Rarefaction MCP server

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_

The walk itself takes tens of milliseconds; the language server's cold start is paid once per session.

What it helps with

several whole-file reads and searches. In a measured spot check the answer was 3.4% of the bytes of reading its live files once; the broader task-level measurement is still to be done (see docs/measurement.md).

• Reducing token usage in AI coding agents. One bounded answer replaces

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.

• Call graph and impact analysis. "What calls this function, and what

only these" command for the project's test runner.

• Choosing which tests to run. Tests that reach the symbol, with a "run

exact path:line:column positions; ambiguity comes back as a list of exact positions rather than a guess.

• Orientation in an unfamiliar codebase. Bare names, qualified names or

and servers without call hierarchy are reported in the answer, not hidden.

• Honest answers about coverage. Stale indexes, servers still indexing

Why an MCP server for LSP call graphs

Shortened. The full README is on GitHub.