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mcp server

mcpslim

Transparent MCP proxy that compresses tool outputs. Cuts Claude Code / MCP token usage 12–60% lossless, up to 90% aggressive.

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Use tool

Set up

Runs on your computer. Pick your agent and operating system.

Before you start

  • Node.js 20 or newer
~/Library/Application Support/Claude/claude_desktop_config.json
{
  "mcpServers": {
    "mcpslim": {
      "command": "npx",
      "args": [
        "-y",
        "mcpslim"
      ]
    }
  }
}

same format Cursor, GitHub Copilot CLI, Junie, JetBrains AI Assistant, Augment (Auggie CLI), Factory Droid, Kiro, Devin CLI +12

Values in angle brackets are placeholders. Restart Claude Desktop after editing its config.Claude Desktop docs

README

synced Oct 21, 2026

mcpslim

License: Apache--2.0 Stars

Cut Claude Code / Cursor token usage on the outputs other tools can't touch.

RTK compresses bash command output (git status, npm test, cat). That's great — but it can't see two of the biggest token drains in modern agent workflows:

  1. MCP tool responses — database dumps, API JSON, browser tools, search results
  2. Bloated / pretty-printed JSON that ships indented straight into your context

mcpslim is a transparent MCP proxy that sits between your agent and your MCP servers and compresses tool results on the way back. It complements RTK — run both.

Honest numbers (measured on real captured MCP payloads, not estimates)

PayloadSafe mode (default, lossless-leaning)Aggressive mode (browse/list tools)
A real 52 KB video-API list response−12%−90%
A 516 KB pretty-printed JSON file−59%−99.8%

We do not claim "90% on everything." That's only achievable by truncating data — which silently breaks any workflow that needs the full payload (e.g. "download all 50 results"). Here's the real model:

  • Safe mode (default): minify JSON, drop empty/null fields, strip base64 blobs, truncate giant strings. Fully reversible-aware, never drops list items. Typical 12–60% with zero risk.
  • Aggressive mode (opt-in per tool): also truncates long arrays. Up to 90% — only enable it on list/search/browse tools where you're scanning, not consuming.

You pick per-tool with a one-line profile. Safe is the default precisely so it never breaks you.

How it works

Claude Code  ──JSON-RPC──▶  mcpslim-proxy  ──JSON-RPC──▶  real MCP server
             ◀─compressed──                ◀──full────────
  • Forwards every request byte-for-byte (requests are tiny — we never touch them).
  • On tools/call results, compresses content[].text.
  • Fail-open: anything it can't parse is forwarded verbatim. It never makes a payload bigger.
  • initialize, tools/list, errors, notifications — all pass through untouched.
  • Newline-delimited JSON-RPC stdio transport.

Install

git clone https://github.com/suncal/mcpslim.git
cd mcpslim
npm link        # exposes `mcpslim-proxy` and `mcpslim` on your PATH

Not yet on the npm registry — install from source for now. npm i -g mcpslim is coming once it's published.

Quick sanity check (compress a captured payload):

mcpslim compress some-tool-output.json            # safe mode
mcpslim compress some-tool-output.json --aggressive

Use — wrap an existing MCP server

In your .mcp.json / Claude Code MCP config, change the server's launch command to go through the proxy:

{
  "mcpServers": {
    "higgsfield": {
      "command": "mcpslim-proxy",
      "args": [
        "--mode", "safe",
        "--profile", "profiles/higgsfield.json",
        "--",
        "node", "/path/to/original-server.js"   // original command + args
      ]
    }
  }
}

That's it. The server behaves identically; outputs are just smaller.

Per-tool profile (profiles/higgsfield.json)

{
  "tools": {
    "show_generations": "aggressive",
    "default": "safe"
  }
}

list/browse tools → aggressive; everything else stays safe.

Modes

ModeStrategiesUse for
safe (default)minify, drop empties, strip blobs, truncate huge stringseverything — zero risk
aggressive+ truncate long arrays to first Nlist/search/browse tools only
offpassthroughdebugging

What it does NOT do (so you're not surprised)

  • It can't compress images/screenshots as text (those are vision tokens). Screenshot downscaling is on the roadmap as a separate pass.
  • It can't touch your agent's native Read/file tool (not an MCP call). Use RTK's cat wrapper for that.
  • It won't recover tokens already spent earlier in the conversation.

Roadmap

  • Screenshot/image downscaling pass for browser & computer-use MCP tools
  • Auto-profiling: learn per-tool safe truncation from observed usage
  • Savings dashboard (tokens saved per server/tool/day)
  • Hosted proxy + team config sync (paid tier)

License

Apache-2.0. Independent project; concept inspired by RTK (also Apache-2.0). No RTK code is used.

If this is useful to you, a ⭐ on the repo helps other people find it. Issues and pull requests are welcome.

Built by Priyankar "Sunny" Chakraborty · everbuiltstudio.com

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