mcp-lens, which launches (and proxies to) the rest.

You can also run it directly to smoke-test it:

npx -y @m8t-jacob/mcp-lens ./mcp-lens.config.json

It logs a one-line summary of the definition-size savings to stderr on startup, then sits waiting for JSON-RPC requests on stdin — that's expected; it's meant to be driven by an MCP client, not used interactively.

Programmatic use

import { buildServer, ToolCatalog, ToolProxy, loadConfig } from '@m8t-jacob/mcp-lens';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';

const config = loadConfig('./mcp-lens.config.json');
const proxy = new ToolProxy(config.servers);
const catalog = new ToolCatalog();
await catalog.build(proxy.serverNames(), (name) => proxy.getClient(name));

const server = buildServer({ getCatalog: () => catalog.list(), proxy });
await server.connect(new StdioServerTransport());

searchTools (the ranking function behind search_tools) and ToolCatalog are also exported directly, e.g. for building your own UI over the fleet catalog. See examples/basic.ts.

Design notes

• Built on the official @modelcontextprotocol/sdk — McpServer + StdioServerTransport on the host-facing side, Client + StdioClientTransport on the downstream-facing side (mcp-lens is both an MCP server and an MCP client).
• Downstream connections are lazy and reused: a server is only spawned when its tools are first listed or called, and the same connection is reused afterwards. If a downstream call fails, that connection is dropped so the next call reconnects instead of repeatedly hitting a broken pipe.
• search_tools's ranking (src/search.ts) is a small TF-like text scorer behind a Scorer interface, so it can be swapped for an embeddings-based scorer later without touching any calling code.
• One broken downstream server doesn't take down the fleet: catalog building skips (and logs) any server that fails to connect or list its tools, the rest still work.
• Strict TypeScript, dual ESM + CJS builds with .d.ts, zero real network calls in the test suite (an in-process InMemoryTransport end-to-end test and a real-subprocess StdioClientTransport test cover the full proxy path instead).

🇵🇱 Po polsku

@m8t-jacob/mcp-lens to proxy/router MCP, który staje przed flotą serwerów MCP i eksponuje do hosta (Claude Desktop, Claude Code, Cursor) tylko garść meta-narzędzi (search_tools, describe_tool, call_tool, list_servers) zamiast definicji wszystkich narzędzi każdego serwera. Power-userzy podłączający 10+ serwerów MCP, z których każdy wrzuca do kontekstu modelu definicje 100-200 narzędzi, zyskują dzięki temu 80-95%+ oszczędności tokenów definicji (zmierzone: 98.3% redukcji przy 100 narzędziach) — model wyszukuje potrzebne narzędzie przez search_tools, a następnie wywołuje je przez call_tool, zamiast widzieć od razu całą listę. Konfiguracja floty downstreamowych serwerów ma taki sam kształt jak mcpServers w .mcp.json/claude_desktop_config.json.

Contributing

Contributions are welcome! See CONTRIBUTING.md for the development workflow and GOOD_FIRST_ISSUES.md for ideas if you're looking for a place to start. This project follows the Contributor Covenant.

License

MIT © 2026 Jakub Jagiełło