Rarefaction

Name: Rarefaction
Author: ip2k

by ip2kversion 0.4.2i★ –iRuns on your computer

Registry nameio.github.ip2k/rarefaction-lsp-mcp

Install

Overview

Description from the maker (original English)

LSP call graph for coding agents: a symbol's callers, callees, tests and live files in one call.

PopNx MCP’s three pillars

These three are never merged into one score: opinions, measurements and popularity are different things.

User voiceiNo reviews yetReviews open together with accounts in a later step
Our basic checksi1 of 1 passedChecked from public information; code not scanned yet
UsageiNot measured yet★ –

What you can do with it

We have not written a plain-language summary for this one yet. Below are the facts we read from the registry.

Connection: Runs on your computer
Transport: stdio
Category: Other

Install

Pick the AI app you use, then copy the settings.

Before you install

Basic checks: 1 of 1 passed
Runs on your computer with your user’s permissions (files, network, apps).
The registry lists no key to fill in.

You need

npmNode.js installed (for the npx command)

Which app do you use?

ClaudeClaude CodeCursorVS CodeChatGPT

Pick your app above. We remember it, so next time the button says “Install in …”.

Open Claude Desktop → Settings → Developer → Edit Config, and paste this into claude_desktop_config.json

{
  "mcpServers": {
    "rarefaction-lsp": {
      "command": "npx",
      "args": [
        "-y",
        "rarefaction-lsp-mcp"
      ]
    }
  }
}

Run this in your terminal

claude mcp add rarefaction-lsp -- npx -y rarefaction-lsp-mcp

Open in Cursor

Or copy the settings yourself

Paste into .cursor/mcp.json in your project, or ~/.cursor/mcp.json

{
  "mcpServers": {
    "rarefaction-lsp": {
      "command": "npx",
      "args": [
        "-y",
        "rarefaction-lsp-mcp"
      ]
    }
  }
}

Your browser asks to open the app, then the app shows the settings and asks you to confirm. Key values are left empty for you to fill in.

Open in VS Code

Or copy the settings yourself

Paste into .vscode/mcp.json in your project

{
  "servers": {
    "rarefaction-lsp": {
      "type": "stdio",
      "command": "npx",
      "args": [
        "-y",
        "rarefaction-lsp-mcp"
      ]
    }
  }
}

Your browser asks to open the app, then the app shows the settings and asks you to confirm. Key values are left empty for you to fill in.

This one runs on your computer, and ChatGPT can only connect to servers on the internet. Pick another app.

Your choice stays on this device in a cookie; no account needed.

These settings are generated from the maker’s registry entry. If the README says otherwise, follow the README.

What it can do

Runs on your computer; the tool list appears in your AI app after you install it.

Basic checksi

We check information anyone can see; we have not read the code. Checks that do not apply are left out of the count. · Checked Nov 7, 2026

✓ Pass
Maker matches the repo
The registry name was verified with the GitHub account ip2k, which owns the repo
… Not checked yet
Still maintained
We have not read this repo from GitHub yet
… Not checked yet
Has a licence
We have not read this repo from GitHub yet
– Not applicable
Endpoint responds
This one runs on your computer; there is no internet endpoint
– Not applicable
Sign-in with OAuth
Runs on your computer and uses local settings instead
… Not checked yet
No hidden instructions in tool descriptions
Runs on your computer; we can only read tool lists from internet endpoints

Compared with others in Otheri

More stars than 0% of the other 32678 servers in this category
Published to the registry more recently than 69% of the other 32678 servers in this category
Basic checks equal to or better than 100% of the other 32678 servers in this category

Compared with every active server in this category in the registry. Stars count only when the publisher owns the repo.

Common questions

Is Rarefaction safe?

It passed 1 of 1 applicable basic checks. We have not read the code. Before adding a key or token, see what access the server asks for and use a key with only the permissions you need.

What keys or codes does it need?

The registry does not list a required key, though some servers ask you to sign in on first use.

Does it run on my computer or in the cloud?

It runs on your computer. Your AI app starts it when needed, and data does not pass through the maker’s servers (unless it calls an online service itself).

Is it still maintained?

The GitHub repo last changed –, and the latest version (0.4.2) reached the registry 17 days ago.

Which apps can use it?

Apps that support MCP, such as Claude Desktop, Claude Code, Cursor and VS Code. You also need Node.js on your computer.

What can I use instead?

In the same category: Spec Driven Development, Remote Desktop Commander, Starreel

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README from the maker

Open the README

Copied from the maker’s repo as written. PopNx MCP has not reviewed or endorsed it. HTML, images and scripts are removed. Links lead off this site.

