LAIN-mcp

Lain

A code graph for coding agents. Lain maps callers, dependencies, symbols, and Git co-changes, then exposes them through the Model Context Protocol (MCP). Before an agent edits a function, it can inspect what depends on it and how the rest of the code reaches it.

The demo traces a symbol across the bytes and tokio repositories. Agents query the same graph through MCP; the browser shows what Lain indexed and where an answer came from.

Ask your agent:

If I change validate_token, what breaks?
Show me the call chain from entry to helper_a.
Which files usually change with auth.rs?
Is another agent already editing this code?

Lain answers with symbol names and source paths from a local index. The index stays on disk between runs, updates when the code changes, and can cover more than one repository. Semantic search is optional; graph queries work without an embedding model.

Install

Using the release installer:

curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | bash
source ~/.zshrc   # or ~/.bashrc
lain --version

Rust 1.89 or newer is only needed when building from source. The quickstart covers non-interactive installation and the optional local embedding model.

Connect an agent

Run one setup command from the repository you want Lain to index:

lain setup --agent claude-code
lain setup --agent codex
lain setup --agent cursor
lain setup --agent vscode
lain setup --agent continue

Use lain setup --agent generic for another MCP host. It writes this project-level .mcp.json, with the absolute path of the lain binary that ran setup as command:

{
  "mcpServers": {
    "lain": {
      "command": "/home/you/.local/bin/lain",
      "args": ["mcp"]
    }
  }
}

Restart the agent after setup. Lain walks up from the agent's working directory to find .git, builds .lain/graph.bin, and serves the repository over stdio. No repos.yaml is needed for one repository.

Languages

Parsers for Rust, Python, TypeScript, JavaScript, Go, Java, C, C++, C#, Ruby, Swift, Kotlin, Scala and PHP are built into the binary, as are the <script> blocks of Vue and Svelte components. Every one gets definitions and a call graph with nothing else installed.

Language servers are optional and only add precision. lain setup lists the languages it found and asks which servers to install; pressing Enter installs none. Non-interactive runs install none unless you name them:

lain setup --lsp python,go      # just these
lain setup --lsp detected       # every missing one for this repository
lain setup --lsp none

First query

Ask the connected agent:

Use Lain to show the blast radius of validate_token.

Or call the same tool from a terminal:

lain oneshot get_blast_radius validate_token

Use a symbol from your repository in place of validate_token. If the graph isn't ready or an answer looks stale, run lain doctor.

Multiple repositories

lain repos add bytes https://github.com/tokio-rs/bytes.git
lain repos add tokio https://github.com/tokio-rs/tokio.git
lain workspaces create tokio-stack --members bytes,tokio
lain server --config ./repos.yaml --transport http --port 9999

Open http://localhost:9999 for the Command Center. The server listens on localhost only; set LAIN_BIND_ADDR=0.0.0.0 (with LAIN_API_KEYS=key1,…) to expose it on the network. Federation adds organization-wide search and cross-repository blast-radius queries. The federation guide covers configuration, source types, and failure states. After a federation schema upgrade, run lain reindex to rebuild the on-disk graph from the sources.

Check the graph

From a source checkout:

scripts/demo.sh --quick

The script creates a small repository with a known call graph, starts a real Lain server, and compares the answers with that graph. It exits non-zero when a check fails. A full run also records timings against this repository.

What Lain provides

Need	Tool or feature
See callers before changing a symbol	get_blast_radius, assess_change
Trace how two functions connect	get_call_chain
Find architectural entry points	find_anchors, list_entry_points
Search by name or meaning	find_symbol, search_code
Follow a symbol across repositories	get_cross_repo_blast_radius, search_org
Keep agents from editing the same code blindly	intents, presence, and advisory file claims

Static analysis can't prove every dynamic call through a message bus, dependency injection container, or reflective router. explain_dispatch combines static callers with convention matches, runtime traces, and Git co-change evidence; an empty result becomes insufficient_evidence, not a claim that the edit is safe.

Documentation

Start here	Covers
Quickstart	Installation, first query, and first aid
User manual	CLI reference, tuning, operation, and troubleshooting
Tool guide	MCP tools, arguments, outputs, and limits
Skill bundle	Agent-loadable skills and role recipes
Federation	Multi-repository configuration and queries
Multiplayer	Agent presence, intents, claims, and hooks
Architecture	Graph construction and design choices
Documentation index	Every maintained document

Project status

License

MIT — Copyright (c) 2026 spuentesp