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 and can cover more than one repository. It follows
commits, not uncommitted edits: after committing, a re-index pass (lain reindex or a fresh session) brings it current — and answers that may miss
recent work say so instead of going quiet. 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
Or from npm (macOS, Linux, and x64 Windows):
npm install -g @spuentesp/lain-mcp
lain --version
Two npm notes:
npm install -gfails withEEXISTwhen an unrelatedlainfile already sits in npm's global bin directory — for example an older manual copy in~/.local/binwhen your npm prefix is~/.local. The npm error itself cannot tell the file is unrelated; move or remove it, or install under a different prefix:npm install -g --prefix ~/.local/lain-npm @spuentesp/lain-mcp(then add~/.local/lain-npm/binto yourPATH).- If npm install scripts were disabled (
--ignore-scripts, a locked-down CI), the platform binary is not fetched during install. The firstlainrun downloads and verifies it, so that first run needs network access; once cached, lain works offline.
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. Each adapter
writes the client's own configuration — claude mcp add for Claude
Code, ~/.codex/config.toml for Codex, ~/.cursor/mcp.json for
Cursor, .vscode/mcp.json (or the user-scoped mcp.json) for VS
Code, a block file under the repository's .continue/mcpServers/ for
Continue. generic writes this project-level .mcp.json:
{
"mcpServers": {
"lain": {
"command": "/home/you/.local/lain/lain",
"args": ["mcp"]
}
}
}
command is always the absolute path of the lain binary that ran
setup — not the bare name — so the agent runs the same binary no matter
which PATH it inherits. That path depends on the install: the release
installer puts the binary at ~/.local/lain/lain, while the npm
package registers the verified cache binary it launches
(~/.cache/lain/<version>/<target>/lain on Linux,
~/Library/Caches/lain/… on macOS, %LOCALAPPDATA%\lain\Cache\… on
Windows). Setup finishes by starting exactly that command once and
verifying the MCP handshake.
On a terminal, lain setup without --agent asks which client to
configure (Enter picks Claude Code), and without --lsp asks which
optional language servers to install (Enter installs none).
Non-interactive runs default to generic and no language servers, so
scripts should pass --agent and --lsp explicitly.
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
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
Project status
License
MIT — Copyright (c) 2026 spuentesp