Rarefaction — LSP call graph MCP server for AI coding agents

Rarefaction is a Model Context Protocol (MCP) server that gives a coding agent the call graph and code context for one symbol straight from the project's language server (Language Server Protocol, LSP). Ask about a function, method or type and get, in one call: its signature and doc, its body, who calls it (tests marked), what it calls, who implements it, the tests that reach it and the short list of files a change can affect. The agent reads that instead of opening whole files and grepping, which keeps its context window small and spends fewer tokens on exploration.

Works with Claude Code, Codex and any MCP client over stdio. Profiles cover Swift, Rust, C/C++, TypeScript/JavaScript, Python and WebAssembly text (WAT).

Run against the bundled TypeScript fixture (fixtures/SampleTypeScript):

# scale — engine.ts:2:17

function scale(value: number): number

Double one input sample.

Callers (2, 1 in tests)

render — render.ts:2 (calls at 3)
· testScale — engine.test.ts:2 (calls at 3) [test]

Tests that reach it (1)

testScale — engine.test.ts:2

Run only these:

npm test -- "engine.test.ts"

Live files (3 of 3)

engine.test.ts
engine.ts
render.ts

_11 LSP requests, 6430 ms (includes language-server cold start); index on-open_

Once the language server has indexed, a call takes a fraction of a second (median 80 ms on ripgrep, 220 ms on the 800-file IronRDP workspace); the server's cold start, 6–16 s on those projects, is paid once per session.

What it helps with

Reducing token usage in AI coding agents. One bounded answer replaces several whole-file reads and searches. Measured on 50 real commits in ripgrep and IronRDP, the answer was a median 6–7% of the bytes of reading the files it names once. How much that saves in a whole agent session is not measured yet; see docs/measurement.md, including where the live set falls short.
Call graph and impact analysis. "What calls this function, and what would break if I change it?" By default two levels of callers and one of callees, plus implementations and type hierarchy, each bounded and marked when truncated.
Choosing which tests to run. Tests that reach the symbol, with a "run only these" command for the project's test runner.
Orientation in an unfamiliar codebase. Bare names, qualified names or exact path:line:column positions; ambiguity comes back as a list of exact positions rather than a guess.
Honest answers about coverage. Stale indexes, servers still indexing and servers without call hierarchy are reported in the answer, not hidden.

Why an MCP server for LSP call graphs

Language servers already know the call graph precisely — that is what "find references" and "call hierarchy" are. Editors and LSP tools expose them one hop at a time, so an agent pays a tool call and a round of context per hop. An agent needs the *closure*: everything within a couple of hops, filtered to the project, with the tests picked out, small enough to read in one go. That composition is what this server does. It is deliberately language-agnostic: the core speaks LSP and nothing else, and everything a language needs is a small profile.

How it relates to nearby tools:

approach	what the agent gets
reading files and grep	whole files; text matches, not calls
per-request LSP tools (definition, references, call hierarchy)	precise answers, one hop per tool call
Serena	symbol navigation and semantic editing over LSP; works well alongside this
Rarefaction	the bounded multi-hop call graph, tests and live files for one symbol in one call

Install

Requires Node 22.22.2 or later; used on macOS and Linux (CI runs on Ubuntu), untested on Windows. The TypeScript and Python language servers are bundled. Install the others you need and put them on the PATH your client launches with — see Languages and configuration. Swift needs macOS with Xcode (or a Swift toolchain with sourcekit-lsp).

As a plugin (Claude Code and Codex)

The plugin bundles the server, launched with npx -y rarefaction-lsp-mcp, and an orient-first skill that tells the agent to call status and orient before reading whole files, and scope before editing.

# Claude Code

claude plugin install rarefaction --marketplace ip2k/rarefaction

Codex

codex plugin marketplace add ip2k/rarefaction codex plugin add rarefaction@rarefaction

In a Claude Code session, /plugin install rarefaction --marketplace ip2k/rarefaction does the same. Restart or reconnect the session afterwards.

As an MCP server

claude mcp add --scope user --transport stdio rarefaction -- npx -y rarefaction-lsp-mcp

codex mcp add rarefaction -- npx -y rarefaction-lsp-mcp

Add --env PATH="$HOME/.local/bin:$HOME/.cargo/bin:$PATH" before the -- if rust-analyzer or wat_server live outside the client's PATH. The tools are status, orient and scope (in Claude Code, mcp__rarefaction__status, mcp__rarefaction__orient and mcp__rarefaction__scope); all three are read-only, so you can allow them in your permissions to avoid repeated prompts. For Codex, give the first index time in ~/.codex/config.toml:

[mcp_servers.rarefaction]

startup_timeout_sec = 20 tool_timeout_sec = 180

The server takes --root and --profile . Explicit --root takes precedence over Claude Code's CLAUDE_PROJECT_DIR, then the working directory. Leave both unset in a user-wide registration so every project can use all supported profiles. See the MCP documentation for Claude Code and Codex.

