LAIN-mcp

LAIN builds a map of how all the code in your project connects — what calls what, what depends on what, which files tend to change together. Then it lets your AI coding assistant ask questions about that map. So instead of the AI just looking at one file and guessing, it can ask "if I change this function, what else breaks?" and get a real answer. It plugs into any AI agent that supports MCP and runs in the background while you work.

See it run

Download MP4 · Download WebM

• Federation overview, repo health, and the call graph — answered in well under a second.
• Edit repos.yaml from the Repos tab; the server hot-reloads without dropping a request.
• Try any MCP tool straight from the Tools tab; Copy as cURL hands the agent a shareable snippet.

[!NOTE] The hero GIF is large (~4 MB) so it autoplays inline on GitHub. For sharper playback, the MP4 and WebM siblings sit alongside it in docs/screenshots/.

How it fits together

flowchart LR
    A["AI Agent<br/>(Claude Code / Kimi / Agy / Codex)"] -->|MCP<br/>JSON-RPC| L["lain"]
    L -->|reads| FS[".lain/<br/>graph.bin"]
    L -->|runs| ENG["LSP / NLP / git<br/>engines"]
    L -->|answers| T["MCP tools<br/>(get_blast_radius,<br/>explain_symbol, …)"]
    A --> T

lain is a long-running MCP server that indexes your code once and keeps it fresh while you work. The agent speaks MCP (JSON-RPC over stdio or HTTP); the server answers structural questions across one repo (lain mcp) or many repos (lain server --config repos.yaml).

Documentation

TL;DR — install in 30 seconds

# Install (interactive — adds `lain` to PATH)
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | bash

# Reload your shell, then verify
source ~/.zshrc   # or ~/.bashrc
lain --version

See QUICKSTART.md for the full install matrix (Homebrew, build-from-source, non-interactive flags, ONNX model).

What is Lain?

Lain is a persistent code-intelligence MCP server. The headline is lain server: a long-running process that reads a repos.yaml config, indexes every registered repository (locally, by clone, or by shallow fetch), and answers structural questions across them through MCP tools. The server also serves a Command Center dashboard at GET / for humans who want to inspect the federation, edit the config, run queries, and exercise the MCP tool surface directly.

The value over LSP-only or RAG-based approaches is cross-file structural reasoning: agents can ask about blast radius, transitive dependency traces, anchor identification, co-change correlation, and contextual build failure decoration, so they reason about callers rather than just the failing line. Written in Rust, persists across sessions, and hot-reloads its repos.yaml / workspaces.yaml config without a restart.

The commands

After install, lain exposes these subcommands:

The cut surface (agents, hook, projects, top-level use) is gone — those concerns are reached through the commands above. server plus the two config CLIs (workspaces, repos) cover everything the prior surface did, scoped to a single project directory that owns a repos.yaml.

This table is checked against lain --help by tests/cli_surface.rs, so it cannot drift from the binary again.

Quick Start

1. Install — see QUICKSTART.md § Install.
2. Configure — see QUICKSTART.md § Federation (multi-repo).
3. Wire your agent — see QUICKSTART.md § Single-repo (recommended default).

Command Center

For a narrated tour of every tab, see command-center.md § Tour.

When lain server runs with --transport http, it serves the Command Center dashboard at GET /. It's a self-contained vanilla-JS SPA that talks back to the running server over the same JSON-RPC endpoint the MCP tools use. No separate API, no auth portal.

Tabs:

• Overview — get_health + get_federation_health in one view.
• Graph — D3 force-directed graph of the active workspace.
• Repos — per-repo table (id, path, health, node/edge counts).
• Query — runs query_graph against the federation.
• Tools — auto-generated MCP tool tester. Calls tools/list, then renders a form per tool by introspecting its inputSchema. Copy as cURL copies a curl -X POST http://localhost:9999/mcp ... snippet to the clipboard.

The status bar in the footer polls every 2 s for get_server_status and get_reload_status so hand-edits to repos.yaml / workspaces.yaml show up live.

See docs/command-center.md for the full walkthrough.

Hot Reload

lain server watches repos.yaml and workspaces.yaml and rebuilds its federation state when they change — no restart needed. Both the notify watcher (for hand-edits) and the CLI (via lain repos add or lain workspaces create) trigger the same ReloadBus.

When you run lain repos add my-repo …, the CLI writes the YAML atomically (write to temp file, then rename), then signals the running server over a Unix socket at ~/.local/lain/run/<repos-stem>.sock. The server's rebuild task diffs the new file against the live federation and applies add / remove operations against FederatedIndex. get_reload_status reports the state (idle / rebuilding / failed); the Command Center status bar shows it live.

See docs/hot-reload.md for the full picture (internals, observability, failure modes, caveats).

Federation mode