From a clone

To run your own changes, build and register the compiled entry instead:

git clone https://github.com/ip2k/rarefaction.git

cd rarefaction npm ci && npm run build claude mcp add --scope user --transport stdio rarefaction -- "$(command -v node)" "$PWD/dist/index.js"

Rebuild after changing the server, then restart or reconnect (/mcp) the client session.

Check it works

Start a new chat in a project and ask the agent to call status, then orient on a symbol you know. status.root should match the project and the selected entry in status.profiles should have ok: true. One registration serves all supported languages; each language server starts lazily.

Agent guidance

The plugin's skill covers this. Without the plugin, add this to ~/.claude/CLAUDE.md, AGENTS.md or a project instruction file:

Use rarefaction's status tool once in a new project, then orient on the

relevant symbol before reading whole files, and scope on a symbol before

editing it (advisory: also look outside the files it lists). In mixed

repositories pass profile or a workspace-relative path:line:column, and

pass project for a nested package that is not a workspace member. Start

with the default graph bounds; expand only when necessary. Follow freshness

and coverage advice, and inspect FFI/generated bindings separately when

relevant.

Xcode projects

Xcode projects need a build-server config so sourcekit-lsp gets compiler flags (SwiftPM packages need nothing):

brew install xcode-build-server

cd /path/to/App && xcode-build-server config -project App.xcodeproj -scheme App echo buildServer.json >> .gitignore

Then build once in Xcode or with xcodebuild; the index is only as fresh as the last build, and orient says so when a source file is newer. Saved source edits are sent to open LSP documents before later queries, but Xcode projects still need a new build to refresh their cross-file index.

Tools

tool	what
`orient`	`{symbol, profile?, project?, up?, down?, max_nodes?, include_body?, max_body_lines?}` — the bundle above. `symbol` may be `updateLocation`, `Engine.updateLocation`, or `path/File.swift:204:10`. Ambiguous names come back as an error listing exact positions.
`scope`	`{symbol, profile?, project?, up?, down?, max_nodes?}` — the working set for a change to that symbol: the call-graph files from `orient` plus files that have historically changed in the same commits as the symbol's file or the call-graph files (from git history), each with its reason. Advisory: it lists what to read and check, not what is safe to ignore (measured coverage).
`status`	`{profile?, project?}` — what the server can see: the profile inventory and which one is selected, prerequisites such as a build-server config, index freshness, and which LSP features the server offers. Call this first if `orient` finds nothing.

Planned: impacted_tests (the upward walk restricted to tests, as a runnable selection).

Languages and configuration

Profile	Language server	Prerequisites / scope
`swift`	sourcekit-lsp	Xcode or a Swift toolchain; SwiftPM, or Xcode with a build-server config
`rust`	rust-analyzer	Cargo.toml or rust-project.json; embedded/no_std/Wasm target settings supported; a project that pins a toolchain needs `rustup component add rust-analyzer --toolchain` (`status` says so)
`c`	clangd	C/C++; compile_commands.json or compile_flags.txt for accurate include paths and defines
`typescript`	typescript-language-server	TypeScript/JavaScript, TSX/JSX; server and TypeScript bundled as dependencies
`python`	Pyright	Python and stubs; bundled; configure interpreter/import paths for your environment
`wasm`	wat_server	WebAssembly `.wat` text; install the prebuilt server on PATH

The server probes each language server's capabilities. When call hierarchy is absent, orient reports references and enclosing symbols, without claiming those references are calls. Native Rust/C sources compiled to WebAssembly use rust/c; .wasm binaries need disassembly to .wat. WAST test scripts, vendor assembly, indirect runtime dispatch and cross-language FFI edges are not indexed as a unified graph.

Optional .rarefaction.json in the workspace root configures each profile. Paths in project, profile root, and relative position queries are relative to that workspace. A position query without project is answered from the outermost Cargo w

Only the beginning is shown; read the rest in the repo

Rarefaction

Overview

PopNx MCP’s three pillars

What you can do with it

Install

Before you install

What it can do

Basic checksi

Maker matches the repo

Still maintained

Has a licence

Endpoint responds

Sign-in with OAuth

No hidden instructions in tool descriptions

Compared with others in Otheri

Common questions

Alternatives

Spec Driven Development

Remote Desktop Commander

Starreel

README from the maker

Rarefaction — LSP call graph MCP server for AI coding agents

Callers (2, 1 in tests)

Tests that reach it (1)

Live files (3 of 3)

What it helps with

Why an MCP server for LSP call graphs

Install

As a plugin (Claude Code and Codex)

Codex

As an MCP server

From a clone

Check it works

Agent guidance

Xcode projects

Tools

Languages and configuration